Modulation of tendon tap reflex activation of soleus motor neurons with reduced stability tandem stance

Reduced standing stability typically decreases soleus Hoffmann (H-) reflex amplitude, purportedly to prevent the Ia signal from excessively activating spinal motor neurons. H-reflex measures exclude the variably sensitive spindle. Tendon tap (T-) reflex measures, including the spindle, could provide additional information, but are largely unexplored for unstable stance. Thirty adults stood in wide stable stance and unstable tandem stance. T-reflex amplitude decreased 11.6% when in the tandem stance (wide 0.248(0.124) mV, tandem 0.219(0.119) mV, p< 0.05, Cohen’s d = 0.24 small), with no relationship between standing sway and reflex modulation. No significant differences in background soleus and tibialis anterior EMG, and tap force occurred across the stances. Results indicate that during a less stable stance Ia excitation of motor neurons is decreased, thereby avoiding potential instability in the reflex loop or saturating the reflex pathway and possibly interfering with descending control of the involved spinal motor neurons.