Imagine yourself walking on a treadmill that starts at a reasonable pace: say, two and a half miles per hour. Every two minutes, the treadmill increases its speed by 0.2 mph: 2.7 mph, 2.9 mph, 3.1 mph, and so on. If you’re in good physical condition, at some point — usually between about 3.0 and 4.5 mph — you’ll find it more comfortable to start running instead of walking. Different individuals have different thresholds based on their fitness level and other factors, but even taking these things into account, it’s difficult to explain exactly why people start running when they do. Do different people have different thresholds for pain?

Gregory Daniels and Karl Newell paid 12 physically fit college students to walk on a treadmill as it gradually increased in speed. To disguise the real purpose of the study, the students were fitted with fake oxygen consumption meters and cardiographs. They were told to walk, but to begin running as soon as it felt more comfortable. They also rated their physical exertion every two minutes by pointing to a numeric chart on the wall (remember, their mouths were covered with the oxygen consumption meters so they couldn’t talk).

But most importantly, the walkers were also sometimes asked to complete simple addition and subtraction problems. Every 10 seconds, a new tape-recorded problem and answer was played, and the walkers had to raise their right hand to indicate a correct answer and raise their left hand for an incorrect answer. They repeated the experiment four times: two times with no math problems, and once each with easy (single-digit) and hard (double-digit) math problems. Here are the results:

The students transitioned to running at a significantly higher speed (about 4.78 mph) when doing the math problems compared to when there were no math problems (about 4.58 mph). There was no significant difference between the hard math problems and the easy problems, even though the students rated the hard problems as significantly more difficult.

Now take a look at this graph showing the students’ perceived exertion ratings for the body core (lungs, heart):

There’s nothing in this graph to show *why* the students decided to start walking rather than running. They became more and more exerted as the rate increased — whether they were walking or running.

But now look at this graph, which charts perceived exertion of the extremities (arms, legs):

Now you can see that the exertion rating climbs sharply until the students transition from walking to running. Suddenly the rating flattens out. By shifting to running instead of walking, the students are able to stop the increase in their perception of exertion. But remember, this transition occurs at a faster treadmill speed for those doing the math problems. So doing math problems somehow distracts people from experiencing exertion, both in their extremities and in the body core. But transitioning from walking to running can only alleviate the exertion in the extremities, not the core.

One possible way the math distracts people is through cognitive load: some cognitive resources are required even to determine that we are getting exhausted. If we’re using those resources to do math problems instead of thinking about how much we’re exerting ourselves, we don’t notice the exertion quite so much.

So why is there no difference in the walk/run transition between easy and hard math problems? Perhaps the hard problems weren’t hard enough. Or perhaps we’re limited in our ability to be distracted from exertion. Only a new study with more difficult math problems would be able to answer that question.

*Related: Words of encouragement and exercise*

Daniels

, G., Newell, K.M. (2003). Attentional focus influences the walk-run transition in human locomotion. Biological Psychology, 63(2), 163-178. DOI: 10.1016/S0301-0511(03)00024-3