Reactive Strength Index Research Articles

The influence of muscle strength and aerobic fitness on functional recovery in elite female soccer players.

We examined the influence of muscle strength and aerobic fitness on wellbeing and muscle function recovery in female soccer players. Sixteen players from the English Women's Super League (age: 26±4 yrs; height, 1.70±0.07 m; mass, 67.9±6.4 kg) completed an isometric mid-thigh pull test (IMTP) and maximal aerobic speed test (MAST). Using a median split, players were divided by strength into high (H-IMTP) and low (L-IMPT) groups and by fitness into high (H-MAST) and low (L-MAST) groups. Countermovement jump height (CMJ), Reactive Strength Index (RSI) and wellbeing were measured 2 days prematch, then 3 subsequent days post-match (M+1, M+2, M+3). In H-IMTP, CMJ, RSI and wellbeing were higher at M+1-M+3 (P<0.05). At M+3, CMJ in H-IMTP was 30.3±3.5 cm vs. 26.2±3.4 in L-IMTP. CMJ and wellbeing were unaffected by fitness, but RSI was lower at M+1-M+3 in the L-IMTP group (P<0.05). In conclusion, isometrically stronger but not aerobically fitter female soccer players recover quicker after matches.

Caffeine Gum Improves Reaction Time but Reduces Composure Versus Placebo During the Extra-Time Period of Simulated Soccer Match-Play in Male Semiprofessional Players.

This study aimed to determine whether caffeine gum influenced perceptual-cognitive and physical performance during the extra-time period of simulated soccer match-play. Semiprofessional male soccer players (n = 12, age: 22 ± 3years, stature: 1.78 ± 0.06m, mass: 75 ± 9kg) performed 120-min soccer-specific exercise on two occasions. In a triple-blind, randomized, crossover design, players chewed caffeinated (200mg; caffeine) or control (0mg; placebo) gum for 5min following 90min of soccer-specific exercise. Perceptual-cognitive skills (i.e.,passing accuracy, reaction time, composure, and adaptability) were assessed using a soccer-specific virtual reality simulator, collected pre- and posttrial. Neuromuscular performance (reactive-strength index, vertical jump height, absolute and relative peak power output, and negative vertical displacement) and sprint performance (15 and 30m) were measured at pretrial, half-time, 90min, and posttrial. Caffeine gum attenuated declines in reaction time (pre: 90.8 ± 0.8AU to post: 90.7 ± 0.8AU) by a further 4.2% than placebo (pre: 92.1 ± 0.8AU to post: 88.2 ± 0.8AU; p < .01). Caffeine gum reduced composure by 4.7% (pre: 69.1 ± 0.8AU to post: 65.9 ± 0.8AU) versus placebo (pre: 68.8 ± 0.8AU to post: 68.3 ± 0.8AU; p < .01). Caffeine gum did not influence any other variables (p > .05). Where caffeine gum is consumed by players prior to extra-time, reaction time increases but composure may be compromised, and neuromuscular and sprint performance remain unchanged. Future work should assess caffeine gum mixes with substances like L-theanine that promote a relaxed state under stressful conditions.

Unlocking Male Youth Soccer Players’ Peak Performance Potential: Exploring the Impact of Maturation, Age, and Physical Demands on Neuromuscular Injury Risk and Recovery Following Competitive Matchplay

