CONTEXT: Researchers have postulated that coactivation of the hamstrings during active knee extension assists the anterior cruciate ligament in maintaining knee joint stability by exerting an opposing force to anterior tibial translation. OBJECTIVE: To compare the reciprocal coactivation of the hamstrings while performing low and high velocity isokinetic movements and two closed chain movements. DESIGN: Within subject's comparison of isokinetic and closed chain exercises. SETTING: Biomechanics laboratory utilizing a Cybex norm isokinetic dynamometer and Biopac Data Collection system. PARTICIPANTS: 12 healthy women. MAIN OUTCOME MEASURES: The root mean square of the Electromyogram (rmsEMG) was used as a measure of overall muscle activity. RESULTS: The rmsEMG for hamstring coactivation during knee extension showed significant differences between the isokinetic movements and the closed chain exercises with greater coactivation when performing the isokinetic movements. In addition, greater activity was seen at the higher isokinetic velocity and during the one legged squat. CONCLUSIONS: These results suggest isokinetic movements, particularly at high speed, can more effectively increase the coactivation activity of the hamstrings when compared to two closed chain activities.