5.27 Interoception and the Insular Cortex: Reading the Body From Inside

"How hard was that set?" You say seven out of ten. Quick question: where did that number come from?

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It feels like a measurement — as if your body has an effort gauge and you simply read the dial. Here's the thing: there is no gauge.

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That seven was built. Assembled, in a specific part of your brain, from dozens of separate body signals — heart, lungs, muscle chemistry, gut, temperature — and then shaped by what you expected to feel.

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This article is about the machinery that does the building: a hidden fold of cortex called the insula. Get to know it and you'll understand why effort, breathlessness, heaviness, readiness and fatigue are constructions, why belief and context measurably change how hard the same physiological state feels, and why some athletes pace superbly while others report body signals that don't match reality — and can't tell the difference.

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We'll finish with the practical side: pacing, when to trust self-report, and where breath-based and body-scan practices do and don't have support.

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Key takeaways

1. Interoceptive signals travel via small-diameter afferents and the vagus to brainstem nuclei, then the thalamus, then posterior to anterior insula. The pathway is as concrete as the visual pathway.
2. The posterior insula holds a body-mapped representation of your internal state. The anterior insula integrates it with context and value to produce the feeling you can actually report.
3. Interoception is predictive. Your brain forecasts the internal consequences of an action and responds to the mismatch — which is why belief and expectation change perceived effort.
4. Accuracy, sensibility and awareness are three weakly correlated dimensions. Confident self-report is not the same as accurate self-report.
5. Group III and IV muscle afferents carry the metabolic and mechanical state of working muscle. They're a major input to perceived exertion and to central regulation of drive.
6. The insula is consistently among the most activated regions during hard exercise, and it's implicated in the decision to stop, alongside the anterior cingulate.
7. Interoceptive training has reasonable support for improving accuracy on the trained signal — and limited evidence for broad performance transfer.

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Beginner section: Where feelings of effort actually come from

Meet the builder: your insula

The brain region doing all this construction work is the insula, and you've probably never heard of it for a good reason: you can't see it.

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It's a patch of cortex hidden inside the lateral sulcus — folded away underneath the frontal and temporal lobes. Look at a picture of the brain's surface and it's simply not there. It's buried.

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Buried, but busy. Every rep you've ever rated, every run that felt heavier than it should, every "I've got nothing left" — assembled here.

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From raw data to "this is heavy"

Signals from inside your body — how much carbon dioxide is in the blood, how stretched the stomach is, what the chemical environment of a working muscle looks like, what the heart is doing — travel on their own set of nerve fibres. First stop the brainstem, then the thalamus, then the back part of the insula (Craig, 2002).

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At that stage they're still just data, organised by body region and by type. A spreadsheet, not a sensation.

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Then the signals move forward through the insula, getting combined with each other, with context, with memory and with what the situation means. By the time the process reaches the front, the result is a feeling you can put words on: I'm breathless. This is heavy. I'm fine. I have nothing left.

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Notice what this means for training. The feeling of effort is built partly from genuine data about your working muscles — which is why it tracks real fatigue and works as a training tool at all.

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One-line recap: the back of the insula holds the numbers; the front writes the story.

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Your brain guesses before it feels

Here's the second essential idea, and it's the strangest one: this system predicts rather than merely reports.

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Before you start a hard interval, your brain generates a forecast of the internal state that interval should produce. What reaches consciousness is heavily influenced by the difference between the forecast and what actually happens.

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And no, this isn't speculation about how it might work. It's the framework that best explains a whole set of otherwise puzzling findings.

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Two of the strangest: the same power output feels harder when you believe you have further to go. And being told a drink contains carbohydrate changes performance even when it does not.

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You've felt this in the gym too. A weight you expected to fly feels grindy, and suddenly the whole set is harder — the forecast missed, and the mismatch became part of the feeling.

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It cuts the other way as well. Knowing exactly how much work remains — a known course, familiar landmarks, an honest rep count — reduces uncertainty, and that alone improves pacing. Feed the forecast good information and the feeling gets more useful.

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The jargon, translated

• Interoception — the sensing and representation of the internal state of the body.
• Insular cortex — cortex buried inside the lateral sulcus. Posterior regions receive body signals; anterior regions produce the reportable feeling.
• Group III and IV afferents — small muscle nerve fibres that report mechanical and metabolic conditions in the working muscle.
• Interoceptive accuracy — how well someone can objectively detect their own internal signals, measured with tasks such as heartbeat counting.
• Allostasis — the anticipatory regulation of the body's internal state, as opposed to reactive correction after a deviation.
• Perceived exertion — the conscious sense of how hard an effort is. An interoceptive construction, not a direct readout of any single variable.

