Perceived exertion is the conscious sense of how hard your body is working during physical activity, and it turns out to be far more than a vague feeling. Researchers have spent decades studying it as a measurable, reproducible signal that the brain constructs from a mix of motor commands, sensory feedback, expectations, and environmental conditions. The Swedish psychophysicist Gunnar Borg formalized this internal sensation into two widely used rating scales, and since then the science has revealed that perceived effort shapes athletic performance, guides clinical rehabilitation, and can be manipulated by everything from music to placebos.
Where the Feeling of Effort Comes From
For years, scientists debated whether perceived exertion originates “top down” from the brain’s own motor signals or “bottom up” from sensory nerves in the muscles. The evidence now leans heavily toward the top-down explanation. When your brain sends a command to your muscles, it also sends an internal copy of that command, sometimes called a corollary discharge, to sensory areas of the cortex. A study measuring brain electrical activity found that the amplitude of movement-related cortical potentials at the top of the skull tracked closely with how hard people rated the effort of lifting weights, providing direct neurophysiological evidence that perceived effort correlates with the brain’s outgoing motor signal rather than with what the muscles report back.1PubMed. Perception of effort reflects central motor command during movement execution
The strongest test of this idea comes from studies that pharmacologically block the sensory nerves running from the muscles back to the spinal cord. If perceived exertion depended on those signals, blocking them should change how hard exercise feels. It does not. When researchers blocked group III and IV muscle afferents during both dynamic and static exercises, perceived exertion remained unchanged, supporting the view that the brain generates the feeling of effort from its own motor commands rather than from muscle feedback.2PubMed Central. Studies using pharmacological blockade of muscle afferents provide new insights into the neurophysiology of perceived exertion
That said, muscle afferents are not irrelevant to exercise. They still influence how much force the nervous system is willing to produce. Sensory nerves from working muscles can inhibit the output of motor neurons, effectively putting a ceiling on how hard you can push, which in turn affects performance even if the conscious feeling of effort is assembled elsewhere.3PubMed Central. On the Influence of Group III/IV Muscle Afferent Feedback on Endurance Exercise Performance
The Brain Regions Involved
Brain imaging during exercise reveals that perceived effort is not seated in a single brain region but distributed across several. Beyond the expected motor and somatosensory cortices, researchers have found activation of the insular cortex, a region tied to awareness of internal body states and autonomic regulation, during exercise at both low and high intensities.4British Journal of Sports Medicine. Modulation of cortical and subcortical brain areas at low and high exercise intensities Another imaging study reported activation of the anterior cingulate cortex as fatigue accumulated during repeated exercise bouts.5Medicine and Science in Sports and Exercise. Neural Correlates of Perception of Effort: A Brain Imaging Study The anterior cingulate cortex is involved in conflict monitoring and decision-making about whether to continue or stop an action, which fits with the idea that perceived effort is not merely a readout of physiological strain but a signal the brain uses to decide how much more work it is willing to tolerate.
How Heat and Low Oxygen Amplify Effort
Anyone who has exercised on a sweltering day knows it feels harder, but the mechanism is more interesting than simple discomfort. When core body temperature rises during prolonged exercise, brain electrical activity shifts. A study that pushed cyclists to exhaustion under progressive hyperthermia found that ratings of perceived exertion climbed from about 11 on the Borg scale at five minutes to maximal (20) at exhaustion around 50 minutes, whereas the same exercise performed at a stable core temperature never reached that point. The best predictors of the exertion ratings were core temperature and a frequency index of brain waves measured over the frontal cortex.6PubMed. Perceived exertion is associated with an altered brain activity during exercise with progressive hyperthermia
Adding low oxygen to the mix makes things worse in a non-linear way. When people exercised in combined heat and simulated altitude, their average perceived exertion jumped by about 3.3 units, considerably more than the 1.9-unit increase seen with heat alone. The researchers interpreted this synergistic effect as evidence that perceived exertion serves a protective, regulatory function, scaling up to keep effort within safe limits rather than simply tracking how hard the muscles are working.7PubMed. Rating of perceived exertion increases synergistically during prolonged exercise in a combined heat and hypoxic environment This fits with the broader “central governor” concept, which proposes that the brain regulates exercise performance in advance, deliberately throttling output to prevent biological harm rather than waiting for the body to fail.8PubMed. The central governor model of exercise regulation applied to the marathon
Mental Fatigue Makes Physical Work Feel Harder
One of the most surprising findings in this area is that tiredness from purely mental tasks bleeds into physical performance. In a landmark experiment, subjects who spent 90 minutes on a demanding cognitive task before cycling to exhaustion quit about 15% earlier than those who had watched emotionally neutral documentaries instead. Their heart rates, lactate levels, and oxygen consumption were no different between conditions, ruling out any change in the body’s actual capacity. The difference was entirely in how hard the exercise felt: the mentally fatigued group reported higher perceived exertion from the start and hit their personal ceiling of tolerable effort sooner.9PubMed. Mental fatigue impairs physical performance in humans
