Upper limb muscle strength and knee frontal plane projection angle asymmetries in female water-polo players

Water-polo players frequently perform overhead throws that could result in shoulder imbalances. For overhead throws, execution of the 'eggbeater kick' (cyclical movement of the legs) is required to lift the body out of the water. Although a symmetrical action, inter-limb differences in task execution could lead to knee frontal plane projection (FPPA) differences. The present study examined imbalances shoulder and knee FPPA in female players. Eighteen competitive female field players (24.1 ± 5.5 years, 1.68 ± 0.06 m, 72.9 ± 13.3 kg) had their shoulder strength assessed in a shot-mimicking position with a portable dynamometer, standing and seated (isolating the shoulder contribution). Anterior: posterior and shooting: non-shooting shoulder comparison were made. Additionally, players performed a drop jump. Knee FPPA was recorded from digitising and comparing the frames just before landing and at stance phase. During standing, players exhibited higher shooting: non-shooting asymmetry (p = 0.032) in the anterior contraction direction, while during seated the shooting shoulder anterior: posterior asymmetry was higher (p = 0.032). Interlimb knee FPPA asymmetry was higher in the stance phase (p = 0.02). Despite the overhead throwing and egg-beater demands impacting differently on each limb, considerable asymmetries do not develop, suggesting the overall training requirements (e.g. swimming, resistance training) were sufficient to maintain the asymmetry within desirable limits.

A Single Bout of On-Ice Training Leads to Increased Interlimb Asymmetry in Competitive Youth Hockey Athletes

Interlimb asymmetry (ILA) refers to an anatomical or physiological imbalance between contralateral limbs, which can influence neuromuscular function. Investigating the influence of neuromuscular fatigue on ILA may be critical for optimizing training programs, injury rehabilitation, and sport-specific performance. The purpose of this study was to determine if a single bout of ice hockey-specific exercise creates or exacerbates lower-limb ILA. Before and after an on-ice training session, 33 youth ice-hockey athletes (14.9 [1.7] y; 11 females) performed 3 repetitions of a maximal vertical countermovement jump (CMJ), an eccentric hamstring contraction, and maximal isometric hip adduction and abduction contractions. Force- and power-related variables were analyzed to determine limb-specific neuromuscular function. The on-ice session reduced maximal isometric hip adduction (left: 7.3% [10.3%]; right: 9.5% [9.6%]) and abduction (left: 4.9% [6.9%]; right: 5.0% [8.1%]) force, but did not impair (P ≥ .10) CMJ performance (jump height, relative peak power, braking duration, and total duration). After the on-ice session, ILA was greater for CMJ propulsive impulse (6.3% [2.9%] vs 5.1% [2.6%]), CMJ braking rate of force development (19.3% [7.6%] vs 15.2% [6.4%]), and peak isometric hip adduction force (6.7% [5.5%] vs 6.1% [4.1%]). In conclusion, hockey-specific exercise leads to increased ILA for multiple force-related metrics, which may be a compensatory mechanism to maintain bilateral task performance when fatigued.

Inter-limb differences in unilateral countermovement jump height are not associated with the inter-limb differences in bilateral countermovement jump force production

This study aimed to explore the relationship between the inter-limb differences in unilateral countermovement (CMJ) height and the inter-limb differences in bilateral CMJ force production, and to elucidate whether the self-reported preferred leg contributes more to force production than the non-preferred leg. Twenty-three senior basketball players performed in a single session eight unilateral CMJs (four with each leg) and four bilateral CMJs. Impulse, peak force, mean force were recorded during the bilateral CMJ, and jump height during the unilateral CMJ. Small correlations were observed between the inter-limb differences in unilateral CMJ height and the inter-limb asymmetries in bilateral CMJ impulse, peak force, and mean force (p ≥ 0.171; r≤-0.142). The self-reported preferred leg revealed a higher performance in 7 out of 23 participants (Kappa = -0.20) for the unilateral CMJ height, 7 out of 23 participants (Kappa = -0.11) for the bilateral CMJ impulse, 6 out of 23 participants (Kappa = -0.36) for the bilateral CMJ peak force, and 8 out of 23 participants (Kappa = -0.34) for the bilateral CMJ mean force. These results highlight that the asymmetries detected during bilateral CMJs cannot be extrapolated to unilateral CMJs, and that the preferred leg generally contributes less to force production than the non-preferred leg during both bilateral and unilateral CMJs.

Knee muscle strength, body composition, and balance performance of youth soccer players

BACKGROUND

Muscle strength and balance abilities have been related to game performance injury prevention. This study aimed to (1)  assess knee muscle strength performance based on intra- and inter-limb asymmetries, (2)  investigate the relationships between knee muscle strength, body composition, and balance, and (3) analyze the variance in the overall stability index (OSI) explained by knee muscle strength indicators after controlling for age and body composition.

METHODS

The sample included 104 young male soccer players (16.0 ± 1.6 years). Body composition, knee muscle strength, and balance ability were assessed. A paired samples t-test was used to examine differences between the preferred leg (PL) and the non-preferred leg (NPL). Pearson correlations and hierarchical multiple regression were conducted to explore the relationships between strength, body composition, and balance.

RESULTS

No significant inter-limb asymmetries were detected for the knee flexors (KF) and knee extensors (KE) performance. The hamstrings-to-quadriceps ratio does not suggest the existence of intra-limb asymmetries (p = 0.06). Significant and negative correlations were found between KF (r = -0.38, p ≤ 0.01) and KE (r = -0.58, p ≤ 0.01) for the PL. Concerning the NPL, KE also revealed a significant and negative relationship with balance (r = -0.30, p ≤ 0.01). Significant and positive relationships were found between bodyweight, fat-free mass, and balance. After controlling for age and body composition, knee muscle strength could explain between 13% and 30% of the variance observed in the OSI (NPL and PL, respectively).

CONCLUSIONS

The results underline knee muscle strength as an important predictor of balance. Thus, training programs targeting knee muscle strength development should be considered during the season periodization by coaches and their staff throughout the players' developmental stages.

Normal hip strength and range of motion values in youth and adult female national football teams: Data from 504 assessments

OBJECTIVES

To determine normal hip adduction- and abduction strength and range of motion (ROM) values for youth and adult female national team football players, and evaluate if increasing age, playing position and leg dominance were associated with these strength and ROM values.

DESIGN

Cohort study.

SETTING

National football center.

PARTICIPANTS

344 unique asymptomatic female football players.

MAIN OUTCOME MEASURES

Hip internal/external rotation (°), Bent Knee Fall Out test (cm), hip adduction/abduction strength(N) and ratio, and normalised hip adduction/abduction torque (Nm/kg).

RESULTS

A total of 504 assessments were performed. A total of 107 players underwent two (n = 67), three (n = 27) or four (n = 13) assessments. Mean peak hip adduction strength was 39% greater in 20 + Y old players 170 (±53 N) than in 13Y old players 122 (±28 N). Normalised hip adduction torque was 9% greater: 2.5 (±0.8Nm/kg) versus 2.3 (±0.5Nm/kg). A positive association between age and all strength measurements was found, while a negative association between age and hip external rotation and total hip rotation was found. No clinically relevant differences were found for the associations between playing position, leg dominance and hip strength- and ROM values.

CONCLUSION

Normal values for hip strength and range of motion in youth and adult female national football players are presented that can be used as clinical reference values.

Relationships among countermovement vertical jump performance metrics, strategy variables, and inter-limb asymmetry in females

Dependent variables commonly studied during countermovement vertical jump (CMVJ) tests largely stem from male-only studies despite females'distinct energy storage and reutilisation strategies. This could limit progress among females seeking increased CMVJ performance through targeted changes in certain variables. We explored relationships between CMVJ performance metrics (jump height, modified reactive strength index, jump power, and takeoff momentum) and (a) temporal and force application variables and (b) inter-limb force and yank (i.e., rate of force development) asymmetry in 31 recreationally active females. Participants performed eight CMVJs while ground reaction force (GRF) data were obtained. Pearson product-moment correlation coefficients assessed the strength and direction of the associations. Twenty-six significant relationships (r ≥ ±0.357; p < 0.05) were detected across the CMVJ performance variables. The significantly correlated variables were generally isolated to only one of the four performance metrics. Only the percentage of concentric phase inter-limb force asymmetry was significantly associated with CMVJ performance, specifically jump power and takeoff momentum. Coaches and physical performance professionals should be aware of popular strategy variables' association or lack of association with commonly studied performance metrics when seeking to understand or improve specific CMVJ jumping abilities in females.

Relationship between Interlimb Asymmetries and Performance Variables in Adolescent Tennis Players

Tennis is an individual sport characterised by high-intensity action, including sprints and changes of direction (COD). However, there is a gap in the knowledge of limb asymmetries in adolescent tennis players and their association with performance. The aim of this study was to investigate the relationship between asymmetry in vertical and horizontal jump tests (CMJ and HJ) and COD with lower limb performance variables in adolescent tennis players. Forty-five adolescent tennis players (age: 13.81 ± 1.08 years; height: 167.64 ± 9.9 cm; body mass: 57.48 ± 10.94 kg; body mass index: 20.27 ± 2.18 kg/m2) performed the CMJ test (unilateral and bilateral), horizontal jumps (unilateral and bilateral), 25 m sprint, and 180° COD test. The single-leg countermovement jump showed the greatest asymmetries among the different tests (6.62 ± 9.35%). Notable negative relationships were found between CMJ asymmetry and COD asymmetry with unilateral horizontal jump variables (r = -0.30 to -0.53). In addition, CMJ asymmetry showed notable relationships with CMJR (r = 0.49) and COD180R (r = 0.29), whereas COD asymmetry showed a notable relationship with COD180L (r = 0.40). On the other hand, HJ asymmetry showed no notable relationships with any variable. The main findings of this study indicate that greater imbalances in the CMJ and COD tests are associated with a reduced capacity to perform unilateral horizontal jumps. These results suggest the incorporation of training protocols to reduce interlimb asymmetries in growing athletes to improve their performance. This practical application is important for coaches and physical trainers who aim to improve the physical performance of adolescent tennis players. Greater asymmetry results in a reduced ability to produce unilateral horizontal force.

