Journal
MUSCLE & NERVE
Volume 42, Issue 5, Pages 749-755Publisher
WILEY
DOI: 10.1002/mus.21228
Keywords
activation capacity; H-reflex; ULLS; unloading; V-wave
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Funding
- European Space Agency [15097/01/NL/SH]
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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. Muscle Nerve 42: 749-755, 2010
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