Why Running Is Hard

What makes running hard? If you ask a random runner this question, you are likely to get answers such as “pain,” “fatigue,” and “lactic acid.” These are all good answers, but none of them captures the essence of why running (as well as other endurance activities) feels so difficult at times. Only very recently have scientists identified the true reason.

Pain certainly is a part of the running experience. Remember how sore your quads felt in the final mile of your last marathon? Yet running is often difficult in the absence of muscle pain. For example, muscle pain is much less of a factor in the 6th second of a steep uphill sprint than it is in the final mile of a marathon, but overall, the experience is not any easier. In both situations you are trying as hard as you can, and you feel it.

Likewise, fatigue is a major factor in the last mile of a marathon but not in the 6th second of an uphill sprint. So fatigue too cannot be the source of the feeling that you are at your limit in both situations. If you doubt the claim that fatigue is not what makes the last mile of a marathon feel so hard, consider this: As soon as you cross the finish line of the race and stop, you feel a lot better immediately. Yet stopping has no immediate effect on your level of fatigue. Therefore it must be something else—something that stopped as soon as you did—that made the homestretch of the race feel so hard.

As for lactic acid, there is actually no mechanism that enables humans to feel how much lactic acid is present in their muscles or blood. If you were injected with lactic acid while at rest, you would not suddenly feel as if you were running as hard as you could.

What truly makes running feel hard is something that exercise physiologists call perception of effort. Perception of effort is a runner’s general sense of how hard he is working—how close to the limit he is—in any given moment. It is a nonlocalized feeling of general bodily resistance to the mind’s will to run that is completely distinct from all other perceptions, including pain, fatigue, thirst, proprioception, and whatever else a runner may feel while running.

In a 2013 paper, a team of leading researchers on perception of effort described the phenomenon as “the conscious awareness of the central motor command sent to the active muscles.” In other words, perception of effort is determined not by how hard the body is working but rather by how hard the brain is working the body. This explains why quadriplegics feel severe effort and even “hit the wall” when they manipulate a robotic arm with their thoughts. Perception of effort is literally all in your head.

This fact also explains why the last mile of a marathon feels hard in the same way as the 6th second of a steep uphill sprint. When you run as fast a you can up a steep hill, your brain immediately becomes intensely active in driving your body to move. Since perception of effort is a person’s conscious awareness of this sort of brain activity, you experience a very high level of effort right away. In contrast, in a marathon you start off at a moderate intensity. This means your brain is not driving your body especially hard, so your perceived effort level is relatively low. But the farther you go, the more fatigued your body becomes. The more fatigued your body becomes, the less responsive it is to your brain’s commands. And the less responsive your body is to your body’s commands, the harder your brain has to work to get the same level of output from your body. So by the time you reach the last mile, your brain is as intensely active as it is in the 6th second of a steep uphill sprint and your perceived effort level is equal. But it is the brain’s response to fatigue, not fatigue itself, that makes you feel so miserable.

There is an important difference between short sprints and longer challenges, however. Research has shown that in races lasting less than about 30 seconds, performance is in fact directly limited by the body. In other words, the reason an athlete doesn’t go faster in short sprints is that the body lacks the physical capacity to go any faster. In races that last longer than roughly a half-minute, though, athletes always finish with reserve physical capacity. Their performance is not constrained by physiological events such as muscular acidosis that limit performance in short sprints.

Why not? Because in longer races, athletes always encounter a psychological limit—the maximum level of perceived effort they can tolerate—before they ever reach hard physiological limits like muscular acidosis. Put another way, athletes don’t go faster in longer events because they cannot bear the discomfort of going faster.

Some people struggle to accept this idea. They prefer to believe that the toughest athletes can handle any amount of perceived effort and that these athletes do in fact realize their full physical capacity in longer races, just as sprinters do in short races. But it’s just not so. Psychological limits are no less real than physical ones. All of us, no matter how mentally tough we may fancy ourselves to be, can tolerate only so much discomfort. For example, scientists have methods of inducing pain that cause no physical harm to the body. Every single person who has ever been subjected to incrementally increasing amounts of such harmless pain has cried “Enough!” eventually, even knowing that no physical damage is occurring. The same is true of perceived effort.