Inclusion of Game-Based Stimulus During Flywheel Resistance Training Positively Influences Physical Performance in Handball Players

Handball is a body-contact Olympic ball sport that is characterized by fast-paced defensive and offensive actions. Players must coordinate explosive movements (e.g. changing of direction) and handball-specific skills (e.g. passing). Maximizing performance requires a systematic approach to training that includes physical, psychological, technical, and tactical preparation. Purpose: The aim of this study is to determine the effects of movement-based (MOV; unspecific sport stimulus) or game-based (GAM; sport-specific stimulus) flywheel resistance training intervention in highly trained youth handball players. Method:Twenty-five highly trained youth male handball players completed two sessions per week of flywheel resistance training (MOV, n = 12; GAM, n = 13) over the 7-week intervention period. Change-of-direction tests (180º change-of-direction speed test of both legs and test) and handball-throwing test were conducted before and after the intervention. Results: Both groups significantly improved V-cut, and 180º Change-of-direction speed test performance (p < .05; d = 0.79-2.05). Notwithstanding, the GAM group demonstrated greater improvements in V-cut and COD180ASY compared with the MOV group (p < .05) with small effect. Handball throwing speed performance remained unchanged independently of training condition (p > .05). Conclusions: These findings provide further support for the training principle of "specificity" and highlight the importance of including a game-based training stimulus during resistance training. This is a key consideration for coaches wanting to enhance physical performance in youth handball players.

Magnitude and direction of shoulder torque asymmetries between different angular velocities in competitive swimmers

Asymmetries in swimming can be the result of poor technique or coordination between limbs, reducing the ability to produce propulsive force and increasing resistive drag. Therefore, this study aimed to compare the magnitude and determine the consistency of isokinetic peak torque asymmetries between the angular velocities of in the shoulder joint movements of internal and external rotation, flexion, and extension. Twenty-one competitive swimmers performed concentric actions at 60°/s (3 repetitions) and 180°/s (20 repetitions) in the movements of internal and external rotation, flexion, and extension of the shoulders using an isokinetic dynamometer, with the peak torque and asymmetry index being common metrics across the tests. The results showed a greater magnitude of asymmetry in internal rotation (16.86 vs. 9.86; p = 0.007) and flexion (12.06 vs. 7.35; p = 0.008) at 60 vs. 180°/s, respectively. The agreement levels of the direction of asymmetries between angular velocities were fair to substantial (Kappa: 0.40 to 0.69). Evaluating isokinetic torque in different movements and angular velocities resulted in different levels of asymmetry. Muscle force asymmetries can impact propulsion efficiency and movement coordination during swimming. Understanding muscle asymmetries allows the development of targeted and individualised training programmes to correct strength imbalances.

Things to keep in mind when selecting physical assessments in youth soccer: Correlations between test performances, interlimb asymmetries, and effects of maturation

A range of field-based tests have been proposed for inclusion in physical performance assessment batteries. However, there are obvious time and human resources constraints in applied settings. The knowledge of potential relationships between tests on performance, asymmetries, as well as maturation-induced changes, may help select the most informative and least time-consuming testing battery. The purposes of this study were: (1) to determine correlations in performance between different field-based tests, in interlimb asymmetry between those unilateral tests, and between asymmetry scores and test performances, and (2) to determine the influence of players' maturity status on test performances and asymmetries. A total of 309 male youth soccer players completed a nine-test battery including y-balance test, drop vertical jump, countermovement jump, single leg countermovement jump, standing long jump, single leg hop for distance, Illinois agility test, 10 m sprint, and 20 m sprint. The results revealed moderate-to-very large relationships between jump, sprint, and agility performances (r = 0.43-0.94), but weak-to-moderate correlations between these tests and balance scores (r ≤ 0.38). No relevant relationship (r ≤ 0.32) for asymmetries detected through different unilateral tests was found, nor between asymmetries and performance scores (r < 0.29). While maturity status clearly influenced players' performance, a limited impact on asymmetries was evident. Despite the mentioned relationships, the low shared variance between tests indicates that they should not be used interchangeably, and coaches should select those with the greatest specificity for the sport. Asymmetries do not influence performance, but their prevalence and unchanging nature with maturation can be seen as an opportunity for the identification of highly asymmetrical players and the application of interventions to improve the weaker limb, irrespective of the athlete's stage of development. To this end, the single leg countermovement jump might be viewed as an appropriate test in male youth soccer.

Change of Direction Asymmetry in Youth Elite Tennis Players: A Longitudinal Study

Longitudinal investigations into the development of inter-limb asymmetry in sports are scarce. This study examined the development of change of direction (COD) asymmetry magnitude in youth elite tennis players. Dominant (overall best performance) and non-dominant (best performance on the other side) scores and COD asymmetry magnitude were quantified annually (up to six years) in 323 male and 235 female players (ages 6 to13 years). Linear mixed effects regression models examined the development of COD performance and asymmetry magnitude according to players' chronological age and sex. Kappa coefficients examined the consistency for the direction of asymmetry across test occasions. Regardless of sex, COD performance significantly improved (p<0.001) while COD asymmetry magnitude significantly decreased (- 0.17±0.87% / year) with increasing chronological age. Regardless of age, males showed significantly lower COD times (- 0.111±0.091 s) and lower COD asymmetry magnitude (- 0.30±1.00%) compared to females. The slight and poor (k-value=0.02-0.00) kappa coefficients for males and females, respectively, highlight the direction specificity of functional asymmetry. These data show that whereas performance of the COD test improved, the magnitude of COD asymmetry declined across chronological age in youth tennis players.

Does It Matter? Isometric or Isokinetic Assessment of Quadriceps Strength Symmetry 9 Months After ACLR in Collegiate Athletes

BACKGROUND

Greater quadriceps strength symmetry is associated with better outcomes after anterior cruciate ligament reconstruction (ACLR). Isometric and isokinetic assessments of quadriceps strength inform therapeutic exercise prescription and return-to-sport decisions. It is unclear whether isometric and isokinetic measures provide similar information post-ACLR.

HYPOTHESIS

Quadriceps strength symmetry is similar between isometric and isokinetic assessments. Isokinetic and isometric strength symmetries have similar associations to functional knee kinetics and self-reported knee function.

STUDY DESIGN

Cross-sectional study.

LEVEL OF EVIDENCE

Level 3.

METHODS

NCAA Division I athletes (N = 35), 8.9 ± 2.5 months post-ACLR completed isometric and isokinetic quadriceps strength assessments, countermovement jumps (CMJs), and treadmill running. Self-reported knee function was assessed using the International Knee Documentation Committee Subjective Knee Form (IKDC). Agreement between isometric and isokinetic strength symmetry was assessed using Bland-Altman analysis, with associations to functional knee kinetics and IKDC assessed using Pearson correlations and linear regressions.

RESULTS

Mean difference in quadriceps strength symmetry between isokinetic and isometric assessments was 1.0% (95% limits of agreement of -25.1% to 23.0%). Functional knee kinetics during running and CMJ were moderately to strongly associated with isometric strength symmetry (r = 0.64-0.80, P < 0.01) and moderately associated with isokinetic strength symmetry (r = 0.41-0.58, P < 0.01). IKDC scores were weakly to moderately associated with isometric (r = 0.39, P = 0.02) and isokinetic (r = 0.49, P < 0.01) strength symmetry.

CONCLUSION

Isokinetic and isometric assessments of quadriceps strength symmetry in collegiate athletes 9 months post-ACLR demonstrated strong agreement. Quadriceps strength symmetry is associated with functional knee kinetic symmetry post-ACLR.

CLINICAL RELEVANCE

Considerable individual variation suggests mode of contraction should be consistent throughout postoperative assessment. Isometric strength symmetry may be a better indicator of functional knee kinetic symmetry, while isokinetic strength symmetry may be associated more closely with patient-reported outcomes.

A Novel Case Study Approach to the Investigation of Leg Strength Asymmetry and Rugby League Player's Multidirectional Speed

ABSTRACT

Helme, M, Emmonds, S, Low, C, and Till, K. A novel case study approach to the investigation of leg strength asymmetry and Rugby League player's multidirectional speed. J Strength Cond Res 38(5): 941-947, 2024-Leg strength and multidirectional speed qualities have repeatedly been linked with increased performance during Rugby League (RL) match play and associated with career attainment. However, very little of this evidence for strength has been gained through unilateral measurements with no study available that has examined the impact of strength asymmetry, in this population, on speed qualities in RL players. Therefore, this study examined the association between unilateral strength, and as a novel development, the subject with the most extreme asymmetry was identified for further analysis. Fifty RL players undertook the rear foot elevated split squat 5 repetition maximum, 20-m linear sprint, and modified 505 change of direction test. The mean leg strength for the group was 88.92 ± 12.59 kg; when divided by body mass, the mean relative strength (REL) was 1.03 ± 0.17 kg·kg -1 , and the mean asymmetry was 3.21 ± 5.70%. The subject with the greatest asymmetry (subject A) had an imbalance of 33% and a mean leg strength of 75 kg (REL = 0.78 kg·kg -1 ) and a body mass 1 SD above the group mean. Analysis of the group's data and that of subject A did not indicate that leg strength asymmetry was either frequent or harmful, with respect to speed performance. However, relative leg strength was associated with both improved linear and multidirectional speed. Practitioners are recommended to prioritize the development of relative leg strength and disregard the aspiration for between leg performance symmetry.

