Reliability and validity of field-based measures of leg stiffness and reactive strength index in youths

Effects of Fatigue on Lower Limb Biomechanics and Kinetic Stabilization During the Tuck-Jump Assessment

CONTEXT

General and local muscular fatigue is postulated to negatively alter lower limb biomechanics; however, few prospective studies have been done to examine the effect of fatigue on tuck-jump performance. The tuck-jump assessment (TJA) is a criteria-based visual screening tool designed to identify neuromuscular deficits associated with anterior cruciate ligament (ACL) injury. Use of kinetics during the TJA after an intense sport-specific fatigue protocol may identify fatigue-induced neuromuscular deficits associated with ACL injury risk.

OBJECTIVE

To examine the effects of a sport-specific fatigue protocol on visually evidenced (2-dimensional) technical performance of repeated tuck jumps and lower limb kinetic stabilization.

DESIGN

Cross-sectional study.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twelve female netball athletes (age = 20.8 ± 2.6 years, height = 170.0 ± 0.04 cm, mass = 67.5 ± 7.4 kg).

MAIN OUTCOME MEASURE(S)

Participants performed 1 set of a TJA before and after a sport-specific fatigue protocol. Paired t tests and effect sizes were used to evaluate differences and the magnitude of differences in TJA scoring criterion, kinetics, and kinetic stabilization prefatigue to postfatigue.

RESULTS

A small increase was observed for vertical relative lower extremity stiffness postfatigue (P = .005; Hedges g = 0.45). Peak center-of-mass displacement, time of jump cycle, ground contact time, flight time, jump height, and vertical net impulse decreased with small to moderate effect sizes (P < .01; Hedges g range, 0.41-0.74). No differences were observed for TJA composite scores, peak vertical ground reaction force, and stabilization indices of kinetic variables after the fatigue protocol (P > .05).

CONCLUSIONS

Kinetic analysis of repeated tuck jumps after a fatigue protocol identified an altered jumping strategy, which was not identifiable via visual 2-dimensional assessment. However, based on kinetic measures, fatigue induces a stiffer jumping strategy, and practitioners should consider assessing load attenuation strategies that may not be visually evident when evaluating ACL-injury risk factors in athletes who are fatigued.

Little associations exist between the three commonly used functional screening tests in collegiate athletes

Although an abundant number of studies have investigated the predictability of the commonly used functional screening tests and despite their popularity and applicability, the relationships between these tests have rarely been studied and have not been well established. This study aimed to examine the potential association between the Functional Movement Screen (FMS), Y Balance Test (YBT), and Landing Error Scoring System (LESS). Six hundred twenty-seven Iranian collegiate athletes (347 males, age = 22.63 ± 4.07, weight = 75.98 ± 13.79, height = 181.99 ± 10.15, BMI = 22.84 ± 3.16; and 280 females, age = 22.22 ± 3.37, weight = 60.63 ± 9.58, height = 166.55 ± 6.49, BMI = 21.81 ± 2.84) participated in this study. Following a 5-min warm-up, each participant underwent a standardized screening battery including the FMS, YBT, and LESS, and the scores were recorded and live coded for the statistical analysis, except for the LESS. The LESS tests were video recorded and scored by one expert examiner using an open-source 2D video analysis software (Kinovea- version 0.9.5), afterwards. The Spearman correlation was utilized as a measure for the correlation, and the Mann‒Whitney U test with a significance level of 0.05 was used to check the differences between male and female athletes. The statistical analysis was performed with RStudio 2023.03.0 using R 4.3.1. A small correlation (0.364) was observed between the FMS composite score and the YBT in male athletes. All other pairwise correlations were negligible among male and female athletes, ranging from - 0.096 to 0.294. Reducing the FMS to the component scores targeting the lower extremities did not alter the correlation to the other screening scores. The median FMS composite score in female athletes was significantly higher than that in males (p < 0.001). Negligible correlations exist between the FMS, LESS, and YBT; they do not measure the same values and therefore are irreplaceable with one another. A combination of these tests as a standardized screening battery may potentially better identify injury-predisposed athletes than the application of each test as a stand-alone screening test. Females outperformed males in the FMS test significantly, so sex must be considered a key variable in the FMS studies. Males had slightly higher LESS scores (median difference = 0.5) than females, but this difference is not clinically meaningful. Future research should continue to explore the relationships between various functional screening tests and identify the most effective combinations for comprehensive assessment in different populations and sports disciplines.

Acute pre- and post-competitive soccer match-play changes in neuromuscular factors, physical performance, and muscle response in youth male players

There is a paucity of literature analyzing the impact of fatigue from actual competitive soccer match-play on measures of neuromuscular function and muscle damage markers in youth players. Aims The main purpose was to analyze the acute pre- and post-competitive soccer match-play changes in measures of landing mechanics, stretch-shortening cycle capability, physical performance, muscle damage, and match intensity. A secondary purpose was to explore whether the players' maturity status and chronological age were associated with post-soccer match-play responses. Methods Thirty-two male youth outfield soccer players from two chronological competition age groups (U14 and U16) were assessed pre-and post-competitive soccer match-play for 2D dynamic knee valgus at landing (DKV), leg stiffness, reactive strength index, 20 m sprint time, CMJ-Abalakov jump height, creatine kinase and urea activity and visual analogue scale for muscle soreness (VAS). Players' maturity status was also estimated using a previously validated regression equation. Results There were statistically significant (p < 0.05) post-competitive soccer match-play impairments in all measures of stretch-shortening cycle capability and muscle damage markers assessed irrespective of the age group. Likewise, significant post-match play alterations in landing mechanics (DKV of the dominant leg) and physical performance (sprint time and jump height) scores were documented for the U14 players. Significant interactions between maturity and landing mechanics and VAS responses to competitive soccer match-play were also found whereby more mature players demonstrated larger post-match changes. Conclusion The findings of this study suggest the existence of a negative influence of competitive match-play on neuromuscular function and muscle damage in youth soccer.

Maturity status influences perceived training load and neuromuscular performance during an academy soccer season

Commonly we see large within-age-group variations in physique, including body mass, stature, and percentages of predicted adult height, which suggests that age-specified training loads are flawed. Aims were to investigate how maturation impacts training load and neuromuscular response within academy soccer and to provide recommendations for practitioners. Fifty-five male soccer players (age 14.5 ± 1.2 years; stature 172 ± 10 cm; body mass 59.8 ± 10 kg; 94.1 ± 1.8% predicted adult height) reported differential ratings of perceived exertion (AU) across a season. Neuromuscular performance (countermovement jump, reactive strength index, absolute and relative leg stiffness) was measured at three time points across the season. Perceived exertion and neuromuscular performance were examined using linear mixed modelling, supplemented with non-clinical magnitude-based decisions. Analysis indicates every 5% increase in maturity status results in players perceiving overall session intensity 6.9 AU lower and 13.9 AU lower for a 10% maturity shift. Both 5% and 10% changes in maturity most likely resulted in higher countermovement jump, with likely to very likely differences observed for RSI and ABS. Maturity substantially influences neuromuscular performance over the season. Therefore, maturity-specific load prescription may prevent significant within age-group differences in accumulated load, possibly reducing injury risk and/or burnout.

The effect of plyometric training and moderating variables on stretch-shortening cycle function and physical qualities in female post peak height velocity volleyball players

Purpose: Although several studies investigated the effect of plyometric training on physical performance, there is a lack of clarity regarding the effectiveness of plyometric training or its moderator variables in youth female volleyball players. The primary aim of this study was to explore the effect of horizontal plyometric training on explosive stretch-shortening cycle hops and jumps in the vertical and horizontal directions in female post peak height velocity (PHV) volleyball players. The secondary aim was to assess the influence of participant and training related moderators on horizontal plyometric training in post-PHV volleyball players. Methods: A total of 23 post-PHV volleyball players participated in this 8-week intervention with horizontal plyometric exercises, twice a week. Pre-testing and post-testing included bilateral and unilateral vertical sub-maximal hopping, horizontal jumping and hopping, and a drop jump test. The effectiveness of the intervention was assessed using a paired t-test. The influence of internal moderators such as age, maturity and body mass and external moderators such as training volume were assessed using regression and correlation analysis. Results: An 8-week plyometric training improved sub-maximal hopping at 2.5 Hz left by 4.4%, bilateral sub-maximal hopping at 2.0 Hz by 9.5% and bilateral sub-maximal hopping at 2.2 Hz by 6.8% in post-PHV female volleyball players. Horizontal jumping and hopping, reactive strength index and other sub-maximal hopping conditions did not improve significantly. Body mass had a large moderating effect on vertical unilateral sub-maximal hopping at 2.5 Hz right (p = 0.010, η 2 = 0.314), vertical unilateral hopping at 3.0 Hz right (p = 0.035, η 2 = 0.170), and vertical unilateral hopping at 3.0 Hz left (p = 0.043, η 2 = 0.203). Training volume together with generalized joint hypermobility moderated right leg triple broad hop performance, whereas maturity and age did not moderate any variables. Conclusion: This study determined that 8 weeks of horizontal plyometric training can improve unilateral absolute leg stiffness in post-PHV female volleyball players, and this training effect can be moderated by body mass. Furthermore, the training effect on triple hopping performance on the right leg can be moderated by combined training volume with generalized joint hypermobility.

