title: Effect of unweighting on skeletal muscle use during exercise.
abstract: Exercise-induced spin-spin relaxation time (T2) shifts in magnetic resonance (MR) images were used to test the hypothesis that more muscle would be used to perform a given submaximal task after 5 wk of unweighting. Before and after unilateral lower limb suspension (ULLS), 7 subjects performed 5 sets of 10 unilateral concentric actions with the quadriceps femoris muscle group (QF) at each of 4 loads: 25, 40, 55, and 70% of maximum. T2-weighted MR images of the thigh were collected at rest and after each relative load. ULLS elicited a 20% decrease in strength of the left unweighted QF and a 14% decrease in average cross-sectional area (CSA) with no changes in the right weight-bearing QF. Average CSA of the left or right QF showing exercise-induced T2 shift increased as a function of exercise intensity both before and after ULLS. On average, 12 +/- 1, 15 +/- 2, 18 +/- 2, and 22 +/- 1 cm2 of either QF showed elevated T2 for the 25, 40, 55, and 70% loads, respectively, before ULLS. Average CSA of the left but not the right QF, showing elevated T2 after ULLS, was increased to 16 +/- 2, 23 +/- 3, 31 +/- 7, and 39 +/- 5 cm2, respectively. The results indicated that unweighting increased exercise-induced T2 shift in MR images, presumably due to greater muscle mass involvement in exercise after than before unweighting, suggesting a change in motor control.
authors.affiliation: Department of Biological Sciences, Ohio University, Athens 45701, USA.
authors.forename: L L
authors.initials: LL
authors.lastname: Ploutz-Snyder
authors.forename: P A
authors.initials: PA
authors.lastname: Tesch
authors.forename: D J
authors.initials: DJ
authors.lastname: Crittenden
authors.forename: G A
authors.initials: GA
authors.lastname: Dudley
mesh_terms: D002370:Casts, Surgical; D005260:Female; D006801:Humans; D007103:Immobilization; D007866:Leg; D008279:Magnetic Resonance Imaging; D008297:Male; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000502:physiology*; D005082:Physical Exertion; D016474:Weight-Bearing