European Journal of Applied Physiology

Background: Caffeine is widely used as an ergogenic aid, yet its effect on neuromuscular function remains heterogeneous across exercise conditions. Although caffeine is known to enhance central nervous system excitability through adenosine receptor antagonism, the extent to which these effects modify spinal motor output remains unclear. Therefore, the present study aimed to characterise acute caffeine-induced alterations in neuromuscular properties during submaximal and sustained fatiguing contractions of the knee extensors. Methods: Seventeen healthy adults completed a randomised, placebo-controlled crossover study (3 mg/kg caffeine). Circulating serum caffeine concentrations were periodically quantified, while motor unit (MU) discharge behaviour, and delta F were derived from high-density surface electromyography. Statistical significance was set at p = 0.05. Results: No intervention effects were observed for MVC, force steadiness, or time to task failure (all p > 0.05). During graded contractions, vastus lateralis MU discharge rate was higher under caffeine (p < 0.001), and discharge variability was lower (p = 0.016). Recruitment and derecruitment thresholds were significantly greater under caffeine at 50% MVC (both p < 0.012), whereas paired MU analyses of delta F was unchanged (p = 0.083). During sustained contractions, caffeine altered fatigue-related trajectories of both MU discharge and discharge variability (intervention * phase interaction, p < 0.011). Conclusions: Acute caffeine ingestion alters motor unit behaviour without enhancing maximal force or fatigue resistance. These effects vary with contraction intensity and fatigue state but are not accompanied by a detectable change in discharge rate hysteresis in the vastus lateralis.

Sex-related differences in force steadiness are often attributed to maximal strength and motor unit (MU) properties, but their independent contributions remain unclear. This study strength-matched females and males to remove the influence of maximal strength and determine whether MU properties are associated with sex-related differences in force steadiness. Twelve young adults (6 females) were matched for elbow flexion strength (females, 188.6{+/-}15.6 N; males, 199.7{+/-}24.8 N, p=0.4). Both groups performed submaximal isometric elbow flexion contractions at 2.5%, 5%, 10%, 15%, and 25% MVC. The MU recruitment thresholds (RT), discharge rates (MUDR), and coefficient of variation of interspike intervals (CVISI) were measured from intramuscular fine wire electromyography (EMG) electrodes. Force steadiness was quantified as the standard deviation (SD) and coefficient of variation (CV) of force. Across forces, SD and CV of force did not differ between females and males (p>0.05). Females had a higher recruitment threshold than males (p<0.05). Females had higher MUDR at 15% and 25% MVC (p<0.02), while males were higher at 5% MVC (p=0.02). The CVISI was greater in females (p<0.001) and positively correlated with SD of force (r=0.2) and negatively with CV of force (r=-0.2) in females and males. When strength was matched, sex-related differences in force steadiness were not evident. However, females exhibited higher MU recruitment thresholds, MUDR and CVISI. Despite greater CVISI in females, these differences did not translate into greater force fluctuations, suggesting that individual MU discharge variability is not a primary predictor of force steadiness when maximal strength is controlled. NEW & NOTEWORTHYO_LIStrength matching eliminated sex-related differences in elbow flexor force steadiness. C_LIO_LIFemales achieved similar force steadiness using higher MU recruitment thresholds and discharge rates, particularly in the short head of the biceps brachii. C_LIO_LIIn females, the greater variability in motor unit discharge was not associated with reduced force steadiness. C_LI

This study aimed to (1) develop an elasticity framework for the sprint force-velocity (F-V) relationship and (2) examine how maximal force (F_{0}), maximal velocity (v_{0}), and sprint distance modulate the four derived elasticity metrics, and (3) explore these elasticity metrics' interrelation. After modelling the F-V relationship differential equation, four elasticity metrics were defined as force elasticity (F_{e}), the elasticity of sprint time to F_{0}; velocity elasticity (v_{e}), the elasticity of sprint time to v_{0}; the force-velocity elasticity norm {(\mathrm{F}-\mathrm{V}}_{\mathrm{EN}}=\sqrt{F_{e}^{2}+v_{e}^{2}}), capturing the combined sprint time sensitivity to proportional changes in F_{0} and v_{0}; and the force-velocity elasticity ratio {(\mathrm{F}-\mathrm{V}}_{\mathrm{ER}}=F_{e}{\div v}_{e}), indicating which variable dominates the sprint time response. Model simulations showed that F_{e} decreased with rising F_{0} and increased with rising v_{0}, while v_{e} showed the opposite pattern. With increasing sprint distance, F_{e} decreased and v_{e} increased. Given its negligible effect on sprint time, ignoring air resistance yields a conservation law (2F_{e}+v_{e}\equiv 1), indicating that a gain in one elasticity metric necessarily diminishes the other in a fixed proportion. This framework also identifies a valley distance (d_{valley}) at {\mathrm{F}-\mathrm{V}}_{\mathrm{ER}}=2, where {\mathrm{F}-\mathrm{V}}_{\mathrm{EN}} is minimized (\sqrt{0.2}) and sprint time is least responsive to changes in F-V relationship variables. Empirical data confirmed that the two theoretical laws still hold approximately when air resistance is considered. By linking changes in F_{0} and v_{0} to sprint time across different distances, the elasticity framework provides a quantitative basis for estimating the theoretical sprint time response to documented changes in F-V relationship variables.