Comparing the effects of akimbo and bent-in-front arm positions on jump metrics: Validity and reliability of a modified 10/5 repeated jump test

The purpose of this study was to compare the effects of two arm positions, akimbo and the newly introduced bent-in-front, on jump metrics in the ten-to-five repeated jump test (10/5 RJT) and to evaluate the reliability and validity of new modified bent-in-front variation. In contrast to akimbo, bent-in-front arm variation allows participants to use their arms freely by holding them with free hands, with parallel arms bent in front of chest and elbows pointing downward without swinging. This new arm position was designed to alleviate postural control difficulties and ensure smooth movement during repeated jumps on the force plate. However, the 10/5 RJT was designed to measure lower-body stretch-shortening cycle (SSC) performance, and it is unknown whether the bent-in-front arm variation would affect jump performance, such as arm swing. If the arms can be freed without interfering with jump performance, it would be possible to determine lower-body SSC performance without the contribution of the arms and have the advantage of postural control assistance. Fifty-five healthy sports science students who regularly participated in intercollegiate or recreational sports performed 10/5 RJT with arms akimbo and bent-in-front during two sessions. Four jump metrics, including the reactive strength index, and four reliability and validity statistics, including intraclass-correlation-coefficients, were estimated. The results indicated no significant differences in the jump metrics between two arm variations. Bent-in-front arm variation can be deemed as a valid and reliable test. Therefore, the newly introduced 10/5 RJT with arms bent-in-front can be used to evaluate SSC performance in this cohort.

Associations Between Motor Competence and Physical Activity, Physical Fitness and Psychosocial Characteristics in Adolescents: A Systematic Review and Meta-analysis

BACKGROUND

Motor competence is an integral component of the health and performance of youth. Numerous studies support the hypothesis that motor competence interacts with perceived motor competence and physical fitness during childhood to induce positive (e.g. healthy weight status) or negative (e.g. reduced physical activity engagement) trajectories. Yet, while adolescence is a key period of rapid growth and maturation, no systematic reviews and meta-analyses have examined the association between motor competence and physical activity, physical fitness and psychosocial characteristics solely within adolescents.

OBJECTIVES

This study aimed to (1) analyse the scientific literature evaluating associations between motor competence and physical activity, physical fitness and/or psychosocial characteristics amongst adolescents; (2) evaluate the associations between motor competence and physical activity, physical fitness characteristics and/or psychosocial characteristics amongst adolescents; and (3) investigate the impact of moderator variables (i.e., age, sex, type of motor competence assessment) on the associations.

METHODS

A systematic search of electronic databases was conducted, followed by a qualitative synthesis of study methods. Random-effects meta-analyses were performed to establish the magnitude and orientation of pooled correlation coefficients between motor competence and physical activity, physical fitness and psychosocial characteristics of adolescents, whilst considering potential moderators (i.e., age, sex, type of motor competence assessment).

RESULTS

Sixty-one studies were included, totalling 22,256 adolescents. Twenty-seven different assessments of motor competence were used, with 31 studies utilising product-orientated (i.e. outcome) motor competence assessments. Meta-analyses of 43 studies showed that motor competence was positively associated with physical activity (r = 0.20 to 0.26), some physical fitness characteristics (e.g. muscular strength, cardiovascular endurance; r = 0.03 to 0.60) and psychosocial characteristics (r = 0.07 to 0.34), and inversely associated with weight status (r =  - 0.36 to - 0.10), speed (r =  - 0.31) and agility (r =  - 0.37 to 0.41). Associations with flexibility were unclear.

CONCLUSIONS

The results of this systematic review and meta-analysis support the hypothesised interactions of motor competence with physical activity (positive), physical fitness (positive except for weight status, speed and agility) and psychosocial characteristics (positive) in adolescence. However, methodological approaches vary considerably (e.g. variety of motor competence assessments utilised), with limitations of the current literature including an inadequate assessment of motor competence, a lack of longitudinal observations and a failure to account for biological maturation. Future research assessing associations between motor competence and physical activity, physical fitness and psychosocial characteristics of adolescents should include longitudinal observations of a combined motor competence assessment (i.e. process and product) and account for biological maturation. Improved evaluation using these recommendations could provide more accurate data, leading to more targeted interventions to improve adolescents' physical and psychosocial outcomes.

CLINICAL TRIAL REGISTRATION

CRD42021233441 (PROSPERO ID).

Sprint and jump performances of female athletes are enhanced in the evening but not associated with individual chronotype

Sprint and jump performances represent performance-determining parameters in individual and team sports. Fluctuations in performance depending on the daytime raise the question of the best time to train and compete. Given the scarce research on females, this study aimed to analyze the influence of daytime on sprint and jump performances and to investigate whether the performance difference is related to the chronotype. Thus, 23 female sports students completed a questionnaire to assess their chronotype followed by performing two 30 m sprints, 5 Repeated Jump Tests (5RJT), and countermovement jumps (CMJ) in the morning (7:00-9:00 h) and evening (17:00-19:00 h). Time after 5 m, 10 m, and 30 m during the sprints, reactive strength index (RSI) during the 5RJTs, and jump height during the CMJs were examined. The performance during the 30 m sprint (t(22) = 5.28, p < 0.01 moderate effect size: 0.50) and the two jump tests (5RJT: t(22) = 8.27, p < 0.01 large effect size: 0.95; CMJ: t(22) = 5.46, p < 0.01 moderate effect size: 0.79) were significantly better in the evening than in the morning. There was no significant correlation between chronotype and the time-of-day effect. The results should be considered when planning training and competition.

Longitudinal changing relationships between growth tempo and vertical stiffness in movement across maturation

PURPOSE

To determine whether average growth tempo influenced longitudinal relationships between maturity status and coordination capability using a 15-s hopping task (Aim 1). To investigate how differences in absolute growth tempo were associated with change in coordination capability within and across peak growth (Aim 2).

METHODS

Participants were N₁ = 110 (Aim 1) and N₂ = 71 (Aim 2) Australian male competitive swimmers, aged 10-15 years, exposed to repeated-measures tracking (2-years, and 12-months respectively) of maturity status, growth tempo and movement coordination capability. Coordination capability was estimated via vertical stiffness (K_V) in a hopping task, reflected by participant mean K_V and between-jump coefficient of variation (CV).

RESULTS

For Aim 1, log-linear mixed model trends identified maturity status and growth tempo were significantly associated with K_V mean and K_V CV. For a given maturity status, mean K_V was 9% lower in the 'High' average growth tempo group than the 'Low' average tempo group. For Aim 2, mixed repeated-measures analyses of variance identified how time points of increased growth tempo were significantly associated with 7-11% reductions in mean K_V, with similar mean K_V decrements irrespective of growth tempo group. Meanwhile, K_VCV only illustrated progressive longitudinal reductions.

CONCLUSIONS

Within maturational progression, short-term accelerations in growth tempo corresponded with short-term decreases in K_V mean, suggesting temporary disruptions to movement coordination capability. Measuring growth tempo and applying hopping tasks in specific movement contexts could help consistently identify disturbances in motor coordination.

