Maximum Force Development in Cardiac Muscle

Abstract

CARDIAC muscle has less strength in comparison with skeletal muscle. It has therefore been suggested that there may be fundamental differences in their processes of force generating^1–3. While skeletal muscle can develop isometric forces ranging from 17 to 40 g/mm² (refs. 4–9), maximum forces of only 2 to 7 g/mm² are reported^2,10–15 for heart muscle. Certain functional differences should be considered, however, before intrinsic differences in the mechanism of contraction of cardiac and skeletal muscle are suggested. Thus the lower isometric tension of cardiac muscle could result from any or all of the following: (1) cardiac muscle cannot be tetanized, and maximum force might be limited by the finite duration of active state during the twitch contraction²; (2) the contractile state of heart muscle is variable and generally less than maxima^12,11, whereas the contractile state of skeletal muscle is essentially constant³; and (3) a large proportion of myocardium consists of non-contractile structures such as mitochondria and nuclei, unlike skeletal muscle¹⁶. It is not known whether correction for these factors would explain discrepancies in force development. Accordingly, the maximum contractile force of cardiac muscle has been examined during maximum potentiation of contractile state, and corrected for non-contractile muscle mass. These corrections permit more meaningful comparisons of the maximum capacity for contraction of cardiac and skeletal muscle.