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Confident is not the same as correct

The third idea is the most practically useful, so lean in for this one.

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Being confident about what your body is telling you and being correct about it are different things — and they don't correlate strongly (Garfinkel et al., 2015).

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Some athletes are objectively excellent at detecting internal signals. Others are highly attentive to their bodies and objectively inaccurate.

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That second group? They're the ones who report feeling terrible on days their output is fine, and feeling great on days they're about to break down.

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Knowing which type an athlete is changes how much weight their self-report deserves. If your training partner's "I'm wrecked" never matches how they actually lift, that's not lying — it's a calibration problem. And it's coachable.

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Want to know which type you are? Try what coaches do: for four to six weeks, jot down a morning readiness score out of ten next to something objective, like a jump or bar speed at a fixed load. If the two track each other, your feelings are usable data. If they don't, trust the output.

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Before the deep anatomy, this short talk is a friendly tour of the whole idea.

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Advanced section: Pathways, prediction, and the construction of effort

The anatomy, and why the insula is a hierarchy

Craig’s work established the modern account of the interoceptive pathway in primates. Small-diameter afferents from throughout the body, including lamina I spinothalamic neurons, project to the nucleus of the solitary tract and parabrachial nucleus, then via the ventromedial nucleus of the thalamus to the posterior insula. This is a genuine primary sensory cortex for the internal body, and in humans it is anatomically distinct and body-mapped (Craig, 2002).

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Information then moves anteriorly through the insula, becoming progressively more integrated and less modality-specific. The anterior insula is densely interconnected with anterior cingulate cortex, orbitofrontal cortex and amygdala, and it is where interoceptive signals meet context, expectation and value. Craig argued that this posterior-to-anterior gradient is what converts a body signal into a subjective feeling (Craig, 2009).

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Two anatomical details are worth noting because they explain otherwise odd findings. The right anterior insula is somewhat specialised for sympathetic and arousing signals and shows stronger relationships with heartbeat detection tasks (Critchley et al., 2004). And the insula contains von Economo neurons, large projection neurons found in only a few species, which is consistent with the region being important for rapid integration across distant networks (Craig, 2009).

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Group III and IV afferents: The muscle’s report to the brain

Perceived exertion during exercise is not generated in the muscle, but the muscle contributes heavily to it (Marcora, 2009; Pageaux, 2016). Group III afferents respond primarily to mechanical stimuli and group IV to metabolic ones, including protons, lactate, adenosine triphosphate (ATP) and bradykinin. Their firing rises with exercise intensity and with accumulated metabolic disturbance.

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The clearest human evidence comes from experiments blocking these afferents pharmacologically. Amann and colleagues used intrathecal fentanyl to attenuate lower limb afferent feedback during cycling and found that participants produced higher power outputs in the early portion of a time trial and then experienced greater peripheral fatigue and impaired locomotion afterwards, with central motor drive no longer appropriately restrained (Amann et al., 2011). The interpretation is that these afferents both contribute to the sensation of effort and participate in a regulatory loop that limits how much peripheral disturbance is permitted.

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The applied reading is that perceived exertion contains genuine physiological information rather than being purely psychological. It is a construction, and it is a construction built partly from real data about the state of the working muscle. That is why it works as a training tool at all, and why it becomes unreliable in exactly the conditions that distort the inputs: Heat, dehydration, illness, unfamiliar exercise modes and high emotional arousal.

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Prediction, expectation, and why belief changes performance

If interoception were a passive readout, expectation could not change it. It clearly does. Several well-replicated findings make the point. Carbohydrate mouth rinsing improves endurance performance without any absorption, apparently through oral receptor signalling and central reward pathways (Chambers et al., 2009). Deception studies in which cyclists are told they are racing against their previous best when in fact the avatar is set slightly faster produce genuine improvements in output (Stone et al., 2012). Menthol in the mouth reduces perceived breathlessness and improves performance in the heat without changing core temperature (Stevens et al., 2016).

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The predictive processing account, developed for interoception by Seth, Barrett and colleagues, handles these findings naturally. Perceived effort is a best estimate that combines afferent evidence with prior expectation, weighted by how reliable each is (Barrett & Simmons, 2015; Seth, 2013). Where afferent evidence is noisy, as it generally is during exercise, priors carry more weight, so manipulating expectation shifts the resulting feeling substantially.

