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This study aimed to investigate the kinematics and kinetics differences in ground reaction force (GRF)-time profiles with uni- and bimodal curves (UNC and BIC) during the concentric phase of the drop jump (DJ). Twenty two male Physical Education college student who met UNC (N = 11) or BIC (N = 11) of the GRF-time profile of were recruited. Two force plates and eight infrared optical cameras were synchronised to collect the GRF and motion data during DJ from a 30-cm height. The Shapiro-Wilk test was used to assess the normality of data. The Wilcoxon test was used when data were not normally distributed. Otherwise, Independent t-tests were used to compare differences between the UNC and BIC groups for each dependent variable. The UNC group demonstrated shorter ground contact time, lower jump height, greater leg stiffness, greater peak power during the eccentric phase, less work during the eccentric and concentric phases, and greater hip and knee joint flexion and extension angle displacements (p < 0.05). No significant intergroup differences were found in reactive strength index (p > 0.05). The UNC and BIC of the GRF-time profiles can indicate whether athletes can practice DJ appropriately. UNC can be representative of a better DJ performance with an efficient stretch-shortening cycle function.

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Soccer players with chronic ankle instability (CAI) may stabilize their supporting leg by the proximal joint to compensate for the ankle instability during kicking motion. This study aimed to investigate the characteristics of leg and joint stiffness of the supporting leg during side-foot kicking in soccer players with CAI. Twenty-four male collegiate-level soccer players with and without CAI participated in this study. The kinematic and kinetic data were obtained using a three-dimensional motion analysis system. Leg stiffness and joint (hip, knee, and ankle) stiffness in the sagittal and frontal planes were calculated and analyzed. The results clarified that soccer players with CAI (0.106 ± 0.053 Nm/°) had greater knee stiffness in knee adduction during the kicking cycle compared to those without CAI (0.066 ± 0.030 Nm/°; p = 0.046), whereas no characteristic differences were observed in knee stiffness in knee flexion and hip and ankle stiffness (p > 0.05). Knee stiffness is believed to occur to compensate for ankle joint instability in the supporting leg. Therefore, adjusting knee stiffness to accommodate ankle joint instability is crucial for maintaining kicking performance. Based on results of this study, it may be important to consider training and exercises focused on joint coordination to improve knee stiffness in soccer players with CAI.

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Typically, animal locomotion studies involve consecutive strides, which are frequently assumed to be independent with parameters that do not vary across strides. This assumption is often not tested. However, failing in particular to account for dependence across strides may cause an incorrect estimate of the uncertainty of the measurements and thereby lead to either missing (overestimating variance) or over-evaluating (underestimating variance) biological signals. In turn, this impacts replicability of the results because variability is accounted for differently across experiments. In this paper, we analyse the changes of a couple of measures of human leg stiffness across strides during running experiments, using a publicly available dataset. A major finding of this analysis is that the time series of these measurements of stiffness show autocorrelation even at large lags and so there is dependence between individual strides, even when separated by many intervening strides. Our results question the practice in biomechanics research of using each stride as an independent observation or of sub-selecting strides at small lags. Following the outcome of our analysis, we strongly recommend caution in doing so without first confirming the independence of the measurements across strides and without confirming that sub-selection does not produce spurious results.

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[This corrects the article DOI: 10.3389/fphys.2022.1044363.].

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Duty factor (DF) and step frequency (SF) were previously defined as the key running pattern determinants. Hence, this study aimed to investigate the association of DF and SF on 1) the vertical and fore-aft ground reaction force signals using statistical parametric mapping; 2) the force related variables (peaks, loading rates, impulses); and 3) the spring-mass characteristics of the lower limb, assessed by computing the force-length relationship and leg stiffness, for treadmill runs at several endurance running speeds. One hundred and fifteen runners ran at 9, 11, and 13 km/h. Force data (1000 Hz) and whole-body three-dimensional kinematics (200 Hz) were acquired by an instrumented treadmill and optoelectronic system, respectively. Both lower DF and SF led to larger vertical and fore-aft ground reaction force fluctuations, but to a lower extent for SF than for DF. Besides, the linearity of the force-length relationship during the leg compression decreased with increasing DF or with decreasing SF but did not change during the leg decompression. These findings showed that the lower the DF and the higher the SF, the more the runner relies on the optimization of the spring-mass model, whereas the higher the DF and the lower the SF, the more the runner promotes forward propulsion.