Interlimb kinetic asymmetries during the tuck jump assessment are more exposed following kinetic stabilization

OBJECTIVE

To analyse interlimb kinetics and asymmetries during the tuck jump assessment (TJA), before and after kinetic stabilization, to identify injury risk in healthy female athletes.

DESIGN

Cross-sectional study.

SETTING

Laboratory.

PARTICIPANTS

Twenty-five healthy females (age 21.0 ± 1.83 yrs; height 1.68 ± 0.06 m; body mass 69.4 ± 10.7 kg).

MAIN OUTCOME MEASURES

Kinetics were measured during 10-s trials of the TJA and absolute asymmetries compared, before and after kinetic stabilization using paired sample t-tests. Statistical parametric mapping (SPM) compared vertical ground reaction force (VGRF) data for each limb during the jumping cycles before and after stabilization.

RESULTS

Small to moderate increases in interlimb asymmetries were observed after stabilization for VGRF, relative vertical leg stiffness, average loading rate, total and propulsive impulse, peak braking and propulsive force (p < 0.05). SPM revealed significant interlimb differences between 77-98% and 83-99% of ground contact for the jumping cycles pre- and post-stabilization respectively.

CONCLUSIONS

Larger asymmetries were evident after kinetic stabilization, with increased VGRF in the non-dominant limb. We speculate that participants sacrificed interlimb landing symmetry to achieve kinetic stability, which may reflect a primal landing strategy that forgoes movement quality. Assessing lower limb biomechanics using the TJA should involve examining kinetic stability and interlimb kinetic asymmetries.

Bilateral Asymmetry of Spatiotemporal Running Gait Parameters in U14 Athletes at Different Speeds

The assessment of leg asymmetries is gaining scientific interest due to its potential impact on performance and injury development. Athletes around puberty exhibit increased gait variability due to a non-established running pattern. This study aims to describe the asymmetries in the spatiotemporal running parameters in developmentally aged athletes. Forty athletes under 14 (U14) (22 females and 18 males) were assessed running on a treadmill at constant speeds of 12 and 14 km·h-1 for 3 min. Step length, step frequency, along with contact (CT) and flight time, both in absolute values and as a percentage of step time, were recorded using a RunScribe sensor attached to the laces of each shoe. U14 runners exhibited high bilateral symmetry in the spatiotemporal parameters of running, with mean asymmetry values (1-5.7%) lower than the intra-limb coefficient of variation (1.7-9.6%). Furthermore, bilateral asymmetries did not vary between the two speeds. An individual-based interpretation of asymmetries identified subjects with consistent asymmetries at both speeds, particularly in terms of CT and contact ratio (%, CT/step time). This study confirms the high symmetry of pubertal runners and paves the way for the application of portable running assessment technology to detect asymmetries on an individual basis.

Effects of a neuromuscular training program on the performance and inter-limb asymmetries in highly trained junior male tennis players

Background

The objective of this study was to assess the effectiveness of neuromuscular training on the performance of highly trainer junior tennis players.

Methods

Twelve male tennis players (age: 13.4 ± 0.36 years; weight: 50.2 ± 6.29 kg; height: 163 ± 4.41 cm) participated and were randomly divided into two groups. The experimental group (EG) performed neuromuscular training that included exercises for speed, strength, throws, agility, jumps and coordination twice a week for a duration of 10 weeks. Performance was evaluated using various variables, including bilateral and unilateral countermovement jump, 30 cm drop jump and horizontal jump, 505 change of direction test conducted with both the right and left legs, 20-m sprint, and overhead 3 kg medicine ball throw. Asymmetries were also evaluated during the unilateral tests. The impact of the training was assessed through the utilization of ANCOVA tests and effect size measurements.

Results

The results indicated a significant enhancement in the EG, specifically in bilateral vertical jump and horizontal jump, as well as explosive strength and speed. Conversely, the control group (CG) did not display similar advancements. Furthermore, there was no increase in asymmetries.

Conclusion

This suggests that the implementation of a neuromuscular training program could prove to be an effective approach in enhancing explosive power in the lower limbs among young competitive tennis players. Finally, this training program could contribute to the enhancement of their physical attributes in lower body of young tennis players.

Exploring the Implications of Inter-Limb Asymmetries on Sprint, Agility, and Jump Performance in Young Highly-Trained Basketball Athletes: Is There a Relevant Threshold?

Background and Objectives: This study aimed to investigate the magnitude of vertical jump inter-limb asymmetries among young highly-trained basketball athletes and to analyze its impact on sport performance, specifically in sprints, agility, and vertical jumps. Materials and Methods: A unilateral countermovement jump (CMJ) was employed to determine Inter-limb Index Asymmetry (IAI) in 320 participants aged from 14 to 18 years, from the Valencia Basket youth academy. IAI was categorized into three groups: 0-9.9%, 10-14.9%, and >15%. The relationship between IAI and performance variables was analyzed through correlation studies (Pearson or Spearman's). The influence of IAI magnitude was assessed using ANOVA or Kruskal-Wallis analysis, with leg dominance as a covariable. SPSS Statistics version 26 was used for analysis. Results: Among all the participants, the mean IAI was 10.6%. Correlation studies revealed non-significant values (p < 0.05) between IAI and sport performance variables. The three IAI magnitude groups did not show statistically significant differences in sprint, agility, and jump outcomes. Leg dominance did not seem to influence performance outcomes, except for unilateral CMJ. Conclusions: The results obtained challenge the assumption that an IAI above 10% negatively affects sprint, agility, or jump performance in young basketball athletes. Notably, the magnitude of IAI did not influence sport performance parameters, suggesting that the 10-15% threshold from previous studies may not be applicable to this population. The study emphasizes the need to understand lower-limb asymmetries in the context of specific sport task performance, considering the potential evolution over time among affected young athletes.

Plyometric Training's Effects on Young Male Karatekas' Jump, Change of Direction, and Inter-Limb Asymmetry

This study analysed the effects of performing a plyometric training programme on different types of jumping and specific changes of direction, and their respective asymmetries in karatekas. Twenty male karatekas (age 19 ± 4 years) were distributed in two groups, the control group (CG) and the experimental group (EG). The EG group (n = 10) performed a 6-week intervention of unilateral plyometric training, performing countermovement jumps (CMJ), drop jumps (DJ), and long jumps (SH). The tests performed at the beginning and at the end of the intervention were a unilateral and bilateral countermovement jump test (CMJ), single-leg hop test (SH), single-leg side-hop test (SSH), triple hop test (TH), and change of direction in a karate position test (MKUKS). The EG group obtained improvements in the CMJ with the stronger (p = 0.01; ES = 0.39) and weaker leg (p = 0.01; ES = 0.59), in the SH with the weaker leg (p = 0.01; ES = 0.45), in the SSH with the weaker leg (p = 0.03; ES = 0.33), in the MKUKS (p = 0.00; ES = 0.98), and improved the asymmetries obtained in the TH (p = 0.02; ES = -0.85). The GC group obtained significant differences in the CMJ with the stronger (p = 0.03; ES = 0.46) and weaker leg (p = 0.00; ES = 0.69), in the bilateral CMJ (p = 0.02; ES = 0.24), in the SH with the weaker leg (p = 0.00; ES = 0.34), in the TH with the stronger (p = 0.00; ES = -0.15) and weaker leg (p = 0.01; ES = 0.09), and in the MKUKS test (p = 0.04; ES = -0.94). A between-group analysis showed improvements of the EG over the GC in the TH with the stronger leg (p = 0.02; ES = 1.05). Performing plyometric training provides improvements in jumping, mainly in horizontal jumps, reducing inter-limb asymmetries in repetitive jumps.

The Effects of Pre-Season and Relationships with Physical, Physiological, Body Composition, and Load Markers: A Case Study Comparing Starters versus Non-Starters from an Elite Female Professional Soccer Team

Background and Objectives: Research on female soccer players that analyzes playing status is scarce and has previously only examined load monitoring, while other markers, such as physical (i.e., strength, power, and agility), physiological (i.e., maximal oxygen uptake), and body composition (i.e., body fat mass, fat-free mass, body water, and phase angle) markers, warrant further investigation. Thus, the study aims were to (a) compare physical, physiological, body composition, and load markers between starters and non-starters; (b) compare measurements pre- and post-training intervention (five weeks); and (c) analyze any relationships between physical, physiological, body composition, and load markers in an elite female soccer team. Materials and Methods: Fourteen first-team players participated in the study (age 23.29 ± 3.19 years, weight 59.14 ± 6.87 kg, height 1.66 ± 0.08 m). Several physical (n = 15), physiological (n = 1), body composition (n = 11), and load markers (n = 14) were collected. In addition, participants were sub-divided into starters (n = 7) and non-starters (n = 7). Results: No differences were revealed between starters and non-starters in any of the examined variables. Moreover, following the training intervention, a significantly lower value was found for total body water/fat-free mass ratio (p = 0.043; ES = 0.582). In addition, there were several correlations detected between load and physical/physiological markers (n = 28); load and body composition markers (n = 6); physical/physiological and body composition markers (n = 34); and physical and physiological markers (n = 42). Conclusions: In conclusion, only a slight tendency of higher load values for starters than non-starters was observed. In addition, no differences in physical, physiological, and body composition markers were found between starters and non-starters, possibly suggesting that five weeks were not enough to improve such variables. Finally, the present results provide novel information assessing the effects of the pre-season in elite female Portuguese soccer players and contribute to a better understanding of the associations between different types of measurements.