Kinaesthetic motor imagery (kMI) is widely used in sport to enhance motor performance by engaging cortical sensorimotor networks. Neurofeedback may further support kMI, but the optimal neural target to reinforce remains unclear. Maximal sensorimotor event-related desynchronisation (SMR-ERD) represents a relevant target as it may index sensorimotor cortex engagement, yet sport expertise has been associated with reduced SMR-ERD, potentially reflecting neural efficiency. The optimal neurofeedback target may therefore depend on sport expertise, movement expertise, and individual kMI ability. This study examined how these factors influence sensorimotor activity during kMI. We compared 17 basketball players (Experts) and 16 individuals without formal basketball training (Novices). kMI ability and frequency of use were assessed using questionnaires, while SMR-ERD was quantified using electroencephalography (EEG) during kMI. Participants imagined either a basketball-specific movement (Free throw), for which only Experts had extensive experience, or a generic movement (Box lifting), familiar to both groups. Experts reported greater kMI ability and more frequent kMI use than Novices. Only Experts exhibited significant and sustained SMR-ERD during kMI. Moreover, SMR-ERD was stronger in Experts than Novices specifically during Free throw kMI, corresponding to their movement of expertise. Nonetheless, within the Expert group, higher kMI ability was associated with reduced SMR-ERD. These findings suggest that sport expertise initially enhances voluntary recruitment of sensorimotor networks during kMI, whereas greater kMI ability may subsequently promote neural efficiency, resulting in reduced overall sensorimotor cortical activation. These results highlight the need to tailor kMI-based neurofeedback training to users' sport expertise and kMI ability levels.

This study aimed to (1) establish the distance-averaged F-V relationship framework and (2) develop elasticity metrics that quantify how F-V relationship variables govern jump height and inform training prescription. Theoretical derivation and experimental validation across 108 F-V relationship models derived from 1578 jumps (countermovement jump and squat jump at three knee angles; 20 well-trained subjects) yielded a standard error of 2.1% and a nearly perfect correlation (r = 0.96, p < 0.001) between measured and predicted jump height. Four elasticity metrics were formulated: force elasticity (F_{e}), the elasticity of jump height to maximal force (F_{0}); velocity elasticity (v_{e}), the elasticity of jump height to maximal velocity (v_{0}); the force-velocity elasticity norm {(\mathrm{F}-\mathrm{V}}_{\mathrm{EN}}=\sqrt{F_{e}^{2}+v_{e}^{2}}), reflecting the overall sensitivity of jump height to changes in F-V relationship variables; and the force-velocity elasticity ratio {(\mathrm{F}-\mathrm{V}}_{\mathrm{ER}}=F_{e}{\div v}_{e}), indicating which variable dominates the jump height response. Simulations and experiments revealed that F_{e} bore an inverse relationship to F_{0}, and v_{e} was inversely related to v_{0}, reflecting diminishing marginal returns. At a fixed jump height, simulations showed {\mathrm{F}-\mathrm{V}}_{\mathrm{EN}} and {\mathrm{F}-\mathrm{V}}_{\mathrm{ER}} displayed a U-shaped relationship; a balanced profile ({\mathrm{F}-\mathrm{V}}_{\mathrm{ER}}=1) did not always correspond to the lowest {\mathrm{F}-\mathrm{V}}_{\mathrm{EN}}. The distance-averaged F-V elasticity framework offers a physically grounded and quantitative tool for linking F-V relationship variables directly to jump performance, providing a basis for informing individualized training decisions.