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Background: Lower extremity stiffness simulates the response of the lower extremity to landing in running. However, its relationship with running economy (RE) remains unclear. This study aims to explore the relationship between lower extremity stiffness and RE. Methods: This study utilized articles from the Web of Science, PubMed, and Scopus discussing the relationships between RE and indicators of lower extremity stiffness, namely vertical stiffness, leg stiffness, and joint stiffness. Methodological quality was assessed using the Joanna Australian Centre for Evidence-Based Care (JBI). Pearson correlation coefficients were utilized to summarize effect sizes, and meta-regression analysis was used to assess the extent of this association between speed and participant level. Result: In total, thirteen studies involving 272 runners met the inclusion criteria and were included in this review. The quality of the thirteen studies ranged from moderate to high. The meta-analysis results showed a negative correlation between vertical stiffness (r = -0.520, 95% CI, -0.635 to -0.384, p < 0.001) and leg stiffness (r = -0.568, 95% CI, -0.723 to -0.357, p < 0.001) and RE. Additional, there was a small negative correlation between knee stiffness and RE (r = -0.290, 95% CI, -0.508 to -0.037, p = 0.025). Meta-regression results showed that the extent to which leg stiffness was negatively correlated with RE was influenced by speed (coefficient = -0.409, p = 0.020, r 2 = 0.79) and participant maximal oxygen uptake (coefficient = -0.068, p = 0.010, r 2 = 0.92). Conclusion: The results of this study suggest that vertical, leg and knee stiffness were negatively correlated with RE. In addition, maximum oxygen uptake and speed will determine whether the runner can take full advantage of leg stiffness to minimize energy expenditure.

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English Premier League soccer players run at multiple speeds throughout a game. The aim of this study was to assess how well the duty factor, a dimensionless ratio based on temporal variables, described running styles in professional soccer players. A total of 25 players ran on an instrumented treadmill at 12, 16, and 20 km/h. Spatiotemporal and ground reaction force data were recorded for 30 s at each speed; video data (500 Hz) were collected to determine footstrike patterns. In addition to correlation analysis amongst the 25 players, two groups (both N = 9) of high and low duty factors were compared. The duty factor was negatively correlated with peak vertical force, center of mass (CM) vertical displacement, and leg stiffness (k leg) at all speeds (r ≥ -0.51, p ≤ 0.009). The low duty factor group had shorter contact times, longer flight times, higher peak vertical forces, greater CM vertical displacement, and higher k leg (p < 0.01). Among the high DF group players, eight were rearfoot strikers at all speeds, compared with three in the low group. The duty factor is an effective measure for categorizing soccer players as being on a continuum from terrestrial (high duty factor) to aerial (low duty factor) running styles, which we metaphorically refer to as "grizzlies" and "gazelles," respectively. Because the duty factor distinguishes running style, there are implications for the training regimens of grizzlies and gazelles in soccer, and exercises to improve performance should be developed based on the biomechanical advantages of each spontaneous running style.

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BACKGROUND: Nike ZoomX Vaporfly (NVF) improves running economy and performance. The biomechanical mechanisms of these shoes are not fully understood, although thicker midsoles and carbon fiber plates are considered to play an important role in the spring-like leg characteristics during running. Leg stiffness (kleg) in the spring-mass model has been commonly used to investigate spring-like running mechanics during running. RESEARCH QUESTION: Does kleg during running differ between NVF and traditional (TRAD) shoes? METHODS: Eighteen male habitual forefoot and/or midfoot strike runners ran on a treadmill at 20 km/h with NVF and TRAD shoes, respectively. kleg, vertical oscillation of the center of mass (∆CoM), spatiotemporal parameters, and mechanical loading were determined. RESULTS: kleg was 4.8% lower in the NVF shoe condition than in the TRAD condition, although no significant difference was observed. ∆CoM was not significantly different between shoe conditions. Spatiotemporal parameters and mechanical loading were also not significantly different between shoe conditions. SIGNIFICANCE: The NVF shoe is well known as improving the running economy and running performance for the cause by characteristics of better spring function. Contrary to expectation, kleg and other parameters were not significantly different during running in the NVF compared to TRAD shoe at 20 km/h. These findings indicate that well-trained runners' spring-like running mechanics would not alter even if wearing the NVF shoes.