Assessing Limb Dominance and Interlimb Asymmetries Over Multiple Angles During Change of Direction Speed Tests in Basketball Players

ABSTRACT

Gonzalo-Skok, O, Dos' Santos, T, and Bishop, C. Assessing limb dominance and interlimb asymmetries over multiple angles during change of direction speed tests in basketball players. J Strength Cond Res 37(12): 2423-2430, 2023-The aims of this study were to establish whether directional dominance is displayed during change of direction (COD) tasks across various angles, to determine the angle-variation data for the asymmetry magnitude and direction, and to analyze the relationships in COD performance (completion time and COD deficit) across tasks. Twenty-four young (U-16 to U-20), highly trained male basketball players performed a 10-m linear sprint test and four 10-m COD tests (45°, 90°, 135°, and 180°) in left and right directions. Change of direction performance was determined via total times and COD deficit, and asymmetry comparisons were made between faster and slower directions and dominant leg (DL) (i.e., first step leg in lay-up) and nondominant leg (NDL). No significant differences (p > 0.05) were found between DL and NDL for any task excluding 45° COD (p < 0.05, effect size [ES] = 0.44-0.78), but significant differences were established between faster and slower sides for all angles (p < 0.05, ES = 0.70-1.28). Levels of the agreement in directional dominance during COD tasks were generally poor to slight (k = -0.14 to 0.14), excluding a fair agreement between COD45 and COD90 (k = 0.34). Correlations between COD total times and COD deficits between angles were moderate to very large (r = 0.32 to 0.81) and moderate to large (r = -0.30 to 0.55), respectively. Players displayed superior COD performance in a particular direction across various angles. This directional dominance is not necessarily consistent between angles, thus highlighting the angle-dependent nature of COD performance. Consequently, practitioners should investigate multiple angles and directions to create a COD angle profile for their athletes.

Cut-off points for isometric handgrip and low limb explosive strength in relation to indicators of overweight/obesity in people with intellectual disabilities: analysis by age groups

BACKGROUND

The prevalence of overweight/obesity has been increasing globally and in people with Intellectual Disabilities (IDs), this problem is exacerbated even more, which added to a low physical condition that contributes to the deterioration of functionality and increases the risk of developing chronic diseases in the course of life. Therefore, the aim of this study was to establish cut-off points for levels of isometric handgrip and low limb explosive strength in children, adolescents and adults, which identify overweight/obesity in people with IDs and their respective associations.

METHODS

The sample was made up of 131 individuals with IDs, belonging to four special and community educational centres in the city of Santiago, Chile. Body mass index (BMI) and waist-to-height ratio (WHR) were used as indicators of overweight/obesity. Handgrip strength was used as a measure of isometric strength, and countermovement jump was used as a measure of low limb explosive strength. For the comparison of variables by age group, the analysis of Ancova, Kruskal-Wallis and chi-square tests were used. The total area under the receiver operating characteristic curve of isometric handgrip and low limb explosive strength was identified as an indicator of overweight/obesity according to age groups. A logistic regression model was used to quantify the effect that strength categories below the cut-off point have on the risk of overweight and obesity.

RESULTS

Significant differences were observed between the age groups for body weight, height, BMI and WHR, as well as in the levels of absolute handgrip strength and vertical jump with countermovement (P ≤ 0.05). Children showed the lowest cut-off points for absolute and relative strength. The adolescent group showed the highest cut-off points for relative strength and countermovement jump and adults showed the highest value for absolute strength as indicators of overweight/obesity. Different associations between cut-off points with BMI and WHR were found.

CONCLUSIONS

Adolescents showed the highest cut-off point for relative strength and countermovement jump, and adults showed the highest value for absolute strength, according to overweight/obesity indicators (BMI and WHR). It is suggested to adjust resistance training programmes according to age categories for the prevention of overweight/obesity in people with IDs.

Acute effects of unilateral and bilateral conditioning activity on countermovement jump, linear speed, and muscle stiffness: A randomized crossover study

PURPOSE

Evidence directly comparing the effects of bilateral and unilateral conditioning activities is limited. Therefore, the aim of this study was to assess the acute effect of unilateral and bilateral conditioning activity on vastus lateralis stiffness, countermovement jump parameters, and 10 m sprint.

METHODS

Twelve semi-professional basketball players participated in this study (age: 23 ± 4 yrs; body mass: 84.7 ± 10.6 kg; body height: 192 ± 6 cm; basketball training experience: 11 ± 4 yrs) performed four experimental sessions to compare the acute effects of bilateral, stronger-only, weaker-only lower limb or no conditioning activity on vastus lateralis stiffness, countermovement jumps variables (height; peak velocity; peak force, contraction time, countermovement depth, and modified reactive strength index and 10 m sprint time. Measurements were performed 5 minutes before and in the 5th and 10th minutes after CA.

RESULTS

Bilateral conditioning activity significantly increase the countermovement jump height (p = 0.002; ES = 0.71) and the reactive strength index modified (p = 0.010; ES = 0.59). Moreover, a significantly higher peak force in the stronger than in the weaker limb was found (p<0.001) without any differences between conditions and time points (p>0.05). However, there were no significant (p>0.05) interactions and effects of conditions or time-point in the case of the other countermovement jump variables, vastus lateralis stiffness, and 10m sprint time.

CONCLUSION

Unilateral and bilateral drop jumps (3 sets of 5 repetitions) did not affect the vastus lateralis stiffness and time of the 10m sprint. However, only bilateral drop jumps effectively enhanced the countermovement jump height and modified reactive strength index. Bilateral drop jumps might be a useful part of a warm-up to improve jumping performance in basketball players.

Effects of Exercise-Induced Neuromuscular Fatigue on Jump Performance and Lower-Limb Asymmetries in Youth Female Team Sport Athletes

The objective of the present study was to examine the effect of acute neuromuscular fatigue on unilateral jump performance and inter-limb asymmetries. Thirty elite youth female team sport athletes (age: U-14 to U-18) performed the Unilateral Countermovement Jump (UCJ) and the Unilateral Drop Jump (UDJ) (18-cm box) tests before and approximately 10 minutes after the 30-15 intermittent fitness test (30-15 IFT). A paired samples t-test showed significant reductions in UCJ jump height in the right leg after the 30-15 IFT (p = 0.018; d = 0.33), but not in the left leg (p = 0.459; d = 0.48). For the UDJ, significant reductions in jump height were shown in both the right (p < 0.001; d = 0.33) and left (p < 0.001; d = 0.33) legs. In addition, for the reactive strength index (UDJ), significant reductions were seen in the left leg after the 30-15 IFT (p < 0.001; d = 0.31), but not in the right leg (p = 0.948; d < 0.001). Only UCJ inter-limb jump height asymmetries increased significantly post 30-15 IFT (p = 0.033; d = 0.46). In conclusion, the current study indicates that the 30-15 IFT provides a sufficient dose of activity for inducing acute fatigue in elite youth female team sport athletes. Therefore, monitoring jump height in unilateral jump testing is recommended given the tests' sensitivity to detect significant differences in physical performance and inter-limb asymmetries under acutely fatigued conditions in healthy youth female athletes.

Change of direction speed and deficit over single and multiple changes of direction: Influence of biological age in youth basketball players

The study analysed the difference in change of direction (COD) deficit among young male basketball players of different age groups and biological ages and analysed the relationships in COD performance (time and deficit) across single and multiple CODs depending on their biological age. Fifty-four highly-trained male players (U-13 and U-17) were tested on sprinting (25-m) and COD ability over single and multiple CODs through total times and the percentage-based COD deficit (CODD). Results showed that older age groups performed significantly (ES = 0.43-2.32, p < 0.05) better in linear and COD times, especially those players with post-peak height velocity (PHV) (ES = 0.89-1.90). Controlling for age at PHV, no significant differences were found in any group or inter-limb asymmetries. Moderate relationships were found between CODD in the pool data (r = 0.36 to 0.50). All CODD and time relationships in any test (i.e., 180º COD and V-cut) were considerably lower (r= -0.27 to 0.32) across individual biological groups. The study highlights the impact of maturation on COD performance, whereas CODD seems to be not affected. Interestingly, the CODD is independent, highlighting this measurement's specific nature. Practitioners should use common distances, angles, and the number of CODs linked to biological status to create a COD profile.

Horizontal jump asymmetries are associated with reduced range of motion and vertical jump performance in female soccer players

BACKGROUND

Performance in jumping and change of direction tests are good proxies to reflect the skill level during soccer-specific actions. Greater inter-leg asymmetries have been identified as a risk factor for developing acute and overuse injuries and jeopardizing soccer performance. The aim of this study was to assess the association between asymmetry in the unilateral vertical and horizontal jump tests, ankle range of motion, linear velocity, and change of direction in a sample of highly trained adult female soccer players.

METHODS

Thirty-eight highly trained female soccer players underwent a testing protocol including ankle dorsiflexion, single leg jumps for height (CMJ), distance (HJ), 40 m sprint and 180° change of direction tests.

