A stepwise increasing exercise protocol to measure the maximal aerobic power output of the quadriceps muscle is proposed for use in rehabilitation medicine. Sixteen healthy volunteers performed isokinetic knee extension/flexion exercises at an angular velocity of 180 degrees x s(-1). The protocol consisted of stages of 200 knee movements from 100 flexion to full extension, starting at 10% peak torque (PT) and increasing by 10% PT each following stage until exhaustion. Quadriceps work, oxygen uptake, heart rate, plasma lactate concentration and surface electromyography were monitored. Quadriceps power output (Pext) was highly related (r = 0.95) to the extension torque at which the subjects were instructed to exercise. The test-retest (r = 0.82) and left to right (r = 0.94) correlations of maximum quadriceps power output (Pmax) were high. Both sexes (males 43+/-9W, females 36+/-8W) achieved their Pmax at 47% PT. At submaximal power output stages oxygen uptake (r = 0.85) and EMG-amplitude (r = 0.88) were linearly related to Pext. Mechanical efficiency (optimum at 60% Pmax) showed a large variation between power output stages and between subjects. The relationship between relative oxygen uptake and heart rate or plasma lactate concentration was similar to that of whole body exercise, e.g. running. We conclude that the proposed test is reliable for assessing the maximal aerobic power output of the quadriceps muscle. This parameter of the muscle function may add useful information for assessing the rehabilitation process after knee immobilization.