Growth of medial gastrocnemius muscle and Achilles tendon in Wistar rats.

Dimensions, architecture, material properties and functional characteristics of medial gastrocnemius muscles and Achilles tendons of young and old Wistar rats were compared. Dimensions associated with length of the leg segment grow isometric with it. Muscle belly transsectional dimensions, volume of muscle tissue and associated functional indices increase out of proportion either with the dimensions of the limb segment or the weight of the animal. As the properties of the contractile tissue remain the same and relative proportions of tissue components alter, changes in architecture result. These are not reflected in transsection of the tendon. Functional characteristics of the muscle tendon complex can be explained by architectural or dimensional-peculiarities emerging in the complex by growth processes of which the main is that the muscle belly of this pennated muscle grows in length mainly by increments in girth of the muscle fibers proper.