Fatigue affects peak joint torque angle in hamstrings but not in quadriceps

Assessing the magnitude and direction of asymmetry in physical tests and morphological measurements in professional female soccer athletes

OBJECTIVES

To analyze the magnitude and direction of inter-limb asymmetries in vertical jump, T-test, isokinetic peak torque, and muscle volume in professional female soccer players.

METHODS

Sixteen female soccer players participated in the study (22.7 ± 3.5 years, 60.4 ± 12.7 kg, 164.2 ± 8.7 cm). The following tests were applied; the unilateral countermovement jump (CMJ), T-test (agility), peak torque of concentric knee extensor (PT_Con_Q), concentric and eccentric knee flexor (PT_Con_H; PT_Ecc_H), and muscle volume of quadriceps (MV_Q) and gastrocnemius (MV_G). The interlimb asymmetry magnitude was calculated as the percentage difference between the stronger and weaker limbs. For the direction of asymmetry, a negative sign (-) was assigned when the non-dominant lower limb was the stronger one, and a positive sign (+) when the dominant lower limb was the stronger one.

RESULTS

The asymmetry (%) values varied from 0.7 to 11.9%, with the highest values of asymmetry found in the CMJ, PT_Con_Q, PT_Con_H, and PT_Ecc_H tests. Moderate levels of agreement were observed for CMJ - Vol_G (K = 0.53; 75% agreement); PT_Con_Q - PT_Con_H (K = 0.48; 75% agreement); T-test - PT_Ecc_H (K = 0.43; 68.7% agreement). Fair levels of agreement were found between the T-test - Vol_Q (K = 0.21; 62.1% agreement); PT_Con_Q - PT_Ecc_H (K = 0.36; 68.8% agreement); and PT_Con_H - PT_Ecc_H (K = 0.31; 68.8% agreement).

CONCLUSIONS

We concluded that of greater asymmetries in the vertical jump and isokinetic torque tests. The direction of the asymmetry seems highly variable, however, the isokinetic tests showed greater agreements with the other tests.

The development of hamstring strength over the course of a simulated soccer match

Hamstring strain injuries are a prevalent burden in soccer. Low strength, muscle fatigue, and inter-limb differences in hamstring strength are associated with hamstring injury risk. Previous research shows increased hamstring injury incidence in soccer at the end of each half or the end of the match, respectively. This study aims at evaluating the aforementioned risk factors of hamstring injury over the course of a simulated soccer match. Ten active soccer players carried out the Loughborough Intermittent Shuttle Test, during which hamstring strength of both legs was assessed on seven occasions via the optimized 90:20 Isometric Posterior Chain Test. Hamstring strength of each limb and inter-limb differences in hamstring strength over the course of the Loughborough Intermittent Shuttle Test were parameters of interest. Repeated measures ANOVA were used to analyze the development of hamstring strength and limb-asymmetries in hamstring strength during the simulated soccer match. Compared to pre-match values, hamstring strength was significantly decreased after 15 and 30 minutes of simulated soccer match for the non-dominant and dominant leg, respectively. There were no further variations in hamstring strength within the simulated soccer match for either leg. We did neither measure significant recovery of hamstring strength to pre-match values at the beginning of the second half, as suspected by previous research, nor inter-limb differences, or a deterioration of limb asymmetries in hamstring strength during the simulated soccer match. Players who only participate for a short period in a soccer match may be exposed to the same risk of suffering hamstring injury like players who compete for a longer duration. Decreasing hamstring strength partly depicts the pattern of hamstring injury incidences during soccer matches. Additional factors may influence the increasing hamstring injury rate at the end of each half or the later stages of a match, respectively.

Exploring the biomechanics and fatigue patterns of eccentric quasi-isometric muscle actions in the knee extensors and flexors

PURPOSE

Eccentric quasi-isometric (EQI) resistance training is emerging as a promising option in sports medicine and rehabilitation. Despite prior research on EQI contractions in quadriceps and biceps brachii, their use in hamstring injury contexts is underexplored. Therefore, our study examines and contrasts the biomechanics and fatigue effects of EQI training on knee extensors and flexors.

METHODS

Following familiarization, 16 healthy, active participants (9 men, 7 women; 23.5 ± 2.6 years, 72.1 ± 12.8 kg, 173.4 ± 10.7 cm) performed, in random order, four EQI contractions for knee extensions and flexions, respectively. EQI contractions were isotonically loaded to 70% of concentric (60°·s-1) maximal voluntary contraction. Rest between repetitions was set at three minutes, while four minutes separated each muscle group. Peak torque, mean torque, and optimal angle were evaluated pre- and post-bouts. Inter-repetition contraction time and angular velocity were also assessed.

RESULTS

Average torque was 160.9 ± 44.2 and 71.5 ± 23.2 Nm for the extensors and flexors. Peak and mean torque significantly decreased for both extensors (p < 0.001, d = 0.70-0.71) and flexors (p ≤ 0.022, d = 0.36) after EQI contractions, respectively. However, the optimal angle increased for extensors (p < 0.001, d = 1.00) but not flexors (p = 0.811, d = 0.06). During EQI contractions, knee flexors exhibited greater intra-repetition velocity than extensors (p = 0.002; η2 = 0.50). Decreases in inter-repetition time and range of motion were more consistent for the extensors.

CONCLUSIONS

Distinct responses exist when comparing EQI contractions of the knee extensors and flexors, particularly their effect on peak torque angles. These findings suggest knee flexors may require lower relative intensities to align more closely with extensor EQI contractions.

Effects of muscle fatigue on exercise-induced hamstring muscle damage: a three-armed randomized controlled trial

PURPOSE

Hamstring injuries in soccer reportedly increase towards the end of the matches' halves as well as with increased match frequency in combination with short rest periods, possibly due to acute or residual fatigue. Therefore, this study aimed to investigate the effects of acute and residual muscle fatigue on exercise-induced hamstring muscle damage.

METHODS

A three-armed randomized-controlled trial, including 24 resistance-trained males, was performed allocating subjects to either a training group with acute muscle fatigue + eccentric exercise (AF/ECC); residual muscle fatigue + eccentric exercise (RF/ECC) or a control group with only eccentric exercise (ECC). Muscle stiffness, thickness, contractility, peak torque, range of motion, pain perception, and creatine kinase were assessed as muscle damage markers pre, post, 1 h post, and on the consecutive three days.

