Meeting Abstract
The modulation of torque about arboreal supports is a critical component of stability for tree-living animals, particularly when moving on narrow branches. Within the primate literature, the combination of grasping hands and feet and diagonal gait patterns (i.e., pairing of contralateral fore/hind feet) is argued to promote stability by facilitating production and modulation of opposing torques about the support. In our continuing investigation into the determinants of primate arboreal stability, we test this hypothesis by presenting kinematic and kinetic data from two squirrel monkeys (Saimiri boliviensis) crossing either broad (5cm), intermediate (2.5cm) or narrow (1.25cm) diameter supports (n = 12 strides per substrate). Kinetic data were collected using a custom-built array of 6 force poles, permitting the measurement of both substrate reaction forces and torques about the support. For each stride we quantified torque production independently from the left- and right-limb pairs. Monkeys exclusively used diagonal sequence gaits across all substrates. During these strides, left and right limbs imparted torques that were equal in magnitude (p=0.62) but opposite in direction, such that each limb pair engendered torques that tended to push the animal centrally over the pole. Within each limb pair, the animals used their grasping extremities to generate active muscular torques that were equal in magnitude (p=0.10), but opposite in direction, to the substrate reaction torques generated passively during contact with the pole, thus mitigating total limb torque. These data suggest that grasping extremities, in combination with diagonally phased gaits, provide for two levels of torque cancelation – both within limbs and between limbs – ensuring low net torque across all supports. Supported by NSF BCS-1126790.
