The biomechanics of load carriage during steady-state human walking
Load carriage presents a significant challenge to balance control and results in increased fall risk and injury. The goal of this research was to understand the effects of load carriage on gait asymmetry, biomechanical demand and muscle contributions to support and propulsion during walking. This research evaluated gradual load changes during pregnancy, as well as sudden load changes in individuals with unilateral transtibial amputation. Over one quarter of pregnant women will experience a fall during their pregnancy. Thus, we sought to identify the relationships between pregnancy progression, multiple pregnancies and gait asymmetry (a predictor of fall risk). We found that gait asymmetry increased as pregnancy progressed, and that the magnitude of asymmetry was greater for the second pregnancy relative to the first. In addition to increased fall risk, individuals with transtibial amputation often develop conditions such as intact limb osteoarthritis and fatigue. Load carriage increases these risks as these individuals lack their residual limb ankle plantarflexor muscles, which are critical for mobility, and depend more heavily on their intact limb. Thus, modeling and simulation was used to identify the effects of five loading positions and five commercially available prostheses on muscle contributions to support and propulsion. We found that the Gluteus Maximus, Soleus, Gastrocnemius and prosthesis responded the most to load carriage; further, the intact-side muscles responded more to load carriage. Finally, we found that carrying a front load resulted in greater metabolic cost than other load carriage positions, and that stiffer prosthetic feet may not be advantageous for load carriage. A final study explored the effects of loading position and various prosthetic feet on measures of biomechanical demand, including metabolic cost, intact knee impulses and total muscle stress. We found that load carriage resulted in increased biomechanical demand relative to the unloaded condition. Carrying a front load resulted in the greatest increase in metabolic cost, while an intact-side load resulted in the greatest increase in intact knee loads and the back, intact and residual-side loads resulted in the greatest total muscle stress. The clinically prescribed foot and prescribed foot with a heel-stiffening wedge resulted in the lowest demand.