title: Increased H-reflex excitability is not accompanied by changes in neural drive following 24 days of unilateral lower limb suspension.
abstract: 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.
authors.affiliation: Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Alsager, UK. o.seynnes@mmu.ac.uk
authors.forename: Olivier R
authors.initials: OR
authors.lastname: Seynnes
authors.forename: Nicola A
authors.initials: NA
authors.lastname: Maffiuletti
authors.forename: Astrid M
authors.initials: AM
authors.lastname: Horstman
authors.forename: Marco V
authors.initials: MV
authors.lastname: Narici
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D001696:Biomechanical Phenomena; D004558:Electric Stimulation; D004576:Electromyography; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D035002:Lower Extremity / Q000294:innervation* / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology; D009476:Neurons, Efferent / Q000502:physiology; D010865:Pilot Projects; D013116:Spinal Cord / Q000502:physiology; D013979:Tibial Nerve / Q000502:physiology; D055815:Young Adult