Changes in Lower Limb Biomechanics Across Various Stages of Maturation and Implications for ACL Injury Risk in Female Athletes: a Systematic Review.

BACKGROUND: Female athletes are four to six times more likely to sustain an anterior cruciate ligament (ACL) injury than male athletes. Jump-landing biomechanics are influenced by maturation, with post-pubertal female athletes at a heightened risk of ACL injuries. OBJECTIVE: The aim of our systematic review was to identify and summarise the current evidence regarding the changes in kinematic and kinetic risk factors associated with ACL injuries during jump-landing tasks in female athletes at various stages of maturity. METHODS: A systematic search was conducted in PubMed, CINAHL, Web of Science, SPORTDiscus, EMBASE and Scopus. Articles were included if they: (1) conducted the research on uninjured female athletes with no restriction on playing level/experience; (2) provided information regarding the stage of the maturity and the scale used for estimating the maturity status of the participants; and (3) reported a biomechanical risk factor associated with ACL injuries during jump-landing tasks across at least two different maturity groups (e.g. pre-pubertal vs post-pubertal). RESULTS: Sixteen articles involving 2323 female athletes were included in our review. A total of 12 kinematic and 8 kinetic variables were identified across these studies. Of the 12 kinematic variables reported in our review, we found strong evidence for higher peak knee abduction angle in post-pubertal female individuals compared with pre-pubertal girls (p < 0.05). With regard to the 8 kinetic variables, we found strong evidence for lower relative peak vertical ground reaction force, higher external knee abduction moment and internal rotation moment in post-pubertal compared with pre-pubertal athletes. The strength of evidence for the remaining kinematic and kinetic variables ranged from conflicting to moderate and, in some instances, could not be determined. CONCLUSIONS: Our study provides an overview of the changes in biomechanical risk factors in female athletes during jump-landing tasks at various stages of maturity. We found moderate-to-limited evidence for most kinematic and kinetic variables, highlighting the need for further research.

The Influence of Maturity Status on Drop Jump Kinetics in Male Youth.

ABSTRACT: Kumar, NTA, Radnor, JM, Oliver, JL, Lloyd, RS, CSCSD, Pedley, JS, Wong, MA, and Dobbs, IJ. The influence of maturity status on drop jump kinetics in male youth. J Strength Cond Res 38(1): 38-46, 2024-The aim of this study was to examine the effects of maturity status on drop jump (DJ) kinetics in young male athletes (categorized as early-pre-peak height velocity [PHV] [<-2.51 years], late-pre-PHV [-1.99 to -1.00 years], circa-PHV [-0.50 to 0.50 years], and post-PHV [>1.00 years]). All athletes performed a DJ from a 30-cm box onto force plates with performance variables (jump height, ground contact time, and reactive strength index) and absolute and relative kinetic variables during the braking and propulsive phases assessed. Subjects were categorized into GOOD (no impact-peak and spring-like), MODERATE (impact-peak and spring-like), or POOR (impact-peak and not spring-like) stretch-shortening cycle (SSC) function. The post-PHV group exhibited significantly greater values for most absolute kinetic variables compared with early-pre-PHV, late-pre-PHV, and circa-PHV (p < 0.05). The differences observed between consecutive maturity groups were similar in magnitude for most absolute variables (Cohen's d = 0.53-1.70). Post-PHV male athletes outperform their less mature counterparts during a DJ, and this may be attributed to the growth and maturity-related structural and motor control strategy changes that occur in children. Stretch-shortening cycle function in boys seems to improve with maturity status reflected by a greater number of post-PHV participants displaying GOOD SSC function (65.8%) and a greater number of early-pre-PHV participants displaying POOR SSC function (54.4%). However, a number of mature boys displayed POOR SSC function (17.8%), suggesting that the development of SSC function is not exclusively related to maturation.

Kinetic Responses to External Cues Are Specific to Both the Type of Cue and Type of Exercise in Adolescent Athletes.

ABSTRACT: Barillas, SR, Oliver, JL, Lloyd, RS, and Pedley, J. Kinetic responses to external cues are specific to both the type of cue and type of exercise in adolescent athletes. J Strength Cond Res 37(3): 597-605, 2023-The purpose of this study was to examine how external cues influence kinetics during isometric and dynamic tasks in adolescent athletes. Fifteen adolescent male soccer players performed an isometric midthigh pull (IMTP), unloaded and loaded squat jumps (SJs) (15 and 30% of body mass), countermovement jump (CMJ), and drop jump (DJ) using a neutral or external force-specific or velocity-specific cues. Cue type had limited effects on outcomes in the IMTP or SJs, with mostly trivial ( g < 0.20), nonsignificant differences ( p > 0.05) across kinetic variables. In the CMJ and DJ, a force cue significantly ( p < 0.05) increased jump height ( g = 0.43 & 0.52) compared with a velocity cue, but in the DJ, a force cue significantly increased jump height ( g = 0.52) compared with both a neutral and velocity cue. However, a velocity cue significantly ( p < 0.05) reduced ground contact time ( g = 0.73-1.52) and time to peak force ( g = 0.50-1.29) in both the CMJ and DJ when compared with a force and neutral cue and increased force and power-related measures ( g = 0.33-1.12) in the CMJ and DJ when compared with a force cue. In adolescent athletes, the type of external cue had limited effects on kinetic measures in an IMTP and SJ but differential effects on both a CMJ and DJ, suggesting cues have more effect with increasing movement velocity. Consequently, practitioners working with adolescent athletes should consider both the type of exercise and the desired outcome when providing external cues.

