The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


Three-dimensional moment arms and architecture of chimpanzee (Pan troglodytes) leg musculature

NICHOLAS HOLOWKA1 and MATTHEW C. O'NEILL2.

1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Department of Anatomical Sciences, Stony Brook University School of Medicine

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Knowledge of the moments produced by the muscles controlling ankle motion is critical to understanding the locomotor biomechanics of chimpanzees. During locomotion, moment arms can change considerably depending on joint position, altering joint moment-producing capabilities. While three-dimensional (3D) muscle-tendon unit (MTU) moment arms are well documented for the human ankle joint, little comparable data exist for chimpanzees. Here, we report measurements from chimpanzee specimens of the 3D moment arms and muscle architecture for 11 leg muscles that act about the ankle to move the foot.

We partially dissected the right hind limbs of two adult, cadaveric chimpanzees (body masses: 63.4 kg and 81.3 kg), and mounted each specimen in a custom-built jig. A 3D motion-capture system simultaneously tracked MTU excursion and hind limb markers as we moved the knee and ankle through their full ranges of motion. We used MTU excursions and 3D joint displacements to calculate plantarflexion-dorsiflexion and inversion-eversion moment arms for 11 MTUs crossing the ankle. We also measured muscle mass, fascicle length, and pennation angle.

MTUs tend to achieve maximum moment arm lengths when the foot is near neutral position, with lengths typically decreasing in a nonlinear manner as the foot moves into larger plantarflexion-dorsiflexion and inversion-eversion angles. This finding suggests that larger muscle forces will be required to counter a given joint moment in behaviors that involve extreme ankle positions, such as climbing. Compared to humans, chimpanzees have relatively short plantarflexor and dorsiflexor moment arms. Muscle architectural data are generally consistent with those reported previously.

Supported by NSF BCS-0935321.

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