Both tension and stiffness as a function of muscle length were measured under relaxing conditions on isolated small bundles of chemically skinned myofibers from normal and dystrophic chicken pectoral muscles. It was shown that the dystrophic muscle was stiffer than normal muscle and developed more tension for the same amount of stretch. A fraction of stiffness was not removed by extraction with either 0.6 M KI or with 5 M guanidine HCl mixed with 1% mercaptoethanol. The stiffness of dystrophic muscle was also unaffected by treatment with bacterial collagenase under conditions that destroyed the stiffness of tendon. Nyquist plots of normal and dystrophic muscles during calcium-activated isometric contraction were very similar and were characteristic of fast-twitch muscle, as evidenced by three clear exponential processes. The normal appearance of the Nyquist plot of dystrophic muscle demonstrates that cross-bridge function is not altered, and the characteristic slowing of contraction and relaxation is not a consequence of a fast-to-slow transformation of muscle types. The increased stiffness of dystrophic muscle may be a very fundamental change in the biomechanics of dystrophy. We postulate that the stiffness is mediated by an altered form of collagen, which is collagenase-resistant by virtue of excessive crosslinking.