I’m not suggesting that runners and endurance athletes do not have physical limits. Of course they do. They are simply unable to ever touch these limits because the level of discomfort required to do so is intolerable. Sprinters, on the other hand, do reach their true physical limits, because even though they experience an extremely high level of effort during their races, they are able to put up with it because they know their suffering will be over with very soon.

What happens when a person tries to defy the psychological limitation that is the maximum tolerable level of perceived effort by attempting to sustain a true maximal effort longer than 30 seconds? Anna Wittekind of the University of Essex answered this question in a 2009 study published in the British Journal of Sports Medicine. Nine subjects were asked to ride stationary bikes outfitted with power meters as hard as they could for 5 seconds, 15 seconds, 30 seconds, and 45 seconds on separate occasions. When she reviewed the results, Wittekind found that the subjects had generated slightly less power during the first 15 seconds of the 45-second test than they had in the 15-second test. In other words, they had not pedaled as hard as they could at the start of the longest test ride, even though they had been instructed to do so. Instead, they had unconsciously paced themselves.

Wittekind speculated that, on the basis of past experience, the subjects recognized that they could not sustain a true maximal effort for 45 seconds without exceeding their maximum tolerance for perceived effort, so they held back just a little without even realizing it. These results suggest that the limit of maximum perceived effort tolerance is so impenetrable that athletes are not psychologically capable of even trying to sustain a maximum exercise intensity longer than approximately 30 seconds.

This proposition—that perception of effort is the true limiter to endurance performance in all circumstances—is the core concept of the psychobiological model of endurance performance, so named by its primary developer, Samuele Marcora of the University of Kent. More than just interesting, the fact that performance in endurance events is always limited by perception of effort rather than by physiology has a profound practical implication, which is this: A runner cannot improve her performance except by changing her relationship with perception of effort. The runner must either increase her perceived effort tolerance so that she’s able to finish races with less reserve capacity or she must find ways to reduce the level of perceived effort she experiences at any given speed, or both.

Training affects perception of effort in both of these ways. Studies have demonstrated that, through the process of gaining experience with perception of effort in workouts and races, runners and other endurance athletes naturally become able to tolerate higher levels of effort. Additionally, training increases the body’s physical capacity, thus reducing its resistance to the brain’s efforts to drive it. The result is that the well-trained runner experiences less effort at any given pace and can go faster for longer periods of time before reaching the limit of his tolerance for perceived effort.

Training is not the only factor that can beneficially alter a runner’s relationship with perception of effort. There is a rapidly expanding body of evidence that a runner’s thoughts, emotions, and perceptions may also alter her relationship with perception of effort in ways that enhance performance. For example, studies have shown that athletes are willing to endure higher levels of perceived effort when they are given incentives that increase their motivation. Other studies have demonstrated that athletes experience less effort when they engage in positive self-talk during a demanding endurance test.

Great athletes have always believed that what distinguishes them from lesser athletes is a mental advantage, not physical superiority. Recent advances in our understanding of the role of the mind and the brain in endurance performance have revealed that psychology has a far greater impact on performance than was previously realized. Consider this: Earlier this year, researchers at the University of Padua subjected a group of runners to a computer test designed to assess their capacity for inhibitory control, or the ability to stay focused on task-relevant stimuli in the presence of distractive stimuli. Then the same group or runners participated in an ultramarathon. Incredibly, those with higher scores for control inhibition—a purely mental ability—performed significantly better in the race.

The psychobiological model of endurance performance has created the need for a new sports psychology, one that is specific to endurance sports and focused on helping athletes alter their relationship with perception of effort in performance-enhancing ways. My forthcoming book, How Bad Do You Want It?, takes a step toward fulfilling this need. Each of its twelve chapters tells a remarkable true story of “mind over muscle” from the world of endurance sports and calls upon the psychobiological model to explain the particular “coping skill” that was used to overcome the challenge that the athlete in question faced. The stories themselves are inspiring, but their scientific interpretation makes them immediately useful to all athletes seeking to break through their existing performance barriers.

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