Introduction: Daytime sleepiness (DS) can compromise athletic performance; however, its influence on Brazilian Jiu-Jitsu (BJJ) athletes remains poorly understood. Daytime naps have been suggested as a strategy to alleviate the effects of DS; however, their impact on physical and specific performance remains inconsistent. Thus, this study investigated the influence of different levels of daytime sleepiness on the physical and specific performance of BJJ athletes and verified the effects of daytime napping on these outcomes. Methods: This investigation employed a randomized, controlled, crossover clinical trial design involving 19 amateur Brazilian Jiu-Jitsu (BJJ) athletes who exhibited excessive daytime sleepiness. Participants performed two experimental conditions, with a 30-minute nap (N30) and without a nap (NN), in a randomized order. Vertical jump (CMJ and SJ), anaerobic power (Wingate ), kimono grip strength (KGST), and specific performance were evaluated using the Jiu-Jitsu Anaerobic Performance Test (JJAPT). Data were analyzed using mixed linear models. Results: Daytime sleepiness significantly influenced all parameters of specific performance (JJAPT), with worse performance observed in athletes classified as having abnormal sleepiness. The opportunity to nap increased the total number of repetitions in the JJAPT of four sets ({approx}20 repetitions; p < 0.001), without consistent changes in physical performance tests. An interaction between napping and sleepiness was observed in relation to isometric endurance strength, indicating that napping may provide greater benefits to athletes experiencing abnormal sleepiness. Conclusion: High levels of daytime sleepiness are associated with reduced specific performance in BJJ athletes. Daytime naps are a simple and potentially effective strategy to mitigate these effects and improve specific performance, reinforcing the importance of monitoring sleepiness and individualizing recovery strategies.

Few studies have focused on plyometric interventions lasting over 24 weeks in team-sport athletes, and little research has validated the practical utility of personalized force-velocity assessment. A total of 36 university volleyball athletes (21.35{+/-}1.87 years) were randomly allocated into either a 32-week plyometric training group (PT, n=18) or a low-intensity active control group (CON, n=18); 34 players completed the trial (17 per group). PT completed three weekly training sessions, while CON maintained routine technical practice and general low-intensity conditioning. CMJ height, RSImod, and force-velocity profiles (F, V, peak power, FV imbalance) were evaluated at baseline and at weeks 8, 16, 24, and 32. PT increased CMJ height from 33.1{+/-}4.8 cm to 38.4{+/-}5.2 cm, a net gain of 5.3 cm (p<0.01, d=0.99, 95% CI [0.57, 1.41]); CON showed negligible change (+0.5 cm, p=0.68). The PT group also improved RSImod by 31.6% (d=0.87, 95% CI [0.45, 1.29], p<0.01) and peak power by 18.4% (d=0.76, 95% CI [0.35, 1.17], p=0.004). Meanwhile, FV imbalance dropped markedly by 31.4%, from 28.3% to 19.4% (d=-0.68, 95% CI [-1.09, -0.27], p=0.024). No significant F-V changes occurred in CON. The reduction in FV imbalance correlated moderately with CMJ gain (r= - 0.53, p=0.024), whereas the change in peak power did not (r=0.21, p=0.39). Overall, 32 weeks of periodized plyometric training enhanced jump performance, reactive strength, and power, and reduced the force-velocity (FV) imbalance by approximately 31%. The magnitude of improvements in jump performance (16%) and FV imbalance reduction (31%) observed in this 32-week periodized program fall within a similar range to previously reported effects of individualized F-V training (14% and 40%, respectively). However, direct numerical comparisons are confounded by differences in study design, population, and methodology; head-to-head trials are needed.

It is well documented that both motoneuron output and the effectiveness of activated Ia afferents to discharge soleus -motoneurons decrease during eccentric (muscle lengthening) contractions. Evidence suggests that these modulations can be explained by recurrent inhibition and greater presynaptic inhibition of Ia afferents. However, the influence of angular velocity on the modulation of the effectiveness of activated Ia afferents to discharge -motoneurons observed during eccentric contractions remains unclear. We investigated the influence of angular velocity on spinal mechanisms involved in the effectiveness of activated Ia afferents to discharge -motoneurons during eccentric plantar flexor contractions using 16 healthy adults. We used both simple and conditioned Hoffmann reflex with different conditioning techniques to assess presynaptic inhibition, heteronymous Ia facilitation and heteronymous recurrent inhibition coupled with electromyography during eccentric contractions of the plantar flexors at three angular velocities. Our results showed that during eccentric contractions, the effectiveness of Ia afferents to discharge -motoneurons was lower at 90{degrees}{middle dot}s-{superscript 1} than 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1} angular velocities. A similar velocity-dependent pattern was observed for heteronymous recurrent inhibition, decreasing at 90{degrees}{middle dot}s-{superscript 1} when compared with 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1}. In contrast, presynaptic inhibition of Ia afferents was not different between the velocities. These demonstrate a differential influence of angular velocity on spinal recurrent inhibitory mechanisms during eccentric contractions and support distinct functional roles of recurrent and presynaptic inhibition in modulating -motoneurons discharge with increasing movement velocity. The findings provide new insights into the velocity-dependent and mechanism-specific modulation of spinal inhibitory circuits during eccentric contractions. KEY POINTSO_LIDuring eccentric contractions in soleus muscle, the effectiveness of activated Ia afferents to discharge -motoneurones decreases with increasing angular velocity, indicating a velocity-dependent modulation. C_LIO_LIPresynaptic inhibition of Ia afferents does not differ between angular velocities, suggesting that it does not contribute to the observed changes. C_LIO_LIHeteronymous recurrent inhibition from the quadriceps to the soleus increases with angular velocity, indicating that increasing movement velocity promotes a functional reorganization of intermuscular recurrent inhibition. C_LIO_LIThese findings suggest a differential functional role of the two spinal inhibitory mechanisms, indicating that increasing angular velocity primarily influences recurrent postsynaptic inhibition rather than presynaptic inhibition. C_LI

