Mechanical weakness of skeletal muscle is thought to contribute to onset and early progression of Duchenne muscular dystrophy, but this has not been systematically assessed. The purpose of this study was to determine in mice: (1) whether the passive mechanical properties of maturing dystrophic (mdx) muscles were different from control; and (2) if different, the time during maturation when these properties change. Prior to and following the overt onset of the dystrophic process (14-35 days), control and dystrophic extensor digitorum longus (EDL) muscles were subjected to two passive stretch protocols in vitro (5% strain at instantaneous and 1.5 L(0)/s strain rates). Force profiles were fit to a viscoelastic muscle model to determine stiffness and damping. The mdx and control EDL muscles exhibited similar passive mechanical properties at each age, suggesting a functional threshold for dystrophic muscle below which damage may be minimized. Determining this threshold may have important clinical implications for treatments of muscular dystrophy involving physical activity.