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Several factors influencing the myoelectric activity of muscles surrounding the knee joint were studied using fine-wire monopolar electrodes. The muscles studied included the vastus lateralis, vastus intermedius, rectus femoris, vastus medialis, gracilis, sartorius, biceps femoris, semimembranosus, semitendinosus, tensor fasciae latae, medial head of the gastrocnemius, and lateral head of the gastrocnemius. Muscle activity was measured in response to unidirectional loads tending to flex and extend the knee, and to combined loads of flexion-adduction, flexion-abduction, extension-adduction, and extension-abduction. Results indicate that the individual muscle responses are dependent upon the direction, magnitude, and combination of external moments, as well as on the flexion angle of the knee joint. Muscle response appeared to be influenced by certain intrinsic mechanical characteristics of the knee joint that tend to change the moment arms of the muscles as the knee moves. For example, the substantial changes in quadriceps myoelectric activity with knee flexion, with constant load applied, can be related to the movement of the tibial-femoral contact changing the lever arm of the quadriceps mechanism. This study indicates that the mechanics of the knee joint must be taken into consideration while attempting to interpret or predict the load response of muscles crossing the knee joint.