Relevant Biomechanical Variables in Skateboarding: A Literature Review.

Skateboarding, once regarded primarily as a means of transportation and entertainment for youth, has become a recognized professional sport, gaining global popularity. With its recent inclusion in the Olympics, a growing imperative exists to comprehensively understand biomechanics explaining skateboarding performance. This literature review seeks to consolidate knowledge within this domain, focusing on experimental and modeling studies about skateboard riding and tricks. The criteria for study selection encompassed content relevance and publication year, spanning from the last two decades and extending further back to 1980 following cross-referencing of seminal works. Peer-reviewed journal articles, conference proceedings, and books were considered, with comprehensive searches conducted on electronic databases, including SCOPUS, PubMed, Scielo, and Taylor & Francis. Comprehending the biomechanical facets of skateboarding is essential in promoting its use and ensuring safety among all practitioners. Insights into factors such as body kinetics, kinematics, and muscle activation represent a foundational step toward understanding the nuances of this sport with implications for both clinical and biomechanical research. Modern data collection systems such as inertial measurement units (IMU) and electromyography (EMG) offer unprecedented insights into human performance during skateboarding, such as joint range of motion, coordination, and muscle activation, whether in casual riding or executing complex tricks and maneuvers. Developing robust modeling approaches also holds promise for enhancing skateboarding training and performance. Crucially, these models can serve as the initial framework for understanding injury mechanisms and implementing strategies to improve performance and mitigate injury risks.

Statistical Tests for Sports Science Practitioners: Identifying Performance Gains in Individual Athletes.

ABSTRACT: Harry, JR, Hurwitz, J, Agnew, C, and Bishop, C. Statistical tests for sports science practitioners: identifying performance gains in individual athletes. J Strength Cond Res 38(5): e264-e272, 2024-There is an ongoing surge of sports science professionals within sports organizations. However, when seeking to determine training-related adaptations, sports scientists have demonstrated continued reliance on group-style statistical analyses that are held to critical assumptions not achievable in smaller-sample team settings. There is justification that these team settings are better suited for replicated single-subject analyses, but there is a dearth of literature to guide sports science professionals seeking methods appropriate for their teams. In this report, we summarize 4 methods' ability to detect performance adaptations at the replicated single-subject level and provide our assessment for the ideal methods. These methods included the model statistic, smallest worthwhile change, coefficient of variation (CV), and standard error of measurement (SEM), which were discussed alongside step-by-step guides for how to conduct each test. To contextualize the methods' use in practice, real countermovement vertical jump (CMJ) test data were used from 4 (2 females and 2 males) athletes who complete 5 biweekly CMJ test sessions. Each athlete was competing in basketball at the NCAA Division 1 level. We concluded that the combined application of the model statistic and CV methods should be preferred when seeking to objectively detect meaningful training adaptations in individual athletes. This combined approach ensures that the differences between the tests are (a) not random and (b) reflect a worthwhile change. Ultimately, the use of simple and effective methods that are not restricted by group-based statistical assumptions can aid practitioners when conducting performance tests to determine athlete adaptations.

Establishing Phase Definitions for Jump and Drop Landings and an Exploratory Assessment of Performance-Related Metrics to Monitor During Testing.

