Ehlers-Danlos Syndrome (EDS) is a familial connective tissue disorder characterized by marked joint hypermobility, skin hyperdistensibility and fragility. To determine the influence of connective tissue hyperdistensibility on muscle function, a young woman with classic EDS underwent extensive muscle function testing which included a controlled strengthening exercise program. Quantitative muscle strength testing revealed generalized muscle weakness characteristic of this disease. The strength-length relationship curve for quadriceps was shifted, implying an increased series elastic component of muscle. No inherent muscle abnormality was evident from electromyography and serum enzyme evaluation. Unusual fluctuation in maximal isometric strength output curves was found in all tested groups. A functional proprioceptive deficit was demonstrated and found correctable with training and visual compensation. Reduced joint proprioception, plus the tendency for minor subluxations from daily activities, resulted in a learned motor pattern that emphasized short jerky motions rather than sustained precise muscular contractions. From these findings, muscle function in EDS was hypothesized to be affected by connective tissue distensibility.