Meeting Abstract
6.3 Friday, Jan. 4 Collision dynamics of bipedal hopping GUTMANN, A.K.*; LEE, D.V.; MCGOWAN, C.P.; University of Idaho; University of Nevada, Las Vegas; University of Idaho agutmann@uidaho.edu
Kangaroos and wallabies are able to maintain a nearly constant metabolic rate across a wide range of speeds when hopping bipedally. The anatomy of large, bipedal hoppers undoubtedly effects locomotion energetics – 30-70% of the mechanical energy required to hop is stored and returned by the long, compliant tendons of the ankle extensors. However, we wanted to determine the effects of collision dynamics. We compared the collision dynamics of two bipedal hoppers, tammar wallabies and desert kangaroo rats, and two quadrupedal trotters, domestic dogs and goats, across a range of speeds. We calculated collision angle (the angle between the ground reaction force vector and C.o.M. velocity vector) and collision fraction (the actual collision angle/maximum collision angle possible for a particular set of ground reaction force and velocity vectors) for all animals. Collision angle was substantially larger for wallabies than kangaroo rats(~6°) or dogs and goats (~9 °) across all speeds. A large collision angle may allow wallabies to take advantage of the potential for elastic strain energy storage and return in their ankle extensor tendons and, thus, allow them to bounce more economically than other species. Kangaroo rats may not use as large a collision angle as wallabies because their shorter, stiffer tendons are less suitable for elastic energy storage and return. Collision fraction was close to one (0.97) and remained constant for bipedal hoppers across all speeds but decreased with increasing speed for quadrupedal trotters. This suggests that bipedal hoppers are able to adjust collision geometry such that they can store and return elastic strain energy in a nearly ideal manner across a broad range of speeds, whereas quadrupedal trotters cannot store and return elastic strain energy as effectively at higher speeds.
