Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units

Prolonged ([≥]60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined, however some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and derecruitment ({Delta}F) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5-min control period and immediately after five 1-min passive plantar flexor stretches. After stretching there was a significant reduction in SOL {Delta}F (-25.6%; 95%CI = -45.1 to -9.1 %, p=0.002) but not GM {Delta}F. These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously-established post-stretch decreases in SOL but not GM EMG amplitudes during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss. SUMMARY STATEMENTMotoneurons require an amplification mechanism to operate within the firing frequencies observed during normal motor behaviour. Here we present evidence that this amplification mechanism is reduced after passive muscle stretching.