Presynaptic inhibition of soleus Ia afferents does not vary with center of pressure displacements during upright standing

- To investigate Ia presynaptic modulation in soleus with center of pressure (CoP) during upright standing.
- Ia presynaptic inhibition is not modulated depending on CoP displacements.
- Excitability of the motor neurons changes in response to CoP displacements.
- Reciprocal inhibition may be involved in changes in motor neuron excitability with CoP displacements.

The present work was designed to investigate the presynaptic modulation of soleus Ia afferents with the position and the direction of the displacement of the center of pressure (CoP) during unperturbed upright standing and exaggerated CoP displacements in young adults. Hoffmann (H) reflex was evoked in the soleus by stimulating the tibial nerve at the knee level. Modulation of Ia presynaptic inhibition was assessed by conditioning the H reflex with fibular nerve (D1 inhibition) and femoral nerve (heteronymous facilitation) stimulation. Leg muscle activity was assessed by electromyography (EMG). The results indicate that in unperturbed standing and exaggerated CoP displacements, the H-reflex amplitude was greater during forward than backward CoP direction (p < 0.05). However, the amplitude of the conditioned H reflex (expressed relative to unconditioned H reflex) did not vary with CoP displacement, regardless of the experimental condition. The soleus EMG was greater during forward than backward CoP direction and during anterior than posterior position in both experimental conditions (p < 0.05). The modulation of the unconditioned H reflex with CoP direction was positively associated with the corresponding changes in soleus EMG (r2 > 0.34). The tibialis anterior EMG did not change during unperturbed standing, but was greater for backward than forward CoP direction during exaggerated CoP displacements. In this experimental condition, soleus EMG was negatively associated with tibialis anterior EMG (r2 = 0.81). These results indicate that Ia presynaptic inhibition is not modulated with CoP direction and position, but rather suggest that CoP displacements induced changes in excitability of the soleus motor neuron pool.