ABSTRACT: Harry, JR, Simms, A, and Hite, M. Establishing phase definitions for jump and drop landings and an exploratory assessment of performance-related metrics to monitor during testing. J Strength Cond Res 38(2): e62-e71, 2024-Landing is a common task performed in research, physical training, and competitive sporting scenarios. However, few have attempted to explore landing mechanics beyond its hypothesized link to injury potential, which ignores the key performance qualities that contribute to performance, or how quickly a landing can be completed. This is because a lack of (a) established landing phases from which important performance and injury risk metrics can be extracted and (b) metrics known to have a correlation with performance. As such, this article had 2 purposes. The first purpose was to use force platform data to identify easily extractable and understandable landing phases that contain metrics linked to both task performance and overuse injury potential. The second purpose was to explore performance-related metrics to monitor during testing. Both purposes were pursued using force platform data for the landing portion of 270 jump-landing trials performed by a sample of 14 NCAA Division 1 men's basketball players (1.98 ± 0.07 m; 94.73 ± 8.01 kg). The proposed phases can separate both jump-landing and drop-landing tasks into loading, attenuation, and control phases that consider the way vertical ground reaction force (GRF) is purposefully manipulated by the athlete, which current phase definitions fail to consider. For the second purpose, Pearson's correlation coefficients, the corresponding statistical probabilities ( α = 0.05), and a standardized strength interpretation scale for correlation coefficients (0 < trivial ≤ 0.1 < small ≤ 0.3 < moderate ≤ 0.5 < large ≤ 0.7 < very large) were used for both the group average (i.e., all individual averages pooled together) and individual data (i.e., each individual's trials pooled together). Results revealed that landing time, attenuation phase time, average vertical GRF during landing, average vertical GRF during the attenuation phase, average vertical GRF during the control phase, vertical GRF attenuation rate, and the amortization GRF (i.e., GRF at zero velocity) significantly correlated with landing performance, defined as the ratio of landing height and landing time ( R ≥ ± 0.58; p < 0.05), such that favorable changes in those metrics were associated with better performance. This work provides practitioners with 2 abilities. First, practitioners currently assess jump capacity using jump-landing tests (e.g., countermovement jump) with an analysis strategy that makes use of landing data. Second, this work provides preliminary data to guide others when initially exploring landing test results before identifying metrics chosen for their own analysis.

Short-term Effects of Static Stretching on Hamstring Passive Stiffness in Young and Older Women.

OBJECTIVES: The objectives of this study were to assess the acute effects of static stretching on hamstring passive stiffness in young and older women. A secondary objective was to compare hamstring muscle size and quality measurements (cross-sectional area and echo intensity) between the two groups and to determine if these characteristics are related to passive stiffness at baseline. METHODS: Fifteen young (23±4 years) and 15 older (73±5 years) women underwent two randomized conditions that included a control treatment and an experimental treatment of four, 15-s static stretches of the hamstrings. Passive stiffness was calculated before (pre-test) and after (post-test) each treatment using a passive knee extension test. Ultrasound imaging was used to measure hamstring muscle cross-sectional area and echo intensity. RESULTS: Passive stiffness collapsed across group decreased from pre- to post-test for the stretching treatment (P=0.001) but not for the control (P=0.467). The older women had lower cross-sectional area (P=0.033) and greater baseline (pre-test) passive stiffness (P=0.042-0.049) and echo intensity (P=0.022) than the young women. Moreover, baseline passive stiffness was significantly related to echo intensity (r=0.430, P=0.018) but not cross-sectional area (r=-0.014, P=0.943). CONCLUSION: An acute bout of static stretching decreased passive stiffness in both young and older women.

Tradeoffs of estimating reaction time with absolute and relative thresholds.

Measuring the duration of cognitive processing with reaction time is fundamental to several subfields of psychology. Many methods exist for estimating movement initiation when measuring reaction time, but there is an incomplete understanding of their relative performance. The purpose of the present study was to identify and compare the tradeoffs of 19 estimates of movement initiation across two experiments. We focused our investigation on estimating movement initiation on each trial with filtered kinematic and kinetic data. Nine of the estimates involved absolute thresholds (e.g., acceleration 1000 back to 200 mm/s2, micro push-button switch), and the remaining ten estimates used relative thresholds (e.g., force extrapolation, 5% of maximum velocity). The criteria were the duration of reaction time, immunity to the movement amplitude, responsiveness to visual feedback during movement execution, reliability, and the number of manually corrected trials (efficacy). The three best overall estimates, in descending order, were yank extrapolation, force extrapolation, and acceleration 1000 to 200 mm/s2. The sensitive micro push-button switch, which was the simplest estimate, had a decent overall score, but it was a late estimate of movement initiation. The relative thresholds based on kinematics had the six worst overall scores. An issue with the relative kinematic thresholds was that they were biased by the movement amplitude. In summary, we recommend measuring reaction time on each trial with one of the three best overall estimates of movement initiation. Future research should continue to refine existing estimates while also exploring new ones.

Anthropometric Predictors of Conventional Deadlift Kinematics and Kinetics: A Preliminary Study.

