KILKENNY, P.J.*; WUNDERLICH, R.E.; James Madison Univeristy; James Madison University: Ground reaction forces during bipedalism in Propithecus verreauxi

The mechanics of bipedal locomotion in hominoids and the mechanics of vertical clinging and leaping (VCL) in indrids have been well documented, however studies addressing the unique form of bipedal locomotion exhibited by select indrid primates, such as Propithecus verreauxi, are limited. Terrestrially, Propithecus moves using a bipedal gallop, where the successive touchdowns of trail and lead limbs are followed by an aerial phase. The stance is rotated 180 degrees every 5-7 strides, which allows swapping of lead and trail limbs. Ground reaction forces (GRFs) were quantified during bipedal galloping in 2 adult P. verreauxi to 1) compare forces during leaping and bipedalism in Propithecus, 2) compare GRFs during bipedalism in Propithecus and other primates, and 3) examine energy exchange during bipedalism in Propithecus. Vertical and horizontal accelerations and velocities, vertical displacement of the center of mass (COM), and kinetic and potential energies (KE and PE) were calculated from the force data. Peak resultant forces during bipedalism were lower than peak forces during VCL. Vertical force curves during bipedal galloping in Propithecus were similar in magnitude and shape to those during bipedalism in other nonhuman primates. Peak propulsive forces were higher on the lead limb than the trail limb, whereas peak vertical forces were similar on lead and trail limbs. The fluctuations of KE and PE were in-phase during bipedal galloping, and % recovery due to exchanges in KE and PE was less than 20%. Propithecus may lower peak vertical GRFs during bipedal galloping by adopting a compliant gait with an acute angle of attack. Switching stance during galloping may reduce muscle fatigue associated with differentiation of the trail and lead limbs into braking and propulsive limbs.