Increased EMG of rat plantaris during locomotion following surgical removal of its synergists.

A chronic EMG electrode implant system was used to determine recruitment patterns of rat plantaris, during treadmill locomotion, before and after surgical removal of its synergists, gastrocnemius and soleus. Bilateral synergist removal resulted in increased plantaris muscle weight and myofibrillar protein content of 59% and 44%, respectively, by 30 days following surgery. Evidence of increased plantaris EMG during treadmill walking (increases in amplitude and integral of EMG bursts to ca. 200% of pre-excision values) occurred at 15-30 days and decreased to non-significant levels at 35 days postsurgery. No corresponding alterations occurred in sham-operated controls. At 25 and 30 days, EMG of plantaris during locomotion was still submaximal, since burst amplitudes were 53% to 67% of those recorded during a dynamic grid-climbing task. The magnitude and time course of changes in EMG of overloaded plantaris during a standardized locomotor task, which reflect increased recruitment and rate of discharge of motor units, are consistent with the chronology of morphological and metabolic events previously described for this model.