The purpose of this preliminary analysis was to determine if there are relationships between anthropometric characteristics (arm length, torso length, thigh length, and shank length) and conventional deadlift (CDL) kinematics and kinetics during a 5 sets of 5 repetitions (5 × 5) CDL routine in resistance-trained males. Eleven males who had experience with the deadlift exercise were included in this analysis (age: 21.5 ± 1.4 y; height: 180.7 ± 5.7 cm; body mass: 89.9 ± 16.0 kg). Anthropometrics were measured by a 3-dimensional optical scanner. The participants underwent a 5 × 5 CDL workout using a self-selected load corresponding to a rating of perceived exertion (RPE) of 8 out of 10. Performance outcomes were measured synchronously using a 3-dimensional 12-camera motion capture system and two force platforms. Outcomes were averaged across all sets and analyzed using multiple linear regression. The selected anthropometric variables were not significantly related to the CDL performance outcomes, except for concentric ankle work. However, in the overall model, anthropometric predictors did not significantly predict ankle concentric work (p = 0.11; R 2 = 0.67; R_2adj = 0.45). Independently, thigh length significantly correlated with ankle concentric work (p = 0.03). In this model, thigh length accounted for 55% of the normalized variance in ankle concentric work. The results from this preliminary study suggest that arm length, torso length, and shank length may not play a clear role in the examined CDL outcomes, but thigh length may be positively correlated with ankle concentric work during a 5 × 5 CDL routine in resistance-trained males.

A Systematic Review of the Different Calculation Methods for Measuring Jump Height During the Countermovement and Drop Jump Tests.

BACKGROUND: The heights obtained during the countermovement jump and drop jump tests have been measured by numerous studies using different calculation methods and pieces of equipment. However, the differences in calculation methods and equipment used have resulted in discrepancies in jump height being reported. OBJECTIVES: The aim of this systematic review was to examine the available literature pertaining to the different calculation methods to estimate the jump height during the countermovement jump and drop jump. METHODS: A systematic review of the literature was undertaken using the SPORTDiscus, MEDLINE, CINAHL, and PubMed electronic databases, with all articles required to meet specified criteria based on a quality scoring system. RESULTS: Twenty-one articles met the inclusion criteria, relating various calculation methods and equipment employed when measuring jump height in either of these two tests. The flight time and jump-and-reach methods provide practitioners with jump height data in the shortest time, but their accuracy is affected by factors such as participant conditions or equipment sensitivity. The motion capture systems and the double integration method measure the jump height from the centre of mass height at the initial flat foot standing to the apex of jumping, where the centre of mass displacement generated by the ankle plantarflexion is known. The impulse-momentum and flight time methods could only measure the jump height from the centre of mass height at the instant of take-off to the apex of jumping, thus, providing statistically significantly lower jump height values compared with the former two methods. However, further research is warranted to investigate the reliability of each calculation method when using different equipment settings. CONCLUSIONS: Our findings indicate that using the impulse-momentum method via a force platform is the most appropriate way for the jump height from the instant of take-off to the apex of jumping to be measured. Alternatively, the double integration method via a force platform is preferred to quantify the jump height from the initial flat foot standing to the apex of jumping.

Eccentric Force Velocity Profiling: Motor Control Strategy Considerations and Relationships to Strength and Jump Performance.

ABSTRACT: Barker, L, Siedlik, J, Magrini, M, Uesato, S, Wang, H, Sjovold, A, Ewing, G, and Harry, JR. . Eccentric force velocity profiling: motor control strategy considerations and relationships to strength and jump performance. J Strength Cond Res 37(3): 574-580, 2023-Currently, no studies exist on the eccentric force-velocity profile (eFVP) during drop landings from increasing drop heights, which may reveal an athlete's braking capacity and control strategies. Therefore, the purpose of this study was to assess the eFVP during bilateral drop landings from increasing drop heights. A secondary purpose was to explore and determine relevant relationships between the eFVP and common metrics like relative strength and jumping performance. Overall, 19 recreationally trained athletes from the university completed a 1-reptition maximum back squat, countermovement jumps, squat jumps, drop jumps, and drop landings from 0.3 to 1.52-m box heights in 0.15-m increments. Average force and velocity from the peak drop landing trial was used to generate an eFVP. The mean linear eFVP was -6.65x + 14.73, and the mean second order polynomial eFVP was -1.37x 2 - 25.84x + 0.17. The second-order polynomial fit the data better with large effect ( dunb = 1.05, p < 0.05). No significant correlations between the eFVP coefficients and the strength and jumping measurements were observed. Future research could investigate how training can influence the eFVP. Eccentric force production during landing may be a unique quality that requires specific development strategies, such has fast or slow eccentric training.

