Muscle

Abstract

Muscles are “machines” for converting chemical energy directly into mechanical energy (work) and heat. Muscular work can easily be measured. When an isolated muscle of a cold-blooded animal — for example, the frog sartorius — is loaded with a light weight and then stimulated electrically with a brief current pulse, it twitches; in lifting the weight, it performs mechanical work (load times distance). A contraction of this sort, in which the muscle shortens under constant load, is called isotonic. By contrast, in an isometric contraction the tendons at the ends of the muscle are held so firmly that although the muscle exerts force it cannot shorten, and thus cannot do external work (it does do work in a physiological sense). The results of long research have made it possible to explain in considerable detail the way this muscle machine operates, at a molecular level and on the basis of physical and chemical laws.