RESULTS

Significant group × time interactions were revealed for muscle thickness (p = 0.02) and muscle contractility parameters radial displacement (Dm) and contraction velocity (Vc) (both p = 0.01), with larger changes in the ECC group (partial η2 = 0.4). Peak torque dropped by an average of 22% in all groups; stiffness only changed in the RF/ECC group (p = 0.04). Muscle work during the damage protocol was lower for AF/ECC than for ECC and RF/ECC (p = 0.005).

CONCLUSION

Hamstring muscle damage was comparable between the three groups. However, the AF/ECC group resulted in the same amount of muscle damage while accumulating significantly less muscle work during the protocol of the damage exercise.

TRIAL REGISTRATION

This study was preregistered in the international trial registration platform (WHO; registration number: DRKS00025243).

Post flywheel squat vs. flywheel deadlift potentiation of lower limb isokinetic peak torques in male athletes

The present study investigated the post-activation performance enhancement (PAPE) of isokinetic quadriceps and hamstrings torque after flywheel (FW)-squat vs. FW-deadlift in comparison to a control condition. Fifteen male athletes were enrolled in this randomised, crossover study. Each protocol consisted of 3 sets of 6 repetitions, with an inertial load of 0.029 kg.m2. Isokinetic quadriceps (knee extension) and hamstrings (knee flexion) concentric peak torque (60º/s) and hamstring eccentric peak torque (-60º/s) were measured 5 min after experimental or control conditions. A significant condition (PAPE) effect was reported (f = 4.067, p = 0.008) for isokinetic hamstrings eccentric peak torque following FW-squat and FW-deadlift, but no significant differences were found for quadriceps and hamstrings concentric peak torques. The significant difference averaged 14 Nm between FW-squat vs. control (95% CI: 2, 28; d = 0.75, moderate; p = 0.033), and 13 Nm between FW-deadlift vs. control (95% CI: 1, 25; d = 0.68, moderate; p = 0.038). This study reported that both FW-squat and FW-deadlift exercises are equivalently capable of generating PAPE of isokinetic hamstrings eccentric torque. Practitioners may use these findings to inform strength and power development during complex training sessions consisting of flywheel-based exercises prior to a sport-specific task.

Short-Term Effects of Lower-Extremity Heavy Resistance versus High-Impact Plyometric Training on Neuromuscular Functional Performance of Professional Soccer Players

BACKGROUND

To compare the effects of short-term 8 week heavy-resistance or plyometric training protocols (HRT or PLY) incorporated into regular soccer practice on measures of neuromuscular functional performance in professional soccer players, a single-blind randomized controlled trial was conducted.

METHODS

Forty-seven participants aged 22.3 ± 3.52 years were assigned to three groups: HRT (n = 15), PLY (n = 15), and control (CON; n = 17). The HRT group performed 3 sets and 10 repetitions twice a week using 80% of their baseline 1-RM (weeks 1-3), followed by 8 repetitions at 85% 1-RM (weeks 4-6), and 6 repetitions at 90% 1-RM (weeks 7-8) of 6 lower-body strength exercises with a 1 min rest period between sets. The PLY protocol involved a preparatory phase (weeks 1-2), followed by two 3-week progressive periods (weeks 3-5 and weeks 6-8). The plyometric sessions consisted of four jump exercises/drills with progressively increasing number of sets and total number of foot contacts. The rest intervals between repetitions and sets were 15 and 90 s, respectively. Outcome measures included tests assessing 10 and 30 m speed (t10m and t30m), one-repetition maximum half-back squat (1-RM squat), isokinetic peak torques for the quadriceps and hamstring muscles (Qcon and Hcon), countermovement jump (CMJ), and squat jump (SJ).

RESULTS

Two-way ANOVA detected main effects of time and group×time interactions for all examined variables, except t30m, 1-RM, and relative 1-RM. Post hoc analyses revealed significant increases in the HRT group (t10m: 6.3%, t30m: 7.1%; absolute 1-RM: 29.6%; relative 1-RM: 30.3%, Qcon: 24.5%; Hcon: 14.4%; CMJ: 5.9%; SJ: 7.2%, all p < 0.001) and the PLY group (t10m: 3.1%; t30m: 4.1%; absolute 1-RM:19.1%; relative 1-RM: 20.3%; Qcon: 12.6%; Hcon: 8.7%; CMJ: 3.3%; SJ: 3.5%, all p < 0.001). HRT was superior compared to PLY in relative 1-RM, Qcon and Hcon (all p < 0.001). In addition, we found knee muscular strength imbalance in 70.5% of participants from the total sample (H/Q ratio < 60%). The HRT and PLY protocols resulted in improved neuromuscular functional performance compared to the regular soccer regime.

CONCLUSIONS

This study showed that during the pre-competitive season, additional HRT and PLY drills/exercises as a substitute for standard soccer training as part of a regular 90 min practice twice a week for 8 weeks, can produce acute physical performance-enhancing effects in professional soccer players.

Peak torque angle, acceleration time and time to peak torque as additional parameters extracted from isokinetic test in professional soccer players: a cross-sectional study

This study investigated additional and traditional variables from isokinetic test of thigh muscles in soccer players across different field positions. One hundred and eighty-nine athletes performed maximal concentric isokinetic knee contractions on dominant (DL) and non-dominant limb (NDL) at 60º/s and 240º/s. The additional outcomes peak torque angle (AngPT), acceleration time (AcT) and time to peak torque (TPT) and traditional outcomes Peak torque (PT), total work (TW) and power (Pw) were extracted from the exam. Goalkeepers (GK), side backs (SB), central backs (CB), central defender midfielders (CDM), central attacking midfielders (CAM) and forwards (FW) were considered. Comparisons between limbs and positions demonstrated that SB extensors of the DL presented TPT lower (p = 0.006) and AngPT higher (p = 0.011) than NDL at 60°/s. CDM extensors of the DL showed lower TPT at 60°/s (p = 0.003) and 240°/s (p = 0.024). CAM flexors of the DL showed lower TPT (p = 0.026) and AcT (p = 0.021) at 240°/s than NDL. CB, CDM and CAM extensors of the NDL showed higher PT, TW and Pw than DL (p < 0.05). In conclusion, there are muscle imbalances between limbs in SB, CDM and CAM and across different field positions.

