Anyone that spends extended periods of time walking or running intuitively understands that for a given gait there is a particular speed that seems most suitable. Experts studying the energetics of human locomotion have shown that these so-called preferred speeds have something to do with the cost of transport (COT), or the energetic cost to move a given amount of body mass a given distance at those speeds. Namely, COT tends to be relatively low at preferred speeds compared with other possible speed choices. Given that the activity of skeletal muscles in the limbs and trunk accounts for most of the energetic cost of locomotion, it makes sense to ask whether large and important muscles involved in walking and running might also work most economically at speeds where COT is minimized. Indeed, as Dave Carrier of the University of Utah and colleagues Christoph Anders and Nadja Schilling of the University of Jena propose in a recent PNAS paper, if the COT was a major selective factor in the evolution of human locomotion, muscles of the limbs and trunk should be tuned to work most effectively at the same speed within each gait, minimizing COT at those speeds.

To test this idea, Carrier and colleagues used surface electromyography to measure electrical activity patterns in 13 leg and trunk muscles during locomotion in 17 men. For each muscle in each subject they summed the integrated electrical activity used to cover ∼20–30 strides for a range of walking and running speeds, and came up with the cumulative muscle activity per distance traveled (CMAPD). These measurements of cumulative activity within a muscle across speeds were then used as a proxy for muscle metabolism.

The researchers found that most muscles exhibited a U-shaped curve relating CMAPD to speed within a gait, which is to say that for both walking and running, most muscles had an intermediate speed at which muscle metabolism was minimized. However, the actual speeds at which these minima occurred differed among muscles. For example, the vastus lateralis worked most economically during running at about 16 km h–1 whereas the gluteus maximus did so at closer to 10 km h–1. This difference is one of the most dramatic observed, but the point nevertheless is clear: not all muscles seem to work best at the same speed within a gait.

The authors use these results to argue that minimizing the COT was not the only important selective factor on limb and trunk muscles in human evolution. It is likely that these muscles evolved to perform a number of tasks well, presumably including things like climbing, throwing and acceleration. One of their more intriguing results was that the U-shaped curve relating CMAPD to speed during running was shallow for most muscles, implying that while there is an optimal speed for minimizing energetic costs, there is also a pretty broad range of speeds over which such costs don’t change by much. Thus, while our muscles may not be specialized solely for economizing locomotion, they do a pretty good job of reining in the costs over a larger range of speeds than would be expected based on work on other animals, like horses. Such an ability to perform well across a range of speeds may well reflect selective pressure that acted on the physiological underpinnings of persistence hunting strategies in our ancestors.

D. R.
The musculoskeletal system of humans is not tuned to maximize the economy of locomotion
Proc. Natl. Acad. Sci. USA