title: Effect of prolonged unweighting of human skeletal muscle on neuromotor force control.
abstract: 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.
authors.affiliation: Interdisciplinary Institute for Neuromusculoskeletal Research, Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, 211 Irvine Hall, Athens, OH 45701, USA. clarkb2@ohio.edu
authors.forename: Brian C
authors.initials: BC
authors.lastname: Clark
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Pierce
authors.forename: Todd M
authors.initials: TM
authors.lastname: Manini
authors.forename: Lori L
authors.initials: LL
authors.lastname: Ploutz-Snyder
mesh_terms: D000328:Adult; D001284:Atrophy; D004576:Electromyography; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D007092:Imagination / Q000502:physiology; D007537:Isometric Contraction / Q000502:physiology; D007866:Leg / Q000033:anatomy & histology / Q000294:innervation / Q000502:physiology; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000294:innervation* / Q000502:physiology*; D010809:Physical Fitness / Q000502:physiology; D011597:Psychomotor Performance / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology; D053828:Stockings, Compression; D014159:Transducers; D018474:Weightlessness Simulation