ECSS Paris 2023: OP-BM04
INTRODUCTION: The neuromechanical determinants of the rate of force development (RFD) have been widely investigated, each time within a different framework, often analyzing only a subset of the relevant neuromechanical variables. Studies have examined central and peripheral factors such as neural drive estimates, motor unit recruitment, muscle contractility and excitability, fascicle behaviour, muscle-belly gearing, and tendon stiffness. However, these factors have rarely been assessed in combination, limiting the ability to understand the factors influencing rapid force capacity comprehensively. In the present study, we aim to provide a wider perspective on the determinants of isometric RFD. METHODS: Right limb knee extensors of 32 healthy young adults (15 F) were tested in isometric conditions. The procedure included: evoked contractions (single and octets stimuli); 5s maximal isometric voluntary contractions to measure maximal voluntary force (MVF); 15 isometric burst-like contractions. HD-EMG from vastus lateralis, vastus medialis and rectus femoris were recorded throughout the session, while B-mode ultrasound data of the patellar tendon and vastus lateralis fascicle were recorded only during voluntary contractions. The root mean square (RMS) of HD-EMG of burst-like contractions was normalized to the peak-to-peak amplitude of the M-wave and then averaged. The patellar tendon stiffness was computed from 0-50% of MVF and 50-100% of MVF. Fascicle and belly shortening velocity and architectural gear ratio were derived from fascicle and belly length changes. Correlation and stepwise multiple linear regressions were run between each time-locked RFD metric and the other neuromuscular variables. RESULTS: RFD at 25 and 50 ms are mainly explained by mean normalized RMS (R2 0.68 and 0.83, respectively). RFD at 75 ms is explained by mean normalized RMS, tendon stiffness up to 50% of MVF and octet’s RFD at 50 ms (R2 =0.81). RFD at 100 ms is explained by octet’s RFD at 50 ms, tendon stiffness in the first 50% of MVF and inversely by muscle velocity (R2 =0.88). RFD at 125 and 150 ms are explained by octets RFD at 50 ms (R2 =0.78 and R2 =0.82, respectively). CONCLUSION: Overall, early RFD is primarily influenced by early muscle activation, whereas late RFD underscores the importance of peripheral force-generating mechanisms, i.e. muscle contractility. Interestingly, mid-phase RFD (RFD at 75 and 100 ms) appears to be shaped by a combination of muscle-tendon properties, muscle activation, and peripheral contractile characteristics.
Read CV Francesco SalvaggioECSS Paris 2023: OP-BM04
INTRODUCTION: Neuromuscular electrical stimulation (NMES) over a nerve or muscle belly can evoke involuntary contractions which allow for the assessment of intrinsic contractile properties. Repeated activation of the muscle via intermittent trains of stimuli can assess fatigability (i.e., force loss), the magnitude of which is influenced by NMES parameters such as stimulation frequency. For example, we determined recently that NMES at the frequency equivalent to the mean motor unit discharge rate (MUDR) expected during a voluntary contraction at 25% of maximal knee extensor force (10Hz) induces less force loss than frequencies that exceed the mean MUDR (i.e.15 and 30Hz). Despite innate MUDR variability between people, it is currently unknown if a stimulation frequency equal to an individual’s mean MUDR, which should optimally match NMES to intrinsic contractile properties, will mitigate fatigability to a greater degree than utilizing a standardized value based on population data (i.e., 10Hz). Thus, the aim of this study was to compare force loss during NMES at 10Hz vs. the frequency tailored to each participant’s mean MUDR. METHODS: Nine healthy, young adults (26+/−3y; 6 females) completed two testing sessions separated by at least 72h, identical in procedure except for stimulation frequency applied to the muscle bellies of the quadriceps. To assess strength and voluntary activation (VA), participants performed brief isometric maximal voluntary contractions (MVCs) of the knee extensors, with a single supramaximal stimulus delivered during and after each MVC. Individual MUDRs were collected using two concentric needle electrodes inserted into the rectus femoris, vastus lateralis, or vastus medialis during repeated 15-s contractions at 25% MVC force, separated by ≥90s. For each participant, all MUDRs (≥24 per muscle) were averaged to determine the individualized NMES frequency. For the 3-min fatiguing NMES protocols, current was set to initially evoke 25% MVC force. The protocols consisted of 144 contractions, each separated by 1.25s (~0.6s on and 0.65s off), with the order of frequencies counterbalanced across participants. RESULTS: Baseline MVC force (538.6+/−189.3N) and VA (95.9+/−1.9%) were not different between sessions (p=0.231). The mean MUDR at 25% MVC force was 11.6±2.2Hz (ranging from 8.5-14.5Hz). At the end of the 3-min protocols, force loss was not different between 10Hz (26.6+/−20.6%) and individualized NMES frequencies (29.8+/−19.0%; p=0.479). CONCLUSION: Despite matching participant knee extensor mean MUDR, NMES at a tailored frequency did not mitigate force loss compared to the standardized 10-Hz session. As the acquisition of MUDRs requires additional equipment, expertise, and time, and there was no functional benefit to a tailored NMES frequency, a standardized NMES frequency of 10Hz appears most appropriate for future studies involving intermittent stimulation of the quadriceps of young, healthy adults.
