Evidence from functional studies in the dog and morphological studies in the human suggests that all of the muscles of the human thigh can, by contraction, produce tension at one joint only. The evidence presented also indicates that movement at the hip without movement at the knee can be produced by a part of the rectus femoris, the sartorius, the gracilis, the semitendinosus, the semimembranosus, and the biceps femoris; and that movement at the knee without movement at the hip can be produced by the gracilis and the biceps femoris. Whereas most of the tension resulting in movement at two joints is transmitted by tendons to attachments to bone, most of the tension resulting in movement at a single joint is transmitted to fascia. Eccentric contraction of the parts of these muscles which act over two joints may facilitate coordinated, simultaneous movements at both the hip and the knee. The muscles which act over two joints are ideally arranged to participate in certain kinds of force couples. When the body weight is raised from a chair, some of the tension produced by the quadriceps femoris can be used by the hamstrings to extend the hip joint, even when the hamstrings are not shortening. Prevention of excessive gliding at the knee joint appears to be one function of two joint muscles.