Neuromuscular changes after a Long Distance Triathlon World Championship

ABSTRACTThis study aimed to determine the neuromuscular changes in muscle contractile properties and countermovement jump (CMJ) performance after a long distance (LD) triathlon, inquiring to what extent it is influenced by gender and the evaluated muscle, and to explore for response differences. Four elite (2 females and 2 males) and 18 well-trained triathletes (3 females and 15 males) who competed in the 2019 LD Triathlon World Championship undergone, prior and post-race, a CMJ and a muscle contractile properties assessment in vastus lateralis (VL) and biceps femoris (BF) using tensiomyography. A mixed ANOVA was carried out to detect triathletes' neuromuscular changes after the competition. A large decrease in jump height and flight time was found in CMJ (p = 0.001, ηp2 = 0.767) with gender differences (ηp2 = 0.308). contraction (Tc), delay (Td) and sustain times (Ts) also showed a large decrease (p = 0.001, ηp2 = 0.174-0.413). However, maximum radial muscle belly displacement was not altered by the competition, but did greatly increase radial displacement velocity (p = 0.031; ηp2 = 0.212). No gender differences were found in contractile properties. Correlation analysis between race performance and the jumping and tensiomyography parameters only showed a moderate negative correlation with Tc of the BF in pre- and post-competition (r = -0.44; -0.43; p = 0.05). In conclusion, triathletes after a LD World Championship suffer a sharp drop in jump performance and variations in VL's and BF's contractile properties, showing decreases in Tc, Td and Ts without gender differences. The regular use of these parameters can allow the monitoring of triathlete's neuromuscular fatigue and aid in the distribution of more efficient loads in their training cycles.HighlightsTrainers and researchers in neuromuscular fatigue can have reference data of elite and well-trained triathletes after performing LD race.The magnitude of change in the CMJ and the contractile properties of BF and VL achieved in competition by these elite reference triathletes can guide to modulate triathletes training.The regular use of the parameters using these references can allow an exhaustive neuromuscular fatigue monitoring during the sessions or training cycles throughout the season.

Building for the Future: A Systematic Review of the Effects of Eccentric Resistance Training on Measures of Physical Performance in Youth Athletes

BACKGROUND

Eccentric resistance training is recognised as an effective stimulus for enhancing measures of muscular strength and power in adult populations; however, its value in youth athletes is currently not well understood.

OBJECTIVE

The aim of this systematic review was to critically appraise the effects of eccentric resistance training on measures of physical performance (i.e. muscular strength, jump, sprint and change of direction) in youth athletes 18 years of age and under.

METHODS

Original journal articles published between 1950 and June 2022 were retrieved from electronic search engines of PubMed, SPORTDiscus and Google Scholar's advanced search option. Full journal articles investigating the acute and chronic effects of eccentric resistance training on measures of physical performance in youth athletes (i.e. a person 18 years of age or under who competes in sport) were included. The methodological quality and bias of each study were assessed prior to data extraction using a modified Downs and Black checklist.

RESULTS

The search yielded 749 studies, of which 436 were duplicates. Three-hundred studies were excluded based upon title and abstract review and a further 5 studies were removed following the modified Downs and Black checklist. An additional 14 studies were identified during backward screening. Accordingly, 22 studies were included in our systematic review. The Nordic hamstring exercise and flywheel inertial training were the most frequently used eccentric resistance training methods in youth athletes. Improvements in physical performance following the Nordic hamstring exercise are dependent upon an increase in the breakpoint angle, rather than training volume (sets and repetitions), and are further elevated with the addition of hip extension exercises or high-speed running. A minimum of 3 familiarisation trials is necessary to elicit meaningful adaptations following flywheel inertial training. Furthermore, an emphasis should be placed upon decelerating the rotating flywheel during the final one to two thirds of the eccentric phase, rather than gradually throughout the entire eccentric phase.

CONCLUSIONS

The findings of this systematic review support the inclusion of eccentric resistance training in youth athletes to improve measures of muscular strength, jump, sprint and change of direction performance. The current eccentric resistance training methods are predominantly limited to the Nordic hamstring exercise and flywheel inertial training; however, the efficacy of accentuated eccentric loading to improve jump performance warrants attention in future investigations.

Alterations in biceps femoris long head fascicle length, Eccentric hamstring strength qualities and single-leg hop distance throughout the ninety minutes of TSAFT90 simulated football match

INTRODUCTION

Football matches show higher hamstring strain injuries (HSIs) than football training. The occurrence of HSIs increases in the last fifteen minutes of both halves of football matches and shows an incremental trend towards the end of the ninety minutes.

OBJECTIVES

This study aimed to examine football-specific fatigue-induced alterations in risk factors of the HSIs, including biceps femoris long head fascicle length via ultrasonography (BFlh FL), single-leg hop distance, hamstrings' maximal eccentric strength, and single-leg hamstring bridge test (SLHB) performance.

METHODOLOGY

During ninety minutes of the TSAFT90 football simulation, the BFlh FL and single-leg hop distance were measured three times (before, at half-time and after 90 minutes of simulated match-play), and maximal hamstrings eccentric strength and SLHB test scores were recorded twice (before and after simulated match-play) for both legs in physically active participants (n = 15).

RESULTS

Maximal eccentric hamstrings' strength (dominant leg (D): p < 0.001, Hedges' (adjusted) g effect size = -0.969; non-dominant leg (ND): p < 0.001, g = -0.929) and the SLHB performance (D: p < 0.001, g = -1.249; ND: p < 0.001, g = -1.108) showed large decrements immediately after the TSAFT90 intervention. There were no significant alterations in the BFlh FL, and the single-leg hop distance.

