Immobilization of the rabbit tibialis anterior muscle in a lengthened position causes addition of sarcomeres in series and extra-cellular matrix proliferation.

Rabbits were immobilized for 3 weeks with the ankle in plantar flexion, midrange position or dorsal extension (n=15). The left leg was used as control. Sarcomere lengths were measured by laser diffraction in vivo in the tibialis anterior (TA) muscle. Legs immobilized in the midrange position showed coherent diffraction patterns through the range of motion, but in those immobilized with TA in the stretched position no diffraction patterns in vivo could be obtained. Morphological analyses revealed increased fibrosis and occurrence of whorled fibers in these muscles. On 15 more likewise immobilized rabbits, a technique of measuring sarcomere lengths in vitro by first digesting the collagen in nitric acid was developed. These in vitro measurements showed shorter sarcomeres in the muscles immobilized in a lengthened position compared to the control, indicating an addition of sarcomeres in series.