RESULTS

Within-session reliability was acceptable (CV ≤ 7.9%), and relative reliability showed good to excellent (ICC: 0.83 to 0.99). The one-way ANOVA reported higher inter-limb differences for change of direction deficit (10.9 ± 8.04%) and single leg CMJ (5.70 ± 5.22%). Pearson correlations highlighted significant relationships between horizontal jump asymmetries and ankle dorsiflexion (r = -0.41), CMJ (r = -0.36 to -0.49) and HJ (r = -0.28 to -0.56).

CONCLUSIONS

Assessing inter-limb asymmetries through different methods can help scientists understand the specificity of their detrimental effects on soccer performance. Practitioners should be aware of these specificities as well as the magnitude and direction of the asymmetries when aiming to improve specific on-field skills.

The effect of lower inter-limb asymmetries on athletic performance: A systematic review and meta-analysis

Inter-limb asymmetry refers to an imbalance in performance between the left and right limbs. Discrepancies throughout asymmetry research does not allow practitioners to confidently understand the effect of inter-limb asymmetries on athletic performance. Therefore, this review summarized the current literature using a meta-analytic approach, conforming to the Preferred Reporting Items for Systematic Review and Meta-Analyses guidelines to identify the association between inter-limb asymmetry and athletic performance. A literature search using PubMed, Web of Science and SPORTDiscus databases yielded 11-studies assessing the effect of inter-limb asymmetries, measured via unilateral jump performance, on bilateral jump, change of direction (COD) and sprint performance in adult sports players. The quality of evidence was assessed via a modified Downs and Black checklist and in compliance with the Grading of Recommendations Assessment Development and Evaluation. Correlation coefficients were transformed via Fishers z (Zr), meta-analysed and then re-converted to correlation coefficients. Egger's regression presented no significant risk of bias. Vertical jump performance was not significantly affected by asymmetry (Zr = 0.053, r = 0.05; P = 0.874), whereas COD and sprint both presented significant weak associations (COD, Zr = 0.243, r = 0.24; Sprint, Zr = 0.203, r = 0.2; P < 0.01). The results demonstrate that inter-limb asymmetries seem to present a negative impact to COD and sprint performance but not vertical jump performance. Practitioners should consider implementing monitoring strategies to identify, monitor and possibly address inter-limb asymmetries, specifically for performance tests underpinned by unilateral movements such as COD and sprint performance.

The associations between unilateral leg strength, asymmetry and injury in sub-elite Rugby League players

OBJECTIVES

The aim of this study was to analyse the relationship between unilateral leg strength, associated asymmetries and the injuries suffered by sub-elite Rugby League (RL) players in one competitive season.

DESIGN

A prospective cohort design was used.

METHOD

Unilateral leg strength was measured using the rear foot elevated split squat five repetition maximum test. Injuries were recorded using the Orchard classification system and were used to quantify relative risk (RR), mean severity, burden, player availability and survival time.

RESULTS

No measures of leg strength were related to RR, relative leg strength was found to have a significant, but not meaningful correlation with total time lost to lower body injury, lower body injury burden and lower body injury survival time.

CONCLUSIONS

The data from the current study indicates a possible positive effect of increasing relative leg strength for injury outcomes in sub-elite RL players. This supports a heuristic that multi-joint lower body strength training for RL players has a potential dual effect of enhancing physical performance and reducing injury time loss, with minimal risk of harm.

Lower Limb Unilateral and Bilateral Strength Asymmetry in High-Level Male Senior and Professional Football Players

This study sought to assess the relationship between different jumping asymmetries and associated performance variables in high-level male senior and professional football players. Nineteen football players with at least 12 years of training experience (23.2 ± 3.1 years of age; 75.2 ± 4.8 kg of body mass and 181 ± 0.06 cm of height) participated in this study performing countermovement jump (CMJ), squat jump (SJ), single-leg CMJ and drop jump (DJ), associated performance variable eccentric utilization ratio (EUR), stretch-shortening cycle (SSC), bilateral deficit (BLD), and limb symmetry index (LSI) were determined. High correlations were observed between different methodologies of jump tests and associated performance indicators (SSC, BLD, EUR), except LSI. Moreover, CMJ and SJ results were different (p < 0.05), but no differences were found between interlimb in CMJ (p = 0.19) and DJ (p = 0.14). Between the same limbs and different jumps differences were detected in CMJ and DJ (p < 0.01), and it has also been found that the laterality effect size on strength was small in CMJ (ES = 0.30) and DJ (ES = 0.35). LSI between CMJ and DJ was not different despite higher mean values in CMJ, and although mean BLD was positive (>100%), the results highlight the need for individual evaluation since eight players scored negatively. An in-depth and accurate analysis of performance in preseason screening jump tests should be considered, aiming to detect injury risk, specifically evaluating different jumping test methodologies, and determining jumping associated performance variables for each test, namely EUR, SSC, BLD, and LSI. Specific muscle-strengthening exercises could be implemented based on this study results and outcomes, aiming to reduce injury risks and lower extremity asymmetries and to enhance individual football performance in high-level male senior and professional football players. Sports institutions should pay special attention regarding potential health problems in athletes exposed to daily high training loads.

Kinematic and muscle force asymmetry in healthy runners: How do different methods measure up?

BACKGROUND

Comparing the performance of one leg to another is a common means of assessing running gait to help inform clinical management strategies. Various methods are employed to quantify asymmetries between limbs. However, limited data is available describing the amount of asymmetry that may be expected during running and no index has been identified as preferable for making a clinical determination of asymmetry. Therefore, this study aimed to describe amounts of asymmetry in collegiate cross-country runners and compare different methods of calculating asymmetry.

RESEARCH QUESTION

What can be expected as a normal amount of asymmetry in biomechanical variables in healthy runners when using different indices to quantify limb symmetry?

METHODS

Sixty-three (29 male and 34 female) runners participated. Running mechanics were assessed during overground running using 3D motion capture and a musculoskeletal model using static optimization to estimate muscle forces. Independent t-tests were utilized to determine statistical differences in variables between legs. Different methods of quantifying asymmetry were then compared to statistical differences between limbs to determine cut-off values and the sensitivity and specificity of each method.

RESULTS

A large portion of runners demonstrated asymmetry during running. Kinematic variables can be expected to have small differences (2-3 degrees) between limbs while muscle forces may show greater amounts of asymmetry. The sensitivities and specificities for each method of calculating asymmetry were similar, however, different methods led to different cut-off values for each variable investigated.

SIGNIFICANCE

Asymmetry can be expected between limbs during running. However, when assessing asymmetry, practitioners should consider the joint, variable, and method of calculating asymmetry when determining differences between limbs.

Exploring of two different equated instability resistance training programs on measure of physical fitness and lower limb asymmetry in pre-pubertal weightlifters

BACKGROUND

Although previous research in pediatric populations has reported performance enhancements following instability resistance training, the effects of different volumes on measures of balance, strength and power and lower limb asymmetry remain unclear.

OBJECTIVE

To compare the effect of two 8-week instability resistance training programs (IRT1: 2 sets x 8 repetitions, 20% one-repetition maximum (1RM); IRT2: 2 sets x 4 repetitions, 40% 1RM) in balance (center of pressure displacements), 1RM, single-leg jumps, and inter-limb asymmetry of prepubescent weightlifters.

METHODS

Thirty-two male athletes (10.94 ± 0.47 yrs, 17.05 ± 0.14 kg/m², and 6.44 ± 0.93% bodyfat) voluntarily participated and combined IRT (front and back squat, lunge, and deadlift) with their usual weightlifting training. Nonparametric tests evaluated the differences between pre- and post-intervention.

RESULTS

Significant improvements were encountered in all the variables for both groups (effect sizes [ES] from 0.46 to 2.60), except the inter-limb asymmetry and the velocity of displacement of the center of pressure, which did not improve in IRT2. It is also worth highlighting that in the single-leg jumps and center of pressure displacements, whereas no significant differences were observed between IRT1 and IRT2 in the baseline, significant differences appeared in the follow-up measurements.

CONCLUSION

IRT combined with weight-lifting training improves prepubescent weightlifters' performance; therefore, strength and conditioning coaches may consider including unstable devices with low loads into an overall conditioning program and warm-ups for prepuberal male weightlifters to promote their physical fitness and potentially decrease inter-limb asymmetry.

TRIAL REGISTRATION

This study does not report results related to healthcare interventions using human participants; therefore, it was not prospectively registered.

More than just a side effect: Dynamic knee valgus and deadbug bridging performance in youth soccer players and alpine skiers have similar absolute values and asymmetry magnitudes but differ in terms of the direction of laterality

From a preventative perspective, leg axis and core stabilization capacities are important for soccer players and alpine skiers; however, due to different sport-specific demands, the role of laterality clearly differs and may result in functional long-term adaptations. The aims of this study are 1) to determine whether there are differences in leg axis and core stability between youth soccer players and alpine skiers and 2) between dominant and non-dominant sides, and 3) to explore the outcomes of applying common sport-specific asymmetry thresholds to these two distinct cohorts. Twenty-one highly trained/national-level soccer players (16.1 years, 95% CI: 15.6, 16.5) and 61 alpine skiers (15.7 years, 95% CI: 15.6, 15.8) participated in this study. Using a marker-based 3D motion capture system, dynamic knee valgus was quantified as the medial knee displacement (MKD) during drop jump landings, and core stability was quantified as the vertical displacement during deadbug bridging exercise (DBBdisplacement). For the analysis of sports and side differences, a repeated-measures multivariate analysis of variance was used. For the interpretation of laterality, coefficients of variation (CV) and common asymmetry thresholds were applied. There were no differences in MKD or DBBdisplacement between soccer players and skiers or between the dominant and non-dominant sides, but there was an interaction effect side*sports for both variables (MKD: p = 0.040, η2p = 0.052; DBBdisplacement: p = 0.025, η2p = 0.061). On average, MKD was larger on the non-dominant side and DBBdisplacement laterality on the dominant side in soccer players, whereas this pattern was reversed in alpine skiers. Despite similar absolute values and asymmetry magnitudes of dynamic knee valgus and deadbug bridging performance in youth soccer players and alpine skiers, the effect on the direction of laterality was opposite even though much less pronounced. This may imply that sport-specific demands and potential laterality advantages should be considered when dealing with asymmetries in athletes.

