Spastic muscle properties are affected by length changes of adjacent structures.

Recent animal experiments have shown that up to 37% of muscle force may be transmitted to adjacent structures rather than reach the insertion of the muscle's tendon, and that the extent of such force transmission depends on the length and relative position of these structures. We tested whether the force-length characteristics of the distally tenotomized human flexor carpi ulnaris muscle (FCU) of nine patients with cerebral palsy varied with the change of relative length of adjacent structures induced by a change of wrist position. In four patients, the FCU exerted up to 40% more active force in a flexed wrist position at short FCU length, whereas the active force was not significantly higher in the other five. In the same manner, passive force-length characteristics of the spastic FCU changed upon changes in wrist position. Variability in myofascial force transmission may partly explain the variability in success of the FCU-transfer.