The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Electromyography of pedal and crural muscles in Cebus apella: implications for the evolution of the anthropoid grasping foot

BIREN A. PATEL, SUSAN G. LARSON and JACK T. STERN JR..

Department of Anatomical Sciences, Stony Brook University

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A foot capable of strong grasping with an opposable hallux has been recognized as an important adaptation in euprimate evolution. Along with relatively long lateral toes, ‘prosimians’ have morphological adaptations in their first metatarsal (Mt1) and Mt1-entocuneiform joint that reflects their ability to abduct/adduct their hallux through a large range, thereby allowing them to effectively utilize small-diameter substrates. In fact, electromyography studies in lemurs and lorises show that the extrinsic toe flexors and the intrinsic hallucal adductor are recruited more when using small diameter supports compared to when walking on large branches. In contrast, anthropoids tend to have relatively shorter lateral toes and a hallux that likely has a smaller range of abduction/adduction, based on morphological comparisons. This would suggest that pedal grasping capabilities in anthropoids are reduced, and their feet are more suitable for locomotion on large-diameter supports. To test this hypothesis, we analyzed new electromyography data from the adductor hallucis, flexor digitorum fibularis, flexor digitorum tibialis, peroneus longus, and peroneus brevis of two capuchin monkeys (Cebus apella) walking on both wide and narrow diameter supports. As documented in lemurs and lorises, our results show that capuchins increase recruitment of their adductor hallucis, but not peroneus longus, during narrow branch locomotion. However, in distinction to prosimians, capuchin toe flexors do not substantially increase in activity on narrow substrates; in fact, flexor digitorum tibialis is not particularly active during walking regardless of substrate size. These results demonstrate fundamental similarities and differences in pedal grasping mechanics among prosimians and anthropoids.

Supported by NSF BCS 0935321

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