CARLSON, K.J.*; DEMES, B.; FRANZ, T.M.; Stony Brook University; Stony Brook University; Stony Brook University: Mediolateral forces associated with quadrupedal gaits of lemurids

Mediolateral (ML) peak forces and impulses during locomotion often are described as trivial or erratic. To evaluate these assertions, we quantified substrate reaction forces (SRFs) of forelimb (FL) and hind limb (HL) contacts in two primates � Eulemur fulvus and Lemur catta � using quadrupedal gaits. Three individuals of each taxon traversed a force transducer incorporated into terrestrial or simulated arboreal substrates. Mediolateral (ML) peak forces and impulses were largely independent of speed. Arboreal ML peak forces were consistently lower than terrestrial, which may reflect reduced peak forces associated with �compliant� gait during arboreal locomotion. However, ML peak forces did not always follow a primate trend towards higher HL than FL force components. Average ML peak force at times exceeded average FA peak force. Thus, ML peak forces are not trivial. Limbs often encountered a laterally-directed SRF during arboreal locomotion and a medially-directed SRF during ground locomotion, but variation was observed, particularly in HLs, and on arboreal substrates. ML impulses often included direction changes during contacts. We attribute these to balance requirements. Direction of FL ML net impulse was consistent between braking and propulsion phases across substrates, as were HL net impulses on the ground. However, HL ML net impulse changed direction between phases in arboreal locomotion. ML peak forces and impulses associated with lemur quadrupedal gaits are not erratic. ML forces appear important for evaluating joint moments and bending regimes of long bones during lemurid locomotion, and perhaps during locomotion of other mammals. Supported by NSF BCS 0411489.