Validity of the Baiobit Inertial Measurements Unit for the Assessment of Vertical Double- and Single-Leg Countermovement Jumps in Athletes

Jump tests are simple, quick to execute, and considered the most reliable tool to measure lower extremities power and explosiveness in athletes. Wearable inertial sensors allow the assessment of jumping performance on any surface. The validity of inertial sensors measurements is a pivotal prerequisite to reliably implement their utilization in the clinical practice. Twenty-seven athletes (20 M/7 F, age: 27 ± 7 years old) performed five double-leg countermovement jumps (CMJs) and three single-leg CMJs per side with their hands on their hips. Jump height was measured/computed simultaneously with the optoelectronic system, force platforms, and the Baiobit inertial sensor system. The athletes completed the international physical activity questionnaire (IPAQ). When comparing the methods (Baiobit vs. force platforms), a non-statistically significant bias of 1.8 cm was found for two-leg CMJs and -0.6 cm for single-leg CMJs. The intraclass correlation coefficients (ICCs) was "excellent" for double-leg CMJs (ICC = 0.92, 95% CI = 0.89-0.94) and "good" for single-leg CMJs (ICC = 0.89, 95% CI = 0.85-0.91). When comparing the methods (Baiobit vs. force platforms + optoelectronic system), a non-statistically significant bias of -0.9 cm was found for two-leg CMJs and -1.2 cm for single-leg CMJs. The intraclass correlation coefficient (ICC) was "good" for both double-leg CMJs (ICC = 0.80, 95% CI = 0.73-0.85) and for single-leg CMJs (ICC = 0.86, 95% CI = 0.80-0.89). Baiobit tends to overestimate double- and single-leg CMJ height measurements; however, it can be recommended in the world of rehabilitation and sport analysis.

Test–Retest Reliability and Sensitivity of Common Strength and Power Tests over a Period of 9 Weeks

This study evaluated the reliability and sensitivity of a set of different common strength and power tests in a healthy adult population in a span of 9 weeks. Seventeen subjects (24.2 ± 2.2 years, 1.75 ± 0.10 m, 68.6 ± 14.2 kg, seven women) participated in the study. We tested countermovement jumps, reactive hops, and the maximal voluntary contraction (MVC) of handgrip and isometric knee extension. The tests were conducted in three separate sessions across a nine-week period, with one week between the first two sessions and eight weeks between the second and the third. Reliability and sensitivity statistics for each test were calculated for both the average of three trials and the best result during each session. The MVC of isometric knee extension and handgrip, as well as the countermovement jump test, demonstrated very high reliability and sensitivity over the nine-week period. The peak force of the reactive hops demonstrated high reliability but high sensitivity only for the average but not for the best result. The average contact time of reactive hops was neither a sensitive nor reliable measurement. In conclusion, isometric maximal knee extension and handgrip tests, as well as countermovement jumps and peak force of reactive hops, can be used as reliable and sensitive measurements of isometric and reactive strength and power over time periods of up to eight weeks. We recommend the use of the average results of three trials instead of the best performance value for longitudinal studies, as this procedure produces more consistent results and a lower measurement error.

School-Based Comprehensive Strength Training Interventions to Improve Muscular Fitness and Perceived Physical Competence in Chinese Male Adolescents

Purpose. This research was to see how effective and feasible school-based comprehensive strength training programs are in improving muscular fitness and perceived physical competence in Chinese male adolescents. Methods. A total of 123 participants ( $13.46\pm 0.60$ years) were randomized to comprehensive strength training intervention group (CST) ( $n=62$ ) and the control group (CON) ( $n=61$ ). The training sessions were performed three times a week for ten weeks in CST. Muscular fitness (i.e., muscular strength, power, and muscular endurance) and perceived physical competence were assessed at initial testing and final testing. Results.The subjects in the CST significantly improved their mean performance in standing long jump ( $p<0.05$ ), vertical jump ( $p<0.05$ ), 1 min push-ups ( $p<0.05$ ), 1 min sit-ups ( $p<0.05$ ), handgrip strength ( $p<0.05$ ), and perceived physical competence ( $p<0.05$ ) after the intervention. Moreover, the CST were greater in standing long jump ( $p<0.05$ ), vertical jump ( $p<0.05$ ), 1 min sit-ups ( $p<0.05$ ), handgrip strength ( $p<0.05$ ), and perceived physical competence ( $p<0.05$ ) compared to the CON, but no in 1 min push-ups ( $p>0.05$ ). Conclusions. The comprehensive strength training interventions designed in this study can significantly increase male adolescents’ muscular fitness, especially in the lower extremity muscle power and abdominal core endurance, and can enhance their perceived physical competence.

Poor Reliability of Measurement Instruments to Assess Acute Responses to Load in Soccer Players Irrespective of Biological Maturity Status

PURPOSE

To assess the short-term reliability of measurement instruments to quantify the acute psychophysiological response to load in adolescent soccer players in relation to biological maturity.

METHODS

Data were collected from 108 U12 to U17 soccer players on 2 consecutive weeks (pre, n = 32; at, n = 34; and post, n = 42 estimated peak height velocity). Measurements consisted of the Short Recovery and Stress Scale, a countermovement jump, assessment of leg stiffness, and a submaximal run to assess exercise heart rate and heart rate recovery. Test-retest reliability was assessed with the coefficient of variation (CV) and intraclass correlation coefficient (ICC).

RESULTS

Items of the Short Recovery and Stress Scale showed poor reliability across maturity groups (CV = 7.0%-53.5%; ICC = .28 to .79). Only few countermovement jump variables (jump height, concentric impulse, and concentric velocity) possessed good reliability. For most variables of the countermovement jump, reliability was better for the post peak height velocity group followed by at-peak height velocity and prepeak height velocity. Very high levels of reliability across maturity groups were observed for exercise heart rate (CV < 1.8%; ICC > .94), while heart rate recovery was more variable (CV < 16.5%; ICC > .48).

CONCLUSION

Results suggest that the majority of investigated variables have poor reliability, questioning their ability to detect small, yet meaningful changes in acute responses to load in adolescent soccer players.

How to Improve the Reactive Strength Index among Male Athletes? A Systematic Review with Meta-Analysis

The reactive strength index (RSI) describes the individual’s capability to quickly change from an eccentric muscular contraction to a concentric one and can be used to monitor, assess, and reduce the risk of athlete’s injury. The purpose of this review is to compare the effectiveness of different training programs on RSI. Electronic searches were conducted in MEDLINE, PubMed, Scopus, SPORTDiscus, and Web of Science from database inception to 11 February 2022. This meta-analysis was conducted in accordance with the recommendations of the preferred reporting items for systematic reviews and meta-analyses (PRISMA). The search returned 5890 records, in which 39 studies were included in the systematic review and 30 studies were included in the meta-analysis. Results from the randomized studies with the control group revealed that plyometric training improved RSI in adult athletes (0.84, 95% CI 0.37 to 1.32) and youth athletes (0.30, 95% CI 0.13 to 0.47). Evidence withdrawn from randomized studies without a control group revealed that resistance training also improved the RSI (0.44, 95% CI 0.08 to 0.79) in youth athletes but not in adults. Interventions with plyometric training routines have a relatively large, statistically significant overall effect in both adult and youth athletes. This supports the implementation of this type of interventions in early ages to better cope with the physical demands of the various sports. The impact of resistance training is very low in adult athletes, as these should seek to have a more power-type training to see improvements on the RSI. More interventions with sprint and combined training are needed.

Reliability, Validity, and Performance Characteristics of Elite Adolescent Athletes at Different Stages of Maturity in the 10 to 5 Repeated Jump Test

PURPOSE

To examine the reliability, validity, and performance characteristics of the 10 to 5 repeated jump test (10-5 RJT) in adolescent male athletes. The 10-5 RJT has been shown to be a valid and reliable test of reactive strength index (RSI) in older adolescents (age 17-19 y), but less is known in younger adolescent athletes at different stages of maturity.

METHODS

Athletes (age 11-17 y) completed the 10-5 RJT on 2 days, 1 week apart, to examine the reliability (n = 41), validity (n = 18) of the test. Athletes were classified as pre, circa, or post peak height velocity (PHV) height velocity using maturity offset to examine the effect of maturation status on RSI, flight time (FT), ground contact time (GCT), and jump height (JH) (n = 68) using a cross-sectional design.