Shilajit is a naturally occurring resinous exudate with centuries of documented use in Ayurvedic medicine. Its principal bioactive constituents -- fulvic acid and a proprietary aromatic compound complex termed shilarathenes, comprising urolithin metabolites, phenolic acids, and flavonoids -- are proposed to enhance mitochondrial ATP synthesis, attenuate exercise-induced muscle damage, and modulate androgen biosynthesis. Prior controlled trials have demonstrated increases in testosterone and retention of muscular strength with purified shilajit, though large trials in resistance-trained populations have been absent. In this randomised, double-blind, placebo-controlled, multicenter trial (CTRI/2025/07/091323), 100 healthy males aged 21-50 years (BMI <30 kg/m{superscript 2}; [&ge;]1 year of resistance training) were enrolled across four sites. Participants received 250 mg twice daily of standardised TruBlk Shilajit resin ([&ge;]60% fulvic acid; [&ge;]10% shilarathenes) or matched placebo for 90 days, and were instructed to maintain habitual training and dietary practices throughout. Primary endpoints were changes from baseline in 1RM leg press, muscle endurance, RPE, and DOMS. Secondary endpoints included VO2max, testosterone, creatine kinase, lactate dehydrogenase, handgrip strength, and global improvement ratings. Ninety-nine participants completed the trial (99% retention). All primary outcomes showed significantly steeper improvement in the active group, assessed by treatment x time interaction: 1RM leg press (+26.0% vs +18.4%; p=0.0003, d=0.74), muscle endurance (+110.8% vs +80.5%; p=0.0017, d=0.60), RPE (p=0.002), and DOMS (p=0.036). Dominant handgrip strength showed the largest secondary effect (+12.5% vs +7.9%; p=0.0002, d=0.77). The active group also demonstrated greater reductions in creatine kinase (-33.6% vs -6.9%; p=0.030, {delta}=-0.26) and lactate dehydrogenase (-17.8% vs -4.2%; p=0.039, {delta}=-0.24), greater increases in free testosterone (+21.8% vs -2.9%; p=0.030, d=0.45) and total testosterone (+15.6% vs -3.8%; p=0.038, d=0.43), and a modest improvement in VO2max (+3.8% vs +2.9%; p=0.032, d=0.41) compared with placebo. No adverse events were recorded in the active group. Ninety days of TruBlk Shilajit resin supplementation produced robust improvements in muscle strength and endurance in resistance-trained males, with moderate effect sizes for the strength outcomes. These data support TruBlk Shilajit resin as a safe and promising ergogenic agent meriting further clinical evaluation.

Physical activity (PA) is effective to counteract age-related declines. However, inter-individual variability in training adaptations may limit prediction of individual responses and intervention optimization. Daily PA behavior may contribute to this variability. The objective was to examine daily PA behavior through a 12-week supervised power training (PT) and its associations with clinical adaptations in 36 older men. Time of PA, step count, total (TEE) and active (AEE) energy expenditure, metabolic equivalents of task (METs), and sedentary time were assessed using a 3D accelerometer for three days surrounding PT sessions (Day-PT-1, Day-PT and Day-PT+1) at pre-(T0) and mid-intervention (6-week; T6). Physical performance, body composition and muscle function were assessed pre-(T0) and post-(T12) intervention. PA behavior outcomes did not change between T0 and T6, except for METs which decreased by 0.2 METS (T0:1.44{+/-}0.34 vs. T6:1.25{+/-} 0.18; p=0.003 [13%]). At T6, day-to-day fluctuations were observed for PA behavior on Day-PT-1 and Day-PT+1 compared with Day-PT (all ps<0.05). Following the intervention, body composition and muscle characteristics improved significantly. Between T6-Day-PT-1 and T6-Day-PT, greater increases in time of PA were associated with lower Fast Timed Up and Go speed adaptation (p=0.02), and greater reductions in step count were associated with greater reductions in muscle power (p=0.02). Finally, increases in step count and time of PA were associated with greater upper limb strength (p=0.04) and muscle pennation angle (p=0.02). PA behavior appears modulated during a 12-week PT intervention in healthy older men. Thus, PA behavior surrounding training sessions could be an important compensatory factor contributing to adaptations variability.

