The purpose of this study was to quantify the effect of combining voluntary effort with passive muscle stretch in the major flexors and extensors of the elbow and knee joints in order to test the validity of the assumption that stretching a muscle in conjunction with voluntary effort enhances the production of muscle activity more than muscle stretch or voluntary effort alone. This was accomplished by using a computer-controlled test instrument to produce the conditions necessary for measuring stretch reflex activity, voluntary effort, and stretch reflex activity plus voluntary effort, and then quantitatively examining the EMG activity associated with each test condition. In general, greater EMG activity was produced by the combination of muscle stretch and voluntary effort than by either muscle stretch or voluntary effort alone. Clinical implications of these findings are discussed.