Adaptive Modification in Agonist Common Drive After Combined Blood Flow Restriction and Neuromuscular Electrical Stimulation

Neuromuscular electrical stimulation (NMES) combined with blood flow restriction (BFR) has garnered attention in rehabilitation for its ability to enhance muscle strength, despite the potential to accelerate training-related fatigue. This study examined changes in force scaling capacity immediately following combined NMES and BFR, focusing on motor unit synergy between agonist pairs. Fifteen participants ( $23.3~\pm ~1.8$ years) trained with combined BFR and NMES on the extensor carpi radialis longus (ECRL) muscle, with maximal voluntary contraction (MVC) of wrist extension, along with force and EMG in the ECRL and extensor carpi radialis brevis (ECRB), measured during a designate force-tracking before and after training. Factor analysis identified latent modes influencing motor unit coordination between the ECRB and ECRL. The results showed a significant decrease in MVC after training ( $\text {p}\lt 0.001$ ). Post-test force fluctuations increased (p =0.031), along with a decrease in the mean inter-spike interval (M_ISI) in the ECRL (p =0.022). Factor analysis revealed an increase in the proportion of motor units (MUs) jointly regulated by the neural mode for both ECRB and ECRL, coupled with a decline in independently regulated MUs. Specifically, the proportion of MUs governed by the ECRL mode decreased, while those regulated by the ECRB mode increased. In conclusion, force generation capacity and force scaling are impaired after receiving combined NMES and BFR treatment. It involves redistribution of the common drive to MUs within two agonists, affecting the flexible coordination of muscle synergy and necessitating compensatory recruitment of MUs from the less fatigable agonist.