This study investigated the 7-day recovery period post-soccer matches in elite male academy players. We analyzed changes in physical performance, stretch-shortening cycle capability, landing mechanics, muscle damage, and perceived well-being while also considering the influence of players’ maturity status, chronological age, and physical demands on post-match responses. In a prospective, observational, mixed longitudinal study design, twenty-six players (U14 = 14 [age = 13.9 ± 0.2 y, and U16 = 12 [age = 15.1 ± 0.2 y]) undertook testing at baseline (1 h pre-match), immediately post-match (0 h), and 48-, 72-, 96-, and 120 h post-match for measures in creatine kinase (CK), urea (UR), CMJ height, 20 m sprint time, reactive strength index (RSI), leg stiffness (LS), landing mechanics, and perceived well-being. Players were also tested pre [168-h] and post the subsequent match. Results showed significant alterations 0 h post-match in CK (+71.3%), UR (+12.8%), CMJ height (−5.3%), 20 m sprint time (+3.8%), RSI (−9.6%), LS (−11.5%), and perceived well-being (−7.7%), with landing mechanics being unaffected. All parameters returned to baseline at 48 h, except for CK and UR, which remained elevated until 168 h. The players’ initial scores influenced how they responded after the match during the week. In conclusion, coaches should focus on post-match strategies to enhance muscle recovery, especially for youth players with a lower training status, given the extended recovery period observed for muscle damage markers.

Effects of Hormonal Contraception and the Menstrual Cycle on Maximal Strength and Power Performance.

To evaluate the effects of oral contraceptive (OC) and hormonal intrauterine device (H-IUD) use, compared to an eumenorrheic (EUM) cycle, on maximal strength and power between hormone phases. One repetition max (1RM) leg press and bench press, peak force (PF) from knee extension and upright row isometric dynamometry, and power from vertical jump height (VJH) and reactive strength index (RSI; cm/s) were measured in 60 healthy, active women (mean ± standard deviation [SD]; Age: 26.5 ± 7.0 yrs, BMI: 22.5 ± 3.7 kg/m2) who were monophasic OC users for ≥6 months (n = 21), had a H-IUD for ≥6 months (n = 20), or had regularly naturally occurring menstrual cycle for ≥3 months or were using a non-hormonal IUD (EUM; n = 19). Participants were randomly assigned to begin in the follicular phase/placebo pill (low hormone phase; LHP) or in the luteal phase/active pill (high hormone phase; HHP) and were tested once in each phase. Estimates of total lean mass (LM), leg LM, and arm LM were measured via dual energy x-ray absorptiometry. Separate univariate ANCOVAs were used to assess the change from HHP to LHP between groups, with LM and progesterone as covariates. Leg press 1RM was significantly different across phases between groups (p = 0.027), with higher leg press 1RM in the HHP for the OC group (mean difference[∆HHP-LHP] ± standard error: ∆7.4 ± 15.9 kg; p = 0.043) compared to the H-IUD group (∆-8.9 ± 23.8 kg; p = 0.043). All groups demonstrated similar bench press 1RM, PF, VJH, and RSI between phases (p > 0.05). Lower body strength was greater in the HHP for OC users (5.6% increase) suggesting lower body maximal strength outcomes may be influenced by hormonal contraception type.

The influence of external loads on post-match neuromuscular fatigue in international rugby union: A partial least squares correlational analysis

ABSTRACT The aims were to determine the relationship(s) between match-play external load and post-match neuromuscular fatigue as latent constructs, the contribution of the specific measured variables to these latent constructs, and how these differ between forwards and backs in elite rugby union. Forty-one elite male rugby union players (22 forwards and 19 backs) from the same international rugby union team were tested, with data included from the 2020 and 2021 international seasons (11 matches; 146 player appearances). Player’s match-play external loads were quantified using microtechnology (for locomotor activities) and video analysis (for collision actions). Neuromuscular fatigue was quantified using countermovement jump tests on force plates which were conducted ~ 24 to 48 hours pre- and post-match. Partial least squares correlation (PLSC) leave one variable out (LOVO) procedure established the relative variable contribution to both external load (X matrix) and neuromuscular fatigue (Y matrix) constructs. Linear mixed-effects models were then constructed to determine the variance explained by the latent scores applied to the variables representing these constructs. For external load, both locomotor and collision variables were identified for the forwards and the backs, although the identified variables differed between groups. For neuromuscular fatigue, jump height was identified as a high contributor for the forwards and the backs, with concentric impulse and reactive strength index high contributors only for the backs. The explained variance between the external load and neuromuscular fatigue latent constructs at the individual player level was 4.4% and 32.2% in the forwards and the backs models, respectively. This discrepancy may be explained by differences in match-play external loads and/or the specificity of the tests to measure indicators of fatigue. These may differ due to, for example, the activities undertaken in the different positional groups.