Seasonal Variation of Physical Performance, Bilateral Deficit, and Interlimb Asymmetry in Elite Academy Soccer Players: Which Metrics Are Sensitive to Change?

ABSTRACT

Bishop, C, Abbott, W, Brashill, C, Loturco, I, Beato, M, and Turner, A. Seasonal variation of physical performance, bilateral deficit, and interlimb asymmetry in elite academy soccer players: Which metrics are sensitive to change? J Strength Cond Res 37(2): 358-365, 2023-This study aimed to report seasonal variations for (a) physical performance, (b) interlimb asymmetry, and (c) bilateral deficit (BLD) data over the course of a competitive soccer season and determine which metrics are sensitive to change. This study used a repeated-measures observational design for 19 under-20, elite, academy, soccer players (age: 17.58 ± 0.61 years; height: 1.81 ± 0.09 m; body mass: 74.36 ± 7.58 kg), who conducted bilateral and single-leg countermovement jumps (CMJ and SLCMJ), linear speed (5, 10, 20, and 30-m) and 505 change of direction (COD) speed tests, at pre, mid, and end of season time points. For jump tests, jump height, reactive strength index modified, time to takeoff, and countermovement depth metrics were monitored, with interlimb asymmetry and the BLD also calculated for each. Significant improvements ( p < 0.05) in performance were evident in all fitness tests: CMJ (effect size [ES]: 0.61 to 1.03), SLCMJ (ES: 0.60 to 2.25), linear speed (ES: -0.54 to -1.96), and COD speed (ES: -0.68 to -1.14). Significant reductions in asymmetry (ES: -0.68 to -1.07) and significant increases in the BLD (ES: 1.15 to 1.57) were also evident throughout the season. Additionally, kappa coefficients were used to determine consistency in limb dominance throughout the season, but only poor to fair levels of agreement (kappa: -0.17 to 0.37) were evident, highlighting the fluctuating nature of limb dominance throughout the season. Despite all tests exhibiting meaningful change, the SLCMJ and linear speed showed the most frequent and largest differences in performance, highlighting their usefulness in the ongoing monitoring process of physical capacities in elite male academy players across a competitive soccer season.

The Effect of Unilateral Versus Bilateral Strength Training on Isometric-Squat Peak Force and Interlimb Asymmetry in Young, Recreationally Strength-Trained Men

PURPOSE

To compare the effects of bilateral strength training (BLST) versus unilateral strength training (ULST) on changes in peak force (PF) and interlimb asymmetry (ILA) in the isometric squat at a 120° knee angle (ISq120).

METHOD

A total of 31 young, recreationally strength-trained men performed either BLST (n = 18) or ULST (n = 13), twice per week for 6 weeks. The total number of repetitions, duty cycle, and effort were standardized between training groups (ie, differing only in the exercises performed). Changes in PF and ILA were assessed pretraining and posttraining.

RESULTS

Comparable increases in PF were observed in the BLST group (mean [SD] change; 17.4% [20.5%], P = .001, standardized mean difference [SMD] = 0.45) and the ULST group (11.4% [19.1%], P = .042, SMD = 0.25). No significant changes in symmetry index (SI) scores were observed following BLST (mean [SD] change; 0 [5.7], P = .526, SMD = -0.12) or ULST (+3 [6.0], P = .702, SMD = 0.4). Individual analyses of subjects with marked ILA (ie, baseline SI score > baseline coefficient of variation) revealed a trend toward BLST being more effective at attenuating SI scores in the ISq120.

CONCLUSIONS

Overall, both BLST and ULST are effective for increasing ISq120 PF. However, it appears that BLST may be more effective at reducing SI scores in those with marked ILA.

Neuromuscular and Biomechanical Jumping and Landing Deficits in Young Female Handball Players

Neuromuscular and biomechanical imbalances that exist in jumping and landing actions should be examined in order to intervene to decrease the risk of ACL injury. The main aim of this study was to analyse and compare, by chronological age, jumping and landing deficits in young female handball players using the Tuck Jump Assessment (TJA). A secondary aim was to relate the qualitative asymmetry values detected using the TJA to the quantitative asymmetry values detected starting from the single leg countermovement jump (SL-CMJ). Sixty-one young female handball players (age: 14.3 ± 1.5 years) were distributed into three groups: U12, U14 and U16 and performed the TJA test and the single leg countermovement jump (SL-CMJ). The female U12 category players obtained the highest scores in the TJA and there were significant differences between the U12 (12.11 ± 1.97) and U14 (10.89 ± 1.74) categories (p = 0.017; ES = 0.374). In the U12 category, the female players presented larger interlimb asymmetry magnitudes in the SL-CMJ test; they also obtained higher scores in the qualitative criteria of the TJA test that referred to asymmetry (r = 0.43; p = 0.027). The analysis of the jumping and landing pattern using TJA allowed us to identify that the lower extremity valgus at landing, foot contact timing not equal and landing contact noise are the main biomechanical deficits in young female handball players. Furthermore, the asymmetry values assessed qualitatively (TJA) are associated with the asymmetry values assessed quantitatively (difference in jump achieved with each limb in the SL-CMJ test) in younger categories.

Experience, Training Preferences, and Fighting Style Are Differentially Related to Measures of Body Composition, Strength, and Power in Male Brazilian Jiu Jitsu Athletes-A Pilot Study

To examine relationships between Brazilian Jiu Jitsu (BJJ) descriptors (belt rank, experience, gi preference, and fighting style), resistance training (RT) experience, and measures of body composition, strength (maximal handgrip, 3-5-repetition maximum [RM] in barbell glute bridge [GB], prone bench row [PBR], and bench press [BP]), and velocity (GB, PBR, and BP at 7 kg and 30−60% 1-RM), 13 experienced (4.3 ± 3.4 years) BJJ athletes were recruited for this cross-sectional, pilot study. Significant (p < 0.05) Kendall’s tau and Bayesian relationships were seen between belt rank and body fat percentage (τ = −0.53, BF10 = 6.5), BJJ experience and body fat percentage (τ = −0.44 to −0.66, BF10 = 2.6−30.8) and GB velocity (τ = −0.45 to −0.46, BF10 = 2.8−3.1), RT experience and strength (τ = 0.44 to 0.73, BF10 = 2.6−75.1) and velocity (τ = −0.44 to 0.47, BF10 = 2.6−3.3), gi preference-training and relative PBR strength (τ = 0.70, BF10 = 51.9), gi preference-competition and height and lean mass (τ = −0.57 to 0.67, BF10 = 5.3−12.4) and BP velocity (τ = −0.52 to 0.67, BF10 = 3.5−14.0). The relevance of body composition and performance measures to sport-specific training and research interpretation are differentially affected by a BJJ athlete’s experience (BJJ, belt rank, RT), gi preferences, and fighting style.

Pivot Step Jump: A New Test for Evaluating Jumping Ability in Young Basketball Players

Jumping ability in basketball is usually assessed using standardized vertical jump tests. However, they lack specificity and do not consider the player’s basketball skills. Several studies have suggested performing specific jump tests, which are tailored to the movement patterns and requirements of a basketball game. The pivot step jump test (PSJT) is a novel test designed to evaluate the specific jumping abilities of basketball players by combining a pivot step on one leg with a maximum bilateral vertical jump. This study had two aims: to determine the reliability and validity of the PSJT using typical jump tests as the criterion measure and to demonstrate the PSJT as a practical test to evaluate specific jumping ability in young male and female basketball players. Twenty female (EGA; 14.0 ± 0.7 years, 59.3 ± 7.9 kg, 162.1 ± 5.5 cm) and fifteen male (EGB; 14.0 ± 0.7 years, 58.1 ± 7.7 kg, 170.3 ± 6.4 cm) basketball players participated in the study. The test−retest reliability of the PSJT within sessions (intrasession reliability) and across sessions (intersession reliability) was assessed within EGA. For the evaluation of validity, EGB performed the PSJT and a series of criterion jumping tests. For EGA, no changes (p > 0.05) were found in PSJT performance between test sessions and excellent intra- and intersession reliability was observed (ICCs > 0.75). Correlation coefficients indicated high factorial validity between the jumping tests and PSJT (r = 0.71−0.91, p < 0.001). The PSJT appears to offer a valid assessment of jumping ability in basketball and is a practical test for assessing sport-specific jumping skills in young basketball players.

