Regulation of sarcomere number in skeletal muscle: a comparison of hypotheses.

Sarcomere lengths from 16 locations in the muscles of mastication were measured from pigs fixed in maximum excursion (wide jaw opening) and in postural position (near occlusion). These data, plus published data on sarcomere lengths in rabbit jaw muscles, were used to evaluate conflicting hypotheses about the factors which regulate serial sarcomere number in striated muscles. According to the most successful hypothesis, sarcomere number is adjusted so as to achieve an optimum sarcomere length when the muscle is experiencing a high level of tension. Most often, this occurs at jaw positions where the muscle is electrically active.