Momentum-Based Load Prescriptions: Applications to Jump Squat Training.

ABSTRACT: Harry, JR, Krzyszkowski, J, Harris, K, Chowning, L, Mackey, E, Bishop, C, and Barker, LA. Momentum-based load prescriptions: Applications to jump squat training. J Strength Cond Res 36(9): 2657-2662, 2022-Velocity-based training is often applied to ballistic exercises, like the barbell jump squat, to improve vertical jump performance. However, determining the ideal training load based on velocity data remains difficult because load prescriptions tend to be limited to subjective velocity loss thresholds, velocity ranges, or both. Using data from jump squats performed with 0, 15, 30, 45, and 60% of the 1-repetition maximum squat, we explored subjective and objective methods to determine the ideal training load. Specifically, we explored takeoff velocity and a related metric only recently discussed in the literature, system momentum (i.e., takeoff velocity multiplied by the mass of the athlete-load system). At the group level, an ideal training load could not be revealed objectively using takeoff velocity. With individual subjects, the process remained challenging using takeoff velocity. Conversely, an ideal training load could be revealed easily and objectively using system momentum at the group average and individual subject levels. System momentum at takeoff is well-suited to assist practitioners seeking to identify appropriate training loads for jump squat training and potentially other ballistic exercises. We suggest a pivot from velocity to system momentum when seeking to objectively establish training loads for the jump squat and related exercises.

Examining the Mechanisms of Internal and External Focus of Attention With Donders' Subtractive Method.

The goal of the current study was to measure the processing demands on the stages of information processing with internal and external foci of attention. Participants completed simple and two-choice reaction time tasks with internal and external foci of attention. Donders' subtraction method was used to isolate the cumulative duration of stages unique to simple and choice reaction time tasks. Mean reaction time was comparable with internal and external foci of attention in simple and two-choice reaction time tasks. These results suggest that processing demands were comparable with internal and external foci of attention. We hypothesize that there was not a processing advantage for an external focus in simple reaction time because the required movements had low movement complexity.

Low-Pass Filter Effects on Metrics of Countermovement Vertical Jump Performance.

ABSTRACT: Harry, JR, Blinch, J, Barker, LA, Krzyszkowski, J, and Chowning, L. Low-pass filter effects on metrics of countermovement vertical jump performance. J Strength Cond Res 36(5): 1459-1467, 2022-Countermovement vertical jump (CMVJ) studies using ground reaction force (GRF) data analyze either unfiltered (i.e., raw) or filtered data while providing little-to-no justification for the selected filtering process. Inappropriate filter choices can lead to inaccurate study results and erroneous interpretations. We examined the effects of not filtering GRF data in comparison with filtering data with various objectively and subjectively selected cutoff frequencies. Twenty-one collegiate male basketball players completed 3 maximal-effort CMVJ trials while GRF data were obtained from 2 force platforms. Countermovement vertical jump performance, explosiveness, power output, and neuromuscular function variables were compared among the following methods using one-way repeated-measures analyses of variance (α = 0.05): no filtering (raw data), a standard 50-Hz cutoff (50 Hz), a visually determined cutoff frequency describing the frequency band containing the majority of the summed (visual inspection 1) or not-summed (visual inspection 2) GRF signal's frequency content, filtering the summed (99% signal power 1) or not-summed (99% signal power 2) GRF using a cutoff frequency retaining 99% of the signal power. The raw data method produced significantly shorter concentric phase times and significantly greater center of mass flight heights (∼3%), modified reactive strength indices (RSIMOD; ∼4%), power outputs (∼6%), and push-off distances (∼4%) than 99% signal power 1 and 2 (p < 0.05). Discrete GRF and phase-specific yank magnitudes were not different among methods (p ≥ 0.05). Importantly, no differences were detected between the raw data and 50 Hz methods for any variable (p > 0.05). Low-pass filtering is not necessary when analyzing GRF data from the CMVJ. However, a low-pass filter with a 50-Hz cutoff can remove noise without altering results when compared with raw data. Explicit methodological descriptions of filtering processes should always be provided to improve the integrity of future CMVJ analyses, comparisons among various studies' results, or both.