Background: Newly available arm angle data offers a new dimension to understand rising rates of arm injury in MLB pitchers. Purpose: To evaluate the relationship between arm angle, pitch characteristics, and elbow and forearm injury in MLB pitchers.<br><br> Study Design: Retrospective cohort study; Level of evidence, 3 Methods: Statcast data from 2020 to 2025 and MLB injured list (IL) data were used to evaluate arm angle and pitch characteristics in relation to elbow and forearm injuries. Results are presented with and without requirements on prior season workload and for same-season and next-season injury incidence. A generalized additive model (GAM) was used to capture non-linear dependence and interactions between selected features and injury incidence to the elbow or forearm. Average marginal effect (AME) odds ratios are reported for main effect terms. Results: N = 3,812 pitcher-seasons were included. 29% pitchers who underwent UCLR did so in the same season as a forearm injury (tmean=44, tmedian=27 days to surgery). Arm angle, fastball usage, and their interaction were the three most predictive features. Arm angle was positively related to incidence of injury (ORmeanAME=1.014), fastball usage was inversely related to incidence of injury (ORmeanAME=0.243), and arm angle moderated the effect fastball usage at high arm angle, where increased usage was no longer protective. Slider velocity (ORmeanAME=1.072), spin rate (ORmeanAME=1.001), and usage (ORmeanAME=2.039) also significantly predicted injury risk. Fastball velocity was not significant in any fit, with ORmeanAME=0.999 across all fits. Fit-level Nagelkerke R2 values ranged from .019 to .052. Conclusion: Fastball usage and arm angle, not velocity, predicted elbow and forearm injury risk among MLB pitchers, and arm angle was the single most predictive feature. The heterogeneity of risk factors as a function of arm angle, and the novelty of MLB arm angle data, may explain why fastball usage has been previously underexplored as a risk factor. Keywords: baseball; arm angle; fastball velocity; fastball usage; spin rate; UCL; ulnar collateral ligament; elbow injury; forearm injury; Statcast

Lifestyle behaviors influence cardiometabolic and cognitive health through multiple interconnected processes, involving metabolic, cardiovascular, behavioral and cognitive factors. This study aimed to assess associations between multivariate plasma and physiological profiles, and behavioral parameters, and their potential association with reaction time (RT) in active young adults. Data on physical activity levels were collected from wearable devices and GPAQ questionnaire, along with anthropometric and clinical assessments, cognitive tests, and blood analyses. Principal component analysis was performed separately for physiological and plasma parameters, retaining the first two principal components (PCs) from each analysis. Multiple linear regression models were used to evaluate the associations between plasma, sex, physical activity levels (METs), daily step count, and alcohol/marijuana use on physiological PC1/PC2 scores, as well as RT. Continuous independent variables were previously categorized, and combined variables were created to represent specific combinations between plasma/physiological PC1 categories and sex. Physiological PC1 (blood pressure) was associated with combined plasma PC1 (insulin/HOMA-IR)-sex variable, total METs and alcohol consumption (adjusted R2 = 0.30). Physiological PC2 (handgrip strength) was mainly explained by combined plasma PC1-sex variable (adjusted R2 = 0.54), with minimal contribution from behavioral factors. RT was associated with physiological PC1 variable and sex (adjusted R2 = 0.35), with slower responses observed among women with lower physiological PC1 scores or hypotension; these associations were supported by RT-physiological PC1 and RT-blood pressure correlations. These exploratory findings suggest sex-dependent associations between physiological, metabolic and lifestyle-related factors, supporting a holistic approach in future research, particularly among young adults.

Dietary supplements are increasingly being adopted by physically active individuals to support exercise performance and recovery. This randomised, double-blind, placebo-controlled trial evaluated the ergogenic potential of SpinoleRG, an HPLC-PDA-standardised combination of Amaranthus tricolor L. and Spinacia oleracea L. in healthy, recreationally active adults over 12 weeks. Participants received a total of 1,600 mg of the extract or a placebo daily before exercise. Primary endpoints included cardiopulmonary exercise testing, muscle strength, and endurance. Secondary endpoints included treadmill-based time to exhaustion (TTE), body composition, repetitions to failure, biochemical markers, delayed onset muscle soreness (DOMS), and vitals. Routine investigations and safety monitoring were also conducted. Four participants per group were withdrawn due to non-compliance and 64 participants completed the study. Compared to the placebo, the test group showed a significant increase in the ventilatory threshold, but not in VO2 max or maximal ventilation. They also showed significantly improved one-repetition maximum strength, fatigue indices, TTE, repetitions to failure, plasma nitrite, blood lactate, creatine kinase, lactate dehydrogenase, and DOMS outcomes. Vital signs or routine parameters were not impacted meaningfully. Three minor adverse events across both groups were observed, considered unrelated to the interventions and fully resolved. Overall, the findings support the tolerability and ergogenic potential of the extract.

