Experimental Physiology

Skeletal muscle hypertrophy results from the integrated regulation of anabolic and proteolytic processes in response to mechanical loading. Although increases in resistance training (RT) volume are used to increase mechanical stress, it remains uncertain whether large and abrupt volume progressions could exceed muscle adaptive capacity by disrupting the balance between anabolic and catabolic signaling. The present study investigated whether a large increase in weekly RT volume (+120%) leads to impaired hypertrophic outcomes and intracellular regulatory responses compared with a modest increase (+20%). Twenty-five resistance-trained men and women (18-35 years old) completed an 8-week randomized, single-blind, within-subject unilateral intervention. Each participant trained both legs twice weekly, with one leg assigned to the large (VOL120) and the contralateral leg to the modest (VOL20) weekly volume progressions relative to habitual training volume. Vastus lateralis muscle cross-sectional area (mCSA) was assessed by ultrasonography before and after training. Muscle biopsies were obtained at baseline, post-intervention, and 24 h after the last session to quantify muscle fiber cross-sectional area (fCSA), satellite cell myonuclear content, and anabolic/catabolic signaling markers. Both protocols induced increases in mCSA over time (p<0.001), with no protocol vs. time interaction. No significant effects were observed for fCSA nor satellite cell number or myonuclear content. Additionally, molecular responses related to translational regulation and protein degradation were largely similar between protocols. Collectively, these data indicate that a large, abrupt increase in weekly set volume does not impair hypertrophic adaptations or meaningfully alter the anabolic-catabolic signaling profile in resistance-trained individuals.

When running, metabolic cost increases as muscles are simultaneously fatigued. However, the contribution of an individual muscle group to fatigue-related increase in metabolic costs remains unclear. We investigated the metabolic consequence of running with local plantar flexor or knee extensor fatigue and associated neuromuscular control strategies. Recreational and experienced male runners (N=20) completed two sessions (one per muscle group), with each including two 10 min running bouts: without and with local fatigue ([~]20% reduction in peak joint torque). Net metabolic power and muscle activity (initial and final minutes) were determined. Metabolic power was unaffected by plantar flexor (p=0.367) or knee extensor (p=0.607) fatigue in both cohorts. Plantar flexor fatigue recovered during the fatigued run (p=0.033), while knee extensor fatigue only recovered for the recreational cohort (p=0.009; experienced: p=0.826). With plantar flexor fatigue, plantar flexor muscle activity was unchanged between runs (p[&ge;]0.312), however initial soleus activity was greater in the unfatigued than fatigued run for experienced runners (p=0.022), and initial medial gastrocnemius activity was greater in the unfatigued than fatigued run for the combined cohort (p=0.009). With knee extensor fatigue, knee extensor muscle activity was mostly lower in the unfatigued than fatigued run (p[&le;]0.009), except for final vastus lateralis activity, which was unchanged between runs (p=0.061). Therefore, muscle groups respond with different activation strategies when fatigued. Running with plantar flexor or knee extensor fatigue, at levels like those induced by prolonged running (10-42 km), does not increase metabolic power and thus, submaximal running energetics may be maintained despite local muscle fatigue. NEW & NOTEWORTHYWhile muscle fatigue is suggested to increase the metabolic cost of running, the individual contributions of key lower limb muscle groups have not been explored. We examined responses after fatigue of only the plantar flexors or the knee extensors. Results indicate that local fatigue did not affect the metabolic power of male runners for either fatigued muscle group. These findings enhance our understanding of running performance and the interaction between fundamental criteria dictating human locomotion.