The Safe Landing warm up technique modification programme: An effective anterior cruciate ligament injury mitigation strategy to improve cutting and jump-movement quality in soccer players

The objective of the study was to evaluate the effectiveness of the Safe Landing (SL), a 6-week technique-modification (TM) programme, on cutting and jump-landing movement quality in football players. In a non-randomized design, 32 male semi-professional football players from two Spanish clubs participated in the study: one served as the control group (CG, n = 11), while the other performed the SL (n = 15). Performance and movement quality of drop vertical jump and 70º change of direction (COD70) were evaluated through 2D video footage pre- and post-intervention. In such tasks, the Landing Error Scoring System for first (LESS1) and second (LESS2) landings, and the Cutting Movement Assessment Score (CMAS) were used for assessing movement quality. Pre-to-post changes and baseline-adjusted ANCOVA were used. Medium-to-large differences between groups at post-test were shown in CMAS, LESS1 and LESS2 (p < 0.082, ղ2 = 0.137-0.272), with small-to-large improvements in SL (p < 0.046, ES=0.546-1.307), and CG remaining unchanged (p > 0.05) pre-to-post. In COD70 performance, large differences were found between groups (p < 0.047, ղ2 = 0.160-0.253), with SL maintaining performance (p > 0.05, ES=0.039-0.420), while CG moderately decreasing performance (p = 0.024, ES=0.753) pre-to-post. The SL is a feasible and effective TM program to improve movement quality and thus potential injury risk in cutting and landing, while not negatively affecting performance.

Relationship between Asymmetries and Functional Autonomy in Older Chilean Adults

The objectives of this study were: (a) to determine asymmetries, both lower limb (LL) and upper limb (UL), in Chilean older adults, and (b) to relate asymmetries to FA in both LL and UL. Forty-one older adults voluntarily participated in this study (mean ± standard deviation [SD]: age 72.0 ± 8.0 years, LL asymmetries 13.78 ± 14.87%, UL asymmetries 10.70 ± 8.85%, FA 40.35 ± 16.26 points). The variables were: (1) asymmetries of LL and UL, assessed through a force platform and handgrip, respectively; (2) FA, assessed through the Latin American Group for Maturity (GDLAM) and the GDLAM index of autonomy (GI) protocol. The relationship between the variables was performed through Spearman's correlation. The analysis showed that 39% of the participants presented asymmetries above 15% in the LL. Likewise, this 39% of older adults presented a lower FA than their peers with asymmetries below 15% in the LL (≤15%: 35.64 ± 12.26 points vs. >15%: 47.69 ± 19.23 points, p = 0.003). The analysis showed a small correlation between LL and GI asymmetries (r = 0.27, p = 0.07) and a small but negative correlation between UL and GI (r = -0.21). The mean values of asymmetries of both LL and UL are within 'normal' parameters. However, several older adults were identified as being at risk. In parallel, older adults who presented a higher level of asymmetries in LL showed a lower level of FA.

Differential Repeated Sprinting Training in Youth Basketball Players: An Analysis of Effects According to Maturity Status

The differential learning approach, which includes fluctuations that occur without movement repetitions and without corrections has received growing interest in the skill acquisition field. This study aimed to determine the effects of a 9-week training intervention involving differential repeated sprint training on a series of physical tests in youth basketball players. A total of 29 participants with different maturity statuses (pre-peak height velocity (PHV), n = 7; mid-PHV, n = 6; post-PHV, n = 16) completed 2 sessions per week of differential repeated sprint training for a period of 9 weeks. Sessions consisted of 2 × 10 repetitions sprints of 20-m whereby participants were instructed to perform various additional fluctuations for each repetition. Before and after the training intervention, participants completed jumping tests (countermovement jump (CMJ), single-leg CMJs, the modified 505 agility test, and straight sprinting tests (0-10 splits time), and maturity status was evaluated as well. Within-group analysis showed improvement in CMJ asymmetries and changes in direction asymmetries and 10-m sprint performance for the pre-, mid-, and post-PHV groups, respectively (p < 0.05), with large to very large effects. Analysis of covariance demonstrated that changes in sprint time in post-PHV players were greater than in the pre- and mid-PHV groups (p < 0.05), with moderate effect. Adding random fluctuations during repeated sprint training appear to be a suitable and feasible training strategy for maintaining and enhancing physical performance in youth basketball players, irrespective of maturity status. Furthermore, the present findings encourage practitioners to implement the present approach in youth athletes to improve their physical performance, but they should be aware that training response can vary according to maturity status.

Influence of the sport specific training background on the symmetry of the single legged vertical counter movement jump among female ballet dancers and volleyball players

Introduction

Vertical jumps are the key components of performance in the classical ballet and volleyball. Asymmetry of performance between the lower extremities is a potential risk factor for injury.

Purpose

The purpose of this study was to analyse the symmetry of the unilateral vertical countermovement jump (CMJ) in a group of female ballet dancers and in a group of female college volleyball players.

Methods

We tested the CMJ with the dominant and nondominant leg and the bilateral CMJ among 15 female ballet dancers and 15 female volleyball players aged 18–24 years. Ground reaction forces were recorded with the force plate and five variables were analysed - jump height, power, energy, and time to flight and time to maximum force during landing.

Results

2 × 2 repeated measures of ANOVA indicates that type of sport is influencing some of the single leg CMJ variables (energy used and time to maximal force in landing), there was a significant asymmetry between dominant and non-dominant leg in some of the vertical CMJ variables (CMJ height, energy used and the average power was marginally significant). The interaction between the type of sport and leg dominance however was not significant for all of the analysed CMJ variables indicating no difference in asymmetry between the dominant and non-dominant leg in the two investigated sports. The results expressed in the percentage differences between both legs that is widely used in the scientific literature showed that ballet dancers exhibited more symmetrical CMJ height, power, and energy compared to volleyball players. The average percent difference in CMJ height between the dominant and non-dominant leg was 4.26 (10.60) % and 13.36 (14.72) %, respectively. On average, volleyball players jumped slightly higher at the bilateral CMJ (p < 0.001).

Conclusion

Sport-specific training background could explain the observed contralateral deficit differences between two sport groups. The elements of ballet training could be introduced into the volleyball training to overcome observed this contralateral deficit.

Assessing Eccentric Hamstring Strength Using the NordBord: Between-Session Reliability and Interlimb Asymmetries in Professional Soccer Players

ABSTRACT

Bishop, C, Manuel, J, Drury, B, Beato, M, and Turner, A. Assessing eccentric hamstring strength using the NordBord: between-session reliability and interlimb asymmetries in professional soccer players. J Strength Cond Res 36(9): 2552-2557, 2022-The aims of the current study were to (a) determine between-session reliability of peak force during the Nordic hamstring exercise (NHE) in professional soccer players during the preseason and (b) determine the subsequent magnitude and direction of peak force asymmetry from the NHE between all test sessions. The NHE was used as a weekly monitoring tool in 14 professional soccer players, throughout a 6-week pre-season period to record peak force from the NordBord device (in Newtons-N) across an average of 6 repetitions for the dominant and nondominant limbs. The first 2 weeks were used as familiarization sessions, with 4 data collection sessions thereafter. When considering relative reliability, intraclass correlation coefficients (ICCs) showed moderate levels of between-session reliability across the 4 weeks on both limbs (dominant = 0.54-0.73 and nondominant = 0.68-0.73) and for the sum total (0.69-0.73). For absolute reliability, both limbs showed coefficient of variation (CV) values <10% across all 4 weeks, with the exception of the dominant limb between sessions 1-2 (10.06%). When determining systematic bias, no meaningful changes were evident in peak force on either limb ( g = -0.59 to 0.40) or for the magnitude of asymmetry ( g = -0.24 to 0.35). However, when assessing consistency in the direction of asymmetry, levels of agreement ranged from 43 to 57% across the data collection period, indicating substantial variation in the superior performing limb from session to session. The findings from this study indicate that 2 familiarization sessions is enough to elicit reliable peak force data during the NHE in professional soccer players during preseason, despite a very small learning effect across the testing period.

Bilateral vs. Unilateral Countermovement Jumps: Comparing the Magnitude and Direction of Asymmetry in Elite Academy Soccer Players

ABSTRACT

Bishop, C, Abbott, W, Brashill, C, Turner, A, Lake, J, and Read, P. Bilateral vs. unilateral countermovement jumps: comparing the magnitude and direction of asymmetry in elite academy soccer players. J Strength Cond Res 36(6): 1660-1666, 2022-The aims of this study were to compare the magnitude and direction of asymmetry in comparable bilateral and unilateral countermovement jumps (CMJs). Forty-five elite academy soccer players from under-23 (n = 15), under-18 (n = 16), and under-16 (n = 14) age groups performed bilateral and unilateral CMJs as part of their routine preseason fitness testing. For the magnitude of asymmetry, no significant differences were evident for any metric between tests. However, the eccentric impulse asymmetry was significantly greater than mean force and concentric impulse in both bilateral and unilateral tests (p < 0.01). For the direction of asymmetry, Kappa coefficients showed poor levels of agreement between test measures for all metrics (mean force = -0.15, concentric impulse = -0.07, and eccentric impulse = -0.13). The mean jump data were also presented relative to the body mass for each group. For the bilateral CMJ, significant differences were evident between groups but showed little consistency in the same group performing better or worse across metrics. For the unilateral CMJ, eccentric impulse was the only metric to show meaningful differences between groups, with the under-18 group performing significantly worse than under-23 and under-16 players. This study highlights that despite the magnitude of asymmetry being similar for each metric between comparable bilateral and unilateral CMJs, consistency in the direction of asymmetry was poor. In essence, if the right limb produced the larger force or impulse during a bilateral CMJ, it was rare for the same limb to perform superior during the unilateral task. Thus, practitioners should be aware that bilateral and unilateral CMJs present different limb dominance characteristics and should not use 1 test to represent the other when measuring between-limb asymmetries.