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Knowledge of uncertainty is valuable mainly in correctly appraising measured effects. In lower limb stiffness, which affects injury risk and athletic performance, uncertainty is often related to vertical (Kvert) and leg (Kleg) stiffness. Imprecisions in measurements of body mass (M), leg length (L), contact (tc) and flight (tf) time propagate through the calculations, augment stiffness uncertainty and inflate relevant effects. This study estimated the limits of this uncertainty as probable (Eprob) and upper bound (Eupper) errors by applying Taylor series expansion on Monte-Carlo simulated data. Eprob and Eupper were 1285 ± 221 N/m (3.9 ± 0.2%) and 1441 ± 248 N/m (4.4 ± 0.3%) in Kvert, and 222 ± 61 N/m (2.1 ± 0.1%) and 375 ± 109 N/m (3.6 ± 0.3%) in Kleg, respectively. To avoid the complexities of full Taylor series expansion, Eprob was predicted (R2 ≈ 1) more simply as 0.89Eupper in Kvert and 11 + 0.56Eupper in Kleg. These uncertainties reflect mostly errors in tc and tf, and uncertainty in Fmax, at kinematic sampling of 300 Hz and running at 4-5 m/s. With slower sampling or faster running these uncertainties rise, and their impact on similar lower limb stiffness effects could be substantial. Applying Taylor series expansion for error propagation on Monte-Carlo simulated data is valid for uncertainty analysis in any multivariable functional relationship.

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Purpose: Individuals with mechanical ankle instability (MAI) and functional ankle instability (FAI) present with residual movement dysfunctions after an initial lateral ankle sprain. This study investigated leg stiffness control to determine how behavioural characteristics between MAI and FAI differ during single-leg drop landing movement. Methods: Thirty individuals (10 with MAI, 10 with FAI, and 10 healthy controls) participated in this study. During single-leg drop landing movement, we estimated dimensionless leg stiffness, peak vertical force (PVF), change in the displacement of leg movement, loading rate, angular joint movement, and internal joint moment for impaired and unimpaired legs, respectively. Univariate one-way analysis of variance was used for each dependent variable across leg conditions. Results: The MAI group had lower dimensionless leg stiffness with greater change in the displacement of leg movement for the impaired leg than the FAI and control groups. Moreover, reduced leg stiffness was associated with greater hip joint flexion movement in the MAI group, whereas ankle dorsiflexion movement increased to decrease the leg stiffness in the FAI and control groups. Conclusion: These findings indicate that lower leg stiffness in the MAI group than in the FAI group may be attributed to compensatory actions to minimise reliance on their ankle movement after landing.

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PURPOSE: This study aimed to determine the contribution of metabolic, cardiopulmonary, neuromuscular, and biomechanical factors to the energy cost (ECR) of graded running in well-trained runners. METHODS: Eight men who were well-trained trail runners (age: 29 [10] y, mean [SD]; maximum oxygen consumption: 68.0 [6.4] mL·min-1·kg-1) completed maximal isometric evaluations of lower limb extensor muscles and 3 randomized trials on a treadmill to determine their metabolic and cardiovascular responses and running gait kinematics during downhill (DR: -15% slope), level (0%), and uphill running (UR: 15%) performed at similar O2 uptake (approximately 60% maximum oxygen consumption). RESULTS: Despite similar O2 demand, ECR was lower in DR versus level running versus UR (2.5 [0.2] vs 3.6 [0.2] vs 7.9 [0.5] J·kg-1·m-1, respectively; all P < .001). Energy cost of running was correlated between DR and level running conditions only (r2 = .63; P = .018). Importantly, while ECR was correlated with heart rate, cardiac output, and arteriovenous O2 difference in UR (all r2 > .50; P < .05), ECR was correlated with lower limb vertical stiffness, ground contact time, stride length, and step frequency in DR (all r2 > .58; P < .05). Lower limb isometric extension torques were not related to ECR whatever the slope. CONCLUSION: The determining physiological factors of ECR might be slope specific, mainly metabolic and cardiovascular in UR versus mainly neuromuscular and mechanical in DR. This possible slope specificity of ECR during incline running opens the way for the implementation of differentiated physiological evaluations and training strategies to optimize performance in well-trained trail runners.

