The gluteus medius (GM) muscle in quadrupedal primates has long been thought to mainly act as a hip extensor. However, previous reports argue that it may be a prime hip medial rotator and functions to rotate the pelvis in the horizontal plane, suggesting the functional differentiation between the GM and other hip extensors as hamstrings. In this study, we aim to quantify the muscle actions of the GM and hamstrings using muscle moment arm lengths and discuss the functional differentiation among hip extensors. Muscle attachment sites of eight specimens of Japanese macaque (Macaca fuscata) were digitized, and musculoskeletal models were constructed.Flexor-extensor, abductor-adductor, and medial-lateral rotator moment arms were calculated as the models were moved following the experimentally acquired kinematic data during walking on a pole substrate. Using electromyography, we also recorded the pattern of muscle activation. The GM showed a larger medial rotator moment arm length than the extensor moment arm length when it was activated, suggesting this muscle acts mainly as a hip medial rotator rather than as a hip extensor. The medial rotator action of the GM in the early support phase may rotate the pelvis in the horizontal plane and function to help contralateral forelimb reaching as a previous study suggested and facilitate contralateral hindlimb swinging to place the foot near the ipsilateral forelimb's hand.

Objectives: Diagonal-sequence, diagonal-couplet (DSDC) gaits have been proposedas an adaptation to travel on discontinuously arranged arboreal branches. Only a few studies have examined primate gait adjustment to support discontinuity. We analyzed the gaits of Japanese macaques walking on the "ground" and two discontinuous conditions, "circle" and "point," to better understand the advantages of DSDC gaits on discontinuous supports.Materials and Methods: Seventy-eight vertical posts, each with a circular upper surface, were arranged in four rows at a spacing of 200 mm. The diameter of the circular upper surface was 150 mm ("circle condition") or 50 mm ("point condition"). We calculated the limb phase, duty factor, and time interval from hindlimb touchdown to ipsilateral forelimb liftoff. The supports the fore-and hindlimbs landed on during walking were identified in the circle and point condition. Results:The macaques predominantly used DSDC gaits in the ground and circle conditions and lateral-sequence, diagonal-couplet (LSDC) gaits in the point condition.The macaques usually placed their hindlimbs on the same supports as their ipsilateral forelimbs during the gait cycle.Discussion: Japanese macaques overlapped the ipsilateral fore-and hindlimb stance phase in all DSDC and some LSDC gaits to proximate the ipsilateral limbs on the discontinuous support, allowing the forelimb to guide the hindlimb placement to the support. The overlap duration of the ipsilateral limb stance phases may be extended by DSDC gaits longer than by LSDC gaits, allowing for a direct pass of the support being held by the prehensile hand to the prehensile foot.

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