Phase-Specific Predictors of Countermovement Jump Performance That Distinguish Good From Poor Jumpers.

ABSTRACT: Krzyszkowski, J, Chowning, LD, and Harry, JR. Phase-specific predictors of countermovement jump performance that distinguish good from poor jumpers. J Strength Cond Res 36(5): 1257-1263, 2022-The modified-reactive strength index (RSImod) is commonly examined during the countermovement vertical jump (CMJ) to assess neuromuscular characteristics (i.e., explosiveness, fatigue, adaptation, etc.) of an athlete. However, both phase-specific variables explaining RSImod and corresponding differences between good and poor jumpers are not well understood in trained populations. This study sought to (a) identify predictors of RSImod during the CMJ based on phase-specific temporal and rate of force development (RFD) variables, and (b) identify differences in those predictors between performers with high and low RSImod performances from a sample of collegiate male basketball players (n = 22; 20 ± 2 years; 1.99 ± 0.06 month; 93.8 ± 7.5 kg). Subjects performed 3 maximal effort CMJ trials while ground reaction force data was recorded using 2 force platforms. Phase-specific temporal and RFD variables were calculated and entered into separate stepwise regression models using backward elimination to identify predictors RSImod. Individuals were then categorized into high (n = 11; RSImod = 0.68 ± 0.10) and low (n = 11; RSImod = 0.48 ± 0.04) RSImod groups according to the overall median RSImod (RSImod = 0.55). Independent t-tests (α = 0.05) were conducted and supplemented by Cohen's d effect sizes (d ≥ 1.2, large) to compare groups relative to significant predictors identified by the linear regression models and related variables. The temporal regression model (R2 = 0.530) retained unloading time and concentric time, whereas the RFD regression model (R2 = 0.429) retained unloading RFD and braking RFD. The high RSImod group exhibited significantly greater RSImod scores (d = 2.51, p < 0.001) and jump heights (d = 1.58, p < 0.001), shorter times to takeoff (d = 1.27, p = 0.007) and concentric times (d = 1.51, p = 0.002), and a greater braking RFD (d = 1.41, p = 0.005) than the low RSImod group. Individuals targeting enhanced CMJ performance may consider exploring strategies or interventions to develop quicker unloading and concentric phases and increasing eccentric RFD abilities.

Optimization-based subject-specific planar human vertical jumping prediction: Effect of elbow flexion and weighted vest.

Jumping strategies differ considerably depending on athletes' physical activity demands. In general, the jumping motion is desired to have excellent performance and low injury risk. Both of these outcomes can be achieved by modifying athletes' jumping and landing mechanics. This paper presents a consecutive study on the optimization-based subject-specific planar human vertical jumping to test different loading conditions (weighted vest) during jumping with or without elbow flexion during the arm-swing based on the validated prediction model in the first part of this study. The sagittal plane skeletal model simulates the weighting, unweighting, breaking, propulsion phases and considers four loading conditions: 0%, 5%, 10%, and 15% body weight. Results show that the maximum ground reaction forces, the body center of mass position, and velocities at the take-off instant are different for different loading conditions and with/without elbow flexion. The optimization formulation is solved using MATLAB® with 35 design variables with 197 nonlinear constraints for a five-segment body model and 42 design variables with 227 nonlinear constraints for a six-segment body model. Both models are computationally efficient, and they can predict ground reaction forces, the body center of mass position, and velocity. This work is novel in the sense that presents a simulation model capable of considering different external loading conditions and the effect of elbow flexion during arm swing.

Military Body Composition Standards and Physical Performance: Historical Perspectives and Future Directions.