RESULTS

Paired samples t tests showed no significant differences (P ≥ .05), and Bland-Altman analysis showed no bias and close limits of agreement for RSI, JH, FT, and GCT between the contact mat and force plate. Interday reliability was rated excellent for RSI (intraclass correlation coefficient = .91) and good for GCT, FT, and JH (intraclass correlation coefficient = .81-.85). All variables had a coefficient of variation ≤ 10%. RSI increased across maturation groups, with significant differences between pre-PHV and post-PHV groups (P = .014, d = 1.00).

CONCLUSION

The 10-5 RJT is a valid and reliable test for adolescent male athletes. Greater RSI with advancing maturity was primarily due to increased FT and JH, with GCT remaining similar.

Maturity Has a Greater Association than Relative Age with Physical Performance in English Male Academy Soccer Players

This study aimed to: (1) examine differences in physical performance across birth-quartiles and maturity-status, and (2) determine the relationships among relative age, maturation and physical performance in young male soccer players. The sample included 199 males aged between 8.1 and 18.9 years, from two professional soccer academies in the English Football League. Data were collected for height, weight, self-reported biological parent heights, 30 m sprint time and countermovement jump (CMJ) height. Relative age was conveyed as a decimal, while maturity status was determined as the percentage of predicted adult height (PAH). There were no significant differences in any measure between birth quartiles, however early maturers outperformed on-time and later maturers in most performance measures. Pearson-product-moment correlations revealed that maturation was inversely associated with 30 m sprint time in U12 to U16 (r = -0.370-0.738; p < 0.05), but only positively associated with CMJ performance in U12 (r = 0.497; p < 0.05). In contrast, relative age was unrelated to sprint performance and only significantly associated with superior CMJ performance in U16. This study indicates that maturity has a greater association with sprint performance than relative age in English male academy soccer players. Practitioners should monitor and assess biological maturation in young soccer players to attempt to control for the influence on physical performance, and avoid biasing selection on absolute performance rather than identifying the most talented player.

Biomechanical Characteristics of Vertical Jumping of Preschool Children in China Based on Motion Capture and Simulation Modeling

Vertical jumping is one of the basic motor skills, and it is an essential part of many sports. The main purpose of this paper is to investigate characteristics of vertical jumping of children. This paper uses a motion capture system, three-dimensional platforms, and a simulation modeling system to analyze the kinematics and dynamics performance of children's vertical jumping. The compression time increases from 3 to 4 years old, and flight height and time increases with age and stage gradually. In the compression phase and pushing phase, the hip and knee joint play a major role; in the landing phase, the knee and ankle joint play a major role. Muscle forces are mainly affected by age, and the three types of muscle force had two different trends. The muscle force of the shank and thigh increased with age, and the pelvic girdle muscles showed an "low-high-low" trend. The regression model suggests that the force of GMiP and the hip angular velocity have a great influence on jumping ability. Therefore, if we want to improve the jumping ability of preschool children, we should pay more attention to hip exercises. We should integrate the hip exercises into interesting games, which are more in line with their physical and mental health.

Evaluation of the Functional Movement Screen (FMS) in Identifying Active Females Who are Prone to Injury. A Systematic Review

Background

The validity of the Functional Movement Screen (FMS) in identifying active females who are predisposed to injury has not been specifically reviewed. This study aims to synthesize the literature on the ability of the FMS to identify at-risk active females.

Methods

Six online databases, including PubMed, Medline, Web of Science, Science Direct, SPORTDiscus and Google Scholar, were searched for the period of April 2006 to September 2021. Out of the 61 potential references, 17 were reviewed in detail with respect to the inclusion criteria; ten were ultimately included. The risk of bias, applicability and level of the studies were then identified using the QUADAS-2 and a checklist for assessing methodological quality. The following data were obtained from the included studies: year of publication, title, study type, participants’ demographic, sample size, FMS cutoff point, injury definition, statistical analyses used, FMS results and study level.

Results

Generally, the quality of eight studies was poor to moderate due to both small sample sizes and short follow-up periods. Except for a study on military members, all studies were carried out on team sports players. The overall bias of the studies was low, but there was an unclear amount of bias for participant selection. Two studies reported no predictive validity for the FMS, while three defended its predictive validity; the rest partially supported the FMS as a valid diagnostic tool. The reliability of the recommended cutoff point was confirmed, though cutoffs higher than 14 were significantly associated with the predictive ability of the FMS.

Conclusion

Although the FMS is reliable for clinical practice, and the current literature shows promise regarding the predictive ability of the FMS among active females, concerns remain regarding its validity in identifying at-risk females. Given the lack of clarity in the literature on the use of the FMS in females, further well-organized studies with larger sample sizes and longer monitoring periods are highly recommended. The sensitivity and specificity of the recommended cutoff of ≤ 14 has considerably decreased , and higher cutoff values should be applied to increase the FMS predictive ability.

Level of evidence The level of evidence was determined to be 2b.

Leg stiffness and muscle power vs manometer in sportswomen with symptoms of stress urinary incontinence

BACKGROUND

Pelvic floor muscle training can cure or alleviate stress urinary incontinence. This study aimed to evaluate maximum voluntary contractions of the pelvic floor muscle in sportswomen and verify the association with leg stiffness and muscle power, both maximal and submaximal.

METHODS

The sample consisted of 41 sportswomen between 18 and 42 years of age. Pelvic floor muscle strength was measured by the manometer. The sportswomen were instructed to perform 3 maximum voluntary contractions of the perineum, held for 3 seconds. Maximal and submaximal leg stiffness and muscle power were measured with a force platform, in two conditions: 1st condition was the sub-maximal, double leg hop test, which was performed allowing sportswomen to self-select their preferred frequency and 2nd condition was the maximal double leg hop test, which was performed asking athletes to maximize hop height and minimize contact time on the top of the force platform for 6 consecutive hops.

FINDINGS

Maximal and submaximal leg stiffness values increase with increasing age, weight, height, and body mass index, showing positive and significant (p <0.05) or close correlations. There are strong positive correlations between maximal and submaximal leg stiffness (r = 0.759) and between maximal and submaximal muscle power. Maximum voluntary contractions values decrease with increasing leg stiffness: the correlation is significant with maximal leg stiffness.

INTERPRETATION

Maximum voluntary contractions values decrease with increasing leg stiffness and increase with increasing muscle power values. If the training program aims to increase muscle power, it may also increase maximum voluntary contractions.

Association and Predictive Ability of Jump Performance with Sprint Profile of Collegiate Track and Field Athletes

This study examined the relationship between broad jump (BJ), countermovement jump (CMJ) and light load countermovement jump (LL-CMJ) performance and sprint performance and Sprint Profile measures in athletes. Additionally, this study aimed to determine the predictive ability of jump measures on Sprint Profile components. Twenty-five athletes performed BJ, CMJ, LL-CMJ, 30-metre acceleration and 30-metre maximal speed fly-by sprints. Results revealed moderate to very large correlations between BJ, CMJ and LL-CMJ performance with acceleration sprint completion times (r = -0.423 to -0.807; p < 0.05), fly-by sprint completion times (r = -0.452 to -0.838; p < 0.05) and maximal sprint speed (r = 0.424 to 0.794; p < 0.05). Additionally, associations were observed with multiple jumping measures and components of the Sprint Profile (r = 0.431 to 0.777; p < 0.05) during acceleration sprints. Furthermore, the BJ distance was the best predictor of Sprint Profile components during acceleration sprints (R2 = 0.57-0.76; p < 0.01) and maximal speed fly-by sprints (R2 = 0.775; p < 0.001). The forces and the manner of force application during the BJ to propel the athlete forwards and upwards are similar to those necessary to exhibit superior sprint performance. This may be due to the rapid generation of forces and orientation of force application during both movements.

Effects of Training Frequency During a 6-Month Neuromuscular Training Intervention on Movement Competency, Strength and Power in Male Youth

Background:

Youth populations benefit from neuromuscular training; however, the extent to which training frequency, baseline fitness, and maturity status moderate the response to long-term neuromuscular training in male youth remains unclear.

Hypothesis:

Twice-weekly training (G2x) would induce larger improvements in movement competency and strength and power kinetics compared with once-weekly training (G1x). Maturity status and baseline fitness would also moderate the training response in strength and power kinetics.

Study Design:

Nonrandomized, repeated-measures design.