The Acute Effects of Accentuated Eccentric Loading on Drop Jump Kinetics in Adolescent Athletes.

ABSTRACT: Lloyd, RS, Howard, SW, Pedley, JS, Read, PJ, Gould, ZI, and Oliver, JL. The acute effects of accentuated eccentric loading on drop jump kinetics in adolescent athletes. J Strength Cond Res 36(9): 2381-2386, 2022-The purpose of the study was to determine the acute effects of accentuated eccentric loading (AEL) on drop jump kinetics in adolescent athletes. Fifteen male rugby players (mean ± SD: age = 16.2 ± 1.01, body mass = 73.38 ± 9.94, and height = 178.91 ± 6.74) completed 3 trials of a drop jump with and without an accentuated eccentric load using a dumbbell release technique that equated to 15% body mass. The addition of the AEL stimulus resulted in significant, moderate increases in jump height (p < 0.05; Wilcoxon r = 0.47) and braking and propulsive impulse (p < 0.05, Hedges' g = 0.43; p < 0.05, g = 0.61, respectively) but did not significantly influence reactive strength index. Concurrently, there were significant, moderate increases in ground contact times (p < 0.05; r = 0.45) and reductions in spring-like correlations (p < 0.05; g = 0.94). Neither the magnitude nor timing of peak landing force were significantly altered in the AEL protocol (p < 0.05, g = 0.38; p < 0.05, r = 0.10, respectively). Cumulatively, these data indicate that implementation of AEL resulted in meaningful improvements in jump height, driven by significant increases in braking and propulsive impulse. Despite extensions in ground contact times and reductions in spring-like behavior, neither peak landing force nor time to peak force were negatively altered by the AEL. These results indicate that adolescent athletes can realize superior vertical jump heights during drop jumps using AEL, without displaying marked changes in the magnitude or timing of peak landing force.

The Influence of Competitive Level on Stretch-Shortening Cycle Function in Young Female Gymnasts.

This cross-sectional study investigated how stretch-shortening cycle (SSC) function and kinetic variables differed between young female gymnasts of varying competitive levels. Drop jump (DJ) force−time profiles were examined in 118 female gymnasts, sub-divided by competitive level (n = 21 recreational, n = 41 regional and n = 50 elite). DJ force−time data were analyzed to calculate performance and kinetic variables. Participants' SSC function was categorized as poor, moderate, or good, depending on the presence of an impact peak and spring-like behavior. A high proportion of gymnasts across each group were categorized as having "good" or "moderate" SSC function (i.e., >94.8%), with a trend of increasingly better SSC function observed with competitive level. Significant differences in reactive strength index, contact time, time of landing peak force, relative propulsive peak force, impulse, and ratio of braking: propulsive impulse were found between the elite and recreational group (p < 0.05). While SSC function was generally good to moderate, elite gymnasts had a more desirable kinetic jump-landing strategy than recreational level gymnasts. Drop jump kinetic variables appear to distinguish between elite and recreational gymnasts but not between regional standard gymnasts. Practitioners should consider the kinetic profile of gymnasts when benchmarking and setting training objectives.

Programming Plyometric-Jump Training in Soccer: A Review.

The aim of this review was to describe and summarize the scientific literature on programming parameters related to jump or plyometric training in male and female soccer players of different ages and fitness levels. A literature search was conducted in the electronic databases PubMed, Web of Science and Scopus using keywords related to the main topic of this study (e.g., "ballistic" and "plyometric"). According to the PICOS framework, the population for the review was restricted to soccer players, involved in jump or plyometric training. Among 7556 identified studies, 90 were eligible for inclusion. Only 12 studies were found for females. Most studies (n = 52) were conducted with youth male players. Moreover, only 35 studies determined the effectiveness of a given jump training programming factor. Based on the limited available research, it seems that a dose of 7 weeks (1−2 sessions per week), with ~80 jumps (specific of combined types) per session, using near-maximal or maximal intensity, with adequate recovery between repetitions (<15 s), sets (≥30 s) and sessions (≥24−48 h), using progressive overload and taper strategies, using appropriate surfaces (e.g., grass), and applied in a well-rested state, when combined with other training methods, would increase the outcome of effective and safe plyometric-jump training interventions aimed at improving soccer players physical fitness. In conclusion, jump training is an effective and easy-to-administer training approach for youth, adult, male and female soccer players. However, optimal programming for plyometric-jump training in soccer is yet to be determined in future research.

