Developing and utilising musculoskeletal models to predict the locomotor and energetic performance of hopping macropods

Lauren Haylee Thornton

doi:10.25907/00899

Macropods such as kangaroos look and move unlike any other animal. These two characteristics are tightly linked, but they are often studied independently. This thesis uses morphologically-informed musculoskeletal models to evaluate both experimental and novel simulated hopping locomotion. It mechanistically links the underlying morphology to the limits of speed and energetic performance, which is of particular interest in macropods because metabolic cost of locomotion is uniquely independent of speed. The first chapter is a meta-analysis of the phylogenetic scaling of skeletal, muscle and tendon morphology within Macropodoidea (kangaroos, wallabies and potoroos), as well as a review of hopping locomotion, including gait parameters and performance. The review identified knowledge gaps where experimental methods reached their limits; these questions, in particular, would benefit from a new modelling approach. The second chapter explored why speed-independent metabolic cost is unique to macropods larger than 3 kg, given that the two largest likely contributions to cost or cost savings (high strain tendons and features of the hopping gait) are found among various other taxa. To do this, I constructed an OpenSim musculoskeletal model of a grey kangaroo. I used the model to analyse empirical kinetic and kinematic data, in the process uncovering a change in hindlimb posture that could disproportionately increase elastic strain energy storage with speed, and thereby contribute to constant metabolic cost for the slow hopping speeds that were measured. Macropods have a unique combination of adaptable posture and appropriate tendon morphology, which may explain why constant cost is unique to these species. Kangaroo metabolic cost has been predicted to increase with stride frequency, but the natural increase in stride frequency would occur at speeds faster than those for which their oxygen consumption has been successfully measured. The third chapter developed the musculoskeletal model to operate within a predictive simulation framework to achieve faster speeds. I found that the cost remains linear, challenging the prediction that metabolic cost is constant at low speeds due to constant stride frequency. The simulation results were verified against the dataset compiled in the meta-analysis, which was independent of the model inputs, and to the empirical kinetic and kinematic data analysed in chapter two, for the range of speeds and sizes that overlapped. The fourth chapter arose from the morphological portion of the review. Previous scaling studies identified Achilles tendon stress as the predominant limit on kangaroo maximum speed and maximum body mass because the tendons were projected to fail at approximately 150 kg. However, previous research was unable to consider gait parameters for possible avenues of stress reduction, as such large kangaroos are now extinct. By geometrically scaling the model, I discovered that tendon stress did not prevent hopping as the expected increase in force was distributed over longer ground contact durations. Tendon stress increasingly limited maximum speed at greater masses, but the more pressing scaling problem for giant kangaroos was muscle strength. Additionally, giant kangaroos, unlike other species, may not benefit from lower mass-specific metabolic costs as they increase in size. Consequently, their comparative energetic performance declined. This likely leads to a maximum viable size for economical hopping.

Developing and utilising musculoskeletal models to predict the locomotor and energetic performance of hopping macropods

Files and links (1)

Abstract

Details

Metrics