Effect of unweighting on skeletal muscle use during exercise. Ploutz-Snyder, Lori L., Per A. Tesch, Daniel J. Crittenden, and Gary A. Dudley. Department of Biological Sciences, Ohio University, Athens, OH 45701; Department of Physiology and Pharmacology, Karolinska Institute, S-17 177 Stockholm, Sweden; Department of Health Sciences, University of Central Florida, Orlando, FL 32816; and Biomedical Operations and Research Office, National Aeronautics and Space Administration, Kennedy Space Center, FL 32899
APStracts 2:0103A, 1995.
Unweighting has consistently been shown to decrease strength more than muscle size. One result of these adaptations should be that more muscle would be used to perform a given submaximal task. This study tested this hypothesis using exercise-induced T2 shifts in magnetic resonance (MR) images. Seven subjects performed ambulatory activity on crutches for five weeks with a 10 cm sole on the right shoe, thereby unweighting the left lower limb. Before and after unilateral lower limb suspension (ULLS), five sets of 10 unilateral, concentric actions were performed with the quadriceps femoris muscle group (QF) at each of four loads; 25, 40, 55, and 70% of the pre-ULLS maximum load that could be raised for five sets of 10 actions. Multiple echo, transaxial T2 weighted MR images (1.5T, TR/TE:2000/30,60) of the thigh were collected at rest and after each relative load to assess QF cross-sectional area (CSA) and CSA of the QF showing elevated T2, and thus considered to have been involved in exercise. ULLS elicited a 20% decrease (p<0.05) in strength of the left, unweighted QF that was greater (p<0.05) than its 14% decrease (p<0.05) in average CSA (p<0.05). There were no changes in the right, weight bearing QF (p>0.05). Average CSA of the left or right QF showing exercise -induced T2 shift increased (p<0.05) 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 right QF, showing elevated T2 after ULLS was increased (p<0.05) to 16+/-2, 23+/-3, 31+/-7, and 39+/-5 cm2, respectively. The results indicate that unweighting increases exercise-induced T2 shift in MR images. This was interpreted to mean that more skeletal muscle was used to perform a given exercise task after than before unweighting. It is suggested that this reflects a change in motor control because unweighting has not been reported to reduce specific tension of muscle. 

Received 6 July 1994; accepted in final form 3 March 1995.
APS Manuscript Number A671-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.