1BackgroundHuman locomotion is a highly adaptive motor skill that adjusts to new environmental demands through learning. Split-belt treadmill paradigms have advanced our understanding of gait adaptation. Most studies have examined gait when the belts move at different speeds in the same direction. We are studying muscle activation patterns during an asymmetric gait, when the treadmill belts move at equal speed in opposite directions, i.e., bidirectional walking (BDW). MethodsTwelve healthy volunteers performed a single session on a split-belt treadmill. We simultaneously collected ground reaction forces via treadmill force plates, joint kinematics via motion capture, and surface electromyography (EMG) from bilateral soleus (SOL) and tibialis anterior (TA) muscles. Participants started with 2 min of forward walking (FW), followed with four 5-min blocks of BDW separated by 1-min standing rest intervals, and finished the session with 2 min of FW (washout). ResultsAll participants successfully completed the protocol. We analyzed EMG signals for temporal activation patterns (rhythm generation) and amplitude characteristics (pattern formation). EMG recordings revealed antiphasic activation of SOL and TA muscles bilaterally throughout all trials. During BDW, the backward-moving legs TA showed prolonged activation patterns that persisted during washout FW, suggesting retention of adaptive changes. Burst-to-cycle duration ratios showed transient changes during early adaptation but remained relatively stable across conditions, demonstrating robust rhythm generation despite adaptive modulation of activation patterns during BDW. DiscussionThese findings demonstrate that BDW induces asymmetric adjustments in muscle activation patterns. Rhythm generation (timing) did not significantly differ between BDW and FW. However, we did observe changes in pattern formation (i.e., EMG profiles) during FW pre- and post-BDW training. Burst-to-cycle duration ratios, as a measure of rhythm generation, showed changes during early adaptation, particularly the increase in right SOL and right TA during block 1, though these changes did not reach statistical significance and largely returned to baseline during washout. The underlying pattern formation structure, was maintained across all conditions, with selective amplitude modulations rather than fundamental reorganization of activation patterns. The substantial temporal adjustments in the backward-moving legs SOL and phase shifts in TA provide the neuromuscular mechanism driving the bilateral step-length reduction, altered inter-limb phasing, and asymmetric double stance timing. These results extend our understanding of locomotor control by suggesting how the central nervous system (CNS) dynamically recalibrates muscle timing and amplitude to maintain satisfactory locomotion under new environmental demands.

This study investigated the spinal neural mechanisms underlying post-activation potentiation in ten healthy young males (21.9 {+/-} 4.8 years). Participants performed a 10-second maximal isometric plantarflexion, after which we measured twitch torque and assessed spinal excitability using the soleus H-reflex, D1 presynaptic inhibition and heteronymous Ia facilitation (HF). High-density surface EMG was decomposed to track single motor unit responses. The conditioning contraction increased twitch torque by 12.2 Nm (p < 0.001) immediately and returning to baseline within nine minutes. This mechanical potentiation was accompanied by a 29% reduction in H-reflex amplitude (p < 0.001), which recovered within three minutes. Paradoxically, neurophysiological indices of presynaptic inhibition, D1 and HF were significantly increased (D1: p<0.017; HF: p<0.001), resulting in spinal facilitation. Single MU analysis revealed increased discharge probability, particularly in higher-threshold units indicating overall spinal facilitation. These results demonstrate that post-activation potentiation involves a complex dissociation: H-reflex pathway inhibition along with facilitation of presynaptic spinal mechanisms. This paradox can be explained by either post-activation depression (caused by depletion of neurotransmitter at the Ia-motoneuron synapse) or muscle thixotropy, a contraction history-dependent decrease in muscle spindle sensitivity, which reduces the efficacy of the Ia afferent volley independently of spinal inhibitory mechanisms. Our findings highlight a dissociation between spinal presynaptic facilitation and the decreased H-reflex, underscoring the need for future studies to explicitly test the roles of post-activation depression and muscle thixotropy during post-activation potentiation. New & NoteworthyThis study provides evidence that post-activation potentiation reduces the soleus H-reflex amplitude while concurrently facilitating presynaptic spinal mechanisms. By combining global EMG and single motor unit analyses extracted from high-density surface EMG, we reveal a dissociation between spinal disinhibition and reflex depression. These findings suggest that acute post-contraction reflex suppression might be mediated by mechanisms other than presynaptic inhibition, potentially involving post-activation depression spinal mechanisms or changes in muscle spindle sensitivity.