Isometric Shoulder and Hip Strength Impact on Throwing Velocity and Reactive Strength.

The aim of the study is to examine relationships between shoulder extension-flexion and internal-external rotation strength of professional male handball players with throwing velocity, and the hip extension-flexion strength with horizontal and vertical reactive strength index. Fifteen professional male handball players participating in 1st League matches took part in the study. The results showed that isometric shoulder flexion and extension strength significantly predicted stable throwing velocity (r2=between 0.27-0.73) and dynamic throwing velocity (r2=between 0.30-0.62). In addition, isometric internal and external rotation strength significantly predicted stable throwing velocity (r2=between 0.32-0.54) and dynamic throwing velocity (r2=between 0.31-0.44). Moreover, isometric hip extension and flexion strength significantly predicted vertical reactive strength index (r2=between 0.31-0.45) and horizontal reactive strength index (r2=between 0.26-0.42). In conclusion, it was observed that shoulder strength has an enhancing effect on handball players' throwing velocity, while hip strength is a determining factor for the reactive strength index. In this context, it is assumed that shoulder weakness during throwing will affect the accuracy and velocity of throwing by disrupting the angular momentum, while weakness in the hip during the jump phase of the throwing motion will negatively affect the explosive power by limiting stabilization during landing.

Considerations for vertical jump testing in collegiate basketball players: The impact of immediate individual versus social comparison feedback and team leaderboard engagement

Recent literature has shown that the provision of feedback can enhance vertical jump performance acutely, as well as chronically when implemented during phases of training. The aim of our study was to investigate the influence of two types of visual feedback on performance and variability of countermovement jump-derived force-time characteristics in a cohort of male and female National Collegiate Athletic Association Division 1 basketball players. Specifically, individual visual feedback (IVF) was compared to a form of social comparison feedback (SCF), and authors hypothesized there to be performance increases and more stable measures in the SCF condition. In line with this hypothesis, findings suggested significantly enhanced performance in the SCF condition for seven out of eight force-time metrics (e.g. jump height and reactive strength index modified). However, given the small between-condition effect sizes, differences between conditions may lack practical significance. Furthermore, findings suggested less between-jump variability in the SCF condition, compared to the IVF condition, making for a more stable assessment. This in particular makes for more reliable measures, for which when studied over time, more subtle changes in performance may be observed. In summary, our findings highlight acutely enhanced vertical jump performance, and more stable measures, when athletes are exposed to an SCF condition, compared to a normal IVF condition. Practitioners are encouraged to consider these findings when planning vertical jump assessments and are discouraged from implementing different types of feedback at random, especially when measuring performance over time.

Return to Play Assessment After Lateral Ankle Sprains - German Male Elite Youth Football (Soccer) Academy Baseline Data.

