ECSS Paris 2023: OP-BM21
INTRODUCTION: Fatigue impairs explosive force production and compromises performance in many sport-specific actions. It is commonly defined as a time-related reduction in voluntary force-producing capacity arising from peripheral and/or central alterations. At the cortical level, voluntary movements are preceded by the Movement-Related Cortical Potential (MRCP), a slow negative potential detectable via electroencephalography (EEG) and known to be sensitive to fatigue. However, the contribution of electrocortical activity to fatigue-induced reductions in rate of torque development (RTD) remains unclear. The present study investigated cortical and peripheral changes induced by repeated rapid isometric contractions. METHODS: Nineteen healthy volunteers (10 females, 9 males) performed 120 rapid isometric knee extensor contractions while 64-channel EEG recorded cortical activity and high-density surface electromyography (HD-sEMG) recorded muscle activation of the vastus lateralis and medialis. Torque was quantified in 25 ms epochs (from 25 ms to 150 ms), together with root mean square (RMS) and conduction velocity (CV) derived from HD-sEMG, as well as MRCP amplitude for each contraction. Before and after the fatiguing task, maximal voluntary torque (MVT) was recorded, and femoral nerve electrical stimulations (M-wave and octet) were delivered to evaluate peripheral contractile function. The best 20 of the first 30 and last 30 contractions were averaged to obtain PRE and POST measurements. RESULTS: MRCP amplitude was significantly reduced at POST (p = 0.0002, n2p = 0.57). Similarly, voluntary torque and RMS (normalized to MVT and M-wave amplitude, respectively) were significantly reduced from 100 ms to 150 ms (p = 0.007, n2p = 0.08; p = 0.0179, n2p = 0.04). M-wave-normalized CV remained stable over time. Octet-evoked torque, normalized to MVT, showed an early increase within the first 25 ms (P = 0.0001, d = 1.561) and subsequently (50, 75ms) returned to pre-fatigue levels (p = 0.0537; p = 0.4166). Neural efficacy (i.e., the ratio between voluntary and evoked torque) significantly decreased at POST (p < 0.0001, n2p = 0.23). CONCLUSION: Repeated explosive contractions induced an impairment in RTD. However, the preservation of CV and evoked responses suggests that peripheral muscle properties did not account for this impairment. The reduction in neural efficacy and MRCP amplitude instead points toward diminished net neural drive and altered cortical motor preparation as primary contributors to the decline in explosive force production.
Read CV Tommaso BerbenniECSS Paris 2023: OP-BM21
INTRODUCTION: Motor imagery (MI) and neuromuscular electrical stimulation (NMES) were shown to limit the decrease of muscle strength during the early stages of limb immobilization (1,2). Recently, a study revealed that corticospinal excitability was enhanced when MI and NMES were combined (MI+NMES), compared with either modality alone (3). These findings raised a pertinent issue regarding the potential impact of MI+NMES during a short-term immobilization. Therefore, the purpose of this study was to evaluate the effect of MI+NMES during a 48-hour arm immobilization on the neuromuscular system. We hypothesized that MI+NMES would attenuate the immobilization-induced strength loss and associated alterations in corticospinal excitability. METHODS: Ten healthy participants (age: 25 ± 5 years) completed three experimental conditions, each lasting 48h, performed in a randomized crossover design: control (CON), immobilization (IMMO) and immobilization with intervention (INTER). During IMMO and INTER, participants had their dominant arm immobilized with splints. Additionally, during INTER, participants performed one session of 10 minutes of MI+NMES targeting wrist flexion every two hours. NMES intensity was set at 20% of maximal voluntary contraction (MVC). For each experimental condition, maximal force (MVC) and corticospinal excitability of flexor carpi radialis (FCR) were measured before (PRE), at 24h (POST24) and immediately after each condition (POST48). Corticospinal excitability was assessed at rest by evoking motor evoked potentials (MEP) using transcranial magnetic stimulation applied over contralateral motor region of flexor carpi radialis muscle. MEPs were normalized to the maximal M-wave (Mmax) evoked by median