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 yr) 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 in a randomized manner (e.g., E, R, E-R, and R-E), separated by 72 h. 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 with R. Although 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 after 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.NEW & NOTEWORTHY All exercise modalities reduced maximal isometric strength; however, endurance exercise led to greater decreases in the rate of torque development. Resistance exercise negatively impacted supraspinal central neural drive, whereas both endurance and concurrent exercise significantly impaired spinal motoneuron responsiveness. Endurance and concurrent exercise also significantly reduced twitch contractile properties and evoked octet responses, with the most pronounced impairments observed following endurance-only exercise.