Force production relies on the interaction between neural control of spinal motoneurons and the mechanical properties of the muscles. Changes in muscle length provide a useful model for exploring this interaction; however, studies typically assess these properties in the same muscle undergoing length change. This study investigated how altering the length of one muscle influences motor unit discharge behavior of its synergists. Eighteen healthy participants performed submaximal isometric knee extensions with the hip joint positioned at 90{degrees} (shortened rectus femoris, RF) and 180{degrees} (lengthened RF). At each hip position, participants followed trapezoidal force profiles at 10% and 30% of maximal voluntary contraction (MVC), while high-density surface electromyography (HDsEMG) was recorded from the synergistic vastus medialis and vastus lateralis (VL). Motor unit spike trains were decomposed from HDsEMG, tracked across hip positions, and analyzed for mean discharge rate and coefficient of variation of interspike interval (CoV-ISI). Lengthening the RF led to increased discharge rates of vasti motor units at 10% MVC, but not 30% MVC, with no changes in CoV-ISI. To further explore these force-dependent changes in discharge rate, two sets of experiments were conducted. The first showed that the discharge rate at recruitment during ramp-up contractions increased with RF lengthening, but only for vasti units recruited below 20% MVC. In the second, electrically evoked twitch contractions in the vasti revealed reduced twitches at 180{degrees} during low-frequency, but not high-frequency stimulation. These findings collectively suggest that the force-dependent changes in the vasti motor unit discharge rates are likely driven by RF-length dependent changes in the vasti muscles contractile properties.

Motivational music has been shown to improve running performance through delaying fatigue and increasing run duration. Previous studies have highlighted the effect of music tempo, that matching tempo to the runners cadence delays running fatigue. It remains unclear whether the motivational content in music lyrics is also responsible for delaying running fatigue. We designed a cross-sectional study and investigated the effect of tempo and motivational content on running biomechanics, and had participants run at a moderate intensity for up to ten minutes, or until exhaustion. Fifteen adults (age=20.9{+/-}1.3 years, weight=71.2{+/-}12.1 kg, height=174.7{+/-}11.0 cm) participated. Participants finished three trials, starting with running without any stimulus as a baseline trial, and ran with a visual metronome that flashed at a rate that matched their running cadence in the visual stimulus trial (VST). In the visual-auditory stimulus trial (VAST), participants ran with the visual metronome (as described in VST) while listening to a non-rhythmic motivational speech. We recorded run duration, perceived exertion, center of pressure sway during standing before and after each trial, and measured trunk acceleration to obtain root-mean-square (RMS) of acceleration during each minute of the run. Compared to baseline, participants reduced perceived exertion by 0.87 and 0.85 rating during the VST and VAST, respectively, though these changes did not reach significance (p=0.05). Stimulus affected the RMS of acceleration in anterior-posterior (p=0.011), vertical (p=0.008), and resultant directions (p=0.006). Our linear mixed effect model suggested that compared VST, VAST further lowered RMS of acceleration by 0.026g (anterior-posterior), 0.028g (vertical), and 0.036g (resultant). Our results showed that motivational content played an important role in lowering RMS of trunk acceleration, with the potential to delay running-induced fatigue. To maximize the effect of music on running performance, runners should listen to music that they find motivational and that is close to their natural running cadence.

Sleep plays a key role in both physical recovery and welfare. However, sleep patterns remain poorly documented in animals, particularly in athletic horses. This study aimed to provide a detailed description of sleep quantity and quality in training Thoroughbred racehorses and to investigate their relationships with age, abnormal behaviours, and cortisol. Thirteen Thoroughbreds (2-7 years old) were continuously monitored in their home environment over three consecutive days. An ethogram was used to quantify the two main phases of sleep: Non Rapid Eye Movement sleep (NREM) and Rapid Eye Movement sleep (REM), as well as sleep interruptions (from day 1 at 12:00 a.m. to day 3 at 12:00 a.m.). Sleep Quality Indices (SQI), defined as the quantity of sleep divided by the number of sleep interruptions (SI), were calculated. Behavioural observations of four indicators of poor welfare (alertness, stereotypies, inactivity, aggressiveness towards humans) were performed using scan sampling, and salivary cortisol was measured each morning. Linear models were used to assess the links between sleep quantity and quality, age, mean cortisol, and abnormal behaviours. Sleep quantity was significantly associated with age: positively for total NREM sleep (ANOVA: {chi}{superscript 2} = 5.26, p < 0.05) and, negatively for total REM sleep (ANOVA: {chi}{superscript 2} = 4.46, p <0.05) and total recumbency duration (ANOVA: {chi}{superscript 2} = 5.68, p < 0.05), suggesting an age-related shift favouring NREM over REM. Morning cortisol concentrations and the frequency of abnormal behaviours were significantly higher in horses with lower sleep quality (cortisol: Total SQI, ANOVA: F = 5.26, p < 0.05; Combined SQI, ANOVA: F = 5.40, p < 0.05; abnormal behaviours: Total SQI, ANOVA: F = 4.07, p = 0.074), pointing to a potential link between stress or altered welfare and poorer sleep quality. These findings suggest that, whilst the type and duration of equine sleep may be mainly affected byage, sleep quality is associated with both cortisol levels and the expression of abnormal behaviours, indicating that poor sleep quality may be linked to poor welfare in this population of horses. Thus, sleep appears to be closely linked with racehorse welfare, highlighting the need for further investigation into how it is influenced by factors such as husbandry, training load, recovery, and performance.

