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Am J Physiol Regul Integr Comp Physiol 295: R1585-R1592, 2008. First published August 27, 2008; doi:10.1152/ajpregu.00045.2008
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EXERCISE AND RESPIRATORY PHYSIOLOGY

Detrimental effects of reloading recovery on force, shortening velocity, and power of soleus muscles from hindlimb-unloaded rats

J. J. Widrick, G. F. Maddalozzo, H. Hu, J. C. Herron, U. T. Iwaniec, and R. T. Turner

Department of Nutrition and Exercise Sciences, Oregon State University, Corvallis, Oregon

Submitted 22 January 2008 ; accepted in final form 26 August 2008

To better understand how atrophied muscles recover from prolonged nonweight-bearing, we studied soleus muscles (in vitro at optimal length) from female rats subjected to normal weight bearing (WB), 15 days of hindlimb unloading (HU), or 15 days HU followed by 9 days of weight bearing reloading (HU-R). HU reduced peak tetanic force (Po), increased maximal shortening velocity (Vmax), and lowered peak power/muscle volume. Nine days of reloading failed to improve Po, while depressing Vmax and intrinsic power below WB levels. These functional changes appeared intracellular in origin as HU-induced reductions in soleus mass, fiber cross-sectional area, and physiological cross-sectional area were partially or completely restored by reloading. We calculated that HU-induced reductions in soleus fiber length were of sufficient magnitude to overextend sarcomeres onto the descending limb of their length-tension relationship upon the resumption of WB activity. In conclusion, the force, shortening velocity, and power deficits observed after 9 days of reloading are consistent with contraction-induced damage to the soleus. HU-induced reductions in fiber length indicate that sarcomere hyperextension upon the resumption of weight-bearing activity may be an important mechanism underlying this response.

muscle atrophy; rehabilitation; hindlimb suspension; eccentric contractions; muscle damage


Address for reprint requests and other correspondence: J. J. Widrick, Dept. of Nutrition and Exercise Sciences, Oregon State Univ., Corvallis, OR 97331 (e-mail: jeff.widrick{at}oregonstate.edu)




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