ABSTRACT: Harty, PS, Friedl, KE, Nindl, BC, Harry, JR, Vellers, HL, and Tinsley, GM. Military body composition standards and physical performance: historical perspectives and future directions. J Strength Cond Res 36(12): 3551-3561, 2022-US military physique and body composition standards have been formally used for more than 100 years. These metrics promote appropriate physical fitness, trim appearance, and long-term health habits in soldiers, although many specific aspects of these standards have evolved as evidence-based changes have emerged. Body composition variables have been shown to be related to many physical performance outcomes including aerobic capacity, muscular endurance, strength and power production, and specialized occupational tasks involving heavy lifting and load carriage. Although all these attributes are relevant, individuals seeking to improve military performance should consider emphasizing strength, hypertrophy, and power production as primary training goals, as these traits appear vital to success in the new Army Combat Fitness Test introduced in 2020. This fundamental change in physical training may require an adjustment in body composition standards and methods of measurement as physique changes in modern male and female soldiers. Current research in the field of digital anthropometry (i.e., 3-D body scanning) has the potential to dramatically improve performance prediction algorithms and potentially could be used to inform training interventions. Similarly, height-adjusted body composition metrics such as fat-free mass index might serve to identify normal weight personnel with inadequate muscle mass, allowing for effective targeted nutritional and training interventions. This review provides an overview of the origin and evolution of current US military body composition standards in relation to military physical readiness, summarizes current evidence relating body composition parameters to aspects of physical performance, and discusses issues relevant to the emerging modern male and female warrior.

Phase-Specific Verbal Cue Effects on Countermovement Jump Performance.

ABSTRACT: Krzyszkowski, J, Chowning, LD, and Harry, JR. Phase-Specific Verbal Cue Effects on Countermovement Jump Performance. J Strength Cond Res 36(12): 3352-3358, 2022-The aim of this study was to determine whether countermovement vertical jump (CMVJ) phase-specific cues can improve jump performance and phase-specific force-time characteristics. Twenty-nine subjects (14 males and 15 females) performed 15 total CMVJ trials (5 per condition) while being provided with a control and phase-specific (unloading phase and eccentric braking phases) foci of attention. Jump height, reactive strength index-modified, countermovement depth, time-to-takeoff, and CMVJ subphase force-time characteristics were compared between each phase-specific verbal cues and the control condition using paired samples t-tests ( α = 0.05) and Cohen's d effect sizes ( d ; large >1.2). Female ( d = 0.242; p = 0.012) and male ( d = 1.96; p = 0.047) subjects achieved greater jump heights in the control condition compared with the unloading phase condition. Females demonstrated a faster unloading phase, less unloading force, greater unloading yank, and greater braking force during the unloading condition, as well as greater eccentric braking force during the eccentric braking condition compared with the control condition ( p ≤ 0.014; d ≥ 0.242). Males exhibited less body mass unloading, greater unloading yank, faster eccentric braking time, greater eccentric braking force, and greater eccentric braking yank for both the unloading and eccentric braking conditions compared with the control condition ( p ≤ 0.047; d ≥ 0.196). Collectively, these results suggest that phase-specific foci of attention do not acutely improve jump performance but can enhance phase-specific force-time characteristics in recreationally active individuals. Specifically, practitioners should consider using an eccentric braking phase instruction for individuals need to improve eccentric braking force generation.

Biomechanical Comparison of Dominant and Non-Dominant Limbs During Leap-Landings in Contemporary Style Female Dancers.

The execution strategy of technical dance movements is constrained by aesthetic and qualitative artistic requirements. As such, there are limited leap-landing strategies that may be used by dancers when executing a grand jeté or saut de chat. The purpose of this study was to determine potential differences in lower extremity angular positioning and joint loading when performing a dance-style leap landing. Fifteen female dancers (age: 20 ± 1 years; height: 1.61 ± 0.13 m; weight: 58.00 ± 11.89 kg) completed six leap-landing trials during which three-dimensional kinematics and kinetics data were collected. Paired-samples t-tests (α = 0.05) and Cohen's d effect sizes (ES; large ≥ 0.8) were used to compare the following variables: jump height; peak vertical ground reaction force; loading time; loading rate; joint angular positioning of the ankle, knee, hip, and trunk in the frontal and sagittal planes; and joint angular impulse of the ankle, knee, and hip in the frontal and sagittal planes between the dominant and non-dominant limbs. Frontal plane hip angular impulse was significantly greater in the dominant limb (p = 0.023, ES = 1.53). While no other statistically significant differences were observed between dominant and non-dominant limbs, moderate effect sizes were observed for the hip and trunk angles in the frontal plane along with hip impulse in the sagittal plane. This study indicates that dancers might slightly alter their landing strategy at the hip joint when leap-landing onto the dominant limb. Frontal plane hip mechanics should be considered to minimize overuse injury potential in the dominant limb.

