The effect of position of immobilisation on resting length, resting stiffness, and weight of the soleus muscle of the rabbit.

The extent of immobilisation-induced adaptations of muscle length and muscle weight is known to be profoundly influenced by the position (or length) at which the muscle is immobilised. However, the effect of the full range of positions of immobilisation on subsequent adaptations of muscle length and weight has not yet been investigated. To examine further the effect of position of immobilisation, we used cast-immobilisation of the hind limbs of 23 rabbits, in various positions between full plantar flexion and full dorsiflexion. Six muscles from non-immobilised rabbits were used as the controls. After 10 days of immobilisation, the wet weight of the soleus muscle and the resting length and resting stiffness of the soleus muscle-tendon unit were determined. Immobilisation in a shortened position was associated with a significantly greater decrease in length and weight than was immobilisation in a lengthened position. In addition, immobilisation produced significant increases in the resting stiffness of muscle-tendon units, although there was no evidence of a position-dependent increase in stiffness. Muscle weight was influenced by the position of immobilisation in a nonlinear way. The data support the views that the pre-immobilisation resting length of the muscle represents a threshold length and that immobilisation at lengths longer than this retards immobilisation-induced atrophy.