This study investigates closed kinematic chain biomechanics in cycling with a focus on knee joint loading. Data from 16 cyclists collected on a standardized ergometer were analyzed in OpenSim using inverse dynamics, static optimization, and joint reaction analysis. To keep the pipeline consistent across all subjects, the report summarizes right-knee outputs over a steady-state interval between 120 and 124 s. Peak knee joint moments ranged from 15.79 to 44.85 Nm (mean 30.49 {+/-} 7.66 Nm), while peak resultant knee reaction forces ranged from 1187.61 to 3309.04 N (mean 2317.19 {+/-} 728.19 N). Static optimization showed strong contributions from the rectus femoris and vastus lateralis during power production, with additional stabilization from the biceps femoris long head and gastrocnemius medialis. Mean peak muscle activation was highest for the rectus femoris (0.72 {+/-} 0.19), followed by the biceps femoris long head (0.66 {+/-} 0.20). Mean peak muscle force was highest for the vastus lateralis (1078.1 {+/-} 305.8 N) and rectus femoris (994.1 {+/-} 379.2 N). The results confirm substantial inter-subject variability in knee loading and support the use of personalized training or rehabilitation strategies when cycling is used for performance development or joint recovery.

Motor unit (MU) firing is affected by motoneuronal persistent inward currents (PICs), which heavily contribute to gain control of motor output. PICs are highly sensitive to inhibition; for instance, Ia reciprocal inhibition via antagonist muscle vibration drastically reduces discharge rate hysteresis ({Delta}F), an estimate of PIC magnitude. A direct link between sensitivity of PICs to inhibition and voluntary force control, however, has not been established. To determine whether force control is altered with inhibition of PICs, we recorded high-density surface EMG from the tibialis anterior, while 11 participants (5F; 6M) completed and isometric force reproduction task. Tendon vibration was applied to the agonist or antagonist muscle during the first (with visual feedback) or second contraction (without visual feedback) and participants were asked to match percieved effort across contractions, in an attempt to match neural drive to the motor pool. In support of our hypothesis, torque and MU firing rates were reduced when vibration was applied to the antagonist (torque: p < .0001; MU firing rate: p < .0001), but not agonist (torque: p = .9980; MU firing rate: p = .312) muscle tendon in the second contraction, compared to control. Conversely, when vibration was applied during the first contraction, opposite effects were observed. These results suggest that PICs play a role in the proprioceptive sense of force, offering a potential link between PICs and voluntary force control, which may be important for understanding and treatment of motor impairments. KEY POINTSO_LIMotoneuronal persistent inward currents amplify synaptic currents and therefore heavily contribute to motor output, however they are extremely sensitive to Ia reciprocal inhibition induced by muscle tendon vibration. C_LIO_LIWe show that modulation of PICs severely impacts human force sense using an effort-based force reproduction paradigm which enabled us to manipulate combinations of tendon vibration and visual feedback. C_LIO_LIThese findings provide a link between PICs and functional motor output, which may be important for understanding neurological impairments and informing rehabilitation strategies. C_LI

Intervertebral disc (IVD) injury is a major cause of low-back pain and can lead to structural deficits and mechanical instability. When the IVD is compromised, neuromuscular compensation by paraspinal muscles, such as the multifidus (MF) and longissimus (ML), is critical for maintaining spine stability. However, it is unknown how IVD injury and its interaction with nociception affect neuromuscular control. This study assessed the effects of IVD injury and additional muscle-derived nociception on trunk motor control during locomotion in a rat model. IVD injury was induced via needle puncture at L4/L5. One week later, hypertonic saline was injected into the lumbar MF to induce nociception. Trunk and pelvic kinematics, bilateral EMG activity of MF and ML were recorded during treadmill locomotion at baseline, one week after IVD injury, and immediately following hypertonic saline injection. Trunk and pelvic kinematics and bilateral muscle activation patterns remained largely consistent across conditions. No significant changes were found in stride duration, pelvic, lumbar and spine angle changes, variability, or movement asymmetry. MF activation was bilaterally synchronized, whereas ML showed left-right alternating activation patterns. Following IVD injury, right MF mean activation and EMG variability increased significantly compared to baseline. When muscle-derived nociception was added in the unstable spine (IVD injury) condition, left MF minimum amplitude was significantly reduced, and instability-related increases in right MF mean activation and variability were attenuated, but not fully reversed. These findings suggest that IVD injury, alone or in combination with muscle-derived nociception, elicits localized neuromuscular adaptations without disrupting the global locomotor patterns.

