Why Running in the Dark Feels Harder

If you recently did a hard workout in the dark, and found that hitting your normal times was harder than usual, don’t worry, you haven’t become a head case. Nor are you suddenly in worse shape. Instead, you were experiencing the effects of fast optic flow, as did several runners in new research on how varying visual experience influences perceived exertion.

Optic flow is your visual sensation of moving through an environment. When you run in the dark, you see only objects that are close to you. This sensation is analogous to “fast” optic flow, in which scenery seems to be speeding by you more quickly than normal.

Dave Parry and Dominic Micklewright, of the University of Essex, UK, started by having 12 young men do a self-paced 5K time trial on a treadmill. Over the next few weeks, the men did three more self-paced 5K treadmill time trials. During each, they got no feedback on how far they’d run or how much time had passed. The runners indicated when they thought they’d completed each kilometre of the 5K, and what their rating of perceived effort was at that point (using the famous Borg scale of perceived exertion). The researchers noted elapsed time and distance covered when the runners thought they’d reached each kilometer mark, as well as the real time when the runners reached each kilometer. If the runners thought they’d finished the 5K before they had, the researchers told them they were wrong and had the runners keep going until they’d reached the finish.

During the time trials, the men watched video footage of a forest path. During one time trial, the footage was synced with the runners’ pace. Unknown to the runners, on one of the other time trials, the footage was sped up by 25%, creating fast optic flow, and on the other it was slowed by 25%, creating slow optic flow.

The runners wound up hitting roughly the same time in their three time trials, but what running that time felt like was significantly different depending on optic flow. In the fast optic flow time trial, which is analogous to running in the dark, “we found that … your [perceived exertion] scales more quickly with time, i.e., you get ‘fatigued’ more quickly,” Parry told Runner’s World. That is, although the runners were running the same pace during the three time trials, they spent more of the fast optic flow trial feeling tired.

Optic flow also affected how far the runners thought they’d gone. When the video footage was accurately synced with the runners’ pace, they thought they’d run 5 kilometres when they’d covered 5.4 kilometres. But in conditions of fast optic flow, by the time they’d covered 4.6 kilometres, they thought they’d run 5 kilometres.

“The important thing is that [perceived effort] ‘fitted’ their belief about distance covered,” Parry said. “How hard exercise feels (or if you like, how ‘fatigued’ you feel at any point in a run) follows your belief about your speed (and therefore how far through a training run you believe are) and not the distance that you have actually completed. The simple take-home message is that in conditions of slow optic flow (e.g., daytime) you will be capable of running further for the same feelings of ‘fatigue’ compared with conditions of fast optic flow (e.g., night-time).”

Parry previously investigated the effects of varying optic flow on cyclists. His earlier findings are consistent with the new research – that when you work out in conditions of fast optic flow, a given effort feels harder.

Something to keep in mind the next time you head out for some kilometres in the dark.

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