Effect of prolonged unweighting of human skeletal muscle on neuromotor force control

The purpose of this study was to determine the effect of 4 weeks of unilateral lower limb suspension (ULLS) on the fluctuations in motor output and the associated physiological changes. Subjects (n = 17) performed steady isometric plantarflexion (PF) and knee extension (KE) tasks, and KE shortening and lengthening contractions (intensity = 25% maximum). Spinal excitability of the soleus muscle was assessed via the H-reflex, muscle cross-sectional area (CSA) via MRI, along with EMG activity during the PF tasks. Following ULLS, isometric force fluctuations increased approximately 12% for the PF, and 22% for the KE (P < 0.05), with no difference in the pattern of PF muscle activation (P = 0.46). The unsteadiness of lengthening KE contractions increased 25% following ULLS (P = 0.03), while KE steadiness during shortening contractions was not altered (P = 0.98). Significant correlations were observed between the percent changes in PF isometric force fluctuations and H-reflex (r = 0.49, P = 0.04), and between the PF isometric force fluctuations and PF CSA (r = -0.61, P < 0.01). These findings suggest the effects of unweighting on neuromotor performance are muscle group and contraction type dependent, and that the disuse-paradigm altering muscle CSA and spinal excitability may serve to mediate the associated loss of steadiness.