Lateral ankle sprain (LAS) is one of the most common types of injury in football (soccer). Normative baseline data of performance tests for Return to Play (RTP) decision are still lacking. The primary aim of this study was to generate baseline values for uninjured elite youth football players for a multifactorial RTP assessment and compare with previously published data. A secondary aim was to investigate the use of the Limb Symmetry Index (LSI) as a method to determine whether an athlete passes a performance test or not. Observational Cohort study. Baseline data of performance tests (Y-Balance [YBT-LQ], Heel Rise [HRT]; Singe Leg Squat [SLST]; Single Leg Drop Jump [SLDJ]; Side Hop [SHT]; Figure of 8 Hop [F-8]; Modified Agility T-Test [MAT]) were assessed in 20 elite youth football players, aged 16-21 years. Additionally, the traditional LSI (dividing the result of the non-dominant leg by the result of the dominant leg and multiplying by 100) and directionally corrected LSI (the worst value is divided by the better value and multiplied by 100) were calculated. The test values were compared to previously reported study results. LSI and side-to-side comparisons between dominant and non-dominant leg sides were analyzed using the Wilcoxon test. Male elite youth football players achieved better results in the dynamic performance tests (SHT, F-8, and MAT) compared to reference values of the cohorts previously described in the literature: YBT-LQ total score (cm) dominant (dom) 99.3±8.3, non-dominant (ND) 99.5±10.4; HRT (average number) dom. 27.1±5.4, ND 25.2±5.1); SLDJ height (cm) dom 15±5, ND 15±5 and contact time (sec) dom 0.29±0.08, ND 0.29±0.07, Reactive Strength Index (RSI) dom 0.52±0.12, ND 0.50±0.13); SHT (sec) dom 7.12±0.73, ND 7.39±0.93; F-8 (sec) dom 10.52±1.02, ND 10.37±1.04; and MAT (sec) 5.82±0.22. Directionally corrected LSI differed significantly from the traditional calculated LSI (p<0.05). The findings of this study highlight the need to determine specific baseline data for RTP testing in male elite youth football players after LAS. The traditional LSI should not be used as a "stand alone method" for determining RTP. LSI calculations should consider the direction of asymmetry to determine passing a performance test or not. 3b.

An investigation on sex-based disparities in male and female collegiate athlete reactive strength index

Reactive strength is the ability to effectively use a fast stretch shortening cycle (SSC) with the eccentric into a concentric contraction (Rebelo et al., 2022, Healthcare, 10, 593). A fast SSC is determined to be &lt;200 ms due to a preliminary study by (Schmidtbleicher &amp; Komi, 1992, Encyclopedia Sports Med. Vol. 3 Strength and Power in Sport) reviewing muscular contractions. Reactive strength is determined through two different metrics used within the literature which are ground contact time (GCT) and jump height (JH) allowing for understanding on how an athlete can utilise the myotatic reflex to react with the floor. Reactive Strength Index (RSI) is typically measured via drop jump (DJ) and 10/5 reactive pogo jumps. A novel way to measure RSI is via a rebound countermovement jump (RCMJ) which utilises RSI-Modified and includes time to take off instead of ground contact time. RSI is an important metric to measure because it has implications within sport-specific metrics like sprinting and jumping (Jarvis et al., 2022, Sports Med, 52, 301-330). However, some of the limitations of RSI are it is a ratio metric, and therefore RSI can mask underpinning metrics. As a result, additional metrics such as such a force, velocity and power can help better understand athlete development and the underpinning key performance indicators. However, many metrics within reactive jumps remain unsearched. Data will be collected from around 16 university athletes aged 18-25 currently competing in sport with at least one year’s experience in resistance training. Data will be collected using Hawkins dynamic force plates, sampling at 1000 Hz, and has recently been reported to be valid and reliable (Badby et al., 2023, Sports, 23, 4820) Participants will be asked to complete 3 DJ, RCMJ and 10/5 jump trials on two separate occasions separated by a minimum of 48 hours. Athletes will be instructed to perform a quiet standing period for 1 second before and after each jump to calculate body weight by the 5 × standard deviation method. Therefore, the study aims to compare the disparities between all three jumps aswell, sexes and the potential implications on performance. Previous studies displayed significant disparities between male and female participants. Therefore, the null hypothesis for this study is to display a non-significant difference across sexes aswell as between jumps. A limitation to this study is controlling the standardisation of variables such as squat depth during all tests.

Do lower limb kinetic and kinematic differences in drop jump landings persist when players have returned to sport following knee ligamentous injuries?

