Increased H-reflex excitability is not accompanied by changes in neural drive following 24 days of unilateral lower limb suspension.

The purpose of this study was to determine whether the gain in soleus H-reflex excitability induced by unilateral lower limb suspension (ULLS) is associated with changes in neural drive to the plantar flexor muscles. Six male subjects (23 ± 2 years, 187 ± 7 cm, 79 ± 9 kg) underwent 24 days of ULLS of the dominant limb. Plantar flexor maximal voluntary contraction (MVC) torque, activation capacity (twitch interpolation), soleus maximal electromyographic (EMG) activity, Hoffman (H)-reflex, and the first volitional (V) wave normalized to the compound muscle action potential (M-wave) were quantified before and after ULLS. Following ULLS, MVC torque decreased by 15% (P < 0.05). However, neither activation capacity nor EMG activity was significantly altered after the suspension. The V-wave remained unchanged consistently after ULLS, whereas the H-reflex increased significantly (+20%). Furthermore, there was no significant relationship between changes in H-reflex and V-wave over the ULLS period. These findings indicate that 24 days of ULLS can result in a substantial reduction of muscle strength without any apparent change in voluntary activation capacity. H-reflex and V-wave findings suggest that the spinal adaptations that underlie the unloading-induced increase in resting soleus H-reflex excitability did not significantly affect the efferent motor output to the plantar flexor muscles.