title: Detrimental effects of reloading recovery on force, shortening velocity, and power of soleus muscles from hindlimb-unloaded rats.
abstract: 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 (P(o)), increased maximal shortening velocity (V(max)), and lowered peak power/muscle volume. Nine days of reloading failed to improve P(o), while depressing V(max) 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.
authors.affiliation: Dept. of Nutrition and Exercise Sciences, Oregon State Univ., Corvallis, OR 97331, USA. jeff.widrick@oregonstate.edu
authors.forename: J J
authors.initials: JJ
authors.lastname: Widrick
authors.forename: G F
authors.initials: GF
authors.lastname: Maddalozzo
authors.forename: H
authors.initials: H
authors.lastname: Hu
authors.forename: J C
authors.initials: JC
authors.lastname: Herron
authors.forename: U T
authors.initials: UT
authors.lastname: Iwaniec
authors.forename: R T
authors.initials: RT
authors.lastname: Turner