Changes in H-reflex, V-wave and contractile properties of the plantar flexors following concurrent exercise sessions - the acute interference effect

The interaction between muscle strength and endurance impacts athletic performance. Integrating both modalities into concurrent exercise (CE) is challenging due to the interference effect. This study explored the acute effects of resistance-only (R), endurance-only (E) and CE sessions on voluntary muscle strength, evoked neurophysiological parameters and contractile properties of the plantar flexors. We also explored whether the sequence of CE (E-R vs. R-E) affects these parameters. Ten males (23.5±2.4 years) experienced in resistance and endurance training underwent neuromuscular baseline assessments, including plantarflexion maximal voluntary isometric contraction (MVIC) and soleus evoked responses (M-wave, H-wave, V-wave, evoked octet and twitch contractile properties). Then, participants completed four different exercise sessions on a randomized fashion (e.g. E, R, E-R and R-E), separated by 72 hours. Exercise sessions were immediately followed by the same assessments completed at baseline. MVIC and the rate of torque development (RTD) were reduced after all sessions. The E session induced a greater decrease in RTD compared to R. While the V-wave amplitude decreased after all sessions, the electromyographic activity of the soleus muscle remained unchanged during MVIC. The normalized amplitude of the H-reflex was reduced after E and both CE sessions. The gain of the H-reflex ascending limb (H_slope) exhibited a larger decrease after CE, irrespectively of exercise sequence. The twitch contractile properties were similarly impaired post-all sessions. The E session induced a larger reduction of the evoked octet response. These findings provide new insights into the neuromuscular etiology of the acute interference effect resulting from CE.