When humans walk at a given speed, they tend to select an intermediate step frequency, the preferred step frequency, which minimizes the metabolic demand. Previous studies have shown that longer step lengths increase the metabolic demand with high costs of step-to-step transitions, responsible for redirecting the COM between steps. The goal of this thesis was to determine the mechanism by which metabolic cost depends upon step frequency. We hypothesized that the increase in metabolic cost at higher step frequencies is due to muscles generating short bursts of force.; This thesis consists of one theoretical and two experimental studies. In the first study (Chapter 2), passive dynamic walking simulations showed that the preferred step frequencies can be described by a tradeoff between the costs of step-to-step transitions and forced leg motion. The results showed that the former cost increased with longer step lengths. The cost of forced leg motion, proportional to the hip torque amplitude and inversely proportional to the step period, increased with higher step frequencies. The second study (Chapters 3--5) addressed the effect of swing frequency on metabolic cost. A simple pendulum model predicted that metabolic power increases in proportion to frequency to the fourth power. The model was tested by isolated leg swinging experiments in healthy young adults. Kinematic, kinetic, and respired oxygen consumption measurements during single leg (12 adults) and double leg swinging (7 adults) revealed metabolic behaviors agreeing with the prediction (coefficients of determination greater than 0.91). In the third experiment (Chapter 6), a metabolic cost model of step-to-step transitions and forced leg motion was successfully developed to describe the energetics of treadmill walking in nine young adults at various speeds and step frequencies (coefficient of determination, 0.86).; We conclude that humans select preferred step frequencies by trading off step-to-step transitions and forced leg motion costs. The hypothesis was supported that swinging the legs at high frequencies costs significant metabolic energy due to muscles generating short bursts of high forces. At fast walking speeds, this cost may account for over 50% of the total cost of walking.
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