Gait is a highly coordinated motor behavior that integrates musculoskeletal and neuromotor control. Analysis of gait parameters is widely used in murine studies as functional biomarkers of locomotor development and disease progression. However, there has yet to be a comprehensive characterization of how gait matures during the rapid growth preceding skeletal maturity, an age range often used in preclinical gait studies. We analyzed gait longitudinally in healthy C57BL/6J mice from 6 to 16 weeks of age, timed to the onsets of sexual and skeletal maturity, respectively. More than 30 gait parameters were quantified weekly and organized into functional groupings reflecting growth, stride, coordination, paw placement, propulsion, and parameter variability. Through a combination of univariate and multivariate analyses, we identified robust age- and sex-based differences across these functional groupings of individual gait parameters. Univariate analysis revealed that many age-associated parameters exhibit a ramp-to-plateau trajectory in the 6- to 16-week age range, with many individual gait parameters plateauing within 8-10 weeks of age. Through multivariate analysis, we identified significant age- and sex-effects on principal components that aligned to functional groupings of gait parameters. Together, these results demonstrate that functional groupings of gait parameters tend to plateau at different phases of skeletal maturation and are differentially impacted by sex, also highlighting the importance of interpreting both univariate and multivariate analysis in longitudinal and comprehensive gait analysis. Understanding these patterns of gait maturation can inform better murine gait study design and more nuanced data analysis that considers potential interactions with age and sex effects.

EMG-based muscle force predictions are often inaccurate following active muscle stretch or shortening because of residual force enhancement (rFE) or depression (rFD), respectively, which can alter the neural drive to a muscle. However, the extent of neural drive modulation due to rFE or rFD remains unknown, making it difficult to correct EMG-based force predictions. Therefore, seventeen participants performed dorsiflexion contractions at 20 and 40% of maximum voluntary torque (MVT) in three conditions: stretch-hold, shortening-hold, and fixed-end reference (REF) conditions. The ankle dorsiflexion torques and angles were matched using dynamometry to the REF condition over a 10-s steady state following a 1-s 25{degrees} stretch or shortening, during which we recorded and decomposed tibialis anterior individual motor unit action potentials from high-density surface EMG recordings to gain insights into neural drive. Normalized EMG amplitudes were 2% lower following stretch and 1 or 3% higher following shortening relative to REF at 20 versus 40% MVT (p[&le;].008), respectively. Discharge rates (DRs) from 19 matched motor units per person on average obtained via DEMUSE and MUedit were similar (p=.871). Following stretch and shortening, DRs were [~]1 Hz lower (p[&le;].004) and 0 (p=.966) to 1 Hz higher relative to REF (p=.003), respectively. More unique motor units were also detected following shortening versus REF and in REF versus following stretch. These findings indicate that to account for rFE or rFD, neural drive is respectively decreased or increased via reduced or additional motor unit recruitment and DR modulation, with a contraction-intensity specific discharge rate modulation following active shortening.