Knee injuries (KI) are identified as one of the most burdensome lower limb injuries in football with long and complicated rehabilitation and high re-injury risk. Current research often describes technique deficits being present upon RTS, often being a result of poor return to sport (RTS) testing procedures. More recently, research has advocated the use of single leg vertical jump assessments, due to the specificity of the movement to injury mechanisms and the ability to consider different phases of the task to inform practitioners of joint contributions to the propulsion and landing phases. The aim of this research is to identify whether these functional deficits exist 12-24 months post RTS authorisation, to help inform future RTS in applied practice. Twenty-four semi-professional male footballers (mean ± SD: age 25 ± 4 years; stature 182 ± 7 cm; body mass 80 ± 8 kg) were recruited into a previously injured and a control group. Participants completed 5 repetitions of single leg drop jump (SLDJ). Jump height, ground contact time, reactive strength index, peak moments, angles and power contributions for the hip, knee and ankle were measured using Qualisys Track Manager then exported to Visual 3D. This study was approved by the Edge Hill University Ethics Committee. No significant findings were identified for landing one however, during the propulsion phase there was a difference between groups (P = 0.026, d=1.14) for knee flexion at take-off. Most differences were identified during landing two bilaterally in both groups. Within the injured group, differences were identified for peak hip (P = 0.011, d = 0.028) and knee powers (P &lt; 0.001, d = 1.269), hip (P &lt; 0.001, d = 1.924) and knee power contribution % (P &lt; 0.001, d = 1.269). No differences were identified for the performance measures or through performing statistical parametric mapping for hip and knee powers for all three phases. Joint coupling analysis was conducted for the total ground contact time amalgamating the landing and propulsion phase, this was analysed as Hip-Knee, Hip-Ankle and Knee-Ankle, with significant findings in Hip-Knee and Hip-ankle. Both groups elicited bilateral differences however, no differences between the injured limb and dominant control limb were identified. To better apply this form of analysis to RTS testing, functional movements such should be undertaken when athletes are injury free to provide baseline measurements to refer to if undertaking rehabilitation from KI. Likewise, the current study identified that joint contributions could be identified in a more applied setting using the range of motion data from each joint.

The Integral Reactive Strength Index: An Accurate Measurement for Evaluating Drop Jump Performance in Sprinter Athletes

The Integral Reactive Strength Index: An Accurate Measurement for Evaluating Drop Jump Performance in Sprinter Athletes

Microdosing Plyometric Training Enhances Jumping Performance, Reactive Strength Index, and Acceleration among Youth Soccer Players: A Randomized Controlled Study Design.

Microdosing can facilitate better accommodation to the training stimulus while aligning with the scheduling needs of teams. In this study, the effectiveness of microdosing exposure was investigated by comparing the effects of microdosing plyometric jump training (microPJT) with those of regular plyometric jump training (regPJT) and a control group not exposed to plyometric training. The comparison focused on the effects on jumping performance, reactive strength index (RSI), and acceleration over a 10-meter distance. Fifty-two male youth soccer players (16.3 ± 0.6 years old) from under-17 teams participated in a randomized controlled study, with interventions lasting 8 weeks. Assessments were conducted twice, before and after the intervention, measuring squat jump (SJ), countermovement jump (CMJ), RSI during drop jumps, and acceleration in a 10-meter sprint test. The regPJT group completed 34 bilateral jumps and 48 unilateral jumps per week over two weekly sessions, totaling 82 jumps. Conversely, the microPJT group performed 17 bilateral jumps and 24 unilateral jumps weekly over 4 sessions week, totaling 41 jumps. Significant interactions between groups and time were observed concerning SJ (p < 0.001; η2= 0.282), CMJ (p < 0.001; η2= 0.368), RSI (p < 0.001; η2= 0.400) and 10-m sprint time (p < 0.001; η2 = 0.317). Between-group analysis indicated that both the microPJT (p < 0.001) and regPJT (p < 0.001) groups exhibited significant better results compared to the control group in post-intervention evaluation of SJ, CMJ, RSI and 10-m sprint time, while no significant differences were found between experimental groups (p > 0.050). In conclusion, this study has revealed that both microPJT and regPJT are equally effective in enhancing jumping performance and acceleration time in soccer players. This suggests that a smaller training volume, distributed more frequently across the week, can effectively induce improvements in soccer players.

