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The electromyographic activities of six muscles of the thigh were recorded, using bipolar surface electrodes, during active extension of the knee by six healthy men. The signal amplitudes were normalized to those recorded during isometric maximum voluntary contractions. Extension of the knee from 90 to 0 degrees (full extension) was performed at the rate of 10 degrees per second with the leg unimpeded and with weights of 1.8, 3.6, 5.4, or 7.2 kilograms attached to the ankle. The hamstrings were found to coactivate with the quadriceps during the terminal phase of extension. Coactivation of all three hamstrings was found to occur at joint angles of as much as 9 degrees, with the maximum at full extension of the knee and the strength of the signals ranging to as much as 20 per cent. The signals of all of the flexors and extensors increased with increasing loads on the ankle and, with the exception of the rectus femoris at 9 degrees of flexion, they also increased as the knee extended. The results of this study support the hypothesis that the hamstrings function synergistically with the anterior cruciate ligament to prevent the anterior tibial displacement that is produced by active contraction of the quadriceps in the terminal degrees of extension of the knee. This information is important for the physical conditioning of healthy individuals in preparation for athletic endeavors. Furthermore, if coactivation of the hamstrings with the quadriceps is mediated by sensors other than, or in addition to, those of the anterior cruciate ligament, then strengthening of the hamstrings appears to be an important adjunct to rehabilitation programs after repair or reconstruction of that ligament.