Countermovement Jump in Female Sprinters: Kinetic Parameters and Asymmetry

Analyzing asymmetry from biomechanical parameters of the lower extremities has become a standard practice for accurate assessment of athletic performance, injury prevention, and rehabilitation. The aims of this study were (1) to determine differences between explosive strength and asymmetry of the lower extremities using kinetic parameters of the CMJ jump test in young female athletics, and (2) to investigate correlation between speed and asymmetry, as speed and kinetic parameters. The two groups of nine female sprinters (mean ± SD; G1-age 15.6 ± 1.34 years, height 170.1 ± 0.57 cm, body mass 62.54 ± 7.73 kg, and BMI 21.6 ± 2.05; G2-age 16.2 ± 1.3 years, height 168.4 ± 0.61 cm, body mass 57.69 ± 3.12 kg, and BMI 20.37 ± 1.38) performed the CMJ test without using an arm swing, as well as a 100 m test. Two tensiometric platforms were used for the kinetic parameters and asymmetry. Asymmetry was calculated by an AI equation, and the values of the takeoff velocities and jump height parameters were obtained by integral formula and the trapezoidal rule of impulse-momentum methods. The results show differences in four kinetic parameters: height (G1- 26.82 ± 3.56 cm, vs. G2- 17.45 ± 2.01 cm), concentric impulse, (G1- 96.05 ± 16.95 N⋅s, vs. G2- 68.41 ± 4.77 N⋅s), takeoff velocity, (G1- 2.29 ± 0.14 m/s, vs. 1.83 ± 0.12 m/s), and concentric velocity, m/s (G1- 1.5 ± 0.175 m/s, vs. 1.17 ± 0.122 m/s), as well as a negative strong and very strong correlation between asymmetry and kinetic parameters for three parameters: Peak Force (G1- r = −0.878, and all subjects r = −0.633), Eccentric Impulse of left leg (G1- r = −0.865) and Concentric Impulse of right leg (G2- r = −0.878), (p <.05). The younger sprinters did not show the principle of muscle activation in the form of a longer preparatory phase of contact time, eccentric and concentric phase, as well as a force impulse that is optimal. There was no correlation between asymmetry and sprint performance.

Interday Reliability of a Testing Battery to Assess Lateral Symmetry and Performance in Well-Trained Volleyball Players

ABSTRACT

Iglesias-Caamaño, M, Álvarez-Yates, T, Carballo-López, J, Cuba-Dorado, A, and García-García, O. Interday reliability of a testing battery to assess lateral symmetry and performance in well-trained volleyball players. J Strength Cond Res 36(4): 895-901, 2022-This study aimed to determine the relative and absolute interday reliability of a testing battery for lateral asymmetry detection and performance in well-trained volleyball players and to determine whether retaining for analysis the best or the average value of 3 attempts would modulate the interday reliability and the magnitude of asymmetry. Thirteen Superleague-2 male volleyball players performed on 3 different days a 5-test battery composed of 4 tests for assessing muscle asymmetries: active knee extension, Y-balance test (YBT), modified 20-yard shuttle run (M-20Y), and single-leg countermovement jump; and one for volleyball sports performance: attack jump (AJ). The intraclass correlation index and coefficient of variation (CV) together with the standard error of measurement (SEM, %SEM) and the minimum detectable change (MDC, %MDC) were taken to calculate relative and absolute reliability. Two-way repeated-measures analysis of variance was used to establish the differences between the best value and the average value in each session. The low %MDC (<10%) suggests that all the tests that make up the testing battery would have good sensitivity to detect possible asymmetries. All tests have shown an excellent interday absolute reliability (CV <4.5%; %SEM < 5%), both when retaining the best value and the average value of the 3 attempts. In addition, AJ and YBT (on both legs) obtained a good-excellent relative reliability, whereas the M-20Y test seems to show a lower relative reliability. Retaining the best value or the average value to determine the magnitude and direction of asymmetry seems to be equally reliable with no differences between both values.

Magnitude or Direction? Seasonal Variation of Interlimb Asymmetry in Elite Academy Soccer Players

ABSTRACT

Bishop, C, Read, P, Chavda, S, Jarvis, P, Brazier, J, Bromley, T, and Turner, A. Magnitude or direction? Seasonal variation of interlimb asymmetry in elite academy soccer players. J Strength Cond Res 36(4): 1031-1037, 2022-Previous research has highlighted a distinct lack of longitudinal data for asymmetry. The aims of this study were to provide seasonal variation data for the magnitude and direction of asymmetry. Eighteen elite male academy soccer players (under-23) performed unilateral countermovement jumps (CMJs) and unilateral drop jumps (DJs) during pre-season, mid-season, and end of season time points. Recorded metrics for asymmetry included jump height and concentric impulse for the CMJ, and jump height and reactive strength index for the DJ. The magnitude of asymmetry showed trivial to small changes throughout the season (CMJ effect size [ES] range = -0.43 to 0.05; DJ ES range = -0.18 to 0.41). However, Kappa coefficients showed poor to substantial levels of agreement for the direction of asymmetry during the CMJ (Kappa = -0.06 to 0.77) and DJ (Kappa = -0.10 to 0.78) throughout the season. These data show that when monitoring asymmetry, the magnitude alone may provide a false impression of consistent scores over time. By contrast, monitoring the direction of asymmetry highlights its task and variable nature and is suggested as a useful tool for practitioners who wish to monitor asymmetry over the course of a competitive soccer season.

Temporal and Spatial Characteristics of Pacing Strategy in Elite Women's 400 Meters Hurdles Athletes

The main objective of the study was to assess the pacing strategy of running 400 m hurdles of the world-level female athletes over the past 40 years based on the functional asymmetry -temporal and spatial characteristics. The data were collected from 1983 to 2019 using the review of scientific literature. Over the 35 years of the study, 37 top-level competitions with 283 finalists-competitors were included. The analysis of the 400 m hurdle covered mainly spatial and temporal factors of the run, related to those technical skills, the level of motor skills, and somatic structure. In addition to the basic statistics, the ANOVA analysis of variance, regression analysis, Pearson correlation, the principal component analysis (PCA), and Kaiser’s criterion was used for the multivariate analysis. The final result in the 400 mH run is determined not by the simple sum of the individual temporal and/or spatial characteristics of the run (the number of steps, the type of attacking leg, but their interaction in the area of functional asymmetry. The decisive factor in the 400 mH run strategy is the second curve, where the emphasis is on the optimal setting of the stride pattern in the context of minimizing the loss of running speed. Additionally, the application of multidimensional statistical methods is a valuable tool that allows to significantly deepen the interpretation of the obtained results, and thus optimize a strategy for a 400 mH run.

Load-Induced Changes of Inter-Limb Asymmetries in Dynamic Postural Control in Healthy Subjects

Inter-limb asymmetries are associated with a higher potential risk for non-contact injuries. Differences in function or performance between the limbs might lead to imbalances and promote instability, increasing the potential risk for injuries. Consequently, an investigation of inter-limb asymmetries should be included in injury risk assessment. Furthermore, since non-contact injuries mainly occur under loaded conditions, an investigation of load-induced changes of inter-limb asymmetries can provide additional information on the athlete's potential injury risk. Therefore, the current study aimed to investigate the influence of physical load on inter-limb asymmetries in dynamic postural control, which is essential in situations with a high risk for non-contact injuries such as landing, cutting, or stopping. In total, dynamic postural control of 128 active and healthy subjects (64 males and 64 females, age: 23.64 ± 2.44, height: 176.54 ± 8.96 cm, weight: 68.85 ± 10.98 kg) was examined. Dynamic postural control was tested with the Y-Balance Test (YBT) before and after a loading protocol on a bicycle ergometer or a treadmill. The results showed no significant increase of the inter-limb asymmetries in anterior direction [F (1, 126) = 4.44, p = 0.04, η2 p = 0.03]. Moreover, there is high variation between the subjects regarding the magnitude and the direction of the asymmetries and the changes due to load. Therefore, a more individual analysis considering the magnitude and the direction of the asymmetries is required. Thereby, considering different modifying factors, e.g., sex, injury history, and baseline level of asymmetries, can be helpful. Moreover, an analysis of the changes during load might provide further insights, reveal possible differences, and help detect the reasons and mechanisms underlying inter-limb asymmetries and asymmetrical loading.

Associations between differing magnitudes of inter-limb asymmetry and linear and change of direction speed performance in male youth soccer players

Study aim: This study examines the relationship between different magnitudes of asymmetry and their effects on speed performance.

Material and methods: Forty-two sub-elite male youth soccer players performed a 30-m sprint, change of direction, single leg countermovement jump and single leg hop. Subjects were divided into groups with vertical and horizontal asymmetry, and both groups were then divided into three groups according to magnitudes of inter-limb asymmetry (<5%, 5–10%, and >10%).

Results: The results showed no significant correlation between different jump asymmetry magnitudes and the mentioned outcomes of speed performance (p > 0.05). In addition, larger asymmetries resulted in faster linear speed, even if small differences (g range = 0.00 to 0.57; p > 0.05). But this was not similar for change of direction speed (g range = –0.42 to 0.34; p > 0.05).

Conclusions: There are inconsistent findings for the effects of inter-limb asymmetries on speed performance. The results of the present study indicate that the magnitude of asymmetry had no meaningful association with independent measures of performance in soccer players. Therefore, it seems more likely to explain the effects of individual asymmetries on performance rather than the idea that asymmetry negatively affects performance.