Meeting Abstract
119.1 Tuesday, Jan. 7 10:15 Built to hop: functional specialization of the hindlimb of the desert kangaroo rat (Dipodomys deserti) GUTMANN, A.K.*; MCGOWAN, C.P.; University of Idaho; University of Idaho agutmann@uidaho.edu
Kangaroo rats hop bipedally whereas most other rodents exclusively use quadrupedal gaits. One hypothesis is that hopping evolved as a means of producing the large accelerations needed to escape predators. If this is the case, one would expect the musculoskeletal anatomy of the kangaroo rat hindlimb to be extremely specialized for accelerating. We measured the mass, fascicle length, and pennation angle of all major muscles in the kangaroo rat hindlimb, and the mass and length of the ankle extensor tendons. We also measured moment arms for all major muscles in the hindlimb. Based on these data, we calculated muscle physiological cross-sectional area, tendon cross-sectional area, tendon safety factor, elastic strain energy storage, and fiber length factor. We compared these data with published data for the rat (Rattus norvegicus), a quadrupedal generalist. Relative to body weight, the kangaroo rats were noticeably more well-muscled than the rats (1.7 times more hindlimb muscle mass/body mass). The hip extensors and the knee flexors represented the two largest muscle groups in both the kangaroo rats and the rats, but these muscle groups were larger in the kangaroo rats (1.6 and 1.8 times more muscle mass/body mass respectively). (Note: Many muscles belong to both muscle groups.) This is due primarily to relatively longer fascicle lengths. These large muscle groups produce the power kangaroo rats need for high acceleration. Additionally, the kangaroo rat ankle extensors had a high tendon safety factor and high fiber length factor indicating that the ankle extensor tendon can withstand high forces and the ankle extensor muscle-tendon unit is better suited for joint position control than elastic energy storage. Thus, there is substantial evidence that the kangaroo rat hindlimb is adapted for accelerating.
