Meeting Abstract

16-4  Thursday, Jan. 5 10:45 - 11:00  New Insights Into the Hill-type Muscle Model: A Comparison Between Simulated and Directly Measured Muscle Fiber Length Changes During Jumping in Kangaroo Rats RANKIN, JW*; BLASDELL, K; MCGOWAN, CP; University of Idaho; University of Idaho; University of Idaho

Few mammals rely on bipedal hopping as a primary locomotor behavior. Previous work using kangaroos and wallabies have found that, in these species, fast hopping is more economical than quadrupedal gaits, with improved economy likely linked to highly derived muscle-tendon unit (MTU) characteristics (e.g. short-fibered ankle extensors with long, slender tendons). On the other hand, small hoppers such as kangaroo rats have relatively thick ankle extensor tendons, which may reduce hopping economy. Instead, the thicker tendons suggest kangaroo rats may prioritize maximum jumping ability, a key evasive behavior. To investigate how MTU design influences possible trade-offs between hopping economy and jumping performance, a detailed musculoskeletal model of a desert kangaroo rat (D. deserti) hindlimb was created in SIMM from digitized surface scans and muscle dissections. As a first step, the model was used within an optimization framework to generate forward dynamics jumping simulations that reproduce collected kinematic and ground reaction force data. Predicted ankle extensor muscle excitation patterns were compared with experimental electromyogram (EMG) data. Simulated muscle length changes predicted by a Hill-type muscle model were then compared to in vivo fiber length data obtained via sonomicrometry. This presentation will discuss the similarities and differences between the in vivo and simulated muscle data, providing a perspective on the potential implications of and pitfalls in using simulations and traditional Hill-type muscle models when generating and testing inferences in comparative and paleontological biomechanical studies.