CONCLUSIONS

Maximal eccentric strength and the SLHB performance of hamstrings are reduced after 90 minutes of simulated football match-play. Practitioners may consider focusing on improving eccentric strength and the SLHB performance. Future studies should examine alterations in the BFlh fascicles' dynamic lengthening and shortening ability during a football match.

Knee Flexor Eccentric Strength, Hamstring Muscle Volume and Sprinting in Elite Professional Soccer Players with a Prior Strained Hamstring

Simple Summary

Given the link between a prior hamstring injury and the increased risk of suffering a subsequent injury in professional soccer, an understanding of the features displayed by previously injured players is important. The aim of the present study was to investigate if players with a history of hamstring injury exhibit bilateral deficits in knee flexor eccentric strength, hamstring muscle volume, and sprinting performance. The eccentric knee flexor strength was greater in the previously injured limbs compared to both the contralateral uninjured limbs in the previously injured group and the uninjured limbs in the previously uninjured players. Previously injured limbs showed possibly larger short heads of the biceps femoris and likely semitendinosus muscle volumes. Sprinting performances in the 5-m were possibly worse in players with a history of hamstring muscle strain injury compared to the previously uninjured players.

Abstract

The aim was to determine if players with a prior hamstring strain injury (HSI) exhibit bilateral deficits in knee flexor eccentric strength and hamstring muscle volume and differences in sprinting performance compared with players without a history of HSIs. Forty-six male professional soccer players participated in this study. Eccentric knee flexor strength, hamstring muscle volume (MRI), and a 20-m running sprint test (5- and 10-m split time) were assessed at the start of the preseason. Eccentric knee strength of the previously injured limbs of injured players was greater (ES: 1.18–1.36) than the uninjured limbs in uninjured players. Previously injured limbs showed possibly larger biceps femoris short heads (BFSh) and likely semitendinosus (ST) muscle volumes than the contralateral uninjured limbs among the injured players (ES: 0.36) and the limbs of the uninjured players (ES: 0.56), respectively. Players who had experienced a previous HSI were possibly slower in the 5-m (small ES: 0.46), while unclear differences were found in both the 10-m and 20-m times. Players with a prior HSI displayed greater eccentric knee flexor strength, possibly relatively hypertrophied ST and BFSh muscles, and possibly reduced 5-m sprinting performances than previously uninjured players. This can have implication for the design of secondary hamstring muscle injury prevention strategies.

Intra- and Inter-Limb Strength Asymmetry in Soccer: A Comparison of Professional and Under-18 Players

(1) Background: the present study examined the isokinetic peak torque exerted by both knee extensors and flexors, anterior–posterior imbalance and the magnitude and direction of inter-limb asymmetry in professional and academy soccer players. (2) Methods: one hundred soccer players (professional = 50, elite academy = 50) volunteered to take part in this investigation. An isokinetic dynamometer was used to measure the knee extensor (quadriceps) and flexors muscle (hamstrings) torques of the limbs as well as inter-limb asymmetries—using a standard percentage difference equation. (3) Results: professional players exhibited significantly greater (effect size [ES] = large) strength levels in the quadriceps and hamstrings under both testing conditions, significantly higher (small to moderate) intra-limb ratio values for 60°·s⁻¹ but not for the 300°·s⁻¹ test condition, significantly (small to moderate) lower inter-limb asymmetry values for all test conditions, with the exception of the hamstrings at 60°·s⁻¹ and the direction of asymmetry was poor to slight, indicating that limb dominance was rarely the same between groups. (4) Conclusions: this study shows that isokinetic assessments, i.e., peak torque exerted by both knee extensors and flexors and intra-limb ratio, and the subsequent inter-limb asymmetry, i.e., magnitude and direction, can differentiate between professional and academy soccer players.

Effects of Postactivation Potentiation After an Eccentric Overload Bout on Countermovement Jump and Lower-Limb Muscle Strength

ABSTRACT

Beato, M, Stiff, A, and Coratella, G. Effects of postactivation potentiation after an eccentric overload bout on countermovement jump and lower-limb muscle strength. J Strength Cond Res 35(7): 1825-1832, 2021-This study aimed to evaluate the postactivation potentiation (PAP) effects of an eccentric overload (EOL) exercise on countermovement jump (CMJ) performance and isokinetic lower-limb muscle strength. Eighteen active men (mean ± SD, age 20.2 ± 1.4 years, body mass 71.6 ± 8 kg, and height 178 ± 7 cm) were involved in a randomized, crossover study. The subjects performed 3 sets per 6 repetitions of EOL half squats at maximal power using a flywheel ergometer. Postactivation potentiation using an EOL exercise was compared with a control condition (10-minute cycling at 1 W·kg-1). Countermovement jump height, peak power, impulse, and force were recorded at 15 seconds, 1, 3, 5, 7, and 9 minutes after an EOL exercise or control. Furthermore, quadriceps and hamstrings isokinetic strength were performed. Postactivation potentiation vs. control reported a meaningful difference for CMJ height after 3 minutes (effect size [ES] = 0.68, p = 0.002), 5 minutes (ES = 0.58, p = 0.008), 7 minutes (ES = 0.57, p = 0.022), and 9 minutes (ES = 0.61, p = 0.002), peak power after 1 minute (ES = 0.22, p = 0.040), 3 minutes (ES = 0.44, p = 0.009), 5 minutes (ES = 0.40, p = 0.002), 7 minutes (ES = 0.29, p = 0.011), and 9 minutes (ES = 0.30, p = 0.008), as well as quadriceps concentric, hamstrings concentric, and hamstrings eccentric peak torque (ES = 0.13, p = 0.001, ES = 0.24, p = 0.003, and ES = 0.22, p = 003, respectively) after 3-9 minutes of rest. In conclusion, the present outcomes highlight that PAP using an EOL bout improves height, peak power, impulse, and peak force during CMJ, as well as quadriceps and hamstrings isokinetic strength in male athletes. Moreover, the optimal time window for the PAP was found from 3 to 9 minutes.

