Changes of fiber type ratio and diameter in rabbit skeletal muscle during limb lengthening

Changes in the fiber-type ratio and diameter during limb lengthening in 10 adult rabbits were studied using histochemical techniques. Changes in the ratio and diameter of muscle fibers (classified as type 1, 2A, or 2B fibers) in tibialis anterior muscles were examined after 20% gradual distraction of the tibia. There was an increase in the number of type 1 fibers and a decrease in type 2B fibers after tibial lengthening. Moreover, the average diameter of the type 1 fibers increased, whereas that of the type 2B fibers decreased. The diameters of muscle fibers measured immediately after completion of the lengthening showed a tendency to recover to normal levels within 1 month; however, the ratio profile of the muscle fibers changed both immediately and 1 month after lengthening. The above results demonstrate that stretched skeletal muscle adapts differently to bone lengthening according to the type of muscle fibers present, resulting in qualitative changes in the fiber-type profile.