Level of Evidence:

Level 3

Methods:

Ninety-five male athletes of varying maturity status (pre– or post–peak height velocity) were divided into G2x, G1x, or control (CON) groups. Training groups received G2x or G1x for 28 weeks. Back squat assessment (BSA) movement competency and isometric mid-thigh pull (IMTP), squat jump (SJ), and countermovement jump (CMJ) kinetics were measured pre- and postintervention.

Results:

The G2x achieved significantly greater adaptations in BSA total score, IMTP allometrically scaled peak force (PF_allo), IMTP peak rate-of-force development (PRFD), SJ PF_allo and PRFD compared with G1x and CON (P < 0.05; d = 0.3-0.9). Baseline fitness, training frequency and maturity status were significant predictors of IMTP PF_allo (R² = 27.5%) and PRFD (R² = 42.9%), while a combination of training frequency and baseline fitness explained a small amount of variance in SJ PF_allo (R² = 12.2%) and SJ PRFD (R² = 6.5%). There were no significant predictors of CMJ kinetics.

Conclusion:

G2x achieved significantly larger improvements in movement competency, isometric strength and concentric jump variables compared with G1x, with the training response greater in those youth with lower baseline fitness and more advanced maturity status.

Clinical Relevance:

Exposure to neuromuscular training twice-weekly elicits a greater response than G1x in young male athletes. Practitioners should take maturity status, training frequency, and baseline fitness levels of young male athletes into account when interpreting testing data, as these variables appear to moderate training responsiveness.

Reactive Strength Index and its Associations with Measures of Physical and Sports Performance: A Systematic Review with Meta-Analysis

BACKGROUND

Reactive strength index (RSI) is used frequently in the testing and monitoring of athletes. Associations with sports performance measures may vary dependent on the task but a literature synthesis has not been performed.

OBJECTIVES

The aim of this meta-analysis was to examine associations between RSI measured during rebound jumping tasks and measures of strength, linear and change of direction speed, and endurance performance.

METHODS

A systematic literature search with meta-analysis was conducted using databases PubMed, SPORTDiscus, Web of Science, and Ovid. Inclusion criteria required studies to (1) examine the relationship between RSI and an independent measure of physical or sporting performance for at least one variable; and (2) provide rebound test instructions to minimise ground contact time and maximise displacement of the jump. Methodological quality was assessed using a modified version of the Downs and Black Quality Index tool. Heterogeneity was examined via the Q statistic and I². Pooled effect sizes were calculated using a random-effects model, with Egger's regression test used to assess small study bias (inclusive of publication bias).

RESULTS

Of the 1320 citations reviewed, a total of 32 studies were included in this meta-analysis. RSI was significantly and moderately associated with strength (isometric: r = 0.356 [95% CI 0.209-0.504]; isotonic: r = 0.365 [0.075-0.654]; pooled strength measures: r = 0.339 [0.209-0.469]) and endurance performance (r = 0.401 [0.173-0.629]). Significant moderate and negative associations were indicated for acceleration (r = - 0.426 [- 0.562 to - 0.290]), top speed (r = - 0.326 [- 0.502 to - 0.151]), and significant large negative associations were noted for change of direction speed (r = - 0.565 [- 0.726 to - 0.404]). Heterogeneity was trivial to moderate across all measures (I² = 0-66%), and significant for isotonic strength and change of direction speed (p < 0.1). Evidence of small study bias was apparent for both acceleration and change of direction speed (p < 0.05).

CONCLUSIONS

We identified primarily moderate associations between RSI and independent measures of physical and sporting performance, and the strength of these relationships varied based on the task and physical quality assessed. The findings from this meta-analysis can help practitioners to develop more targeted testing and monitoring processes. Future research may wish to examine if associations are stronger in tasks that display greater specificity.

Relationship between Running Spatiotemporal Kinematics and Muscle Performance in Well-Trained Youth Female Athletes. A Cross-Sectional Study

The purpose of this cross-sectional study was to analyse the relationship of neuromuscular performance and spatiotemporal parameters in 18 adolescent distance athletes (age, 15.5 ± 1.1 years). Using the OptoGait system, the power, rhythm, reactive strength index, jump flying time, and jump height of the squat jump, countermovement jump, and eight maximal hoppings test (HT_8max) and the contact time (CT), flying time (FT), step frequency, stride angle, and step length of running at different speeds were measured. Maturity offset was determined based on anthropometric variables. Analysis of variance (ANOVA) of repeated measurements showed a reduction in CT (p < 0.000) and an increase in step frequency, step length, and stride angle (p < 0.001), as the velocity increased. The HT_8max test showed significant correlations with very large effect sizes between neuromuscular performance variables (reactive strength index, power, jump flying time, jump height, and rhythm) and both step frequency and step length. Multiple linear regression found this relationship after adjusting spatiotemporal parameters with neuromuscular performance variables. Some variables of neuromuscular performance, mainly in reactive tests, were the predictors of spatiotemporal parameters (CT, FT, stride angle, and VO). Rhythm and jump flying time in the HT_8max test and power in the countermovement jump test are parameters that can predict variables associated with running biomechanics, such as VO, CT, FT, and stride angle.

Resisted Sprint Training in Youth: The Effectiveness of Backward vs. Forward Sled Towing on Speed, Jumping, and Leg Compliance Measures in High-School Athletes

ABSTRACT

Uthoff, A, Oliver, J, Cronin, J, Winwood, P, Harrison, C, and Lee, JE. Resisted sprint training in youth: the effectiveness of backward vs. forward sled towing on speed, jumping, and leg compliance measures in high-school athletes. J Strength Cond Res 35(8): 2205-2212, 2021-Resisted sprinting (RS) is a popular training method used to enhance sprinting performance in youth. However, research has only explored the effects of forward RS (FRS) training. We examined the effects of FRS and backward RS (BRS) and compared these with a traditional physical education curriculum (CON). One hundred fifteen boys (age 13-15 years) were matched for maturity and allocated to either an FRS (n = 34), BRS (n = 46), or CON (n = 35) group. Training groups towed progressively overloaded sleds (20-55% body mass) 2 d·wk-1 for 8 weeks. Pre-training and post-training data were collected for sprinting times over 10 and 20 m, countermovement jump (CMJ) height, and leg stiffness (KN). Performance remained unchanged for the CON group (all p > 0.05), whereas all variables significantly improved (p < 0.05) after BRS, and all but 10-m performance improved after FRS. Compared with the CON, BRS and FRS significantly (p > 0.05) improved CMJ (Effect size [ES] = 0.67 and 0.38) and KN (ES = 0.94 and 0.69), respectively. No differences were found between training groups. The probabilities of improving sprinting performance after BRS (∼70%) were on average ∼10 and ∼8% better than the FRS and CON groups, respectively. The BRS and FRS showed similar probabilities of improving CMJ (75 and 79%) and KN (80 and 81%), respectively, over the CON group. It seems that BRS may be a means to improve sprint performance, and regardless of direction, RS seems to be a beneficial method for improving jumping height and leg stiffness in youth male athletes.

The Effect of Autoregulated Flywheel and Traditional Strength Training on Training Load Progression and Motor Skill Performance in Youth Athletes

Background: The effects of flywheel resistance training (FRT) on youth are relatively unknown. The aim of this study was to compare the effects of autoregulated FRT with traditional strength training (TST) on jumping, running performance and resistance training load progression in youth athletes. Thirty youth athletes (11.8 ± 0.9 yr) were matched for peak height velocity (PHV) status and block-randomised into two groups: FRT (n = 15, PHV −0.8 ± 1.6) and TST (n = 15, PHV −0.8 ± 1.5). Twelve resistance training sessions over a six-week intervention with flywheel or barbell squats were performed using autoregulated load prescription. Squat jump (SJ); countermovement jump (CMJ); and 10 m, 20 m and 30 m sprints were assessed pre- and post-intervention. The external load increased similarly for FRT and TST (z = 3.8, p = 0.06). SJ increased for both groups (p < 0.05) but running performance was unaffected (p > 0.05). Conclusions: FRT resulted in similar load progression and motor skill development in youth athletes as TST, but the perceived exertion was less. Autoregulation is a practical method for adjusting training load during FRT and should be considered as an alternative to autoregulated TST.

The Influence of Age and Maturation on Trajectories of Stretch-Shortening Cycle Capability in Male Youth Team Sports

PURPOSE

To examine the influence of growth and maturation in the trajectory of stretch-shortening cycle capability.