Read CV Alexander PaishECSS Paris 2023: OP-BM04
INTRODUCTION: During a sustained (e.g., 60s) isometric maximal voluntary contraction (MVC), both torque and human motor unit (MU) firing rates decline by ~50% (1,2). The mechanism(s) for these rate adaptations during muscle fatigue (i.e., torque loss) are unclear, however, modified afferent feedback and alterations in descending drive have been implicated (1,3). The purpose was to assess whether voluntary descending drive is obligatory to reduce maximal MU firing rates with fatigue. To test this, we compared maximal MU firing rates following a sustained 60s MVC and separately following 60s of tetanic peripheral nerve stimulation at similar high torque levels. If changes in descending drive contribute to reductions in maximal firing rates with similar torque losses in both conditions then with electrically induced contractile failure maximal firing rates should not be affected, or to a much lesser degree. METHODS: Participants (7 males and 3 females, 23.7 ± 3 years) were seated upright on a chair with their left leg placed in a custom-made dynamometer to record isometric dorsiflexor torque. A bar electrode was placed over the common fibular nerve and a supramaximal current was used to induce electrically evoked contractions. To record single motor unit (MU) activity two tungsten microelectrodes were inserted into the tibialis anterior by two independent operators. For baseline measures, three 5s dorsiflexion MVCs, separated by 5-min rest, were performed. On separate days, participants completed a 60s sustained MVC or 60s peripheral nerve stimulation. To mimic the firing rate decline observed during MVCs, stimulation frequency was reduced from 40Hz to 20Hz over the 60s (2). Immediately (~2s), 1.5, 3, 5 and 10min post-task, a 5s MVC was performed, and MU activity was recorded. The protocols were repeated twice on separate days to increase MU yield. Two-way repeated measures ANOVAs and mixed linear models were performed. RESULTS: Baseline MVC torque was 40.1±10.8 Nm and tetanic nerve stimulation induced ~80% MVC. Immediately (~2s) after both tasks MVC was similarly reduced (~30%) relative to baseline (P=0.2). Remaining post-task MVCs recovered similarly and by 10min returned to baseline (P=0.3, ~96%). A total of 1531 MUs were analyzed. Baseline maximal MU firing rates of the tibialis anterior were 40.1±11.5 Hz. Firing rates immediately declined by ~30% post-tasks (P<0.001) but recovered similarly (~95%) by 10min with no task differences at any time point (P>0.1). CONCLUSION: The similar reductions and recovery in firing rates in both fatiguing tasks indicate that during a high-intensity 60s task voluntary drive is not a contributor to reductions in firing rates. These findings provide enhanced evidence that peripheral factors likely originating from the fatigued muscle are responsible for firing rate reductions during voluntary efforts. 1) Bigland-Ritchie et al., J Physiol, 1986 2) Zero et al., J Physiol, 2023 3) Gandevia, Physiol Rev, 2001
Read CV Alexander ZeroECSS Paris 2023: OP-BM04