Anticipation of ground reaction force (GRF) in depth jumping requires multisensory integration of exteroceptive, vestibular, and proprioceptive inputs. Vision contributes to the anticipation of GRF in drop landings and may influence depth jump performance when disrupted. The purpose of this investigation was to evaluate the effects of stroboscopic vision on depth jump performance. Thirteen female NCAA Division I volleyball athletes completed a testing protocol consisting of 0.38 m depth jumps under condition of full vision and stroboscopic vision at strobe frequencies of 4 and 1.75 Hz. Depth jump performance was assessed via the Reactive Strength Index (RSI) and time-series vertical GRF (vGRF) data. Main effects of stroboscopic vision were evaluated for statistical significance via Repeated Measures Multivariate Analysis of Variance with post hoc multiple paired t-tests (α = 0.05). RSI (p < 0.001) and rebound jump height (p = 0.006) were lower in the 1.75 Hz stroboscopic condition versus full vision, while ground contact time (p = 0.008), and rate of vertical ground reaction force development (p = 0.016) were greater in the 1.75 Hz stroboscopic condition versus full vision. Stroboscopic vision could be used to modify the intensity of depth jumping and considered for inclusion into plyometric training.

Since the reactive strength index (RSI) and reactive strength index-modified (RSI-mod) share similar nomenclature, they are commonly referred as interchangeable measures of agility in the sports research literature. The RSI and RSI-mod are most commonly derived from the performance of depth jumping (DJ) and countermovement jumping (CMJ), respectively. Given that DJ and CMJ are plyometric movements that differ materially from biomechanical and neuromotor perspectives, it is likely that the RSI and RSI-mod measure distinct aspects of neuromuscular function. The purpose of this investigation was to evaluate the association and agreement between RSI and RSI-mod scores. A mixed-sex sample of NCAA division I basketball athletes (n = 21) and active young adults (n = 26) performed three trials of DJ from drop heights of 0.51, 0.66, and 0.81 m and three trials of countermovement jumping. Using 2-dimensional videography and force platform dynamometry, RSI and RSI-mod scores were estimated from DJ and CMJ trials, respectively. Linear regression revealed moderate associations between RSI and RSI-mod scores (F = 11.0–38.1; R² = 0.20–0.47; p < 0.001–0.001). Bland–Altman plots revealed significant measurement bias (0.50–0.57) between RSI and RSI-mod scores. Bland–Altman limit of agreement intervals (1.27–1.51) were greater than the mean values for RSI (0.97–1.05) and RSI-mod (0.42) scores, suggesting poor agreement. Moreover, there were significant performance-dependent effects on measurement bias, wherein the difference between and the mean of RSI and RSI-mod scores were positively associated (F = 77.2–108.4; R² = 0.63–0.71; p < 0.001). The results are evidence that the RSI and RSI-mod cannot be regarded as interchangeable measures of reactive strength.

Researchers commonly use the ‘free-fall’ paradigm to investigate motor control during landing impacts, particularly in drop landings and depth jumps (DJ). While recent studies have focused on the impact of vision on landing motor control, previous research fully removed continuous visual input, limiting ecological validity. The aim of this investigation was to evaluate the effects of stroboscopic vision on depth jump (DJ) motor control. Ground reaction forces (GRF) and lower-extremity surface electromyography (EMG) were collected for 20 young adults (11 male; 9 female) performing six depth jumps (0.51 m drop height) in each of two visual conditions (full vision vs. 3 Hz stroboscopic vision). Muscle activation magnitude was estimated from EMG signals using root-mean-square amplitudes (RMS) over specific time intervals (150 ms pre-impact; 30–60 ms, 60–85 ms, and 85–120 ms post-impact). The main effects of and interactions between vision and trial number were assessed using two-way within-subjects repeated measures analyses of variance. Peak GRF was 6.4% greater, on average, for DJs performed with stroboscopic vision compared to full vision (p = 0.042). Tibialis anterior RMS EMG during the 60–85 ms post-impact time interval was 14.1% lower for DJs performed with stroboscopic vision (p = 0.020). Vastus lateralis RMS EMG during the 85–120 ms post-impact time interval was 11.8% lower for DJs performed with stroboscopic vision (p = 0.017). Stroboscopic vision altered DJ landing mechanics and lower-extremity muscle activation. The observed increase in peak GRF and reduction in RMS EMG of the tibialis anterior and vastus lateralis post-landing may signify a higher magnitude of lower-extremity musculotendinous stiffness developed pre-landing. The results indicate measurable sensorimotor disruption for DJs performed with stroboscopic vision, warranting further research and supporting the potential use of stroboscopic vision as a sensorimotor training aid in exercise and rehabilitation. Stroboscopic vision could induce beneficial adaptations in multisensory integration, applicable to restoring sensorimotor function after injury and preventing injuries in populations experiencing landing impacts at night (e.g., military personnel).