METHOD

Using a mixed-longitudinal design, absolute and relative leg stiffness and reactive strength index (RSI) were measured 3 times over a 3-year period in 44 youth team-sport players. Maturation was determined as maturity offset and included within the Bayesian inference analysis as a covariate alongside chronological age.

RESULTS

Irrespective of age and maturation, there was no change in absolute leg stiffness, however relative leg stiffness decreased over time. Maturation and age reduced this decline, but the decline remained significant (Bayesian factor [10] = 5097, model averaged R2 = .61). The RSI increased over time and more so in older more mature youth players (Bayesian factor [10] = 9.29e8, model averaged R2 = .657).

CONCLUSION

In youth players who are at/post peak height velocity, relative leg stiffness appears to decline, which could have an impact on both performance and injury risk. However, RSI increases during this period, and these data reinforce that leg stiffness and RSI reflect different components of stretch-shortening cycle capability. Practitioners should consider these differences when planning training to maximize stretch-shortening cycle capability during growth and maturation in athletes on the developmental performance pathway.

Weightlifting Is Better Than Plyometric Training to Improve Strength, Counter Movement Jump, and Change of Direction Skills in Tunisian Elite Male Junior Table Tennis Players

ABSTRACT

Kaabi, S, Mabrouk, RH, and Passelergue, P. Weightlifting is better than plyometric training to improve strength, counter movement jump, and change of direction skills in Tunisian elite male junior table tennis players. J Strength Cond Res XX(X): 000-000, 2020-This study aimed to compare the effect of integrating weightlifting (WL) resistance training or plyometric training for 8 weeks on physical performance among elite male junior table tennis players. Forty-five players were randomly divided into 3 groups. A WL resistance training group (n = 15), plyometric (P) group (n = 15), and a control (C) group (n = 15). Weightlifting group and P group were trained for 8 weeks (2 sessions per week with 3 different exercises each time) in addition to the usual practice of table tennis during the preparatory phase, whereas the C group continued classical table tennis training sessions. Subjects were tested for 5-m sprint time, change of direction (CD) test on a 20-m "T" shape course, jumping tests such as standing long jump, squat jump (SJ) and countermovement jump, hang grip, and 1 repetition maximum strength tests as the bench press and back squat before (T0) and after (T1) the training program. There was no statistical difference between the 3 groups before the beginning of the training program. After the 8-week training program, only P and WL groups significantly improved their performances for all the tests. Combining all the tests, the WL group had larger improvement (12.6 vs. 8.2%) and effect sizes (1.88 vs. 1.22) than the P group. Thus, WL training seems more efficient than P training to improve neuromuscular performance of table tennis players, especially for a sport-specific table tennis change-of-direction test such as the 20-m CD test.

Effects of Combined Surfaces vs. Single-Surface Plyometric Training on Soccer Players' Physical Fitness

Ramirez-Campillo, R, Álvarez, C, García-Pinillos, F, García-Ramos, A, Loturco, I, Chaabene, H, and Granacher, U. Effects of combined surfaces vs. single-surface plyometric training on soccer players' physical fitness. J Strength Cond Res 34(9): 2644-2653, 2020-The aim of this study was to compare the effects of a 8-week plyometric jump training (PJT) performed on different surfaces (grass, land-dirt, sand, wood, gym mat, and tartan-track) vs. a single-surface PJT (grass) on components of physical fitness (muscle power, speed, and change-of-direction speed [CODS] tasks) and sport-specific performance (i.e., maximal kicking velocity [MKV]) in male soccer players aged 11-14 years. Athletes were randomly assigned to a combined surfaces PJT (PJTc, n = 8), a single-surface PJT (PJTs, n = 8), or an active control (CON, n = 7). Although the PJT group trained on grass, the PJTc trained on 6 different surfaces and equally distributed the total jump volume according to the surface. Pre-post tests were conducted on grass. Significant main effects of time were observed for the countermovement jump, the standing-long-jump, the 20-cm drop jump, 30-m sprint time, CODS, and MKV (all p < 0.001; d = 0.53-0.87). Group × time interactions were identified for all jump tests, MKV, 30-m sprint time, and CODS (all p < 0.001; d = 0.58-0.71) in favor of PJTc. No significant pre-post changes were observed in the CON (all p > 0.05; d = 0.07-0.1). In conclusion, PJT is effective in improving physical fitness in young soccer players when conducted in combination with regular soccer training. Although general fitness testing and PJTs were performed on grass, larger physical fitness improvements were found after PJTc. Thus, PJTc is recommended, as it provides a better overload stimulus compared with more conventional training overload (e.g., increase in training volume or intensity). Future studies still have to address the underlying physiological adaptations after PJTc.

Long-term effects of school barefoot running program on sprinting biomechanics in children: A case-control study

BACKGROUND

The acute changes of running biomechanics in habitually shod children when running barefoot have been demonstrated. However, the long-term effects of barefoot running on sprinting biomechanics in children is not well understood.

RESEARCH QUESTION

How does four years of participation in a daily school barefoot running program influence sprint biomechanics and stretch-shortening cycle jump ability in children?

METHODS

One hundred and one children from barefoot education school (age, 11.2 ± 0.7 years-old) and 93 children from a control school (age, 11.1 ± 0.7 years-old) performed 50 m maximal shod and barefoot sprints and counter movement jump and five repeated-rebound jumping. To analyse sprint kinematics, a high-speed camera (240 fps) was used. In addition, foot strike patterns were evaluated by using three high-speed cameras (300 fps). Jump heights for both jump types and the contact times for the rebound jump were measured using a contact mat system. Two-way mixed ANOVA was used to examine the effect of school factor (barefoot education school vs control school) and footwear factor (barefoot vs shod) on the sprinting biomechanics.

RESULTS

Sprinting biomechanics in barefoot education school children was characterised by significantly shorter contact times (p = 0.003) and longer flight times (p = 0.005) compared to control school children regardless of footwear condition. In shod sprinting, a greater proportion of barefoot education school children sprinted with a fore-foot or mid-foot strike compared to control school children (p < 0.001). Barefoot education school children also had a significantly higher rebound jump height (p = 0.002) and shorter contact time than control school children (p = 0.001).

SIGNIFICANCE

The results suggest that school-based barefoot running programs may improve aspects of sprint biomechanics and develop the fast stretch-shortening cycle ability in children. In order to confirm this viewpoint, adequately powered randomised controlled trials should be conducted.

Gender and Age Related Differences in Leg Stiffness and Reactive Strength in Adolescent Team Sports Players

The aim of the present study was to identify potential gender differences in leg stiffness and reactive strength during hopping tasks in 13 to16-year old team sports players. Reactive strength index (RSI) and leg stiffness were obtained in two consecutive seasons from 51 girls (U14: n = 31, U16: n = 20) and 65 boys (U14: n = 32, U16: n = 33). A significant main effect on absolute (U14: p = 0.022, η²= 0.084; U16: p < 0.001, η²= 0.224) and relative leg stiffness (U14 p<0.001; η²= 0.195; U16; p = 0.008, η²= 0.128) for gender was found in both groups with values higher in boys than in girls. For absolute and relative stiffness gender differences in the U14 group were significant in the 1^st year only (p=0.027 and p=0.001), and for the U16s in the 2^nd year only (p < 0.001 and p = 0.022). For RSI, a significant main effect for gender was observed in the U16 group only (p < 0.001 η²= 0.429) with values significantly higher in boys than in girls in both years of measurement (p = 0.001; p < 0.001). Results of this study support previous limited findings, mostly related to non-athletes, suggesting lower stretch-shortening cycle capability in adolescence female compared to male, however our data only partly supports the theory that quality of neuromuscular functions increases with age until post puberty.

Effect of a Simulated Match on Lower Limb Neuromuscular Performance in Youth Footballers-A Two Year Longitudinal Study

The aim of this study was to explore the effects of simulated soccer match play on neuromuscular performance in adolescent players longitudinally over a two-year period. Eleven players completed all measurements in both years of the study (1st year: age 16.0 ± 0.4 y; stature 178.8 ± 6.4 cm; mass 67.5 ± 7.8 kg; maturity-offset 2.24 ± 0.71 y). There was a significant reduction in hamstring strength after simulated match by the soccer-specific aerobic field test (SAFT⁹⁰), with four out of eight parameters compromised in U16s (4.7–7.8% decrease) and six in the U17s (3.1–15.4%). In the U17s all of the concentric quadriceps strength parameters were decreased (3.7–8.6%) as well as the vastus lateralis and semitendinosus firing frequency (26.9–35.4%). In both ages leg stiffness decreased (9.2–10.2%) and reactive strength increased pre to post simulated match (U16 8.0%; U17 2.5%). A comparison of changes between age groups did not show any differences. This study demonstrates a decrease in neuromuscular performance post simulated match play in both ages but observed changes were not age dependent.

