Leg muscles differ in spatial activation patterns with differing levels of voluntary plantarflexion activity in...

The purpose of this study was to investigate the differential activity between and within individual muscles commonly grouped as plantarflexors. Much of the previous information gathered on plantarflexor activity has been attained using electromyographic recordings. In this study, we used magnetic resonance imaging which allowed us to look at spatial differences in activation.

Twenty-two human subjects exercised under four different conditions - combinations of loads of 25 or 65% of maximum voluntary contraction (MVC) and the direction of plantarflexion at a sagittal and off-sagittal angle. Before and after each exercise condition, T2-weighted magnetic resonance images were collected. Regions of interest were drawn around the lateral gastrocnemius (LG), medial gastrocnemius (MG), soleus (SOL), peroneus longus (PER) and tibialis anterior (TA) muscles and analyzed for differences.

Significant increases in T2 relaxation times during 25% MVC conditions were found for PER and, during the 65% MVC, for all four muscles considered plantarflexors (LG, MG, SOL, PER). No significant differences were found between sagittal and off-sagittal conditions. Within LG and MG, greater increases in T2 times with exercise were found in proximal regions compared with distal regions.

These results are consistent with suggestions that individual members of muscle groups are capable of differential activity and that for at least some muscles, such differential activity may exist within subvolumes of individual muscles.