Fiber type composition of monkey forearm muscle.

Histochemical staining methods were applied to selected superficial forearm muscles of Macaca mulatta monkeys. The muscles were analyzed with regard to relative percentage distribution of different fiber types. In extensor carpi radialis brevis, extensor carpi radialis longus, and palmaris longus there was an even dispersion of each fiber type from the superficial to the deep part of the muscle. Extensor digiti communis showed a slightly higher percentage of type I fibers and correspondingly lower percentage of type II fibers in its central as compared to its superficial area. Three muscles, bracioradialis, extensor carpi ulnaris, and flexor carpi radialis, displayed marked differences between their superficial and deep areas. All of them contained a higher proportion of type I fibers (and correspondingly lower percentage of type II fibers) in their deep parts than in their superficial areas. Flexor carpi ulnaris (FCU) differed from the other muscles studied in that it showed distinctly different fiber proportions on either side of a central tendon. While the ulnar head of FCU was dominated by type II fibers (71% compared to 27% type I fibers), the humeral head contained a larger proportion of type I fibers (58% vs. 40% type II fibers). This difference in fiber type distribution suggests different functional demands for the two heads of FCU, with the possibility of more sustained activity in the humeral head.