Lower Limb Muscle-Tendon Unit Properties in Human Locomotion

The muscle-tendon unit (MTU) is the motor responsible for voluntary movement. Biomechanical properties of muscles and tendons can change over the lifespan in response to aging, disease, and physical training. The impact of MTU biomechanical properties on musculoskeletal function is poorly understood but has broad implications for understanding a range of phenomena including clinical gait disorders, human performance, and the evolution of human bipedalism. The purpose of this dissertation was to explore the properties of lower limb MTUs and better understand how they influence the mechanics and metabolic energy cost of human locomotion. We used a musculoskeletal modeling approach in isolation, and integrated with experimental techniques. It is challenging to investigate human MTU properties in vivo, but they are readily modifiable in a musculoskeletal model and can be isolated from confounding factors. The computational demands of biomechanical simulation present substantial limitations, but modern computers with multicore processors can potentially overcome these limitations through parallel computing. Thus, the first study of this dissertation focused on achieving the computational performance necessary to meet the demands of the remaining two studies. In Chapter 2, we analyzed the parallel speed-up in solving optimal control simulations of human movement using multicore parallel computing. The results suggest up to a several-fold speed-up can be achieved across a range of musculoskeletal simulations; however, the actual performance improvement is problem specific and requires careful evaluation. This knowledge was used to facilitate the large number of simulations performed in Chapters 3 and 4 of this dissertation. In Chapter 3, we used musculoskeletal simulation to investigate the effects of optimal muscle fiber length, tendon slack length, and tendon stiffness on whole-body and muscle-specific locomotor performance. We found lower limb MTU properties can be optimized to reduce whole-body neuromuscular and metabolic demands of human walking. However, reducing overall demands typically required increased effort or cost in some muscles that were offset by greater reductions in others. These MTU-specific performance outcomes may reflect evolutionary trade-offs associated with the need to perform tasks besides level walking. In Chapter 4, we explored the relationships of in vivo tendon stiffness of the major lower limb tendons, using ultrasound shear wave elastography, with the metabolic energy cost of locomotion. This experimental approach was complemented by musculoskeletal simulations where tendon stiffness was varied. The experimental findings suggest lower limb tendon stiffness was not associated with whole-body metabolic power. However, the simulation results suggest correlations at the MTU-specific level may not be evident at the whole-body level. While estimating tendon stiffness with ultrasound at rest is common, it may not adequately capture MTU behavior during dynamic contractions. The findings of this dissertation indicate MTU properties are not specialized to reduce the neuromuscular and metabolic demands of locomotion, yet these outcomes were both sensitive to optimizing individual MTU properties. Tendon stiffness uniquely affects the MTU-specific energy demands resulting in a diverse range of responses, possibly contributing to a cancelation effect at the whole-body level. Overall, our findings reinforce the difficulty of establishing a direct link between elements of the musculoskeletal system and the associated demands during locomotion. Despite these challenges, future investigations should continue advancing our fundamental understanding of MTU function in locomotion because it holds the potential to broadly benefit many areas of human movement.PHDMovement Science PhDUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/178051/1/alxdent_1.pd