Impaired Stretch-Shortening Cycle Function Persists Despite Improvements in Reactive Strength After Anterior Cruciate Ligament Reconstruction.

ABSTRACT: Read, PJ, Pedley, JS, Eirug, I, Sideris, V, and Oliver, JL. Impaired stretch-shortening cycle function persists despite improvements in reactive strength followingafter anterior cruciate ligament reconstruction. J Strength Cond Res 36(5): 1238-1244, 2022-Reactive strength index (RSI) during a single-leg drop jump (SLDJ) has been indicated to determine return-to-sport readiness after anterior cruciate ligament (ACL) reconstruction, but only cross-sectional studies are available. Ground reaction force data and characterization of stretch-shortening cycle (SSC) function also remain sparse. Single-leg drop jump performance, ground reaction force, and SSC function were examined in soccer players with ACL reconstruction (n = 26) and matched controls (n = 25). Injured players were tested at 2 time points (32 and 42 weeks postsurgery). Stretch-shortening cycle function was classified as good (no impact peak and spring like), moderate (impact peak but still spring like or no impact peak and not spring like), or poor (impact peak and not spring like). The involved limb displayed lower-jump height, poorer RSI, less spring-like behavior, earlier peak landing force, and a greater ratio of landing peak to take-off peak force compared with the uninvolved limb and controls at the initial assessment (p < 0.001). Proportionally, more involved limbs were categorized as poor or moderate at the initial assessment (69.2%) and follow-up (50%) in comparison with the control limbs (14%). The reactive strength index was the only variable to change significantly between the initial assessment and follow-up on the involved limb (p < 0.05). No changes in the proportion of ACL reconstructed individuals categorized as poor or moderate SSC function at the follow-up assessment were observed. Residual deficits are present in SLDJ performance, SSC function, and ground reaction force characteristics after ACL reconstruction. The reactive strength index should not be the sole metric, as observed increases did not correspond with changes in SSC function.

Comparison of Weightlifting, Traditional Resistance Training and Plyometrics on Strength, Power and Speed: A Systematic Review with Meta-Analysis.

BACKGROUND: Weightlifting training (WLT) is commonly used to improve strength, power and speed in athletes. However, to date, WLT studies have either not compared training effects against those of other training methods, or been limited by small sample sizes, which are issues that can be resolved by pooling studies in a meta-analysis. Therefore, the objective of this systematic review with meta-analysis was to evaluate the effects of WLT compared with traditional resistance training (TRT), plyometric training (PLYO) and/or control (CON) on strength, power and speed. METHODS: The systematic review included peer-reviewed articles that employed a WLT intervention, a comparison group (i.e. TRT, PLYO, CON), and a measure of strength, power and/or speed. Means and standard deviations of outcomes were converted to Hedges' g effect sizes using an inverse variance random-effects model to generate a weighted mean effect size (ES). RESULTS: Sixteen studies were included in the analysis, comprising 427 participants. Data indicated that when compared with TRT, WLT resulted in greater improvements in weightlifting load lifted (4 studies, p = 0.02, g = 1.35; 95% CI 0.20-2.51) and countermovement jump (CMJ) height (9 studies, p = 0.00, g = 0.95; 95% CI 0.04-1.87). There was also a large effect in terms of linear sprint speed (4 studies, p = 0.13, g = 1.04; 95% CI - 0.03 to 2.39) and change of direction speed (CODS) (2 studies, p = 0.36, g = 1.21; 95% CI - 1.41 to 3.83); however, this was not significant. Interpretation of these findings should acknowledge the high heterogeneity across the included studies and potential risk of bias. WLT and PLYO resulted in similar improvements in speed, power and strength as demonstrated by negligible to moderate, non-significant effects in favour of WLT for improvements in linear sprint speed (4 studies, p = 0.35, g = 0.20; 95% CI - 0.23 to 0.63), CODS (3 studies, p = 0.52, g = 0.17; 95% CI - 0.35 to 0.68), CMJ (6 studies, p = 0.09, g = 0.31; 95% CI - 0.05 to 0.67), squat jump performance (5 studies, p = 0.08, g = 0.34; 95% CI - 0.04 to 0.73) and strength (4 studies, p = 0.20, g = 0.69; 95% CI - 0.37 to 1.75). CONCLUSION: Overall, these findings support the notion that if the training goal is to improve strength, power and speed, supplementary weightlifting training may be advantageous for athletic development. Whilst WLT and PLYO may result in similar improvements, WLT can elicit additional benefits above that of TRT, resulting in greater improvements in weightlifting and jumping performance.

