Adaptations in the number of sarcomeres in series in muscle fibers or fascicles (sarcomere number) may be important in pathologie conditions, as weU as normal muscle bction. Despite the importance of sarcomere nurnber adaptations, the signd(s) important in these adaptations remain to be elucidated.

The first purpose ofthis dissertation was to test the hypothesis that increasing muscle excursion results in Uimeased serial sarcomere addition in growing animals. The tibialis anterior (TA) was released fiom its retinacular restmint at the ankle joint to increase muscle excursion in 4-week-old rabbits. Twelve weeks post-release, muscle excursion and sarcomere number were iricreased for released TAS cornpared 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 Uicrease in serial sarcomere addition. Increasing force production of released TAS by parîïal ablation of the extensor digitorum longus did not inhibit the increase in serial sarcomere addition. Thus, the results of this study süpport the hypothesis that increasing muscle excursion resuhs in increased serial sarcomere addition in growing animals.

The second purpose of this dissertation was to use the TA release mode1 as an initial examination of the capability ofa muscle to adapt foliowing 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 contml TAS. Decreased in viwo TA force production compensated for the increased moment arm in released TAS such that in vivo TA de joint torque was sidar between al1 groups. Physiological cross-sectionai area was smder, and sarcomere number was larger, in the released TA compared with the control TA These adaptations were consistent with the srnalier in vnto 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.