Previous research has found that lower limb muscle asymmetries increase with age and are linked to fall and injury risks. However, past studies lack a wide variety of muscle function modes and measures as well as comparison to a comparable younger age group. The purpose of this study was to examine age-related lower limb muscle function asymmetries across a variety of muscle action types and velocities in young and old adults. Lower limb balance, strength, power, and velocity were evaluated with concentric, isometric, isotonic, and eccentric muscle actions during a single-leg stance test and on single- and multi-joint dynamometers in 29 young (age = 21.45 ± 3.02) and 23 old (age = 77.00 ± 4.60) recreationally active men and women. Most (15 of 17) variables showed no statistical (p > 0.05) or functional (10% threshold) limb asymmetry for either age group. There was a significant main effect (p = 0.046; collapsed across groups) found for asymmetry (dominant > non-dominant) for the isotonic peak velocity variable. There was a significant (p = 0.010) group × limb interaction for single-joint concentric peak power produced at a slow (60 deg/s) velocity due to the non-dominant limb of the young group being 12.2% greater than the dominant limb (p < 0.001), whereas the old group was not asymmetrical (p = 0.965). The findings of this investigation indicate there is largely no age-related asymmetry of the lower limbs across a range of muscle function-related variables and modes, with a couple of notable exceptions. Also, the significant asymmetries for the isotonic peak velocity variable perhaps show the sensitivity of this uncommonly used measure in detecting minimally present muscle function imbalances.

The depth jump (DJ) is commonly used to evaluate athletic ability, and has further application in rehabilitation and injury prevention. There is limited research exploring sex-based differences in DJ ground reaction force (GRF) measures. This study aimed to evaluate for sex-based differences in DJ GRF measures and determine sample size thresholds for binary classification of sex. Forty-seven participants from mixed-sex samples of NCAA athletes and young adults performed DJs from various drop heights. Force platform dynamometry and 2-dimensional videography were used to estimate GRF measures. Three-way mixed analysis of variance was used to evaluate main effects and interactions. Receiver operating characteristic (ROC) curve analysis was used to evaluate the combined sensitivity and specificity of dependent measures to sex. Results revealed that reactive strength index scores and rebound jump heights were greater in males than females (p < 0.001). Additionally, young adult females showed greater peak force reduction than young adult males (p = 0.002). ROC curve analysis revealed mixed results that appeared to be influenced by population characteristics and drop height. In conclusion, sex-based differences in DJ performance were observed, and the results of this study provide direction for future DJ investigations.

Context: Cold water immersion (CWI) is a common aid in exercise recovery. CWI effectiveness depends on the magnitude of muscle and core cooling. Individual cooling responses to CWI are variable and likely influenced by CWI dose and individual physiological characteristics. Objective: Evaluate body fat percentage and thigh anthropometrics as predictors of intramuscular and skin cooling responses to CWI. Design: Interrupted time-series. Setting: Sports medicine research center. Participants: Sixteen young adults (8 male, 8 female, age=24.3±1.84 years, height=176.4±12.7 cm, mass=86.6±29.4 kg). Intervention: Body fat percentage was measured using a three site skinfold. Thigh length, thigh circumference, anterior thigh adipose thickness, anterior thigh muscle thickness, and thigh volume were estimated using manual and ultrasound methods. Using sterile techniques, thermocouple probes were approximated in the belly of the rectus femoris (2 cm deep to sub-adipose tissue) and on the anterior mid-thigh surface. Participants cycled on an ergometer for 30 minutes at a target heart rate between 130 and 150 beats per minute. Post-exercise, participants were placed in CWI (immersion depth: iliac crest; 10°C) until intramuscular temperature was 7°C below pre-exercise baseline temperature, with a maximum immersion duration of 30 minutes. Main Outcome Measure(s): Intramuscular rectus femoris and thigh skin temperatures measured post exercise, after 10 and 15 minutes of CWI, and post-CWI. Results: Body fat percentage significantly predicted rectus femoris cooling magnitude and rate after 10 minutes of CWI, 15 minutes of CWI, and post-CWI (p <0.001; R² = 0.58–0.64). Thigh anthropometrics significantly predicted thigh skin cooling rate post-CWI (p = 0.049; R² = 0.46). Conclusions: A simple three site skinfold assessment may improve the efficacious prescription of CWI through estimation of the dose required for minimal muscle tissue cooling.