Validity of the Hawkin Dynamics Wireless Dual Force Platform System Against a Piezoelectric Laboratory Grade System for Vertical Countermovement Jump Variables.

Dos'Santos, T, Evans, DT, and Read, DB. Validity of the Hawkin dynamics wireless dual force platform system against a piezoelectric laboratory grade system for vertical countermovement jump variables. J Strength Cond Res 38(6): 1144-1148, 2024-The aim of this study was to determine the criterion validity of the Hawkin Dynamics (HD) wireless dual force platform system for assessing vertical countermovement jump (CMJ) variables, compared with those derived from a Kistler piezoelectric laboratory grade force platform system. During a single testing session, HD force platforms were placed directly on top of 2 adjacent Kistler force platforms to simultaneously collect vertical ground reaction forces produced by 2 male recreational soccer players (age: 29.0 ± 2.8 years, height: 1.79 ± 0.01 m, mass: 85.6 ± 4.7 kg) that performed 25 vertical CMJs each. Sixteen vertical CMJ variables pertaining to jump height (JH), flight time (FT), time-to-take off (TTT), countermovement depth, body weight (BW), propulsive and braking mean, and peak powers, forces, and impulses were compared between systems. Fixed bias was observed for 6 of 16 variables (peak and mean braking power, mean propulsion force, TTT, FT, and BW), while proportional bias was present for 10 of 16 variables (peak and mean propulsive and braking force, TTT, FT, peak and mean braking power, mean propulsive power, and BW). For all variables regardless of fixed or proportional bias, percentage differences were ≤3.4% between force platform systems, with near perfect to perfect correlations (r or ρ = 0.977-1.000) observed for 15 of 16 variables. The HD dual wireless force platform system can be considered a valid alternative to a piezoelectric laboratory grade force platform system for the collection of vertical CMJ variables, particularly outcome (i.e., JH, reactive strength index modified) and strategy variables (countermovement depth).

Acute Effects of Slow, Moderate and Fast Tempo Dynamic Stretching Exercises on Power in Well-Trained Male Wrestlers.

Due to the potential detrimental effects of static stretching exercises on subsequent muscle power performance, athletes and trainers have started to replace static stretching with dynamic stretching exercises in their training routines. However, there are no well-accepted guidelines regarding dynamic stretching variables, including tempo/velocity, volume (reps and sets), etc. Therefore, this study aimed to evaluate the acute effects of slow, moderate, and fast tempo dynamic stretching exercises on jump height, relative power, the reactive strength index, and leg stiffness in well-trained male wrestlers. Seventeen wrestlers (aged 20.00 ± 4.06 years, wrestling experience 6.00 ± 3.09 years, and training volume per week 10.00 ± 5.69 hours) took part in the experiment under four conditions (control session, slow tempo dynamic stretching, moderate tempo dynamic stretching, and fast tempo dynamic stretching) on separate days with a 72-h interval in between, following a randomized, crossover study design. The control session consisted of a 10-min jog on a motor-driven treadmill at 6 km/h and a 0% slope. Dynamic stretching sessions consisted of seven dynamic stretching exercises performed at 50 bpm, 100 bpm, and 120 bpm, following a 5-min warm-up on a treadmill at 6 km/h and a 0% slope. After each condition, wrestlers performed a 2 x 30-s repeated vertical jump test with 5-min rest intervals in between. The best results for jump height, relative power, the reactive strength index, and leg stiffness were registered for statistical analysis. One-way repeated ANOVA results demonstrated that there were no significant differences in pairwise comparisons of all variables obtained after the four different conditions (p > 0.05). Overall, none of the slow, moderate, and fast tempo dynamic stretching exercises led to a change in repeated jump performance of well-trained male athletes. Further studies are needed to clarify the acute effects of different tempo dynamic stretching on muscular performance in well-trained wrestlers.