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This should not be read as evidence that effort is imaginary or that limits can be believed away. The same framework predicts that the influence of expectation is bounded by the strength of the afferent evidence, which is why deception effects are measurable but small, and why they disappear as the athlete approaches genuine physiological limits.

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The insula in the decision to stop

Imaging studies of exhausting exercise consistently show insular and anterior cingulate activation increasing as the task progresses, and the pattern maps better onto the subjective experience than onto any single physiological variable. Hilty and colleagues reported changes in communication between the insula and motor cortex approaching exhaustion during sustained isometric work (Hilty et al., 2011).

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A reasonable synthesis, consistent with the material in 5.39 and 5.40, is that the decision to stop or slow down is a value computation: The anterior insula supplies the current and predicted cost of continuing, the anterior cingulate weighs that cost against the benefit, and prefrontal and striatal systems determine whether the effort continues. Exhaustion in most sporting contexts is a decision made under a cost estimate rather than a mechanical failure of the muscle.

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This is not a claim that physiological limits do not exist. It is a claim about where the operative limit usually sits. In a maximal 400 metre run, peripheral disturbance is close to genuinely limiting. In a five-hour ultramarathon or a long team-sport match, the limit is almost always the cost computation.

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Measuring interoception, and the trouble with heartbeat counting

The dominant measure of interoceptive accuracy has been the heartbeat counting task, and it has significant known problems. Performance is influenced by beliefs about resting heart rate, by time estimation ability and by the instruction wording, and a substantial number of participants appear to be reporting an estimate rather than a detection (Garfinkel et al., 2015). Heartbeat discrimination tasks, in which participants judge whether a tone is synchronous with their heartbeat, are less confounded but harder to administer.

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Garfinkel and colleagues proposed the three-dimensional framework in Figure 3 specifically to resolve confusion in the literature, and it has proved useful (Garfinkel et al., 2015). The important applied point is that questionnaire measures of body awareness are measuring sensibility, not accuracy, and correlate weakly with objective detection (Mehling et al., 2012).

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For practitioners the implication is straightforward: Do not infer an athlete’s interoceptive accuracy from how much they talk about their body. Infer it empirically, by checking their self-reports against objective output over weeks. An athlete whose subjective readiness reliably predicts their jump height or their pacing is worth listening to closely. One whose reports do not track anything measurable needs objective anchors instead.

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What interoceptive training can and cannot do

Interventions that train attention to internal signals, including body scans, breath-focused practice and heartbeat detection training, do improve accuracy for the trained signal, with reasonable evidence from controlled studies. There is also good evidence that slow-paced breathing acutely increases heart rate variability and reduces subjective arousal, discussed in 5.33.

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• Well supported: Improved detection of the specific trained signal; acute reduction in arousal from slow breathing; reduced perceived breathlessness from cooling and menthol strategies.
• Plausible but not established: Transfer of improved interoceptive accuracy to better pacing or earlier detection of overreaching in athletes.
• Not supported: Claims that body awareness practices raise physiological capacity, or that increased attention to internal signals is always beneficial.

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The last point deserves emphasis because it runs against intuition. Excessive attention to internal signals is a feature of anxiety disorders rather than of elite performance, and in endurance sport an associative attentional strategy focused on body signals is not universally superior to a dissociative one. The goal is calibration, not vigilance.

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Practical section: Calibrating an athlete’s internal instrument

1. Validate self-report against something objective before trusting it. For four to six weeks, record a one-to-ten readiness score each morning alongside a thirty-second objective measure such as a counter-movement jump or a fixed-load bar velocity. If the two track each other, the athlete’s reports are usable. If they do not, use the objective measure and teach the athlete what the objective measure feels like.
2. Teach calibration explicitly. Have the athlete predict a rating of perceived exertion before a set or an interval, then compare it to their rating afterwards and to the actual output. Prediction with feedback is what improves calibration; simply asking for ratings does not.
3. Anchor perceived exertion to a known reference. A rating scale is meaningless without a fixed point. Establish what a genuine ten looks like once, in a controlled maximal effort, so subsequent ratings have a scale.
4. Do not use perceived exertion as the primary tool in distorting conditions. Heat, dehydration, illness, altitude, unfamiliar exercise modes and high emotional arousal all shift the construction. Use objective load prescription in those conditions.
5. Use expectation deliberately and honestly. Accurate information about how much work remains, a known course, and familiar landmarks all reduce uncertainty and improve pacing. Deception has measurable effects and is not a sustainable coaching strategy because it destroys the calibration you are trying to build.
6. Use slow-paced breathing for acute arousal reduction where it is appropriate. Roughly six breaths per minute with a longer exhalation than inhalation reliably increases heart rate variability and lowers subjective arousal within a few minutes. This is a state tool, not a fitness intervention.
7. Be cautious about increasing body vigilance in anxious athletes. For an athlete who already over-attends to symptoms, more internal focus is likely to make things worse. External focus and objective anchors are better.

