If you ever run late to the bus, you might decide to break into a sprint to catch it at the expense of entering with damp armpits, glowing cheeks and gasping for air. Had you had plenty of time, you could have strolled aboard and saved yourself the effort. Erik Summerside and his colleagues from the University of Colorado in Boulder, USA, wondered whether the trade-off between how people perceive the value of their time versus the energy that they exert to move quicker can explain how people decide whether to walk or run. They hypothesized that people would use a strategy that was sensitive to saving both time and energy and that the specific combination of the two strategies would vary between people.

To test this hypothesis, the team identified pairs of distances where individual athletes equally preferred walking (DW) and running (DR). The scientists determined DW/DR pairs by making the athletes run and walk to pairs of marker cones that were initially the same distance (e.g. 40 m) away from the start line. The team then adjusted the distance that the athletes walked to the second cone until the athletes' preferences for running to the 40 m cone and walking the adjusted distance to the second cone were identical. The team then repeated the process with each athlete over initial distances of 60, 80, 100 and 120 m before plotting the pairs of distances on a graph and looking at the slope of the line that they produced. Computer simulations by the team had already shown that a steep DW/DR slope indicated that the athlete had selected an energy-conserving strategy while a shallow slope indicated that the athlete had elected a time-saving strategy. By comparing the slopes of the athlete's plots with the simulations, the team could tell whether the athletes preferred to conserve energy when moving or save time.

After comparing the slope of the DW/DR plots for all 20 athletes with the simulated DW/DR plots for energy- versus time-conserving strategies, the team found that eight athletes tried to conserve energy, 11 tried to shorten movement time and a single athlete tried to move as fast as possible. This means that, as the team expected, the group of athletes didn't use the same single strategy. But when the team tested whether the individual strategies were based on only saving energy or only saving time, they found that the athletes used strategies that covered a range of combinations that were specific to each of them.

Next, the team wondered whether the athletes who wanted to save time had decided to move faster during the experiment. When the team compared the athletes’ strategy with their speed, they found that athletes ran faster – but didn't walk faster – if they had put emphasis on saving time rather than energy. Interestingly, the finding that walking speeds were confined to a narrow interval could be explained by the steep energy cost of walking at a speed that is not the individual's preferred speed – contrary to running, which only exerts a little extra energy away from people's preferred speed.

The observations made by Summerside and colleagues show that people consider saving both time and energy when they decide whether to walk or run. Their approach might also be a powerful way to understand how humans and animals perceive costs and rewards when they decide whether to invest time and energy into diverse movements such as foraging, exercise or even catching the bus.

Summerside
,
E. M.
,
Kram
,
R.
,
Ahmed
,
A. A.
(
2018
).
Contributions of metabolic and temporal costs to human gait selection
.
J. R. Soc. Interface
15
,
20180197
.