Influence of Fatigue on the Rapid Hamstring/Quadriceps Force Capacity in Soccer Players

The objective of this study was to examine the effect of fatigue on maximal and rapid force capacities and muscular activation of the knee extensors and flexors. Seventeen professional soccer players volunteered to participate in this study. Peak torque (T_peak) and rate of torque development (RTD) of knee flexor (90°. s^-1, -30°. s^-1) and extensor (90°. s^-1) muscles were measured before and after fatigue (i.e., 30 maximal knee extension and flexion repetitions at 180°s^-1) performed on an isokinetic dynamometer. Hamstring to quadriceps peak strength and RTD ratios were calculated. Besides, using surface EMG, the mean level of activation (RMS_mean), Rate of EMG Rise (RER), and EMG Frequency-Time maps were measured on quadriceps and hamstring muscles. Following fatigue, T_peak, RTD, RER declined significantly in the two muscle groups (all p < 0.05) without modification of RMS_mean. No decrease in conventional and functional H/Q ratios was observed after fatigue except for a significant increase in the H _ecc30/Q _con180 ratios (1.03 ± 0.19 vs. 1.36 ± 0.33, p < 0.001). Besides, the RTD H/Q ratios decreased significantly after fatigue, and the statistical parametric mapping analysis (SPM) performed on the EMG/angle curves, and EMG Frequency-Time maps showed that fatigue strongly influenced the muscle activation during the first 100 ms of the movement, following the higher EMG frequency component shift toward the lower frequency component. Our results show that the reduction of RTD and RER during the first 100 ms of the contraction after fatigue exercise makes more sense than any H/Q ratio modification in understanding injury risk in soccer players.

A 1-Minute Re-warm Up at High-Intensity Improves Sprint Performance During the Loughborough Intermittent Shuttle Test

Although a 3- to 7-min re-warm up (RW) elicits performance and physiological benefits after half-time (HT), a time-efficient and feasible RW protocol is required for the use of an RW in the athletic setting. This study aimed to investigate the effect of a 1-min RW at high-intensity on the performance and physiological responses during the Loughborough Intermittent Shuttle Test (LIST). In a randomized and counterbalanced cross-over design, 12 male amateur intermittent team sports players (soccer, basketball, handball, and lacrosse; age, 22 ± 2 years; height, 1.70 ± 0.08 m; body mass, 65.1 ± 8.3 kg; body mass index, 22.4 ± 1.9 kg m⁻²; VO_2max, 53.5 ± 4.5 ml kg⁻¹ min⁻¹) performed the LIST. The LIST comprised two 45-min halves separated by a 15-min HT. Each half comprised repetitions of exercise cycles consisting of 3 × 20-m walking, 1 × 20-m maximal sprint, 3 × 20-m jogging, and 3 × 20-m running. During the HT, the participants were assigned to a control trial (CON; 15-min seated rest) or an RW trial (1-min running at 90% of the maximal oxygen uptake after a 14-min seated rest). Compared to the CON, the RW prevents reductions in sprint performance at the fourth and sixth periods of the LIST (fourth: 2.4%, p = 0.002, d = 1.68, sixth: 3.6%, p = 0.012, d = 1.74) and a decrement of gastrointestinal temperature during HT (0.5°C, p = 0.010, d = 1.41). Moreover, the RW decreased the electromyogram amplitude of maximal voluntary contraction (MVC) after HT (12%, p = 0.017, d = 1.12) without a decrease of maximal voluntary contraction force, suggesting an increased neuromuscular efficiency (9%, p = 0.048, d = 0.58). The RW also increased the mean heart rate in the initial part of the second half (4 bpm, p = 0.016, d = 0.38). In conclusion, the RW improved sprint performance, core temperature, muscle activation, and heart rate in the second half of the LIST. The findings suggest that the RW should be recommended for intermittent team sports players when longer RWs are not possible.

Lower-Limb Muscle Strength, Anterior-Posterior and Inter-Limb Asymmetry in Professional, Elite Academy and Amateur Soccer Players

Given the importance of the lower-limb strength and strength balance in soccer players and its relationship with injury prevention and performance, the present study compared quadriceps and hamstrings strength, the conventional (H_conc:Q_conc), functional (H_ecc:Q_conc) hamstrings-to-quadriceps ratio and inter-limb strength asymmetry in professional, elite academy and amateur male soccer players. In this cross-sectional study, two hundred-six soccer players (professional = 75, elite academy = 68, amateurs = 63) volunteered to participate. Quadriceps and hamstrings isokinetic peak torque was investigated at 60^° _.s^-1 in both the concentric and eccentric modality and at 300^°.s^-1 in the concentric modality. The conventional H_conc:Q_conc, functional H_ecc:Q_conc ratio and quadriceps and hamstrings inter-limb strength asymmetry were then calculated. Professional players presented greater quadriceps and hamstrings strength than elite academy (effect size from small to moderate) and amateur players (moderate to very large). Both the conventional H_conc:Q_conc and functional H_ecc:Q_conc ratio were greater in professional than elite academy and amateur players (small to moderate). Overall, quadriceps and hamstrings inter-limb strength asymmetry was greater in amateurs than professional (small to very large) and elite academy (trivial to large) players. The present findings provide coaches and medical staffs with normative lower-limb muscle strength data on professional, academy and amateur soccer players. Overall lower-limb muscle strength and inter-limb strength asymmetry could be used to evaluate possible inference on injury prevention and performance. The hamstrings-to-quadriceps ratio poorly differentiates between the soccer players background and offers limited prediction for injury prevention and performance.

A Maximal Incremental Test in Cyclists Causes Greater Peripheral Fatigue in Biceps Femoris

Purpose: The aim of this study was to determine the changes in neuromuscular contractile properties in elite cyclists associated to the peripheral fatigue produced in a maximal incremental test in cycle ergometer. Methods: Forty-eight volunteers' well-trained male cyclists were evaluated during the competition season within a recovery microcycle. Tensiomyography was used before and after performing an incremental test until exhaustion in cycle ergometer to measure the radial muscle belly displacement of the vastus lateralis (VL), rectus femoris (RF), and biceps femoris (BF) on the dominant leg. Maximum radial muscle belly displacement (Dm), contraction time (Tc), delay time (Td), sustain time (Ts), and radial displacement velocity (Vrd) were measured. Mixed-design factorial analysis of variance was used to detect changes in the mechanical and neuromuscular characteristics after a maximal incremental exercise test. Results: The results show in the evaluated muscles a moderate-large decrease (p < .01, η2 = 0.06-0.51) in Tc, Td, Dm, Ts, and Vrd. This decrease occurs more sharply in the BF than in RF or VL. Conclusion: An incremental effort until exhaustion produces peripheral fatigue associated with a decrease in Dm, Tc, Td, Ts, and Vrd, being more pronounced in biceps femoris than in vastus lateralis and rectus femoris. Coaches can use these changes found in the contractile properties as a reference to detect the muscle fatigue degree that certain training models pose for the cyclist.

