A reexamination of the biphasic theory of skeletal muscle growth

Because of the importance of fibre diameter measurement is was decided to re evaluate the biphasic theory of skeletal muscle growth and development. This theory proposes an initial monophasic distribution of muscle fibres which changes to a biphasic distribution during development. The theory is based on observations made on certain muscles in mice, where two distinct populations of fibre diameters (20 and 40 μm) contribute to the biphasic distribution. In the present investigation cross sections of frozen biceps brachii of mice in rigor mortis were examined. The rigor state was used to avoid complications produced by thaw rigor contraction. The diameters of the outermost and innermost fibres were found to be significantly different. However, if the outer and inner fibres were combined to form one group, no significant difference between this group and other random groups was found. The distributions of all groups were monophasic. The diameters of isolated fibres from mice and rats also displayed a monophasic distribution. This evidence leads to the conclusion that the biphasic theory of muscle growth is untenable. Some of the variables which may occur in fibre size and shape are discussed.