AbstractSingle skinned fibers were prepared from muscle biopsies of patients with Duchenne muscular dystrophy (DMD) and other neuromuscular diseases. The activation of the contractile system by calcium or strontium ion and the function of the sarcoplasmic reticulum were studied. In many fibers of the biopsy specimens from patients with DMD, activation by strontium revealed a pattern intermediate between those shown by the usual type 1 and type 2 fibers. These fibers were generally of smaller diameter and produced less maximal tension than type 1 and 2 fibers. The study of muscle of newborn animals suggests that this unusual pattern of activation might reflect either arrested development or incomplete regeneration after muscle necrosis in the disease.