RAICHLEN, D.A.; University of Texas at Austin: The effects of distally distributed limb mass on quadrupedal locomotion

The effects of limb mass distribution on the energetic costs of quadrupedalism are currently unclear. Although several researchers have predicted that animals with distally distributed limb mass should have higher energetic costs, Taylor et al. (1974) found that three animals with different limb mass distribution patterns (a cheetah, a gazelle, and a goat) had no differences in their costs of locomotion. This study presents a kinematic mechanism that may mitigate the negative energetic impacts of distally distributed limb mass. Distally distributed limb mass is hypothesized to produce relatively low stride frequencies (due to relatively large natural periods of limb oscillation), which reduce the amount of internal work needed to move the limbs relative to the body. This reduction in internal work should lead to a reduction in energetic costs of locomotion. Primates are an excellent sample to test this hypothesis because their limb muscles have been selected for grasping hands and feet, creating limbs with relatively large mass moments of inertia and distal limb centers of mass. The ontogeny of primate limb shape offers a �natural experiment� because limb mass migrates proximally as the infant�s need to grasp its mother�s fur decreases with age. In this sample of infant baboons ( Papio cynocephalus ), as limb mass migrates proximally during development, their stride frequencies get relatively higher and their values of internal work show either no change, or increase slightly. This study demonstrates a link between limb mass distribution and quadrupedal kinematics, and suggests that this link has an effect on internal work. It is possible that these links mitigate the negative energetic effects of distally distributed limb mass.