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AIM: To determine whether a running intervention utilising plyometric activities improved leg stiffness in youth with cerebral palsy (CP), GMFCS levels I and II. METHOD: This stratified randomised controlled trial examined the lower limb kinetics and kinematics of a sample of youths with CP during sub-maximal hopping and running, prior to and immediately following a 12-week running intervention that incorporated low load plyometric training. Included participants were 13 in the control group (mean age 13 years 2 months [SD 2 years 7 months]; six males; nine GMFCS level I; six unilateral) and 18 in the intervention group (mean age 12 years 9 months [SD 2 years 10 months]; 13 males; 11 GMFCS level I; nine unilateral). Derived variables included three-dimensional leg stiffness as well as resultant ground reaction force and change in leg length. Generalised linear mixed models were developed for statistical analysis. RESULTS: At follow-up the intervention group had greater leg stiffness than the control group during submaximal hopping (Intervention median = 3278Nm-1; Control median = 1556Nm-1; p < 0.01). At follow-up, participants in the intervention group in GMFCS Level I had greater leg stiffness than the control group during jogging (Intervention mean=38.84 (SD=25.55); Control mean=29.38 (SD=11.11); t = 2.61 p = 0.01). INTERPRETATION: A running training intervention which includes plyometric activities can improve leg stiffness in young people with CP, especially those in GMFCS level I.

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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.

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BACKGROUND: The spring-like behavior of the leg and the joints of the lower body during running are thought to influence a wide range of physiologic and mechanical phenomena, including susceptibility to overuse injury. If leg and joint stiffness are associated with running-related injuries, altering joint or leg stiffness may be a useful avenue for injury rehabilitation and injury prevention programs. PURPOSE: To test the associations between running-related injury and leg stiffness, knee stiffness, and ankle stiffness in a prospective study of recreational runners. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: A total of 49 healthy recreational runners took part in a year-long study. Participants completed a 3-dimensional kinematic and kinetic biomechanical assessment at baseline and reported training volume and injury status in a weekly survey during the follow-up period. Relationships between stiffness and injury were assessed at the level of individual legs (n = 98) using spline terms in Cox proportional hazards models. RESULTS: During follow-up, 23 participants (29 legs) sustained injury. The median time to injury was 27 weeks (53.27 hours of training). Relative injury rate as a function of knee stiffness displayed a weak and nonsignificant U-shaped curve (P = .187-.661); ankle and leg stiffness displayed no discernable associations with relative injury rate (leg stiffness, P = .215-.605; ankle stiffness, P = .419-.712). CONCLUSION: Leg and joint stiffness may not be important factors in the development of running-related injuries. Moderate changes in leg and joint stiffness are unlikely to substantially alter injury risk.

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BACKGROUND: Leg stiffness is important during running to increase velocity and maximise efficiency by facilitating use of the stretch-shortening cycle. Children with cerebral palsy who have neuromuscular impairments may have altered leg stiffness. The aim of this study was to describe leg stiffness during running in typically developing children and those with cerebral palsy in Gross Motor Function Classification Scale levels I and II at a range of speeds. METHODS: This cross-sectional study examined kinematic data collected from typically developing children (n = 21) and children with cerebral palsy (Gross Motor Function Classification Scale level I n = 25, Gross Motor Function Classification Scale level II n = 13) during jogging, running and sprinting. Derived variables were resultant ground reaction force, change in leg length and three-dimensional leg stiffness. Linear mixed models were developed for statistical analysis. FINDINGS: Children with cerebral palsy had reduced stiffness when jogging (Gross Motor Function Classification Scale level I affected t = 3.81 p < 0.01; non-affected t = 2.19 p = 0.03; Gross Motor Function Classification Scale level II affected t = 2.04 p = 0.04) and running (Gross Motor Function Classification Scale level I affected t = 3.23 p < 0.01) compared to typically developing children. Affected legs were less stiff than non-affected legs only in Gross Motor Function Classification Scale level I during running (t = 2.26 p = 0.03) and sprinting (t = 2.95 p < 0.01). INTERPRETATION: Children with cerebral palsy have atypical leg stiffness profiles which differ according to functional classification.

