Have you ever thought about walking as a mode of locomotion, I mean really thought about it? What muscles are involved? How much force do limbs exert against the ground? What energetic cost is incurred? How is it different from running? While such questions may not occupy your mind during a leisurely Sunday stroll, they do engage the minds of interested biomechanists, and for good reason. As Gottschall and Kram suggest in the opening line of a recent paper on the energetics and muscular basis of human locomotion: “Walking is simple to do but surprisingly difficult to understand scientifically”.
During pedestrian locomotion, limbs must exert vertical forces against the ground to support body mass and horizontal forces against the ground for propulsion. Limb muscles generate these forces and, in so doing, consume metabolic energy. Gottschall and Kram were interested in isolating and understanding the generation of horizontal propulsive forces during walking. Specifically, they were interested in what muscles contribute to the production of horizontal forces during walking and what fraction of the metabolic cost of walking can be attributed to horizontal force production?
To study horizontal propulsion, the researchers used an experimental apparatus that allowed horizontal forces (5%, 10% and 15% body mass) to be externally applied to an experimental subject walking at a constant speed on a treadmill. Such forces were applied via long rubber tubing stretched from a winch through a series of low-friction pulleys to a belt worn about the subject's waist. The magnitude and direction (i.e. impeding vsaiding) of the horizontally applied forces were varied systematically, and ground reaction forces, O2 consumption and CO2production, and electromyographic data from muscles acting at the ankle were measured.
Applying impeding forces greatly increased the cost of walking but in a linear fashion: the higher the impeding force, the higher the metabolic expenditure. Applying aiding forces decreased the metabolic rate during walking, but the decrement was not a linear function of the applied force's magnitude. Instead, the largest decrease in metabolic rate, nearly a 50%reduction, occurred with the application of an intermediate aiding force (10%body mass). Larger and smaller aiding forces also decreased metabolic rate relative to unassisted walking, but to a lesser extent. So why doesn't walking simply get cheaper and cheaper as more and more propulsive aid is applied? The authors propose that large aiding forces lead to increased braking (to maintain constant speed), which in turn reduces energetic savings.
The intensity of electromyographic activity increased substantially in both the medial gastrocnemius and soleus muscles when impeding forces were applied,but aiding forces only effected a decrease in gastrocnemius intensity. Electromyographic intensity is often used as an estimate of muscle recruitment or muscle force production; hence, as Gottschall and Kram suggest, the decrease in gastrocnemius intensity in response to aiding forces strongly implicate it as critical in providing horizontal force production during walking.
Finally, and what I find most intriguing about this study, the metabolic experiments demonstrate that horizontal force production accounts for a higher fraction of energy expenditure in walking (50%) than in running (33%)! So much for intuition...