nerve stimulation. Each parameter was recorded on both immobilized and contralateral arm. RESULTS: On the immobilized limb, MVC significantly decreased by 15% from PRE to POST24 and from PRE to POST48 (P=0.025 and P=0.008, respectively) only in the IMMO condition. FCR MEP/Mmax significantly decreased from PRE to POST24 (P=0.017) and from PRE to POST48 (P=0.015), only in the IMMO condition. Peripheral measures (Mmax and resting twitches) did not show any significant changes across all conditions. The contralateral limb did not show any significant changes. CONCLUSION: These results showed that a short immobilization period (48h) can result in decreased maximal force, accompanied by a decrease in corticospinal excitability targeted to the immobilized limb, while peripheral markers were preserved. This highlights that a decline in motor function, of central origin, already arises at early phases of immobilization. The lack of decline observed in INTER condition shows that NMES+MI combination is a good candidate to mitigate the early decline in nervous system excitability. REFERENCES : 1. Campbell et al. (2019), Sports Medicine, 49, 981-986 2. Debarnot et al. (2021), Sci Rep, 11:8928 3. Eon et al. (2025), Eur J Appl Physiol, 125(2):561-572
Read CV Sidney GrospretreECSS Paris 2023: OP-BM21
INTRODUCTION: Ia afferent feedback contributes to the stretch reflex during stretch–shortening cycle (SSC) exercise [1]. However, its contribution to performance in specific SSC tasks remains debated [2,3]. One approach to address this question is to evoke additional Ia input via H-reflex stimulation and examine its influence on performance, especially before the natural short-latency reflex response can modulate stiffness. We therefore tested whether H-reflexes during pre-activity or early ground contact (GC) affect performance during hopping. METHODS: Ten healthy adults (2 female, 31 ± 8 years) completed a within-subject repeated-measures study. Participants performed 20 sets of 40 submaximal hops (60 ± 15% of max. hop height) with 2-min rest between sets. For each hop, we randomly applied tibial nerve stimulation (-100 to +30 ms relative to GC) or no stimulation and adjusted stimulation intensity to maximize the Soleus H-reflex while restricting M-wave amplitude to <10% of Mmax. We quantified performance using the Reactive Strength Index (RSI) and analyzed it with a generalized additive mixed model including a smooth interaction of normalized H-reflex amplitude (H/Mmax) and timing, with subject as a random effect and previous flight time and Soleus pre-activity as covariates. Further, we identified connected regions where the modeled stimulation effect differed from zero using 95% simultaneous confidence bands. RESULTS: After quality control, we analyzed 4000 hops from nine participants. One participant was excluded because all stimulated trials exceeded the predefined M-wave threshold. A significant H-reflex amplitude x timing interaction predicted RSI (p < .001) explaining 0.09% of additional variance beyond subject effects and covariates. Within this effect, the model identified: A facilitative region at ~13% of H/Mmax and ~30% of GC (~61 ms after GC), with a median dRSI of +4.2% (range: +3.1% to +6.1%), and A suppressive region at ~18% of H/Mmax and ~3% of GC (~7 ms after GC), with a median dRSI of -2.0% (range: -2.4% to -1.6%). The facilitative region was only covered by data of one participant and should be interpreted cautiously. CONCLUSION: Overall, externally evoked Ia input during pre-activity or early GC-phase had minimal influence on hopping performance. This suggests that a single H-reflex contribution during hopping is of minor functional relevance or that Soleus activity during early SSC phases may already be optimally aligned with task mechanics, leaving little capacity for performance-relevant integration of externally evoked Ia input. Misaligned Ia input may instead slightly disrupt motor output, indicated by the early suppressive region. In the participant showing facilitation, H-reflex timing coincided with the stretch reflex response, suggesting potential temporal summation of spinal reflex activity. [1] Zuur et al., 2010, J Physiol, 588(Pt 5):799-807. [2] Rissmann et al, 2025, J Physiol, 603(14):3987-4004. [3] Taube et al., 2025, J Physiol, 603(22):7397-7398.
Read CV Justin HowaldtECSS Paris 2023: OP-BM21