Exploring the associations between ankle dorsal and plantar flexion strength, reactive strength index, and balance in elite youth soccer players

Exploring the associations between ankle dorsal and plantar flexion strength, reactive strength index, and balance in elite youth soccer players

The Association between Countermovement Rebound Jump Metrics and Independent Measures of Athletic Performance

This study investigates the associations between countermovement rebound jump (CMRJ) performance metrics and various independent measures of athletic performance, including the isometric mid-thigh pull (IMTP), 20 m linear sprint, and 505 change-of-direction (COD) speed tests. Pearson’s correlations were used to analyse the relationship between the CMRJ measures with athletic performance, with significance being set at p ≤ 0.006. Results showed large significant positive relationships between IMTP peak force and force at 300 milliseconds with the first jump height of the CMRJ (JH-1, r = 0.54 to 0.55, p ≤ 0.002). Additionally, inverse relationships were observed between reactive strength index modified (RSImod) and reactive strength index (RSI) with 20 m sprint total and split times (r = −0.55 to −0.66, p ≤ 0.001), and the 10 m and total sprint times were significantly correlated with JH-1 (r = −0.54, p = 0.003), indicating that greater vertical explosive power and reactive strength are associated with faster sprint performance. Finally, a significant inverse relationship was identified between CMRJ metrics (two JH values and RSImod) and 505 COD times in both the left and right sides (r = −0.51 to −0.68, p ≤ 0.006). These findings suggest that CMRJ performance metrics are valuable indicators of lower-limb explosive force production, with a strong link to both linear sprint and COD performance. The finding underscores the importance of including CMRJ assessments in athletic performance evaluations due to their dual assessment capacity of slow and fast stretch–shortening cycle mechanics.

The Modified Reactive Strength Index Is a Valid Measure of Lower-Body Explosiveness in Male and Female High School Athletes.

Witte, BC, Schouten, TC, Westphal, JA, VanZile, AW, Jones, DD, Widenhoefer, TL, Dobbs, WC, Jagim, AR, Luedke, JA, and Almonroeder, TG. The modified reactive strength index is a valid measure of lower-body explosiveness in male and female high school athletes. J Strength Cond Res 38(8): 1428-1432, 2024-The modified reactive strength index (mRSI) is a commonly used metric to quantify lower-body explosiveness during countermovement jump (CMJ) performance. However, few studies have attempted to examine its validity as a measure of explosiveness, particularly among high school athletes. The purpose of this study was to examine the validity of the mRSI as a measure of lower-body explosiveness among a relatively large sample of male and female high school athletes from various sports. As part of this study, male ( n = 132) and female ( n = 43) high school athletes performed CMJs, while ground reaction forces were recorded using a force platform. The vertical ground reaction force data collected during the CMJs were used to derive the following variables: peak force (PF), peak power, time to PF, time to take-off, peak rate of force development, and the mRSI. Principal component analysis was applied and reduced these variables into 2 components related to "force" and "speed." The mRSI loaded on both the force (loading = 0.82) and speed (loading = -0.46) components, indicating that it incorporates elements of both force and speed, although it loaded more strongly on the force component than the speed component. The observed pattern of cross-loading suggests that the mRSI is generally a valid measure of lower-body explosiveness for male and female high school athletes.

A New Method for Evaluating the Reactive Strength Index in Track and Field Sprinting: Relationships with Muscle Architecture