Background: Sweeping sliders with large horizontal break are hypothesized to be associated with arm injury, and prior work suggests a link between slider usage, arm angle, and injury. Purpose: To evaluate whether arm angle and slider horizontal movement are associated with elbow or forearm injury risk in MLB pitchers. Study Design: Retrospective cohort study; Level of evidence, 3 Methods: Statcast data from 2020-2025 and injury data were used to study the relationship between sliders and arm injuries. Pitchers with at least 30 innings pitched (IP) were evaluated for same- and next-season injury incidence to the Elbow, Forearm, or Elbow/Forearm. A generalized additive model (GAM) related pitch-level variables to injury incidence, and average marginal effect (AME) odds ratios are reported for main effects. Results: All fits were significant at p<.01. Fits for same-season (N=1,957, p=.00008, R2=.066) and next-season (N=2,511, p=.00005, R2=.053) Elbow/Forearm injury were significant at p<.001. Same-season (R2=.061) and next-season (R2=.055) Forearm injury fits had p=.001. Main effects for slider glove-side movement and fastball usage, their interactions with arm angle, and arm angle main effects were the most predictive features. Across the three fits where the slider glove-side movement main effect was significant, odds ratios ranged from 1.03 to 1.07, meaning that each additional inch of slider glove-side movement was associated with a 3% to 7% increase in the observed injury incidence. Furthermore, this effect was magnified at high arm angle and muted at low arm angle. Conclusion: We observed that slider horizontal movement, arm angle, and their interaction were significantly associated with incidence of Forearm and combined Elbow/Forearm injury. This effect was weak or non-existent at low arm angles and strong at high arm angles, suggesting that arm angle moderates the risk of slider horizontal movement, and providing evidence that sliders with large horizontal break (e.g. sweepers) may pose an injury risk. Keywords: baseball; sweeper; slider; arm angle; horizontal movement; elbow injury; forearm injury

Background: Force production reflects neuromuscular capacity, central regulation, and sensory feedback, making performance-based testing vulnerable when disease-related limitations could be interpreted as reduced effort. Repeated handgrip strength (HGS) testing may quantify neuromuscular fatigability in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), but interpretation of within-subject variability as an effort-validity marker remains controversial. We tested whether the coefficient of variation (CV) distinguishes ME/CFS-related impairment from deliberate submaximal performance and compared it with the sum of residuals (SR), a trajectory-aware metric. Methods: We analyzed three cohorts using the same repeated-HGS protocol: two 10-trial sessions separated by one hour. We included 211 ME/CFS participants and 170 controls, including 28 instructed to perform at 50% perceived maximum force. CV and SR were determined as HGS variability measures and compared by probability-density overlap and receiver operating characteristic analyses. The primary comparison was ME/CFS versus deliberately submaximal controls. Results: CV distributions showed substantial overlap between groups. SR improved group-level separation, increasing non-overlapping fractions from 0.217 to 0.326 in the Jaekel cohort and from 0.180 to 0.361 in the MIRACLE cohort. In ROC analyses, SR consistently yielded higher discrimination than CV across cohorts, sessions, and sex strata. A commonly used 15% CV cutoff classified 42-48% of ME/CFS participants as submaximal, while correctly identifying only 57-64% of deliberately submaximal controls. Conclusions: CV may conflate ME/CFS-related fatigability with irregular force modulation and should not be used as stand-alone evidence of submaximal effort. Trajectory-aware SR partly reduced this bias and indicated that ME/CFS-related force impairment is distinguishable from deliberate submaximal force production at the group level. However, neither metric should be used alone to diagnose ME/CFS or evaluate sincerity of effort.

