Sarcomere number adaptation after retinaculum transection in adult mice.

Skeletal muscle has been shown to adjust serial sarcomere number in response to chronic static length changes. However, the adaptive responses to alterations in the dynamic environment are less well defined. The adaptations of the adult mouse tibialis anterior (TA) muscle to altered length and excursion were investigated by surgical transection of the flexor retinaculum. TA moment arm and muscle excursion increased by 38 +/- 7% (mean +/- S.E.M.) and fully extended (plantarflexed) muscle length was decreased by 8% after flexor retinaculum transection. In spite of the significant shortening of the muscle in full plantar- and dorsiflexion, serial sarcomere number decreased by 10 +/- 1% after 2 weeks of recovery. Gait analysis of these transected animals revealed a 14 +/- 3% decrease in dorsiflexion angular velocity after transection. The decrease in angular velocity was less than the increase in moment arm and, as a result, muscle velocity was calculated to increase by 20 +/- 4%. These data suggested that the muscle adapted in response to the underlying change in length, irrespective of the altered excursion or velocity.