The effect of four weeks of plyometric training on reactive strength index and leg stiffness is sport dependent

BACKGROUND

Plyometric exercises are often used to develop lower limb strength and performance-related biomechanics such as leg stiffness. However, the effectiveness of plyometric training may depend on participants' own training and performance demands. The purpose of this study was to examine the effect of plyometric training on Reactive Strength Index (RSI) and leg stiffness (Kleg) on young athletes of different sports.

METHODS

Forty eight female athletes (25 Taekwondo (TKD) and 23 rhythmic gymnastics (RG), mean±SD: age: 8.94±2.50 years; mass: 29.73±7.69 kg; height: 138.84±11.90 cm; training experience: 4.62±2.37 years) participated in this study. Participants were randomly assigned to experimental (PT, N.=24) and control (CG, N.=24) groups. The PT group followed a twice-weekly plyometric training program for 4 weeks. Plyometric drills lasted approximately 5-10 s, and at least 90 s rest was allowed after each set. To examine RSI, participants performed trials of five maximal CMJs. Submaximal hopping (20 hops) was performed in order to examine leg stiffness.

RESULTS

Significant interaction effect was found for RSI and the post hoc analysis showed that RSI significantly increased by 35% (P=0.017) in RG athletes, whereas a significantly reduction by 28% (P=0.004) was revealed in TKD athletes. The interaction effect between time and group was statistically significant for Kleg (P<0.05) with Kleg significantly increasing by 31% (P=0.008) in TKD athletes, but remaining unchanged (P>0.05) in RG athletes.

CONCLUSIONS

The results showed that the effect of a 4-week plyometric training program on RSI and leg stiffness is sport dependent. Further, the applied plyometric program was effective in reducing ground contact time and therefore increasing leg stiffness.

Normative Data and Physical Determinants of Multiple Sprint Sets in Young Soccer Players Aged 11-18 Years: Effect of Maturity Status

Selmi, MA, Sassi, RH, Yahmed, MH, Giannini, S, Perroni, F, and Elloumi, M. Normative data and physical determinants of multiple sprint sets in young soccer players aged 11-18 years: Effect of maturity status. J Strength Cond Res 34(2): 506-515, 2020-The aims of the study were: (a) to establish normative data for repeated-sprint sets (RSS) test based on the maturity status (age at peak height velocity [PHV]) and (2) to investigate the relationship between anthropometrical variables (stature, sitting height, body mass, and body fat percentage), RSS (2 × 5 × 20 m with 15-second recovery between sprints and 1-minute recovery between sets), and fitness tests {squat jump, countermovement jump, standing long jump, standing triple jump, 5-jump test, and 20-m shuttle run (multistage shuttle run test [MSRT])}. Young male soccer players (n = 262; age: 14.5 ± 2.9 years) were evaluated and classified into 4 groups according to their maturity status: pre-PHV, circum-PHV1, circum-PHV2, post-PHV. An analysis of variance and Bonferroni post hoc were used to determine maturity group differences (p ≤ 0.05), whereas Pearson's correlation was used between variables. Repeated-sprint sets' indices (sum of sprint times [SST] and best sprint time [BST]) were significantly different between the maturity groups. Significant correlations between SST with body mass (from -0.73 to -0.33) and MSRT (from -0.49 to -0.30) among each maturity group were found. With the different maturity groups, correlations between SST (s), BST (s), and vertical jump (cm) (r = -0.63 to -0.25 and r = -0.68 to -0.23) and horizontal jump (m) (r = -0.70 to -0.38 and r = -0.63 to -0.43) were observed. Repeated-sprint sets' values improve during maturation of young soccer players and the correlations between RSS and fitness tests vary through the maturity groups. This information could be useful for the coach to identify talent and to prescribe specific physical training to improve performance.

Influence of Resisted Sled-Pull Training on the Sprint Force-Velocity Profile of Male High-School Athletes

Cahill, MJ, Oliver, JL, Cronin, JB, Clark, K, Cross, MR, Lloyd, RS, and Lee, JE. Influence of resisted sled-pull training on the sprint force-velocity profile of male high-school athletes. J Strength Cond Res 34(10): 2751-2759, 2020-Although resisted sled towing is a commonly used method of sprint-specific training, little uniformity exists around training guidelines for practitioners. The aim of this study was to assess the effectiveness of unresisted and resisted sled-pull training across multiple loads. Fifty-three male high-school athletes were assigned to an unresisted (n = 12) or 1 of 3 resisted groups: light (n = 15), moderate (n = 14), and heavy (n = 12) corresponding to loads of 44 ± 4 %BM, 89 ± 8 %BM, and 133 ± 12 %BM that caused a 25, 50, and 75% velocity decrement in maximum sprint speed, respectively. All subjects performed 2 sled-pull training sessions twice weekly for 8 weeks. Split times of 5, 10, and 20 m improved across all resisted groups (d = 0.40-1.04, p < 0.01) but did not improve with unresisted sprinting. However, the magnitude of the gains increased most within the heavy group, with the greatest improvement observed over the first 10 m (d ≥ 1.04). Changes in preintervention to postintervention force-velocity profiles were specific to the loading prescribed during training. Specifically, F0 increased most in moderate to heavy groups (d = 1.08-1.19); Vmax significantly decreased in the heavy group but increased in the unresisted group (d = 012-0.44); whereas, Pmax increased across all resisted groups (d = 0.39-1.03). The results of this study suggest that the greatest gains in short distance sprint performance, especially initial acceleration, are achieved using much heavier sled loads than previously studied in young athletes.

Convergent Validity, Reliability, and Sensitivity of a Running Test to Monitor Neuromuscular Fatigue

PURPOSE

To investigate the convergent validity, reliability, and sensitivity over a week of training of a standardized running test to measure neuromuscular fatigue.

METHODS

Twenty male rugby union players were recruited for the study, which took place during preseason. The standardized running test consisted of four 60-m runs paced at  ~5 m·s-1 with 33 seconds of recovery between trials. Data from micromechanical electrical systems were used to calculate a running-load index (RLI), which was a ratio between the mechanical load and the speed performed during runs. RLI was calculated by using either the entire duration of the run or a constant-velocity period. For each type of calculation, either an individual directional or the sum of the 3 components of the accelerometer was used. A measure of leg stiffness was used to assess the convergent validity of the RLI.

RESULTS

Unclear to large relationships between leg stiffness and RLI were found (r ranged from -.20 to .62). Regarding reliability, small to moderate (.47-.86) standardized typical errors were found. The sensitivity analysis showed that the leg stiffness presented a very likely trivial change over the course of 1 week of training, whereas RLI showed very likely small to a most likely large change.

CONCLUSIONS

This study showed that RLI is a practical method to measure neuromuscular fatigue. In addition, such a methodology aligns with the constraint of elite team-sport setup due to its ease of implementation in practice.