Background: A previous meta-analysis by Singh-Ospina et al. (2017) suggested that Gender affirming hormone treatment (GAHT) does not change transgender mens bone mineral density (BMD) at any clinically relevant site; emerging studies and advances in synthesis methods necessitate an updated evaluation. The primary aim was to update the bone measures of Singh-Ospina et al. (2017), with the secondary aim to expand measures to how GAHT affects musculoskeletal health. Methods: A systematic review with meta-analysis was conducted using studies published in English up to 31 July 2024, identified through three electronic databases (PubMed, Embase, SportDiscus), and final cross-referencing in summer 2025. Primary outcomes were longitudinal changes in femoral neck (FN), lumbar spine (LS), and total hip (TH) bone mineral density (BMD). Secondary outcomes included body composition and muscle strength. Standardised effect sizes (Hedges g) were pooled using the inverse heterogeneity (IVhet) model. Results: GAHT (4 years) was not associated with significant longitudinal changes in FN, LS, or TH BMD. In contrast, substantial anabolic effects were observed, including increases in BMI (g = 0.13), body mass (g = 0.18), fat-free mass (g = 0.59), and muscle strength (g = 0.86). Heterogeneity was high for muscle strength, FN and TH BMD, limiting confidence in pooled estimates. Conversely, changes in LS BMD, BMI, body mass and fat-free mass demonstrated low heterogeneity and greater consistency across studies. Conclusion: Masculinising GAHT does not negatively affect clinically relevant BMD sites while reliably increasing lean mass and muscle strength; however, the evidence base remains methodologically weak and highly variable, particularly for FN and TH. The need for continued clinical monitoring of bone health and muscle function, alongside high-quality longitudinal research incorporating advanced imaging modalities such as HR pQCT is emphasised. Strengthening the evidence base will be essential for clarifying long-term skeletal trajectories as transgender men age. PROSPERO registration: CRD42024573102

Plantar tissue adaptation during activity is thought to contribute to diabetic foot ulceration (DFU), yet most existing studies only measure compressive quasi-static properties. This pilot study developed an ultrasound-loadcell measurement tool, PlantarSense, and used an infrared thermometer to measure dynamic compressive and shear energy dissipation ratio (EDR) and temperature of plantar-tissue at the first metatarsal head (1stMTH) and calcaneus in people living with and without diabetes at baseline, post-walk, and post-recovery. People living with diabetes showed significantly greater post-walk temperature increases (11.0 % vs 6.9% in controls at calcaneus, p=0.03) and less complete thermal recovery than controls. Baseline compressive EDR at the 1stMTH was significantly higher in people living with diabetes (67.8% vs 56.0% in controls, p=0.04). EDR modulation was greater from shear loading (21.5%) than compression (5.4%) and post-walk induced reductions in EDR were present in all participants, but people living with diabetes showed a 20% lower recovery than controls. Impaired thermoregulation and tissue adaptation in people living with diabetes was demonstrated by plantar temperature and EDR differences in post-walk and post-recovery. Future work is needed to test more participants with a greater range of diabetes progression to quantify statistically significant plantar tissue differences to inform DFU risk management.

Mental fatigue is induced by prolonged engagement in cognitively demanding tasks and impairs endurance performance. The neuropsychophysiological mechanisms underlying this decreased performance remain unclear, with suggestion that mental fatigue may disrupt motor command and consequently muscle activation. We aimed to test this hypothesis in a repeated cross-over design study in which 18 participants completed two experimental sessions involving a time-to-exhaustion cycling test at 80% of peak power output. Each cycling task was preceded by 1h of a prolonged Stroop task (Stroop session) or a neutral control task (Control session). Perception of effort and surface electromyography from ten lower-limb muscles of the right leg were recorded at regular intervals during cycling. Mental fatigue was higher in the Stroop compared to the Control session (p = .002). Endurance cycling time was 111 {+/-} 160 s shorter in the Stroop than in the Control session (p = .009). No significant differences in electromyography parameters were observed between Stroop and Control sessions, for any muscle (p > .05). Perception of effort was higher in the Stroop session from the onset of the cycling task (p = .006), and the rate of increase in perception of effort was significantly higher in the Stroop than Control session (p = .031). Our findings do not support the hypothesis that mental fatigue alters motor control or increases central motor command, as no changes in muscle activation were detected. Conversely, our results reinforce the notion that prolonged cognitive engagement impairs endurance performance primarily through an increased perception of effort. Future research should consider combining surface electromyography with more sensitive neurophysiological techniques to investigate potential subtle changes in motor drive during dynamic, whole-body tasks under mental fatigue. Impact statementOur study confirms that mental fatigue induced by prolonged cognitive exertion impairs cycling endurance performance. By combining measurements of perceptual responses and multi-muscle surface EMG during the endurance task, we observed that the decreased endurance performance is related to an increased perceived effort in the presence of mental fatigue, not related to alterations in motor command.