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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.

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The present study aimed to assess the differences in leg stiffness and the associated performance variables between athletes from various training backgrounds during tasks relevant to athletic training. Forty-seven female participants (20 nationally identified netballers, 13 high-level endurance athletes and 14 age-matched controls) completed a sprint, anticipated sidestep change of direction and unilateral repetitive hopping task to assess leg stiffness and the relationship of stiffness between the different tasks. Leg stiffness and performance variables were evaluated with a 10-camera motion analysis system and force plate, and leg stiffness was derived through the McMahon and Cheng methodology (1990). Significant differences were evident in leg stiffness, and the contributing performance variables between groups across all assessed tasks (p < 0.001-0.017). Furthermore, results indicated the control group displayed no leg stiffness relationship between the evaluated tasks, while the stiffness relationship between tasks within athletic populations reflected training-specific demands of athletes. The results of this study indicated that the athletic training background of individuals may contribute to inherent leg stiffness differences between groups. Furthermore, the stiffness relationship observed between tasks suggests practitioners should take care in the selection of task used to monitor leg stiffness from a performance or injury risk perspective.

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An inability to pre-plan a side-step cutting maneuver results in a greater reduction in speed and shallower cut angle. Although leg stiffness has not been directly quantified in cutting, indirect evidence suggest that greater stiffness may benefit cutting speed, but lower stiffness may benefit cut angle. No studies have investigated if stiffness causally mediates the relationship between anticipation, cutting speed and angle. The aims of the present study were to determine the influence of anticipatory cues on leg stiffness, and quantify the mediation effects of stiffness on cutting speed and angle. Seventeen healthy participants performed a 45° cut at an approach speed of 4 m/s. Leg stiffness (% bodyweight/leg length [BW/LL]), cutting angle and change in running speed between initial contact and toe-off of the cut were calculated. Causal mediation analysis was performed with anticipatory cues as the independent variable, cutting speed and angle as the dependent variables, and stiffness as the mediator. Unanticipated cutting significantly increased leg stiffness (ß=3.82%BW/LL,P=0.005) compared to anticipated cutting. The average causal mediation effect of stiffness on cutting angle was not significant (P = 0.68). The average causal mediation effect of stiffness on cutting speed was significant (-0.02 m/s [95%CI -0.04 to 0.00 m/s, P = 0.016). Reduced preplanning time in cutting increased leg stiffness. Alterations in leg stiffness only explained the change in speed, and not angle, associated with cutting under different anticipatory cues. Loss of speed when cutting is unplanned may be mitigated by improving leg stiffness.

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The role of trunk orientation during uneven running is not well understood. This study compared the running mechanics during the approach step to and the step down for a 10 cm expected drop, positioned halfway through a 15 m runway, with that of the level step in 12 participants at a speed of 3.5 m s-1 while maintaining self-selected (17.7±4.2 deg; mean±s.d.), posterior (1.8±7.4 deg) and anterior (26.6±5.6 deg) trunk leans from the vertical. Our findings reveal that the global (i.e. the spring-mass model dynamics and centre-of-mass height) and local (i.e. knee and ankle kinematics and kinetics) biomechanical adjustments during uneven running are specific to the step nature and trunk posture. Unlike the anterior-leaning posture, running with a posterior trunk lean is characterized by increases in leg angle, leg compression, knee flexion angle and moment, resulting in a stiffer knee and a more compliant spring-leg compared with the self-selected condition. In the approach step versus the level step, reductions in leg length and stiffness through the ankle stiffness yield lower leg force and centre-of-mass position. Contrariwise, significant increases in leg length, angle and force, and ankle moment, reflect in a higher centre-of-mass position during the step down. Plus, ankle stiffness significantly decreases, owing to a substantially increased leg compression. Overall, the step down appears to be dominated by centre-of-mass height changes, regardless of having a trunk lean. Observed adjustments during uneven running can be attributed to anticipation of changes to running posture and height. These findings highlight the role of trunk posture in human perturbed locomotion relevant for the design and development of exoskeleton or humanoid bipedal robots.

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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.

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