Rising from a chair and navigating stairs, including ascent and descent, are common yet challenging tasks for the elderly and those with lower limb pathologies. Rehabilitation strategies implemented for these populations do not always result in significant improvement in short-term pain or the ability to perform those tasks. To inform clinical practices, we first need to understand how healthy populations use their muscles to rise from a chair and navigate stairs. In particular, while young adults generally navigate stairs faster than older adults and those with lower limb pathologies, the effects of speed and age on muscle function when navigating stairs remain unknown. This information will serve as a baseline that can help better evaluate compensatory movements and deficits in relevant populations (e.g. elderly, individuals with a lower limb pathology).

We investigated muscle forces and their contributions to center of mass (COM) acceleration (muscle function) during the sit-to-stand transfer in a young, healthy population by creating dynamic simulations with a custom musculoskeletal model that captured spine curvature and arm dynamics. The gluteus maximus and soleus largely contributed to forward and upward COM acceleration, respectively, whereas the quadriceps largely opposed forward acceleration. Inter-limb muscle force differences were also observed, demonstrating lower limb symmetry cannot be assumed during this task, even in healthy adults.

Joint torque patterns of young, healthy adults during stair ascent (SA) have been found to contrast those during gait, as they increase at the hip with increasing speed, but not at the knee or ankle. Using dynamic simulations, we investigated muscle forces and function across SA speeds in young, healthy adults to better understand the mechanisms underlying these torque patterns. The vasti force patterns were consistent with the knee torque profiles, as they did not significantly change across SA speeds. None of the hip or ankle muscle force patterns were consistent with their respective torque profiles, except for the iliacus as its force increased from a slow to self-selected speed. However, there was large variability in kinematics and muscle forces across participants across speeds, demonstrating we cannot assume individuals have the same kinematic strategies or use their muscles in the same way to modulate SA speed.

Joint torque patterns of young, healthy adults during stair descent (SD) have been found to also contrast those during gait as they increase at the hip and ankle with increasing speed but not at the knee. To better understand the mechanisms underlying these torque patterns, we investigated muscle forces and function across SD speeds in those young, healthy adults using dynamic simulations. The vastus lateralis and vastus medialis forces were consistent with the knee torque profile as they decreased from slow to self-selected speeds, which illustrated the utility of dynamic simulations to explain underlying mechanisms behind tasks like SD.

As a first step in understanding how muscle function changes with age when navigating stairs, we examined muscle forces and function during SD in healthy older adults and compared them to those of healthy young adults descending stairs at a similar speed. In comparison to young adults, the older adults’ gluteus medius and rectus femoris produced greater forces and contributed more to COM acceleration whereas their vasti produced lower forces and contributed less to COM acceleration, which suggests that muscle function does change with age during SD.

This dissertation advances our understanding of muscle forces and function during the sit-to-stand transfer as well as how muscle forces and function change with speed and age when navigating stairs. This work serves as a foundation from which future research can build on to better understand compensation strategies and deficits in populations that find these tasks challenging and to develop targeted intervention programs that may improve task performance and patient function.