SARCOMERE NUMBER ADAPTATION AFTER RETINACULUM TRANSECTION IN ADULT MICE

Skeletal muscle has been shown to adjust serial sarcomere number in re sponse to chronic static length changes, However, the adaptive respons es to alterations in the dynamic environment are less well defined, Th e adaptations of the adult mouse tibialis anterior (TA) muscle to alte red length and excursion were investigated by surgical transection of the flexor retinaculum. TA moment arm and muscle excursion increased b y 38+/-7% (mean+/-S.E.M.) and fully extended (plantarflexed) muscle le ngth was decreased by 8% after flexor retinaculum transection, In spit e of the significant shortening of the muscle in full plantar-and dors iflexion, serial sarcomere number decreased by 10+/-1% after 2 weeks o f recovery, Gait analysis of these transected animals revealed a 14+/- 3% decrease in dorsiflexion angular velocity after transection, The de crease in angular velocity was less than the increase in moment arm an d, as a result, muscle velocity was calculated to increase by 20+/-4%. These data suggested that the muscle adapted in response to the under lying change in length, irrespective of the altered excursion or veloc ity.