ASSESSMENT OF AVERAGE MUSCLE FIBRE CONDUCTION VELOCITY AND INTEGRATED MYOELECTRIC ACTIVITY DURING HEAVY- AND SEVERE-INTENSITY CYCLING EXERCISE

Introduction: A fundamental tenet of human exercise physiology is the predictable reduction in the capacity of a muscle or muscle group to generate force or power during exercise. This phenomenon of ‘muscle fatigue’ has been extensively studied using surface electromyography (sEMG) during dynamic exercise. In most cases, the assessment of neuromuscular activity was confined to the analysis of sEMG amplitude as an indicator of integrated muscle activation (iEMG). Recent advances in EMG recording techniques, however, have enabled the exploration of a broader range of neuromuscular function parameters such as muscle fibre conduction velocity (MFCV). MFCV, however, has been exclusively characterised during dynamic, incremental exercise. We, therefore, investigated the temporal pattern of iEMG and MFCV during constant-workload exercise performed within the heavy- and severe-intensity domains. Methods: Ten healthy, moderately trained adults participated in this study. After completing a ramp-incremental exercise test on a cycleergometer to determine gas exchange threshold (GET) and peak power output (POpeak), they returned to the laboratory to complete, in a randomised order, either a 30-min constant-load exercise bout in the heavy-intensity domain (15%D between GET and POpeak) or a time-to-exhaustion test in the severe-intensity domain (75%D). HDsEMG signals from the vastus lateralis muscle were continuously recorded throughout each exercise session. MFCV was estimated using the cross-correlation method, and iEMG was computed. Both MFCV and iEMG were normalised to the initial 20 s of activity, which was defined as the baseline (100%). Results: Linear mixed-effects model revealed that, compared to baseline, MFCV increased over time during heavy-intensity domain (P<0.01), while it decreased during severe-intensity domain (P<0.01). A significant main effect of exercise domains was observed (P<0.01), along with a significant time 9 condition interaction (P<0.01). In contrast, iEMG responses significantly increased over time in the severe-intensity domain (P<0.01), but not during the heavy-intensity domain (P = 0.28), with no significant time 9 condition interaction (P = 0.07). Conclusion: Our findings indicate that MFCV and iEMG exhibit distinct temporal patterns during constant-workload exercise in the heavy- and severe-intensity domains. Further research exploring neuromuscular function during dynamic exercise should consider the intensity-dependent characteristics of these neuromuscular parameters.