A systematic review of the broader literature confirmed this pattern, finding that mental fatigue consistently leads to worse endurance performance paired with inflated perceived exertion, without corresponding changes in the body’s physiological markers.10PubMed. The Effects of Mental Fatigue on Physical Performance: A Systematic Review The practical implication is real: if you have spent your day in grinding cognitive work, your evening workout will genuinely feel harder, and you will likely produce less power or run slower, even though your muscles are perfectly fresh. Perceived effort is increasingly understood not just as a marker of intensity but as a factor that actively limits endurance performance.11PubMed Central. Fatigue Induced by Physical and Mental Exertion Increases Perception of Effort and Impairs Subsequent Endurance Performance
Music, Distraction, and the Power of Active Engagement
If mental fatigue inflates perceived exertion, certain psychological inputs can deflate it. Music is the most studied example. During high-intensity exercise, combining music and video lowered perceived exertion ratings compared to exercising without them, and the effect grew larger as the session progressed, precisely when you would expect fatigue to erode motivation most.12PubMed Central. Effects of music and video on perceived exertion during high-intensity exercise Even during something as simple as walking, medium-tempo music reduced perceived exertion compared to a no-music condition, apparently by shifting attention away from internal bodily sensations and toward external, dissociative thoughts.13PubMed Central. Effect of Music Tempo on Attentional Focus and Perceived Exertion during Self-selected Paced Walking
The story gets more interesting when people actively make the music rather than passively listen. In an experiment where participants controlled musical output through their physical movements during strenuous exercise, perceived exertion dropped significantly. The researchers argued that this “musical agency” effect goes beyond mere distraction, suggesting that the feeling of controlling sound production during movement taps into a deeper motivational circuit.14PubMed Central. Musical agency reduces perceived exertion during strenuous physical performance They even speculated that this effect may have been an evolutionary driver of music-making itself: creating rhythmic sound while doing physically demanding work makes the work feel less exhausting.
Placebos, Expectations, and Sensory Illusions
Your expectations about how hard something should feel have a measurable influence on how hard it actually feels. In a study of endurance athletes, those given a placebo supplement increased their objective physiological effort without a corresponding rise in subjective exertion, as if the belief in the supplement allowed them to work harder without noticing.15PLOS ONE. Increasing effort without noticing: A randomized controlled pilot study about the ergogenic placebo effect in endurance athletes and the role of supplement salience A similar phenomenon appeared in children: after drinking a placebo they were told would boost performance, kids exercised for significantly longer, reached higher heart rates, and yet reported lower peak perceived exertion than during a control session.16PubMed Central. The effect of placebo on endurance capacity in normal weight children – a randomized trial
Visual information works in a similar way. When cyclists performed a time trial while watching virtual scenery scrolling past at a slower-than-normal rate, they perceived less exertion and actually produced more power, despite no change in heart rate or cadence. The slower optic flow apparently tricked their brains into underestimating speed, which in turn lowered the sense of effort.17PubMed. Optic flow influences perceived exertion during cycling
Expectation can also inflate effort. When cyclists were falsely told they were breathing low-oxygen air, their ratings of breathlessness increased significantly even though the air was normal. Interestingly, their leg-effort ratings did not change, suggesting that breathlessness and muscular effort are processed through partially separate channels, with breathlessness being more susceptible to top-down expectancy effects.18PubMed. Perceived exertion and dyspnea while cycling during a hypoxic and hyperoxic placebo
Breathlessness and Leg Effort Are Not the Same Signal
That distinction between respiratory effort and peripheral muscular effort deserves its own attention, because they do not always move in lockstep. People often describe exercise as hard because their legs burn, or because they cannot catch their breath, or some combination, and researchers have found that these two sensations respond to different inputs. In a study where participants fatigued their legs before cycling, their breathlessness during subsequent exercise was significantly elevated compared to a non-fatigued condition, even after statistically accounting for the higher ventilation rate. This supports the idea that sensory signals from fatigued leg muscles can amplify the sensation of breathing difficulty through a cross-talk mechanism.19PubMed. Effect of induced leg muscle fatigue on exertional dyspnea in healthy subjects
The practical takeaway is that “how hard does this feel” is not one question but at least two, and training programs, clinical assessments, and research studies increasingly ask people to rate respiratory and limb effort separately. Failing to distinguish them can mask what is actually limiting a person’s performance.
