Electromyographic study of the obturator muscles in non-human primates: implications for interpreting the obturator externus groove of the femur

Abstract The development of an obturator externus groove on the posterior aspect of the femoral neck of certain fossil hominids led Day (1969) and subsequent observers to suggest that the habitual stance and gait of these hominids included full extension and/or hyperextension of the hip joints as part of an adaptation to upright bipedalism. However, assessment of the role played by active contraction of the obturator externus muscle in producing an obturator externus groove is prevented by the complete absence of knowledge about when the obturator muscles are used during any behavior in any animal. Therefore, we undertook a telemetered electromyographic study of these muscles in the spider monkey, gibbon, chimpanzee and orang-utan. The two muscles were found to have separate and distinct roles in both voluntary movements and locomotion. All previous speculation that these muscles maintain hip stability and integrity, or that they act more or less continuously to regulate femoral position, were unsupported. In all subjects, vertical climbing recruited obturator externus in the first half of swing phase to effect the lateral rotation of the thigh accompanying its flexion. In bipedal walking, the muscle acted at lift-off and during the early part of swing phase, probably to counteract the medial rotatory effect of the anterior adductor muscles. Because in all of our subjects the obturator externus acts at that point in the bipedal step cycle when the thigh is most extended, we believe that use of obturator externus at this time is a potential repetitive source of active pressure on the posterior aspect of the femoral neck. If bipedalism was the habitual mode of terrestrial progression in a fossil hominid, this alone was probably sufficient to induce development of an obturator externus groove. There is no reason to assert that the degree of hip extension must have been comparable to that of modern humans.