Preseason assessment of anaerobic performance in elite soccer players: comparison of isokinetic and functional tests

Isokinetic and functional jump tests are frequently performed for assessing the physical qualities of soccer players during preseason. The purpose of this investigation was to explore, in an elite soccer players population, the relationships between isokinetic strength and functional jump performances. Thirty-eight professional soccer players were evaluated as follows: isokinetic knee assessment in concentric (CON) mode (60, 240°/s) for quadriceps and hamstrings, and in eccentric (ECC) mode for the hamstrings only (30°/s); one-leg hop tests for distance (single hop (SH), triple hop (TH) and triple crossover hop (TCH)); one-leg vertical jump tests (countermovement jump, drop jump). Players with a low bodyweight normalised (BWN) quadriceps (Q) strength (<2.71 Nm/kg) performed, for a majority of the measured variables, significantly reduced jump performances compared to the players with high BWN Q strength (>3.14 Nm/kg; p < 0.05). Greater bilateral differences between uninjured and past injured lower limbs were found with isokinetics (Q CON 60°/s (mean bilateral difference (MBD): 10.3%; p < 0.01), Q CON 240°/s (MBD: 9.9%; p < 0.05), H ECC 30°/s (MBD: 16.1%; p < 0.001) than with functional tests (MBD: 2 to 9%; p > 0.05. In conclusion, due to their complementary role and implications for performance, functional and isokinetic tests should be associated in a preseason soccer players assessment.

Injury risk and injury incidence rates in CrossFit

INTRODUCTION

Despite completing complex movements under cardiovascular and muscular fatigued conditions, CrossFit suggested safety of participants is one of the most important factors to consider. Therefore, the aim of this brief review was to provide an overview of the injury incidence and incidence rates reported in the current literature as well as to assess the injury risks associated with CrossFit.

EVIDENCE ACQUISITION

Research papers were searched using the following databases: Scopus, SPORTDiscus, Web of Science, PubMed, and MEDLINE. The search identified 718 potential articles across all databases. After the application of inclusion criteria, a total of 14 articles were included in this review.

EVIDENCE SYNTHESIS

The shoulder, spine, and knee are the most commonly injured areas in CrossFit. Injury incidence of each study ranges from 12.8-73.5% and reported injury rates ranging from 0.27-3.3/1000 training hours.

CONCLUSIONS

Reported findings in the current literature would suggest CrossFit has a relatively low injury risk. The current review highlights three important factors associated with injury incidence and incidence rates in CrossFit: 1) training frequency; 2) duration of CrossFit experience; 3) individuals that compete in CrossFit competitions. Due to recall bias and selection bias further research is warranted over a longitudinal prospective study. Additionally, further research is needed to investigate mechanisms of injury and the relationship between training load and injury risk in CrossFit.

Current Evidence and Practical Applications of Flywheel Eccentric Overload Exercises as Postactivation Potentiation Protocols: A Brief Review

PURPOSE

To summarize the evidence on postactivation potentiation (PAP) protocols using flywheel eccentric overload (EOL) exercises.

METHODS

Studies were searched using the electronic databases PubMed, Scopus, and Institute for Scientific Information Web of Knowledge.

RESULTS

In total, 7 eligible studies were identified based on the following results: First, practitioners can use different inertia intensities (eg, 0.03-0.88 kg·m2), based on the exercise selected, to enhance sport-specific performance. Second, the PAP time window following EOL exercise seems to be consistent with traditional PAP literature, where acute fatigue is dominant in the early part of the recovery period (eg, 30 s), and PAP is dominant in the second part (eg, 3 and 6 min). Third, as EOL exercises require large force and power outputs, a volume of 3 sets with the conditioning activity (eg, half-squat or lunge) seems to be a sensible approach. This could reduce the transitory muscle fatigue and thereby allow for a stronger potentiation effect compared with larger exercise volumes. Fourth, athletes should gain experience by performing EOL exercises before using the tool as part of a PAP protocol (3 or 4 sessions of familiarization). Finally, the dimensions of common flywheel devices offer useful and practical solutions to induce PAP effects outside of normal training environments and prior to competitions.

CONCLUSIONS

EOL exercise can be used to stimulate PAP responses to obtain performance advantages in various sports. However, future research is needed to determine which EOL exercise modalities among intensity, volume, and rest intervals optimally induce the PAP phenomenon and facilitate transfer effects on athletic performances.

Effects of in-season enhanced negative work-based vs traditional weight training on change of direction and hamstrings-to-quadriceps ratio in soccer players

The present study investigated the effects of in-season enhanced negative work-based training (ENT) vs weight training in the change of direction (COD), sprinting and jumping ability, muscle mass and strength in semi-professional soccer players. Forty male soccer players participated in the eight-week, 1 d/w intervention consisting of 48 squat repetitions for ENT using a flywheel device (inertia=0.11 kg·m^-2) or weight training (80%1 RM) as a control group (CON). Agility T-test, 20+20 m shuttle, 10 m and 30 m sprint, squat jump (SJ) and countermovement jump (CMJ), lean mass, quadriceps and hamstrings strength and the hamstrings-to-quadriceps ratio were measured. Time on agility T-test and 20+20 m shuttle decreased in ENT (effect-size =-1.44, 95% CI -2.24/-0.68 and -0.75, -1.09/-0.42 respectively) but not in CON (-0.33, -0.87/0.19 and -0.13, -0.58/0.32). SJ and CMJ height increased in both ENT (0.71, 0.45/0.97 and 0.65, 0.38/0.93) and CON (0.41, 0.23/0.60 and 0.36, 0.12/0.70). Overall, quadriceps and hamstrings strength increased in both ENT and CON (0.38/0.79), but the hamstrings-to-quadriceps ratio increased in ENT (0.31, 0.22/0.40) but not in CON (0.03, -0.18/0.24). Lean mass increased in both ENT (0.41, 0.26/0.57) and CON (0.29, 0.14/0.44). The repeated negative actions performed in ENT may have led to improvements in braking ability, a key point in COD performance. Semi-professional soccer players may benefit from in-season ENT to enhance COD and the negative-specific adaptations in muscle strength and hamstrings-to-quadriceps ratio.