Perceived Exertion in Rehabilitation and Strength Training
Perceived exertion scales have found a major practical home in cardiac rehabilitation. Many cardiac patients take medications that blunt their heart-rate response to exercise, making the usual method of prescribing intensity by heart rate unreliable. In these cases, RPE offers an alternative way to set training loads.20PubMed. A pilot study comparing physiological responses of phase III cardiac patients to recumbent and upright exercise using the RPE scale A pilot study in cardiac rehab found that prescribing resistance training based on RPE (specifically, a scale tied to how many repetitions people felt they had left in the tank) produced strength gains comparable to those achieved with traditional percentage-of-maximum-strength prescriptions, with increases ranging from roughly 19% to 25%.21Journal of Cardiovascular Development and Disease. Rate of Perceived Exertion Based on Repetitions in Reserve Versus Percentage of One-Repetition Maximum for Resistance Training Prescription in Cardiac Rehabilitation: A Pilot Study
Outside clinical settings, the “repetitions in reserve” approach to RPE has gained traction in general strength training. Rather than asking “how hard was that set on a scale of 1 to 10,” this method asks “how many more reps could you have done?” The advantage is that it automatically adjusts for daily fluctuations in strength, sleep quality, and stress. If you slept badly and your usual weight feels heavier, the scale catches that and you load accordingly, rather than grinding through a predetermined number that no longer matches your capacity.22PubMed Central. Application of the Repetitions in Reserve-Based Rating of Perceived Exertion Scale for Resistance Training
RPE-based prescription is not perfect everywhere. A randomized pilot study comparing RPE-guided and heart-rate-guided exercise in cardiac rehab patients who could use heart rate found no differences in outcomes, suggesting that when a reliable heart-rate target is available from a recent exercise test, RPE does not add much.23PubMed Central. Comparison of Ratings of Perceived Exertion and Target Heart Rate Based Exercise Prescription in Cardiac Rehabilitation: Randomized Controlled Pilot Study The tool shines most when objective intensity markers are unavailable or unreliable.
When Perceived Exertion Goes Wrong
In certain medical conditions, the calibration between actual physical demand and perceived effort breaks down. A meta-analysis of studies involving people with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and fibromyalgia found a large effect, with patients reporting substantially higher perceived exertion than healthy controls at matched exercise intensities.24PubMed Central. Elevated Perceived Exertion in People with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome and Fibromyalgia: A Meta-analysis This is not simply poor fitness or low motivation. The inflated effort perception appears to be a core feature of the conditions, consistent with altered central processing of exertion-related signals.
Even more striking, the distortion extends beyond performing exercise yourself. A study found that people with chronic fatigue predicted significantly higher exertion for other individuals they were merely watching exercise, across all intensity levels. Their predictions were strongly influenced by their own clinical symptoms, suggesting a prediction bias where the brain’s model of what effort “should” feel like is systematically skewed upward.25PubMed. Prediction bias for physical exertion in chronic fatigue: Evidence from an observational paradigm This finding matters for clinicians working with these patients: it means the elevated effort perception is not just a complaint during exercise but a pervasive feature of how the nervous system evaluates physical demand in general.
How Age Shapes the Ability to Rate Effort
Perceived exertion is not a fixed ability you are born with. It develops gradually throughout childhood, and this has consequences for how exercise is prescribed to young people. A comprehensive review found that children under about four years old cannot meaningfully rate their exertion at all. Between ages four and seven, the ability emerges in stages: kids first learn to perceive local muscle sensations during tasks like gripping, then gradually become able to rate cardiorespiratory effort during activities like running. By ages eight to twelve, most children can distinguish effort sensations from different parts of the body and use them to regulate exercise intensity, though the scale used and the type of exercise still influence accuracy. During adolescence, the correlation between perceived exertion and heart rate is weaker than in adults.26PubMed. Perceived exertion: influence of age and cognitive development
At the other end of the age spectrum, the news is mostly reassuring. In healthy middle-aged and older adults, the ability to perceive and rate effort appears intact as long as differences in fitness levels are accounted for. Even in physically deconditioned elderly people, a period of rehabilitation training can improve their ability to detect and use muscular sensations to gauge effort. The upshot is that RPE remains a useful tool well into old age.
Social Presence and Encouragement
The people around you during exercise also shape how hard you perceive the work to be, though not always in the direction you might expect. When a social facilitator actively encouraged participants during sprint and endurance exercise, their rated perceived exertion actually went up, not down. Encouragement seemed to unlock a willingness to push harder, and the elevated RPE reflected that genuinely greater effort rather than the exercise feeling worse. The effect was more pronounced during endurance activity than sprinting.27PubMed Central. Impact of active and passive social facilitation on self-paced endurance and sprint exercise: encouragement augments performance and motivation to exercise This is worth keeping in mind when interpreting RPE data in group settings or coached sessions: a higher number does not always mean the experience was less pleasant or the person is closer to their limit. Sometimes it means they were coaxed into accessing a deeper reservoir of effort that they would not have tapped alone.
How Valid Is the Rating Scale Itself?
Borg’s RPE scale, and the later category-ratio variant he developed, remain the most commonly used tools for measuring perceived exertion.28PubMed Central. Perceived Exertion: Revisiting the History and Updating the Neurophysiology and the Practical Applications A meta-analysis of its validity in healthy people found that the correlation between RPE and objective measures of exercise intensity, while positive, may not be as high as earlier estimates suggested. Previous reviews had placed the correlation at roughly 0.80 to 0.90, but the pooled analysis found it somewhat lower, with variability depending on the type of exercise and the conditions under which ratings are collected.29PubMed. Criterion-related validity of the Borg ratings of perceived exertion scale in healthy individuals: a meta-analysis This does not make the scale useless, but it does mean that RPE works best as one input among several rather than a standalone measure. In clinical and training contexts, pairing it with objective markers when possible gives a fuller picture of what is actually happening during exercise.