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1 Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: ortegajd{at}colorado.edu.
A human walker vaults up and over each stance limb like an inverted pendulum. This similarity suggests that the vertical motion of a walker's center of mass reduces metabolic cost by providing a mechanism for pendulum-like mechanical energy exchange. Alternatively, some researchers have hypothesized that minimizing vertical movements of the center of mass during walking minimizes the metabolic cost, and this view remains prevalent in clinical gait analysis. We examined the relationship between vertical movement and metabolic cost by having human subjects walk normally and with minimal center of mass vertical movement ('flat trajectory walking'). In flat trajectory walking, subjects reduced center of mass vertical displacement by an average of 69% (p = 0.0001) but consumed approximately twice as much metabolic energy over a range of speeds (0.7-1.8 m s-1) (p = 0.0001). In flat trajectory walking, passive pendulum-like mechanical energy exchange provided only a small portion of the energy required to accelerate the center of mass because gravitational potential energy fluctuated minimally. Thus, despite the smaller vertical movements in flat trajectory walking, the net external mechanical work needed to move the center of mass was similar in both types of walking (p = 0.73). Subjects walked with more flexed stance limbs in flat trajectory walking (p < 0.001), and the resultant increase in stance limb force generation likely help cause the doubling in metabolic cost compared to normal walking. Regardless of the cause, these findings clearly demonstrate that human walkers consume substantially more metabolic energy when they minimize vertical motion.
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