Effect of muscle length on the modulation of H-reflex and inhibitory mechanisms of Ia afferent discharges during passive muscle lengthening

The effectiveness of activated Ia afferents to discharge a-motoneurons is decreased during passive muscle lengthening compared with static and shortening muscle conditions. Evidence suggests that these regulations are explained by 1 ) greater postactivation depression induced by homosynaptic postactivation depression (HPAD) and 2 ) primary afferent depolarization (PAD). It remains uncertain whether muscle length impacts the muscle lengthening-related aspect of regulation of the effectiveness of activated Ia afferents to discharge a-motoneurons, HPAD, PAD, and heteronymous Ia facilitation (HF). We conducted a study involving 15 healthy young individuals. We recorded conditioned or nonconditioned soleus Hoffmann (H) reflex fl ex with electromyography (EMG) to estimate the effectiveness of activated Ia afferents to discharge a-motoneurons, HPAD, PAD, and HF during passive shortening, static, and lengthening muscle conditions at short, intermediate, and long lengths. Our results show that the decrease of effectiveness of activated Ia afferents to discharge a-motoneurons and increase of postactivation depression during passive muscle lengthening occur at all muscle lengths. For PAD and HF, we found that longer muscle length increases the magnitude of regulation related to muscle lengthening. To conclude, our fi ndings support an inhibitory effect (resulting from increased postactivation depression) of muscle lengthening and longer muscle length on the effectiveness of activated Ia afferents to discharge a-motoneurons. The increase in postactivation depression associated with muscle lengthening can be attributed to the amplification fi cation of Ia afferents discharge.