Unique spatial distribution of in vivo human muscle activation.

The present study examined spatial changes in the muscle activation pattern with different contraction intensity using magnetic resonance imaging (MRI). Transverse relaxation time (T2)-weighted MRI was used to acquire 22 axial slices, which covered the entire medial gastrocnemius muscle, before and immediately after seven sets of 10 plantarflexions with loads that were approximately 20 and 60% of their one-repetition maximum, respectively. Reconstructions of the activated regions from MR images revealed the following: (1) the muscle activation determined by MRI correlated significantly (P < 0.05) with the results of electromyography; (2) the activated area distribution increased with increasing force levels; and (3) differential activation between 20 and 60% load was distributed in a spatially non-uniform manner within the muscle, and some regions indicated reduced activity. These results indicate that at submaximal contraction intensity, activation does not necessarily increase in the whole muscle with increase in contraction intensity.