Regulation of sarcomere number in the growing rabbit tibialis anterior

Adaptations in the number of sarcomeres in series in muscle fibers or fascicles (sarcomere number) may be important in pathologic conditions, as well as normal muscle function. Despite the importance of sarcomere number adaptations, the signal(s) important in these adaptations remain to be elucidated.;The second purpose of this dissertation was to use the TA release model as an initial examination of the capability of a muscle to adapt following tendon transfer. The hypothesis tested was that increasing the moment arm and excursion of the TA muscle (via TA release) results in adaptation of muscle architecture that helps to achieve near normal TA torque at the ankle joint. Twelve weeks post-release, moment arm and excursion were increased in released compared with control TAs. Decreased in vivo TA force production compensated for the increased moment arm in released TAs such that in vivo TA ankle joint torque was similar between all groups. Physiological cross-sectional area was smaller, and sarcomere number was larger, in the released TA compared with the control TA. These adaptations were consistent with the smaller in vivo force for the released TA. In summary, muscle appears capable of adapting to increased moment arm and excursion in a manner that helps achieve near normal TA torque at the ankle joint.;The first purpose of this dissertation was to test the hypothesis that increasing muscle excursion results in increased serial sarcomere addition in growing animals. The tibialis anterior (TA) was released from its retinacular restraint at the ankle joint to increase muscle excursion in 4-week-old rabbits. Twelve weeks post-release, muscle excursion and sarcomere number were increased for released TAs compared with control TAs. However, in vivo TA force production was decreased in released TAs compared to control TAs and may have been responsible for the increase in serial sarcomere addition. Increasing force production of released TAs by partial ablation of the extensor digitorum longus did not inhibit the increase in serial sarcomere addition. Thus, the results of this study support the hypothesis that increasing muscle excursion results in increased serial sarcomere addition in growing animals.