The aim of the present study was to assess a new reactive strength index (RSI RUN) based on contact time and stride length measured in sprint running and then to correlate this index with sprint performance, muscle architecture and echo intensity of the vastus lateralis. Participants included ten elite and sub-elite sprinters (age 24.4 ± 3.1 years, height 177.5 ± 7.7 cm, mass 69.8 ± 11.7 kg) who were tested with a vertical drop jump (VDJ) and a horizontal drop jump (HDJ) from a 30 cm high box, a 20 m straight-leg running drill (SLR) and a 60 m sprint. A nearly perfect correlation (r = from −0.90 to −0.96, p &lt; 0.01) was detected between RSI RUN and sprint performance (30 m, 60 m and 100 m sprint time), and a very large correlation (r = from −0.72 to −0.77, p &lt; 0.05) was found between the traditional RSI from vertical drop jump (RSIDJV) and sprint performance. In addition, the RSI RUN was more correlated to sprint performance than other RSI indices studied in previous research. The echo intensity of the vastus lateralis (VLEI) was largely correlated with maximum running speed (r = from 0.76 to 0.87, p &lt; 0.05) and the RSI RUN (r = −0.80, p &lt; 0.05). No significant correlations were noted between echo intensity and other RSIs. In conclusion, the RSI RUN and VLEI seem to be good predictors for track and field sprinting performance.

Seasonal Variation and Positional Differences in Anthropometry, Strength, and Power Characteristics in English Premiership Women's Rugby Union Players.

Yao, X, Austerberry, A, Bishop, C, Wilson, L, Chiang, C-Y, and Turner, A. Seasonal variation and positional differences in anthropometry, strength, and power characteristics in English premiership women's rugby union players. J Strength Cond Res 38(5): 924-931, 2024-Women's rugby is a collision sport that relies heavily on body composition and physical characteristics of strength and power to achieve competitive success. Furthermore, the seasonal nature presents a variety of physical challenges that can cause fluctuations in a player's physical development. Therefore, the purpose of this study was to determine the differences in anthropometry, strength, and power characteristics between forwards and backs in women's rugby union athletes in England and to identify changes throughout a season. Forty-seven players were recruited from the English premiership women's rugby during the 2020-2021 season. Players were split into forwards and backs and underwent body composition testing by dual-energy X-ray absorptiometry and strength and power tests (countermovement jump, drop jump [DJ], and isometric midthigh pull) on 3 separate occasions (preseason, midseason, postseason). Overall, forwards had significantly ( p < 0.01) higher body mass, fat mass, lean mass [LM], bone mineral content, and take off momentum, and backs had significantly higher ( p < 0.01, d > 0.5) jump height, reactive strength, and shorter DJ contact time. When observing seasonal changes, there were statistically significant differences ( p < 0.01) or moderate-to-large practical differences ( d > 0.5) in LM, reactive strength index modified, time to take-off, and DJ flight time [FT] among forwards when comparing 3 testing time frames. For backs, statistically significant differences ( p < 0.01) or moderate-to-large practical differences ( d > 0.5) were reported in LM and DJ FT throughout the season. In conclusion, the strength and power testing and characteristics shown in this study could support coaches and junior women's rugby athletes to have a basic understanding of English premiership physical standards.

Dose-Response Effect of an Inertia Flywheel Postactivation Performance Enhancement Protocol on Countermovement Jump Performance.

The purpose of this study was to investigate the dose-response effect of a high-load, 6-repetition, maximum effort inertial flywheel (IFw) squat postactivation performance enhancement (PAPE) protocol on countermovement jump (CMJ) performance metrics. Thirteen subjects completed 5 squat testing sessions: 1 session to determine back-squat 6-repetition maximum, 1 session to determine 6-repetition maximum IFw load, and 3 sessions to investigate the dose-response effect of an IFw PAPE protocol set at the load determined in the second session. In the IFw PAPE sessions, subjects completed either 1, 2, or 3 sets of IFw squats, then performed 5 CMJs over 12minutes (1, 3, 6, 9, and 12min post-IFw). All CMJ tests were conducted on a force platform where CMJ performance outcomes and impulse variables were calculated. There was no main time or volume effect for jump height, contact time, reactive strength index, peak force, or any of the impulse variables. A main time effect was identified for flight time (P = .006, effect size = 0.24) and peak power (P = .001, effect size = 0.28). The lack of change in jump height may indicate that too much fatigue was generated following this near-maximal IFw squat protocol, thereby reducing the PAPE effect.