Impact of mechanical load on functional recovery after muscle reloading

The purposes of this study included: 1) to determine the impact of reloading after hindlimb suspension on the in vitro isometric contractile properties of the rat soleus muscle (experiment 1), and 2) to determine the impact of altered mechanical load on the recovery of muscle contractile properties after reloading-induced injury (experiment 2). Male Sprague-Dawley rats (240-270 g) were used for both experiments. In experiment 1, all rats were hindlimb suspended for a period of 7 d and studied either immediately afterward (no reloading, 0D-RL), after 1 d (24 h) of reloading (1D-RL), or after 2 d (48 h) reloading (2D-RL). In experiment 2, all rats underwent hindlimb suspension for 7 d followed by reloading for 2 d. Rats were then randomly assigned to one of three groups: 1) cage-bound, 2) treadmill walking (12m · min^-1 x 20 min daily), or 3) resuspended. Rats were then studied 7 or 14 d later. In experiment 1, maximal isometric specific force (force/cross-sectional area, Po) was significantly decreased by ~32% and ~50% in the 1D-RL and 2D-RL groups (P < 0.05) compared with 0D-RL group. During the recovery phase (experiment 2), Po was significantly higher in the cage-bound and treadmill walking groups compared with the resuspended group both at 7 and 14 d. However, there was no significant difference in Po between the cage-bound and treadmill walking groups at either time point. Results from this study indicate that skeletal muscle reloading (after a period of unloading) results in a significant force decrement. Additionally, removal of the load during the recovery phase significantly impairs functional recovery, whereas mild exercise offers no advantage over cage-bound activity.