Effects of Different Plyometric Training Frequencies on Measures of Athletic Performance in Prepuberal Male Soccer Players

Bouguezzi, R, Chaabene, H, Negra, Y, Ramirez-Campillo, R, Jlalia, Z, Mkaouer, B, and Hachana, Y. Effects of different plyometric training frequencies on measures of athletic performance in prepuberal male soccer players. J Strength Cond Res 34(6): 1609-1617, 2020-This study aimed to compare the effects of 1 vs. 2 sessions of equal-weekly volume plyometric training (PT) across 8 weeks on measures of athletic performance (i.e., sprint time, change of direction [CoD], jumping ability, and muscle strength) in prepuberal male soccer players. Thirty participants were randomly assigned either to 1 session PT group (1SPT [n = 15]) or 2 sessions PT group (2SPT [n = 15]). Plyometric training was integrated into their regular soccer training routine. Pretraining and posttraining tests for the assessment of sprint time (e.g., 5, 10, 20, and 30-m), CoD (e.g., T-test and modified Illinois change of direction test [MICODT]), jumping ability (e.g., standing long jump [SLJ], countermovement jump [CMJ], and squat jump [SJ]), muscle strength (reactive strength index [RSI]), and kicking distance were conducted. Results showed a main effect of time for 5-m sprint-time performance (F(1,56) = 4.00, effect size [ES] = 0.53 [medium], p = 0.05), T-test (F(1,56) = 23.19, ES = 1.28 [large], p < 0.001), MICODT (F(1,56) = 5.72, ES = 0.94 [large], p = 0.02), SLJ (F(1,56) = 16.63, ES = 1.09 [large], p < 0.001), CMJ (F(1,56) = 15.43, ES = 1.04 [large], p < 0.001), SJ (F(1,56) = 20.27, ES = 1.20 [large], p < 0.001), RSI (F(1,56) = 26.26, ES = 1.36 [large], p < 0.001), and kicking distance (F(1,56) = 47.19, ES = 1.83 [large], p < 0.001). There were no training group × time interactions in all the measured outcomes. In conclusion, when an equated moderate volume of jumps is performed, higher PT frequency across 8 weeks has no extra effects on prepuberal male soccer players' measures of athletic performance. The present findings may help optimizing PT interventions dedicated to prepuberal male soccer players.

Lower Extremity Stiffness in Collegiate Distance Runners Pre- and Post-Competition

Previous evidence has suggested that there is a relationship between leg stiffness and improved running performance. The purpose of this investigation was to determine how leg stiffness of runners was influenced in the 24 and 48 hour period following a cross country race. Twenty-two collegiate cross-country runners (13 males, 9 females, 19.5 ± 1.4 yr) were recruited and participated in the study. Leg stiffness was assessed 24 hours before and after a race as well as 48 hours post-race. Three jumping protocols were conducted: 1) a static jump, 2) a countermovement jump, and 3) a vertical hopping test. Two embedded force plates (1000 Hz) were utilized to measure ground reaction forces for each test and a metronome was utilized to maintain hopping frequency (2.2 Hz). A significant main effect was found for a static jump, a countermovement jump and leg stiffness. Leg stiffness was significantly reduced 24 hours post-race (pre-race 36.84 kN·m^-1, 24h post 33.11 kN·m^-1, p < 0.05), but not 48 hours post-race (36.30 kN·m^-1). No significant differences were found in post-hoc analysis for the squat jump, countermovement jump height and the eccentric utilization ratio. Following a cross-country race, leg stiffness significantly declined in a group of collegiate runners in the immediate 24 hours post-race, but returned to baseline 48 hours post-race. Sport scientists and running coaches may be able to monitor leg stiffness as a metric to properly prescribe training regiments.

Effects of a 12-Week Training Program on Isometric and Dynamic Force-Time Characteristics in Pre- and Post-Peak Height Velocity Male Athletes

Dobbs, IJ, Oliver, JL, Wong, MA, Moore, IS, and Lloyd, RS. Effects of a 12-week training program on isometric and dynamic force-time characteristics in pre- and post-peak height velocity male athletes. J Strength Cond Res 34(3): 653-662, 2020-Literature shows that training children and adolescents can enhance strength and power irrespective of their stage of development; however, the development of the kinetic variables that underpin strength and power performance are typically unreported in youth training studies. Twenty-four pre- and 14 post-peak height velocity (PHV) male athletes were divided into maturity-specific experimental (EXP) and control groups (CON), with the EXP groups completing a twice-weekly, 12-week training program. Force-time characteristics during the isometric midthigh pull (IMTP), countermovement jump, and squat jump tests were quantified at both baseline and after the completion of the 12-week program. Alpha level was set at p < 0.05. No changes in total score for back-squat assessment were observed in any group (p > 0.05). Analysis of IMTP data revealed that only the post-PHV EXP group significantly increased absolute isometric peak force (PFabs) and peak rate of force development within the IMTP after training. Both EXP groups displayed significant increases in isometric PF at time epochs 0-90, 0-150, 0-200, and 0-250 ms. Data from the dynamic tests indicated that the pre-PHV EXP cohort improved concentric qualities as reflected by increased squat jump height and countermovement jump concentric power. There were no significant changes for any variables across all tests within either CON group (p > 0.05). Maturity-related differences in response to short-term training affects the kinetic variables associated with strength and power performance, but not movement competency in young male athletes.

The stretch-shortening cycle efficiency is dependent on the maturational stage

Abstract Maturation interferes with physiological and mechanical aspects of muscle contraction. The aim of the present study was to compare the efficiency of the stretch-shortening cycle (SSC) between different maturation stages. One hundred forty six young people, with 72 boys and 74 girls, were recruited to participate in the study. Sexual maturation was measured and the group was stratified into adolescents (> 3 auto-tanner) and children (<2 auto-tanner). Squat jump (SJ) and countermovement jump (CMJ) performance were measured to measure reactive force (RF) capacity, eccentric utilization ratio (EUR) and pre-stretch augmentation (PSAP). The children did not present statistical difference between SJ and CMJ (t(55)= -1.93; ∆ = -0.61; 95% CI = -1.24 to -0.02; p = 0.058), while the adolescents presented higher performance of CMJ (t(89)= -12.28; ∆ = -2.44; 95% CI = -2.84 to -2.05; p <0.001). In addition, adolescents had higher RF performance (t(144)= 5.18; p <0.0001; 95% CI = 2.53 to 1.13), PSAP (t(144)= 4.38; p <0.0001; 95% CI = 14.00 to 5.29) and EUR (t(144)= 4.38; p <0.0001; 95% CI = 0.14 to 0.05). We conclude that more mature young people have better SSC utilization.

Influence of resisted sled-push training on the sprint force-velocity profile of male high school athletes

Sled pushing is a commonly used form of resisted sprint training; however, little empirical evidence exists, especially in youth populations. The aim of this study was to assess the effectiveness of unresisted and resisted sled pushing across multiple loads. Fifty high school athletes were assigned to an unresisted (n = 12), or 3 resisted groups; light (n = 14), moderate (n = 13), and heavy (n = 11) resistance that caused a 25%, 50%, and 75% velocity decrement in maximum sprint speed, respectively. All participants performed two sled-push training sessions twice weekly for 8 weeks. Before and after the training intervention, the participants performed a series of jump, strength, and sprint testing to assess athletic performance. Split times between 5 and 20 m improved significantly across all resisted groups (all P < .05, d = 0.34-1.16) but did not improve significantly with unresisted sprinting. For all resisted groups, gains were greatest over the first 5 m (d = 0.67-0.84) and then diminished over each subsequent 5 m split (d = 0.08-0.57). The magnitude of gains in split times was greatest within the heavy group. Small but non-significant within-group effects were found in pre to post force-velocity profiles. There was a main effect of time but no interaction effects as all groups increased force and power, although the greatest increases were observed with the heavy load (d = 0.50-0.51). The results of this study suggest that resisted sled pushing with any load was superior to unresisted sprint training and that heavy loads may elicit the greatest gains in sprint performance over short distances.

Effects of Eccentric Preloading on Concentric Vertical Jump Performance in Youth Athletes

This study measured peak force (PF), peak rate of force development (PRFD), peak power (PP), concentric impulse, and eccentric impulse during static jump (SJ), countermovement jump (CMJ), and drop jump (DJ) in youth athletes to examine changes in vertical jump power with progressively greater eccentric preloading in relation to age, maturity, and muscle mass. Twenty-one males ranging from 6 to 16 years old performed the following vertical jumps in a random order: SJ, CMJ, and DJ from drop heights of 20, 30, and 40 cm (DJ20, DJ30, and DJ40, respectively). Measurements included PF, PRFD, PP, eccentric impulse, and concentric impulse for each vertical jump condition. Maturity offset was calculated, while ultrasound images quantified thigh muscle cross-sectional area (CSA). PF and PRFD increased from CMJ to DJ20. PP increased from SJ to CMJ. Concentric impulse remained unchanged, but eccentric impulse increased systematically from across jumps. The change in PP from SJ to CMJ was correlated with age, height, weight, maturity offset, and CSA. The CMJ resulted in the greatest concentric PP with the least amount of eccentric preloading. The inability of young athletes to translate the energy absorbed during the eccentric phase of the stretch-shortening cycle of DJs may be influenced by growth and development.