The Conversation Inside the Effort: How We Measure Pain, Fatigue, and Hard Work

Six to Twenty

Gunnar Borg spent much of his career trying to measure something that existed nowhere on an exercise machine's display: how hard the work felt to the person doing it. His best-known scale begins, oddly, at 6 and ends at 20. At 9 the effort is very light, 13 is somewhat hard, 17 very hard, and 20 maximal. The scale became one of exercise science's most enduring tools because perceived exertion, although subjective, tracks meaningfully with physiological measures such as heart rate, oxygen consumption and blood lactate. It is useful precisely because the person exercising is contributing information that the wattmeter cannot. [1]

That information is easily muddled in ordinary language. People say that they are tired, that their legs hurt, that an exercise is uncomfortable, or simply that they cannot continue. Researchers have good reason to keep these experiences apart. Perceived effort concerns how hard or strenuous a task feels; fatigue can describe changes in performance capacity or a subjective state of tiredness; discomfort and pain are sensations of their own. Rating scales become less useful when all of these are quietly compressed into the single question, “How hard was that?” [2]

A stationary bicycle makes the distinction unusually clean. It can require exactly 200 watts from the rider minute after minute. The machine does not know whether those watts feel manageable, hateful or suddenly impossible. The number on the screen describes the external work. The sensation belongs to a biological system continually interpreting what it is being asked to do.

The Burning Quadriceps

Pain is an especially unreliable word if it is treated as a synonym for damage. The International Association for the Study of Pain defines pain as a sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. Its accompanying notes go further: pain and nociception are different phenomena, pain cannot be inferred simply from activity in sensory neurons, and a person's experience is influenced to varying degrees by biological, psychological and social factors. [3]

That leaves plenty of room for the unpleasant sensations manufactured by hard exercise.

Acute exercise-induced muscle pain—the burning, aching experience familiar during sustained endurance work—is treated in the research literature as different from delayed-onset soreness, muscular injury and chronic pain. It can be intense while remaining transient and non-threatening. The quadriceps screaming during a climb are not therefore providing a live percentage readout of damaged tissue. [4]

Athletes appear to become particularly accustomed to this distinction. A 2024 meta-analysis covering 36 studies and 2,492 participants found that athletes tolerated experimentally induced pain for longer than non-athletes and, on average, reported lower pain intensity. Their advantage in pain threshold—the point where a stimulus first becomes painful—was smaller than their advantage in tolerance. Training may therefore be associated less with never receiving the message than with behaving differently once the message has arrived. The mechanisms remain uncertain. [5]

Anyone who has trained seriously will recognise the practical version. The final minute of a hard interval can hurt badly while feeling entirely familiar. A much milder sensation in the wrong place can be more unsettling. Intensity alone does not settle the question.

The Same Wattage

Before an endurance test, researchers have sometimes made participants spend long periods completing demanding cognitive tasks. The muscles have not just run a race or performed a set of squats, yet the exercise that follows can feel more difficult. Research on mental fatigue has repeatedly examined this separation: prior physical exertion can reduce the force-producing capacity of working muscle, whereas mental exertion can impair subsequent endurance without producing the same peripheral muscular fatigue. Perception of effort is one of the variables that can change in both situations. [6]

The size and reliability of the mental-fatigue effect are still being refined. A 2023 systematic review and meta-analysis found a modest average reduction in endurance performance after mental fatigue, alongside substantial differences between individuals. Earlier bias-sensitive analysis had warned that publication and reporting bias might make the effect look larger than it really is. The useful observation survives the dispute without requiring an exaggerated conclusion: the physical workload alone does not uniquely determine what the workload feels like. [7]

Expectation can move performance in an even stranger way. Fifteen trained runners once spent a week giving themselves saline injections while believing they were receiving “OxyRBX,” described to them as having effects similar to erythropoietin. Their 3-kilometre race times improved 1.2 per cent more in the placebo phase than in the control phase. During interviews they also described lower physical effort, greater potential motivation and improved recovery. Nothing in the syringe could increase their red-blood-cell count. The belief attached to it was nevertheless capable of changing what they produced on the track. [8]

Racing the Ghost

Twelve trained cyclists once rode a four-kilometre time trial twice: once alone and once against a virtual opponent. Against the opponent they averaged roughly 290 watts rather than 272 and completed the distance faster. Their knee extensors lost more force by the finish, yet their reported perceived exertion did not significantly increase. Somehow, the presence of another rider—really only a representation on a screen—helped persuade them to spend more of what they had. [9]

The behaviour of the opponent matters too. In another four-kilometre experiment, cyclists started more aggressively when racing virtual competitors, and a faster-starting opponent pulled higher power from them during the opening portion of the trial. Pacing is therefore not simply the body's automatic response to diminishing fuel or accumulating metabolites. It is also behaviour conducted in an environment, with information arriving about distance, rivals and what may be required next. [10]

Music is less confrontational but works on some of the same territory. A meta-analysis of 139 studies involving 3,599 people found that music was associated with improved affect, modestly better physical performance and lower perceived exertion. Heart rate did not show a significant overall reduction. The playlist had not removed the cardiovascular work. It had altered aspects of the experience in which the work occurred. [11]

This is why two identical treadmill sessions can acquire entirely different personalities. One takes place after eight hours of concentrated work, alone, watching the seconds crawl across a display. Another happens rested, with music, beside someone slightly faster. The belt can be moving at precisely the same speed.

A Pain in One Spot

There is an obvious bad lesson to draw from all this: if pain and effort are partly constructed experiences, perhaps the disciplined athlete should simply learn to override them.

Bone stress injuries are a useful correction. They can begin with discomfort in one specific location that appears only toward the end of activity and disappears with rest. As the injury progresses, pain may persist throughout exercise, appear during ordinary walking or at night, and be joined by swelling, bruising or a limp. Continuing to load the bone can turn an early stress reaction into an actual fracture. The early warning can therefore feel considerably less dramatic than perfectly ordinary muscular burning. [12]

Other symptoms leave even less room for experimentation. The American Heart Association lists chest pressure or tightness, light-headedness, dizziness, unusual or extreme shortness of breath, and fast or irregular heartbeats among warning signs during physical activity; its guidance is to stop and seek appropriate medical attention rather than treat them as obstacles to a better workout. [13]

The useful distinction is often found in pattern rather than toughness. Diffuse muscular discomfort that appears predictably under heavy load and disappears when the load ends belongs to a different conversation from new focal pain, pain that changes movement, or symptoms appearing where they ordinarily do not. Experienced athletes accumulate a catalogue of familiar sensations. That catalogue can make novelty more conspicuous.

The Last Repetition

Borg's scale asks a beautifully limited question. It does not ask whether the athlete is damaged. It does not ask how much glycogen remains, whether another repetition is mechanically possible, or how badly the athlete wants the result. It asks how hard the effort feels.

At 17, the answer is “very hard.”

A rival can change what happens at 17. So can expectation, music, attention and the mental work that came earlier in the day. Pain can accompany it without implying injury. Fatigue can be genuine without representing total physical incapacity. In another situation, a small, unfamiliar pain in one precise spot can carry more useful information than the spectacular discomfort of the entire workout.

The barbell still has a mass. The bicycle still demands its watts. The final kilometre remains the same length.

What changes is the conversation taking place while the body does the work.

Sometimes “this hurts” belongs to the last repetition.

Sometimes it belongs to the first warning.