Humans tend to walk at speeds that minimize energy expenditure and time duration. While most walking experiments examine individuals in relative isolation, everyday locomotion frequently occurs in social contexts. We investigated whether walking speed is modulated in response to a social interaction during a cooperative task. Participants completed 96 randomized walking trials where they approached and retrieved boxes varying in distance (2.5-10 m) and mass (0-6.8 kg). Boxes either rested on the ground or were handed off by an experimenter, signaling prosocial effort benefitting the participant. Approach speed was measured with inertial measurement units placed at the feet and fit to a saturating exponential function of walking distance using a nonlinear mixed-effects regression model. Based on the energy-time optimization framework, we hypothesized that participants would approach more quickly when larger boxes were held at farther distances, to reduce energy and time costs of the experimenter, despite exerting more effort themselves. Participants walked 8.5% faster (1.29 m s-1 versus 1.19 m s-1; p = 3.85 x 10-6) when the largest box was held out by the experimenter versus left on the ground. However, the manner in which the box was held had no influence on approach speeds (p = 0.31). Exploratory analyses identified modest trends between experiment responses and individual characteristics, but none reached statistical significance. The findings suggest that locomotor decisions reflect not only an individuals own energy and time costs but also the perceived costs borne by others. This study demonstrates that social context can meaningfully influence walking behavior. Summary StatementHumans walk more quickly in response to the perceived energy and time others spend helping them, even at the cost of exerting more energy themselves.

The contribution of persistent inward currents (PICs) to spinal motor neurons discharge rate is lower in old compared to young adults. However, whether this difference further progresses at very old age ([&ge;]80 years) and differs between males and females remains unknown. Thirty young (26 {+/-} 4 years, 15 females), thirty old (72 {+/-} 5 years, 15 females) and twenty-nine very old adults (85 {+/-} 3 years, 15 females) performed isometric dorsiflexion ramp contractions at 20% of maximal voluntary torque. Individual motor unit spike trains were identified from decomposed high-density surface electromyograms of the tibialis anterior. The paired motor unit technique was used to estimate the prolongation effect of PICs ({Delta}F and normalized {Delta}F), and the quasi-geometric approach to estimate neuromodulatory (brace height) and inhibitory pattern (attenuation slope) contributions. {Delta}F was lower in old and very old adults than in young adults, with no difference between old and very old adults regardless of sex. Compared with young adults, normalized {Delta}F was lower in both old and very old males, whereas in females it is only lower in the very old group. Brace height and attenuation slope did not differ across ages or sexes. These findings indicate that age-related changes in spinal motor neuron excitability are established by the seventh decade and do not progress further in healthy active very old adults, following a nonlinear trajectory that may differ between males and females. The absence of age-related differences in the neuromodulatory index does not support a lower monoaminergic drive in older adults.

The validation of objective neurophysiological monitoring of concussion from diagnosis to return to play has the potential to improve the clinical care of athletes. Extracted from event-related potentials (ERP), brain vital signs offer a non-invasive objective neurophysiological measure of three well established ERP responses that complement existing clinical assessments: 1) the N100 response related to auditory sensation; 2) the P300 response related to basic attention; and 3) the N400 response related to cognitive processing. The N400, in particular, has increasingly been demonstrated to be sensitive to head impact related cognitive impairment. Accordingly, brain vital sign measures were collected in n =165 rugby league players from two Australian professional National Rugby League clubs (NRL), with concussed players (n =26) being subsequently assessed over a total of five separate time points (Baseline, Day 2, Day 5, Day 10, and Day 42). This timeline aligned with acute (Day 2), NRL mandatory minimum return-to-play (RTP) guidelines (Day 10), and an extended post RTP period. Relative to baseline, the results revealed significant N400 cognitive processing latency delays at Day 2 post-concussion (p =0.002). The N400 latency delays persisted past day 5, significantly returning to baseline values at Day 10, consistent with the RTP mandatory minimum requirement. The findings indicated that neurophysiological recovery occurred in this cohort within an approximate 10-day time frame. Future studies monitoring brain vital sign changes across the trajectory of injury to recovery are needed in sports related concussion management.

Cardiovascular markers such as heart rate (HR), heart rate variability (HRV), and blood pressure (BP) are promising for low-intrusive monitoring of mental states relevant to safety-critical performance, yet evidence is mixed regarding which measures are most sensitive to cognitive workload, stress, and fatigue across different task types and time frames. Sixty adults (33 women; M age = 28.1 years, SD = 7.9) completed a 90-120 minute laboratory protocol including four cognitive workload tasks (Judgment of Line Angle and Position, Corsi Block-Tapping, Balloon Analogue Risk Task, Stroop), a modified Trier Social Stress Test (preparation, speech, arithmetic), and a fatigue condition operationalized as time-on-task during a prolonged Stroop (15 min) and Psychomotor Vigilance Task (10 min), each divided into three intervals. Continuous beat-to-beat HR, interbeat interval (IBI), and systolic/diastolic BP were recorded using the CareTaker device; HRV indices (RMSSD, high-frequency power) were derived from IBI using Kubios. Difference scores (task minus baseline) were analyzed using linear mixed models (random intercepts for participants; age and gender covariates), with Tukey-corrected pairwise comparisons to quantify sensitivity across conditions. HR and IBI consistently differentiated multiple workload and stress phases and distinguished Stroop-based fatigue from PVT intervals. HRV-RMSSD showed selective sensitivity, differentiating specific cognitive tasks (notably Stroop relative to other workload tasks), but only demonstrated a trend toward detecting fatigue-related differences at later time intervals. High-frequency HRV showed task-dependent effects only within the cognitive workload condition. Continuous BP did not reliably differentiate tasks, showing at most marginal (uncorrected) effects during TSST phase comparisons. Cardiovascular reactivity patterns depended on task type and temporal dynamics. HR and IBI provided the most robust sensitivity across workload, stress, and fatigue within a prolonged, multi-task protocol, while HRV indices contributed complementary but more task-specific information. The results support multi-metric monitoring approaches for real-time mental state transitions in high-risk operational settings.

