Effects of nine weeks of unloading on neuromuscular activities in adult rats

Effects of 9-week hindlimb suspension and 8-week recovery on locomotor performance and electromyogram (EMG) activities of soleus (Sol), plantaris (Pl), lateral gastrocnemius (LG), and tibialis anterior (TA), were studied in adult rats. Hyperextension of knee and ankle joints, noted after nine weeks of suspension, did not recover during 8-week ambulation. Growth of Sol was fully inhibited by suspension and did not recover completely within 8 weeks of ambulation. The EMG levels in Sol, Pl, and LG 1 day after suspension were 52-76% less than the pre-suspension level (resting on the floor). These activities were recovered to or near the pre-suspension level around 1 week, but decreased again to 10-29% of controls from 7 to 9 weeks. The integrated EMG of TA was elevated during the first week of suspension but then gradually declined to control levels within four weeks. At the end of suspension, the Sol and Pl, not the LG, EMGs remained reduced and the TA EMG remained hyperactive. Co-activation of dorsi- and plantar- flexors occurred often during quadripedal walking following suspension. Such a phenomenon was not observed in the control rats. These phenomena were recovered within 1 week. It is suggested that the unloading-related alterations of neuromuscular activities and/or locomotion, but not the hyperextension of knee and ankle joints, in rats are reversible.