ObjectivePhysical activity is a first-line therapeutic intervention for managing osteoarthritis-related pain and functional impairment. However, the growing literature questions the long-term relevance of exercise-induced improvements in patients, while pre-clinical research evidence base is limited by reliance on stressful, forced exercise paradigms which do not reflect voluntary engagement. Here, we aimed to investigate the effects of voluntary wheel running on the pain experience in mice with joint pain. DesignWe investigated the impact of free access to a running wheel on sensory, functional and affective outcomes following unilateral intra-articular injection of monoiodoacetate in single-housed male and female C57Bl/6J mice. ResultsMonoiodoacetate injection transiently reduced running activity in both sexes; however, females rapidly resumed and sustained high activity levels over a two-month period, while males showed a progressive decline in running distance. Active males and females showed improvements in the monoiodoacetate-induced hindpaw secondary mechanical hypersensitivity. Moreover, mechanical thresholds positively correlated with the distance ran after injury, suggesting a functional relationship between exercise and secondary pain relief. However, access to a wheel temporarily exacerbated several monoiodoacetate-induced gait impairments in both sexes. Finally, while there were no obvious effects of running on anxio-depressive-like behaviours or cognitive functioning, exercise significantly impacted stress-induced faecal output and phenotypic regulation of body weight. ConclusionsOur findings suggest that persistent loading of an injured knee joint may compromise functional outcomes independently of pain relief away from the joint, underscoring a critical consideration for exercise-based therapeutic strategies in osteoarthritis.

ABSTRACTMotion sickness (MS) is commonly hypothesized to arise from sensory conflicts between incongruent sources of sensory information. Different types of sensory conflicts can induce MS, yet it remains unclear whether distinct contexts produce different physiological responses. Moreover, there is a lack of reliable objective predictors of MS, particularly for space motion sickness (SMS), which appears unrelated to motion sickness susceptibility on Earth. This study examined multiple physiological measures as potential objective markers of MS, including heart rate, blood pressure, salivary cortisol, skin conductance, skin surface temperature, and facial skin colorimetry. Subjective motion sickness severity and symptomatology were assessed using standardized questionnaires (SSQ, MSAQ, MSSQ). All measures were collected before and immediately after exposure to two sensory conflict paradigms: virtual reality (visuo-vestibular conflict) and parabolic flight (otolitho-canal conflict). Post-exposure, both paradigms were associated with increased cortisol, skin conductance, and skin greeness. Notably, increased skin greenness was associated with greater MS severity in parabolic flight and strongly correlated with subjective nausea ratings in both paradigms. Skin temperature and systolic blood were affected differently by VR and parabolic flight. No robust new physiological predictors of MS were identified. Overall, our findings suggest that facial skin color -particularly skin greenness- may serve as a simple, non-invasive, and reliable objective indicator of MS severity.

Persistent pain is a common but poorly understood outcome of traumatic burn injury. With increasing numbers of patients surviving their burn injuries, ongoing pain presents a growing complication to patient healing and quality of life. Despite more women reporting chronic pain post-burn than men, preclinical burn research rarely includes female animals. To address this gap, this study examined a diverse set of behavioral outcomes in male and female rats after a unilateral full-thickness burn to the hind paw. Utilizing traditional methods to assess evoked pain behaviors, new technology to assess gait abnormalities, and established techniques to evaluate comorbid anxiety-like behavior, we determined that male and female rats have divergent pain-related behaviors post-burn. Both sexes experienced mechanical allodynia after burn injury, but only males experienced thermal hyperalgesia. In contrast, female rats were acutely resistant to noxious thermal stimulation. While both sexes demonstrated gait abnormalities post-burn when freely ambulating, female rats exhibited a wider range of abnormal gait features, which were more severe and longer-lasting than those in males. However, despite both sexes demonstrating symptoms of persistent pain, only males displayed anxiety-like behavior on the Elevated Zero Maze. In conclusion, our study found that male and female Sprague Dawley rats displayed divergent, sex-specific evoked pain responses, gait dysfunction, and anxiety-like behavior after full-thickness burn injury. Future studies should examine the underlying mechanisms behind these behavioral sex differences. PerspectiveThis article takes a novel approach to pain behavior testing after full-thickness burn injury, capturing behaviors beyond traditional reflexive ("evoked") behaviors. The results of this article provide evidence that preclinical research must expand behavioral testing to capture the full animal pain experience and better model human patient outcomes.

