Difference in stretching of sarcomeres between medial gastrocnemius and tibialis anterior by tibial lengthening: an experiment in rabbits.

PURPOSE To compare the degree of stretching of sarcomeres--in the medial gastrocnemius versus the tibialis anterior--acutely extended by tibial lengthening. METHODS The right lower legs of 6 New Zealand White rabbits were acutely distracted by 6.3 mm using an external fixator, whereas the left lower legs served as controls. Immediately after distraction, the ankle was dorsiflexed at 50 degrees and the knee flexed at 140 degrees, with respect to the posture of resting on the ground. Both legs were immersed in 10% buffered formalin for fixation, and the medial gastrocnemius and tibialis anterior were removed. Length of the 2 sarcomeres was measured by a laser diffraction technique using isolated muscle fibre bundles. RESULTS The mean lengths of sarcomeres in the medial gastrocnemius and tibialis anterior were 3.0 microm and 2.3 microm in the lengthened legs, and 2.4 microm and 2.1 microm in control legs, respectively. The degree of stretching of corresponding sarcomeres was therefore 25% and 10%, this difference being significant (p<0.01). CONCLUSION Sarcomeres of the medial gastrocnemius are more stretchable than those of the tibialis anterior following tibial lengthening.