Acute and Residual Soccer Match-Related Fatigue: A Systematic Review and Meta-analysis

BACKGROUND

Understanding soccer players' match-related fatigue and recovery profiles likely helps with developing conditioning programs that increase team performance and reduce injuries and illnesses. In order to improve match recovery (the return-to-play process and ergogenic interventions) it is also pivotal to determine if match simulation protocols and actual match-play lead to similar responses.

OBJECTIVES

(1) To thoroughly describe the development of fatigue during actual soccer match play and its recovery time course in terms of physiological, neuromuscular, technical, biochemical and perceptual responses, and (2) to determine similarities of recovery responses between actual competition (11 vs. 11) and match simulations.

METHODS

A first screening phase consisted of a systematic search on PubMed (MEDLINE) and SportDiscus databases until March 2016. Inclusion criteria were: longitudinal study with soccer players; match or validated protocol; duration > 45 min; and published in English.

RESULTS

A total of 77 eligible studies (n = 1105) were used to compute 1196 effect sizes (ES). Half-time assessments revealed small to large alterations in immunological parameters (e.g. leukocytes, ES = 1.9), a moderate decrement in insulin concentration (ES = - 0.9) and a small to moderate impairment in lower-limb muscle function (ES = - 0.5 to - 0.7) and physical performance measures (e.g. linear sprint, ES = - 0.3 to - 1.0). All the systematically analyzed fatigue-related markers were substantially altered at post-match. Hamstrings force production capacity (ES = - 0.7), physical performance (2-4%, ES = 0.3-0.5), creatine kinase (CK, ES = 0.4), well-being (ES = 0.2-0.4) and delayed onset muscle soreness (DOMS, ES = 0.6-1.3) remained substantially impaired at G + 72 h. Compared to simulation protocols, 11 vs. 11 match format (CK, ES = 1.8) induced a greater magnitude of change in muscle damage (i.e. CK, ES = 1.8 vs. 0.7), inflammatory (IL-6, ES = 2.6 vs. 1.1) and immunological markers and DOMS (ES = 1.5 vs. 0.7) than simulation protocols at post-assessments. Neuromuscular performances at post-match did not differ between protocols.

CONCLUSION

While some parameters are fully recovered (e.g. hormonal and technical), our systematic review shows that a period of 72 h post-match play is not long enough to completely restore homeostatic balance (e.g. muscle damage, physical and well-being status). The extent of the recovery period post-soccer game cannot consist of a 'one size fits all approach'. Additionally, the 'real match' (11 vs. 11 format) likely induces greater magnitudes of perceptual (DOMS) and biochemical alterations (e.g. muscle damage), while neuromuscular alterations were essentially similar. Overall, coaches must adjust the structure and content of the training sessions during the 72-h post-match intervention to effectively manage the training load within this time-frame.

Specific Adaptations in Performance and Muscle Architecture After Weighted Jump-Squat vs. Body Mass Squat Jump Training in Recreational Soccer Players

Coratella, G, Beato, M, Milanese, C, Longo, S, Limonta, E, Rampichini, S, Cè, E, Bisconti, AV, Schena, F, and Esposito, F. Specific adaptations in performance and muscle architecture after weighted jump-squat vs. body mass squat jump training in recreational soccer players. J Strength Cond Res 32(4): 921-929, 2018-The aim of the present study was to compare the effects of weighted jump-squat training (WJST) vs. body mass squat jump training (BMSJT) on quadriceps' muscle architecture, lower-limb lean-mass (LM) and muscle strength, performance in change of direction (COD), and sprint and jump in recreational soccer players. Forty-eight healthy soccer players participated in an offseason randomized controlled trial. Before and after an 8-week training intervention, vastus lateralis pennation angle, fascicle length, muscle thickness, LM, squat 1RM, quadriceps and hamstrings isokinetic peak torque, agility T-test, 10-and 30-m sprints, and squat-jump (SJ) were measured. Although similar increases were observed in muscle thickness, fascicle length increased more in WJST (Effect size [ES] = 1.18, 0.82-1.54) than in BMSJT (ES = 0.54, 0.40-0.68), and pennation angle increased only in BMSJT (ES = 1.03, 0.78-1.29). Greater increases in LM were observed in WJST (ES = 0.44, 0.29-0.59) than in BMSJT (ES = 0.21, 0.07-0.37). The agility T-test (ES = 2.95, 2.72-3.18), 10-m (ES = 0.52, 0.22-0.82), and 30-m sprints (ES = 0.52, 0.23-0.81) improved only in WJST, whereas SJ improved in BMSJT (ES = 0.89, 0.43-1.35) more than in WJST (ES = 0.30, 0.03-0.58). Similar increases in squat 1RM and peak torque occurred in both groups. The greater inertia accumulated within the landing phase in WJST vs. BMSJT has increased the eccentric workload, leading to specific eccentric-like adaptations in muscle architecture. The selective improvements in COD in WJST may be related to the increased braking ability generated by the enhanced eccentric workload.