Exercise provides broad health benefits, including improved emotional well-being and cognitive function. Emerging evidence suggests that exercising at different times during the day can have differential effects. However, how circadian phase and sex influence behavioral and physiological responses to exercise remains unclear. To address this question, we examined male and female wild-type mice maintained in either regular (REG, lights on/off at 7AM/7PM) or inverted (INV, lights off/on at 10AM/10PM) light cycles. Mice were then subjected to daily 20-min group swimming exercise sessions at ZT2-3 for 3 weeks. Exercised and sedentary controls mice were then subjected to an open field test (OFT) and blood corticosterone (CORT) measurements 24 hours post-exercise. We quantified several behaviors during swimming: escape attempts, floating, climbing and collisions. We also identified a novel swimming behavior: floating with only nostrils-above-water events (NAWEs). We found that expression of these behaviors was differentially modulated by sex, light-cycle and their interaction. Notably, behavioral differences were more pronounced in REG mice (rest phase). REG mice also lost weight after exercise and had elevated CORT levels compared to mice kept in INV conditions (active phase). Interestingly, OFT behaviors showed significant differences primarily in INV mice, particularly females, when comparing exercised vs sedentary groups. Our novel findings reveal that circadian rhythms and sex significantly interact to shape swimming exercise and stereotyped behaviors in mice. This emphasizes the need to consider the animals circadian phase when designing preclinical studies to match intended behavioral and physiological outcomes. HIGHLIGHTSCircadian phase and sex jointly shape swimming behavior patterns. Newly identified swimming behavior is more prevalent during rest-phase Restphase exercise produced stronger behavioral and physiological effects. Rest-phase exercise resulted in weight loss and elevated stress markers. Active-phase exercised females showed the strongest open field behavioral differences.

Cycling is commonly employed in sports performance, rehabilitation, and clinical contexts, while musculoskeletal (MSK) simulations enable the investigation of internal biomechanics that cannot be measured experimentally. Despite growing use, the application, validation, and standardisation of MSK simulations in cycling remain unclear. This review aimed to systematically characterise the application, validation strategies, modelling assumptions, and reporting practices of musculoskeletal simulations in lower-limb cycling biomechanics. Searches were performed in Scopus, PubMed, IEEE Xplore, and Web of Science on 1 August 2024, covering studies from January 2010 to July 2024. Peer-reviewed English-language journal articles applying MSK simulations to lower-limb cycling were included; inverse kinematics-only was excluded. No protocol was registered, and no formal risk-of-bias assessment was conducted, as there were no intervention effects and no quantitative synthesis. Twenty-eight studies met the inclusion criteria. Most of them investigated bicycle-rider configuration, neuromuscular coordination, or electrical stimulation control, with participant cohorts overwhelmingly composed of young men and minimal female representation (272 total). Model reporting was often incomplete, with wide variation in anatomical scope, inconsistent descriptions of degrees of freedom, and limited sharing of models or code. Use of experimental data was uneven across studies: while all incorporated kinematic measurements, only two-thirds included kinetic data, and only one study reported physiological measures. Model validation was generally based on literature values. Seventy-eight per cent of studies used optimisation, mainly with effort-based cost functions, and parameter variations were exploratory rather than systematic. The evidence base is limited by small, predominantly male cohorts, inconsistent reporting standards, and limited physiological validation. These results consolidate current practices and highlight the need for more transparent and open reporting, sex-balanced and clinically diverse participant representation, stronger validation, and more rigorous sensitivity analysis to enhance reproducibility and practical relevance. This review was funded by AGAUR (Spain), CAPES (Brazil) and FAP-DF (Brazil). Author summaryCycling is widely used in sports training, rehabilitation, and clinical practice, and musculoskeletal simulations are increasingly used to study how muscles and joints work during cycling. These simulations allow us to estimate internal biomechanical variables that cannot be directly measured in experiments, such as muscle forces and joint loading. However, it is currently unclear how consistently these simulations are applied, validated, and reported across the literature. In this study, we systematically reviewed research published over the past 15 years that used musculoskeletal simulations to analyse lower-limb cycling. We identified 28 relevant studies and examined their modelling choices, experimental inputs, optimisation strategies, and validation approaches. We found substantial variability in model complexity, limited transparency in reporting, and a strong reliance on simplified literature-based validation methods. Most studies focused on narrow participant groups and explored modelling parameters in an ad hoc rather than a systematic way. Our findings highlight important gaps in current practice and point to clear opportunities for improvement. We provide an overview of common approaches and their limitations, and outline key recommendations to enhance the transparency, reproducibility, and practical relevance of musculoskeletal simulations in cycling research.