Stiffness of knee extensors in Duchenne muscular dystrophy.

The series elastic component (SEC) of Hill's muscle model is sensitive to a modification in muscle functional demand. In this study, SEC stiffness was quantified from quick-released movements in knee extensors of Duchenne muscular dystrophy (DMD) boys to look for possible modifications with the stage of disease. A SEC stiffness index (SI) was defined and was found significantly lower for controls than for DMD. Moreover, a linear relationship was established between mean SI and knee extensors involvement. This suggests a role for this parameter, together with other functional tests, for following patients with DMD.