Strength-Endurance Training Reduces the Hamstrings Strength Decline Following Simulated Football Competition in Female Players

Hamstring strains are the most common injury in multiple sprint sports, with inadequate eccentric hamstring strength and fatigue identified as important risk factors. Resistance training interventions aimed at reducing injury risk typically focus on the development of maximum strength, while little is known about the impact of training on hamstring fatigue resistance. The present study compared the effects of strength endurance (SE) with a strength intervention (S) on the eccentric hamstring strength decline induced by a simulated soccer match. Twenty-one female soccer players were randomly assigned to a S group (n = 10) or a SE group (n = 11). Hamstrings and quadriceps isokinetic concentric and eccentric peak torque (PT) were assessed at 120°.s^-1 and hamstrings-to-quadriceps ratio (HEcc:QCon) calculated, pre- and immediately post a 90-min simulated match (BEAST90). This was repeated following a 7-week intervention of either three to five sets of 6RM leg curl and stiff-leg deadlift with 3-min inter-set rest (S), or the same exercises performed using three sets of 12–20 RM with 45–90 s inter-set rest (SE). At baseline, the simulated match led to significant declines in hamstrings eccentric peak torque (EccPT) in both groups in both dominant (D) and non-dominant (ND) legs [SE: (D: -15.5, ND: -15.6%), P = 0.001 to 0.016; S: (D: -12.3%, ND: -15.5%), P = 0.001 to 0.018]. After the 7-week intervention, we observed a group^∗intervention^∗match interaction such that there was no significant decline in EccPT in the SE group following the simulated match (D: 5.3%, ND: 2.0%), but there remained significant declines in the S group (D: -14.2%, ND: -15.5%, P = 0.018–0.001). Similarly, in the SE group, there was a significant decrease in the HEcc:QCon in D before (-14.2%, P = 0.007), but not after the training intervention, whereas declines were observed in the S group both at baseline, and following the intervention (D: -13.9%, ND: -15.6%, P = 0.045). These results demonstrate that SE training can reduce the magnitude of the EccPT decline observed during soccer competition. As inadequate eccentric strength and fatigue are both risk factors for hamstring injury, SE training should be considered along with the development of peak eccentric strength, as a component of programs aimed at reducing injury risk in multiple-sprint sports.

Effects of Plyometric and Directional Training on Speed and Jump Performance in Elite Youth Soccer Players

Beato, M, Bianchi, M, Coratella, G, Merlini, M, and Drust, B. Effects of plyometric and directional training on speed and jump performance in elite youth soccer players. J Strength Cond Res 32(2): 289-296, 2018-Soccer players perform approximately 1,350 activities (every 4-6 seconds), such as accelerations/decelerations and changes of direction (CODs) during matches. It is well established that COD and plyometric training have a positive impact on fitness parameters in football players. This study analyzed the effect of a complex COD and plyometric protocol (CODJ-G) compared with an isolated COD protocol (COD-G) training on elite football players. A randomized pre-post parallel group trial was used in this study. Twenty-one youth players were enrolled in this study (mean ± SD; age 17 ± 0.8 years, mass 70.1 ± 6.4 kg, and height 177.4 ± 6.2 cm). Players were randomized into 2 different groups: CODJ-G (n = 11) and COD-G (n = 10), training frequency of 2 times a week more than 6 weeks. Sprint 10, 30, and 40 m, long jump, triple hop jump, and 505 COD test were considered. Exercise-induced within-group changes in performance for both CODJ-G and COD-G: long jump (effect size [ES] = 0.32 and ES = 0.26, respectively) and sprint 10 m (ES = -0.51 and ES = -0.22, respectively), after 6 weeks of training. Moreover, CODJ-G reported substantially better results (between-group changes) in long jump test (ES = 0.32). In conclusion, this study showed that short-term protocols (CODJ-G and COD-G) are important and able to give meaningful improvements on power and speed parameters in a specific soccer population. CODJ-G showed a larger effect in sprint and jump parameters compared with COD-G after the training protocol. This study offers important implications for designing COD and jumps training in elite soccer.

Greater fatigability in knee-flexors vs. knee-extensors after a standardized fatiguing protocol

The present study aimed to investigate the effects of a standardized fatiguing protocol on central and peripheral fatigue in knee-flexors and knee-extensors. Thirteen healthy men (age: 23 ± 3 years; height: 1.78 ± 0.09 m; body-mass: 73.6 ± 9.2 kg) volunteered for the present study. Maximal voluntary contraction (MVC), Electromyography (EMG) activity, voluntary activation level (VAL) as an index of central fatigue and twitch potentiation as an index of peripheral fatigue were measured before and after the fatiguing protocol. The fatiguing protocol consisted of a 0.6 duty-cycle to exhaustion (6 s isometric contraction, 4 s recovery) at 70% MVC. After the fatiguing protocol, MVC decreased in both (Effect-size (ES) = 1.14) and knee-extensors (ES = 1.14), and EMG activity increased in both knee-flexors (ES = 2.33) and knee-extensors (ES = 1.54). Decreases in VAL occurred in knee-flexors (ES = 0.92) but not in knee-extensors (ES = 0.04). Decreases in potentiation occurred in both knee-flexors (ES = 0.84) and knee-extensors (ES = 0.58). The greater central occurrence of fatigue in knee-flexors than in knee-extensors may depend on the different muscle morphology and coupled with a greater tolerance to fatigue in knee-extensors. The present data add further insight to the complicated knee-flexors-to-knee-extensors strength relationship and the mechanisms behind the different occurrence of fatigue.

Correlation between quadriceps and hamstrings inter-limb strength asymmetry with change of direction and sprint in U21 elite soccer-players

The aim of this study was to investigate the relationship between in quadriceps and hamstrings inter-limb strength asymmetry and change of direction, sprinting and jumping abilities in U21 elite soccer players. Twenty-seven soccer players volunteered for this study. Isokinetic quadriceps and hamstrings peak torque was measured at high and low angular velocities, both in concentric and eccentric modalities. Performance in agility T-test, 20 + 20 m shuttle-test, 10 m and 30 m sprint, squat jump (SJ) and counter-movement jump (CMJ), were measured. Overall, time on agility T-test and 20 + 20 m shuttle-test was moderately and positively correlated with the quadriceps and hamstrings inter-limb eccentric peak torque asymmetry, both at high and low angular velocities. In addition, time on 10 m and 30 m sprints was moderately and positively correlated with the hamstrings inter-limb high-velocity concentric peak torque asymmetry. SJ and CMJ showed trivial to small correlations with hamstrings and quadriceps inter-limb peak torque asymmetry. The present results provide further information insight the role of lower-limb muscle strength balance in COD, sprinting and jumping performance.