Phase-Specific Force and Time Predictors of Standing Long Jump Distance.

This study sought to identify potential predictors of standing long jump (SLJ) performance using force-time strategy metrics within the unloading, eccentric yielding, eccentric braking, and concentric phases. Fifteen National Collegiate Athletic Association division 1 male soccer players (19 [1] y, 1.81 [0.94] m, 80.3 [22.4] kg) performed 3 maximum-effort SLJs, while 3-dimensional ground reaction force (GRF) data were obtained. Regularized regression models were used to investigate associations between force-time strategy metrics and 2 metrics of SLJ performance (ie, jump distance and modified reactive strength index). Jump height and eccentric yielding time were the only predictors of jump distance that also demonstrated large correlations to jump distance. Anterior-posterior unloading yank, average concentric vertical force, and concentric phase duration were the only predictors of modified reactive strength index that also demonstrated large correlations to modified reactive strength index. To maximize SLJ distance in high-level soccer athletes, human performance practitioners could design interventions to drive changes in strategy to increase jump height and decrease eccentric yielding time. To improve SLJ explosiveness, interventions to drive changes in unloading and concentric force application and decrease concentric time could be emphasized. Importantly, unique variable combinations can be targeted when training for SLJ distance and explosiveness adaptations.

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

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

Optimization-based subject-specific planar human vertical jumping prediction: Model development and validation.

Jumping biomechanics may differ between individuals participating in various sports. Jumping motion can be divided into different phases for research purposes when seeking to understand performance, injury risk, or both. Experimental-based methods are used to study different jumping situations for their capabilities of testing other conditions intended to improve performance or further prevent injuries. External loading training is commonly used to simulate jumping performance improvement. This paper presents the optimization-based subject-specific planar human vertical jumping to develop the prediction model with and without a weighted vest and validate it through experiments. The skeletal model replicates the human motion for jumping (weighting, unweighting, breaking, propulsion) in the sagittal plane considering four different loading conditions (0% and 10% body mass): unloaded, split-loaded, front-loaded, and back-loaded. The multi-objective optimization problem is solved using MATLAB® with 35 design variables and 197 nonlinear constraints. Results show that the model is computationally efficient, and the predicted jumping motion matches the experimental data trend. The simulation model can predict vertical jumping motion and can test the effect of different loading conditions with weighted vests and arm-swing strategy on the ground reaction forces. This work is novel in the sense that it can predict ground reaction forces, joints angles, and center of mass position without any experimental data.

Effects of medial longitudinal arch flexibility on propulsion kinetics during drop vertical jumps.

This study examined the effects of medial longitudinal arch (MLA) flexibility on kinetics during the eccentric and concentric subphases of a drop vertical jump (DVJ). Physically active adults with flexible (n = 16) and stiff (n = 16) MLA completed DVJs onto a force platform from a height of 30 cm. Eccentric and concentric subphases of the DVJ were identified from the vertical ground reaction force (GRF) data. Jump height, ground contact time, reactive strength index (RSI), vertical center-of-mass depth, vertical stiffness and time of the eccentric and concentric subphases were evaluated. Amortization force, peak vertical GRF and vertical impulse were also obtained for the eccentric and concentric subphases of the DVJ. Dependent variables were compared between groups using independent samples t-tests (p < 0.05). Significantly greater vertical stiffness (p = 0.048; ES = 0.63) was found in the stiff arch group (-173.91 ± 99.73 N/kg/m) compared to the flexible arch group (-122.95 ± 63.42 N/kg/m). A moderate-magnitude difference (ES = 0.58) was observed for RSI between flexible (0.89 ± 0.39) and stiff arch (1.20 ± 0.70) groups, but was not significant (p = 0.063). The active and passive structures supporting the MLA may be used differently to achieve similar vertical jump height during a DVJ. Additional research is warranted to further understand the contributions of MLA flexibility to jumping performance.