Why is the preferred walking speed (PWS) of healthy adult humans so predictable and consistent? A commonly given explanation is that we unconsciously select a speed that minimizes the energy expended per unit distance (the energy cost of transport). However, a recent study by Arizona State University researchers suggests that the amount of energy required may not be as important as the source of the energy.
The traditional `minimal energy hypothesis' correctly predicts that there is a U-shaped relationship between the energy cost of transport and walking speed, with the lowest energy cost occurring at the PWS (about 4.5 km h–1, or 2.8 miles h–1) and higher costs occurring at slower and faster speeds. A possible weakness of this hypothesis is the fact that the slope of the U curve is quite shallow; in other words,large deviations from PWS cause only slight increases in energy cost. Could the body really set PWS so precisely if the energetic penalty for straying from PWS is so small?
To explore the issue further, Willis and colleagues studied the perambulations of 12 young, healthy adults. After each subject's PWS was established, he or she walked on a treadmill for 10 min at each of six speeds ranging from 3.2 to 7.2 km h–1. At each treadmill speed,Willis and colleagues measured the subject's expired O2 and CO2 levels and asked the subject to rate his or her perceived exertion. The team then used the expired gas measurements to calculate carbohydrate and fat oxidation rates at each speed.
The researchers were able to reproduce the usual U-shaped relationship between energy cost of transport and walking speed, again noting its shallow slope. However, they found that the rates of carbohydrate oxidation varied dramatically as a function of walking speed, with carbohydrate use increasing substantially at speeds above the PWS. Furthermore, subjects' rates of carbohydrate use correlated with ratings of perceived exertion more strongly than did energy costs of transport.
Based on this analysis, Willis and colleagues propose that PWS is selected not to minimize energy expenditure but to minimize carbohydrate use. Carbohydrates are valuable to the body because they are the primary fuel for intense exercise, yet are stored only in small amounts, whereas fat reserves are ample even in slender individuals. From an evolutionary perspective, then,it may be advantageous to walk at speeds that primarily burn fat, thus saving the body's limited carbohydrate stores for the occasional fight-or-flight moments when they are really needed.
If Willis and colleagues are correct, the relationship between PWS and carbohydrate oxidation should hold true in different subpopulations with different PWSs. Indeed, they found this to be the case when they retrospectively analyzed data from a previous study of young and old active and sedentary people.
If the need to conserve carbohydrates really does dictate PWS, the next logical question to ask is, how does the central nervous system `sense' the rate of carbohydrate oxidation? The tight correlation between this rate and perceived exertion ratings suggests that subjects' subjective sense of exercise difficulty is indeed somehow linked to carbohydrate use. The researchers' speculations on the mechanisms underlying this connection are not entirely convincing, in my opinion. Nevertheless, their carefully designed and well-executed study does offer reasonable evidence that PWS is selected to minimize depletion of the body's carbohydrate stores.