Electromyography of rat soleus, medial gastrocnemius, and tibialis anterior during hind limb suspension.

Hind limb suspension is a model frequently utilized to study muscle plasticity. One reason for its frequent use is that it is thought to mimic in many respects the conditions imposed on some muscles during spaceflight. Changes in muscle properties that follow hind limb suspension generally have been attributed to reductions in the recruitment of these muscles. To determine the validity of this assumption, the electromyographic (EMG) activity of three hind limb muscles, the soleus, a slow extensor, the medial gastrocnemius, a fast extensor, and the tibialis anterior, a fast flexor, was studied. The EMG was recorded in each rat for 25 min of each hour for 24 consecutive hours, 7 and 3 days prior, on the day of, and 3, 7, 14, 21, and 28 days after hind limb suspension. Control rats were treated similarly and their EMG recorded on corresponding days. Compared with presuspension, soleus activity was reduced significantly to 91% on the first day of suspension, but had recovered to 81% of its normal activity by the seventh day. Similarly, there was a significant reduction to 54% in activity of the medial gastrocnemius on the day of hind limb suspension which recovered to 98% of its presuspension values by day 7. In contrast, the tibialis anterior showed a significant increase in activity relative to presuspension values within 3 days of the initiation of suspension. These data indicate that hind limb suspension produced only a relatively short-term reduction in the activity of both the soleus and medial gastrocnemius and results in an increased activity in the tibialis anterior. Collaborative studies showed that significant alterations in muscle mass and metabolic and mechanical properties occurred and persisted in spite of the recovery of activation in the soleus and medial gastrocnemius. In addition, no alterations in mass and mechanical properties were evident in the tibialis anterior during a 4-week suspension even though the EMG increased after hind limb suspension. Together, these data indicate that the adaptations in muscle properties following hind limb suspension are not closely related to changes in the total amount of muscle EMG activity per day.