A Novel Method to Categorize Stretch-Shortening Cycle Performance Across Maturity in Youth Soccer Players.

ABSTRACT: Pedley, JS, Lloyd, RS, Read, PJ, Moore, IS, Myer, GD, and Oliver, JL. A novel method to categorize stretch-shortening cycle performance across maturity in youth soccer players. J Strength Cond Res 36(9): 2573-2580, 2022-This study used a novel method to categorize stretch-shortening cycle (SSC) function during a drop jump (DJ) using the force-time curve. This method was then used to determine the effect of maturity status upon SSC function and effect of SSC function on DJ performance. Prepeak, circa-peak, and postpeak height velocity male youth soccer players completed a preseason 30-cm DJ onto a force plate. Stretch-shortening cycle function was categorized as poor (impact peak and not spring-like), moderate (impact peak and spring-like), or good (no impact peak and spring-like). Interactions between SSC function and maturity status, and SSC function and kinetic variables were explored. Youth soccer players displaying good SSC function were older and more mature than those with poor SSC function; however, 9.9% of post peak height velocity still displayed poor SSC function. Players with good SSC function recorded significantly shorter ground contact times, reduced time between peak landing and takeoff force, reduced center of mass displacement, and significantly greater takeoff forces than players with moderate and poor SSC function (all p < 0.05). SSC function during a standardized DJ improves with maturation, but a portion of mature players still demonstrate poor SSC function. Good SSC function was associated with improved DJ outcome measures except jump height. Tailored training interventions based on SSC competency may be required to optimally enhance SSC function.

Maturity alters drop vertical jump landing force-time profiles but not performance outcomes in adolescent females.

The stretch-shortening cycle (SSC) assists in effective force attenuation upon landing and augments force generation at take-off during a drop vertical jump (DVJ). General performance outcomes such as jump height or peak measures have been used to assess SSC function in youth populations; however, these discrete metrics fail to provide insight into temporal jump-landing characteristics. This study assessed DVJ force-time profiles in 1013 middle and high-school female athletes (n = 279 prepubertal, n = 401 pubertal, and n = 333 postpubertal). Maturity status was determined using the Pubertal Maturation Observation Scale. Ground reaction force data were analyzed to extract a range of variables to characterize force-time profiles. SSC function was categorized as poor, moderate, or good dependent on the presence of an impact peak and spring-like behavior. No differences in jump height or ground contact time were observed between maturity groups (p > 0.05). Significant differences in absolute peak landing and take-off force were evident between all maturational statuses (p < 0.05). Relative to bodyweight normalized forces, only peak take-off force was significantly different between prepubertal and postpubertal groups (p < 0.05; d = 0.22). Spring-like behavior showed small improvements from pubertal to postpubertal (p < 0.05; d = 0.25). Most females displayed poor SSC function at prepubertal (79.6%), pubertal (77.3%), and postpubertal (65.5%) stages of maturity. Large increases in absolute forces occur throughout maturation in female athletes; however, only small maturational differences were found in relative force or spring-like behavior. Consequently, most girls display poor SSC function irrespective of maturity.

The Influence of Growth, Maturation and Resistance Training on Muscle-Tendon and Neuromuscular Adaptations: A Narrative Review.

The purpose of this article is to provide an overview of the growth, maturation and resistance training-related changes in muscle-tendon and neuromuscular mechanisms in youth, and the subsequent effect on performance. Sprinting, jumping, kicking, and throwing are common movements in sport that have been shown to develop naturally with age, with improvements in performance being attributed to growth and maturity-related changes in neuromuscular mechanisms. These changes include moderate to very large increases in muscle physiological cross-sectional area (CSA), muscle volume and thickness, tendon CSA and stiffness, fascicle length, muscle activation, pre-activation, stretch reflex control accompanied by large reductions in electro-mechanical delay and co-contraction. Furthermore, a limited number of training studies examining neuromuscular changes following four to 20 weeks of resistance training have reported trivial to moderate differences in tendon stiffness, muscle CSA, muscle thickness, and motor unit activation accompanied by reductions in electromechanical delay (EMD) in pre-pubertal children. However, the interaction of maturity- and training-related neuromuscular adaptions remains unclear. An understanding of how different neuromuscular mechanisms adapt in response to growth, maturation and training is important in order to optimise training responsiveness in youth populations. Additionally, the impact that these muscle-tendon and neuromuscular changes have on force producing capabilities underpinning performance is unclear.