Long-latency stretch reflex and corticospinal excitability in the tibialis anterior muscle (TA) are facilitated when balance is threatened, even without background TA activity, suggesting supraspinal modulation as preparatory tuning for ankle stabilization. However, it remains unclear whether such tuning is evident at the spinal level and specific to the TA among lower-limb muscles. We therefore examined the effects of height-induced postural threat on multi-segmental monosynaptic spinal reflexes (MMR) in lower-limb muscles during quiet standing. Seventeen healthy young males performed 90-s standing tasks under three postural threat conditions, created by combining real and virtual reality (VR) heights: (1) Low-threat (real ground & VR ground), (2) Medium-threat (real table & VR ground), and (3) High-threat (real table & VR bridge). During each condition, transcutaneous spinal cord stimulation (tSCS) was applied to the lumbar spine to elicit MMR in lower-limb muscles. Electromyograms (EMG) were recorded from six muscles of the right leg: vastus medialis (VM), biceps femoris (BF), TA, soleus (SOL), medial (MG), and lateral gastrocnemius (LG). MMR excitability was quantified as peak-to-peak EMG amplitude. Fear ratings and electrodermal activity were higher in High-threat than Low-threat (all p < 0.05), confirming successful threat induction. Peak-to-peak EMG amplitude in the TA was significantly higher in High-threat than Low-threat (17.1% increase, p = 0.0393), whereas background TA activity remained absent across conditions. These results indicate that TA has unique function to facilitate spinal excitability as a preparatory tuning for ankle stabilization. Key pointsO_LIPrevious studies have shown the supraspinal facilitation of the tibialis anterior muscle without background muscle activation as a preparatory tuning for ankle stabilization. C_LIO_LITo test the hypothesis that such tuning is also evident at the spinal level and specific to the tibialis anterior muscle, this study examined whether height-induced postural threat modulates multi-segmental monosynaptic reflex excitability in lower-limb muscles using transcutaneous spinal cord stimulation. C_LIO_LIElectrodermal activity and fear ratings increased under height-induced postural threat, confirming the successful induction of postural threat. C_LIO_LIUnder height-induced postural threat, the multi-segmental monosynaptic reflex was selectively facilitated in the tibialis anterior muscle, while its background activity remained absent. C_LIO_LIOur findings demonstrate selective facilitation of spinal excitability in the tibialis anterior muscle, which may serve as preparatory tuning for ankle stabilization under threat to balance. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=147 SRC="FIGDIR/small/742625v1_ufig1.gif" ALT="Figure 1"> View larger version (50K): org.highwire.dtl.DTLVardef@ca21f3org.highwire.dtl.DTLVardef@7b1679org.highwire.dtl.DTLVardef@1007191org.highwire.dtl.DTLVardef@1ff88a_HPS_FORMAT_FIGEXP M_FIG C_FIG Abstract figure legendWhen balance is threatened, corticospinal excitability and long-latency stretch reflex in the tibialis anterior muscle (TA) are facilitated even in the absence of background TA activity, suggesting supraspinal preparatory tuning for ankle stabilization. This study tested the hypothesis that such facilitation is also expressed at the spinal level and is specific to the TA. Participants completed 90-s quiet standing trials under three different height-induced postural threat conditions. During each trial, transcutaneous spinal cord stimulation was delivered over the lumbar spine to elicit multi-segmental monosynaptic reflexes (MMR) in multiple lower-limb muscles. High-threat condition increased fear ratings and electrodermal activity, indicating successful threat induction. Moreover, MMR excitability was selectively increased in the TA under High-threat condition despite the absence of background TA activity. These findings suggest that spinal facilitation is selectively expressed in the TA and may reflect preparatory tuning for ankle stabilization under threat to balance.