Quantitative EMG analysis in soleus and plantaris during hindlimb suspension and recovery

Activity levels in rat soleus (SOL) and plantaris (PL) were quantitated electromyographically during control (6 days), hindlimb suspension (HS, 28 days), and recovery (6 days). The number and amplitude of "turns" were determined in each muscle from the complex interference pattern every 2 min for 40 days. Each muscle was sampled for 500 ms consecutively, providing 12 s.2 min-1 x muscle-1 of actual data. Changes in activity were assessed by differences in the total number of turns each 24 h, expressed as a percentage of control. Daily intervals of low (< 5,000 turns/2 min) and high activity (> 30,000 turns/2 min) were determined. The number of turns was significantly reduced throughout HS, to just 39% (SOL) and 35% (PL) of control levels on day 28. Low-activity periods increased significantly during HS while high-activity periods disappeared. During recovery, the number of turns increased to control levels in SOL and to 80-90% of control levels in PL. Large differences in activity observed between the dark and light cycles in control muscles were absent during HS and recovery. Similarly, cyclical periods of low and high activity were common during the control period but disappeared during HS and only partly returned during recovery. In conclusion, not only were activity levels significantly reduced in both SOL and PL during HS, but also the patterns of activity were changed.