Functional heterogeneity in a multipinnate muscle

TLDR

Many mammalian muscles possess complex internal architecture that enables a single muscle to generate diverse force vectors via selective regional contractions. The authors tested this hypothesis by recording electromyographic activity simultaneously from multiple intramuscular sites in the multipinnate pig masseter. The recordings revealed systematic variation in activity across masseter regions during mastication, linked to fasciculus orientation, sarcomere length, and fiber type, demonstrating that assumptions of uniform contraction and constant line of action are invalid for complex muscles like the pig masseter.

Abstract

Abstract Many mammalian muscles have a complex internal architecture. This type of structure could allow a single muscle to produce a variety of force vectors through selective regional contractions. This hypothesis was tested electromyographically in the multipinnate pig masseter by recording simultaneously from several intramuscular sites. It was found that the activity in different portions of the masseter varied systematically during the various phases of mastication. Anatomical correlates of the differential activity included fasciculus orientation and length, sarcomere length in specific jaw positions, and histochemical fiber type. The usual assumptions made about muscles for biomechanical analysis, such as uniform contraction and constant line of action, are inappropriate for complex muscles such as the pig masseter.