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A note on pacing specifically. Good pacing is a well-calibrated predictive model of the internal cost of the remaining work. That model is built by completing the actual event distance, on the actual terrain, at close to the actual intensity, enough times to know what the middle third feels like. No amount of generic body-awareness practice substitutes for that specific experience, which is why race-specific and event-specific sessions matter so much for pacing quality.

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A note on monitoring for overreaching. The interoceptive framework predicts that subjective symptoms will often precede objective performance decline, because the cost estimate rises before the capacity falls. That is consistent with the monitoring literature, in which simple subjective wellness questionnaires often outperform physiological markers at detecting accumulated stress (Saw et al., 2016). The condition is that the athlete’s reports have been validated as accurate first.

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Sport applications

In endurance sport interoception is the sport. Pacing is a predictive interoceptive model, and the practical implication is that the specificity of the pacing model matters as much as the physiology. Athletes who train the distance, terrain and intensity of their event pace better than equally fit athletes who do not.

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In combat sports the relevant use is round management. A fighter’s estimate of what they have left determines whether they commit in the final thirty seconds, and that estimate is built from sparring rounds of the correct duration with the correct work rate. Short rounds build a bad model.

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In team sports the construction is heavily influenced by score, time remaining and context, which is why the same physical state produces very different work rates in a close contest and a decided one. Coaches can exploit this legitimately by supplying accurate information about remaining time and workload.

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In strength sport perceived exertion is used to autoregulate load through repetitions-in-reserve estimates. These are reasonably accurate in trained lifters close to failure and considerably less accurate in novices and at low intensities, which is exactly what a calibration model predicts: Accuracy is best where the afferent evidence is strongest.

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In heat, all sports face the same distortion. Perceived exertion rises faster than any single physiological variable, and cooling strategies including menthol reduce perceived effort without changing core temperature (Stevens et al., 2016). This is genuinely useful and it also means an athlete can be tricked into tolerating a dangerous thermal state, which is why objective heat policies must not rely on how athletes feel.

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Common mistakes

• Treating a readiness score as data before validating it. Check it against an objective measure for a few weeks. Some athletes track beautifully and some do not track at all.
• Collecting ratings of perceived exertion without an anchor. A ten means nothing until the athlete has experienced a genuine maximal effort and labelled it.
• Using perceived exertion in the heat, when ill, or in a new exercise. These conditions shift the construction. Use objective prescription instead.
• Increasing body focus in an anxious athlete. Over-attention to internal signals is a feature of anxiety, not of expertise.
• Using deception to improve output. It works and it destroys the calibration you need. It also fails once the athlete knows.
• Assuming exhaustion is a mechanical limit. In most sporting contexts it is a cost estimate. That has implications for pacing, motivation and race tactics.
• Treating breathwork as a fitness intervention. Slow breathing reliably changes acute state. It does not change aerobic capacity.
• Ignoring subjective wellness because it is subjective. Validated subjective monitoring frequently detects accumulated stress earlier than physiological markers.

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Coaching cues

• Predict the rating before the set, then compare.
• Once a year, find out what a real ten feels like.
• When it is hot or you are ill, use the clock and the load, not the feeling.
• Give athletes accurate information about how much is left.
• Six breaths a minute, longer out than in, for two minutes.
• Pace the event by training the event.
• If their score never matches their output, use the output.

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FAQs

Is perceived exertion actually reliable?

Reasonably, within limits. It correlates well with objective intensity in familiar exercise modes in trained people, and it is sensitive to accumulated fatigue (Zamunér et al., 2011). It becomes unreliable in heat, dehydration, illness, at altitude, in unfamiliar exercise, and under high emotional arousal, because those conditions shift either the afferent inputs or the expectations that shape the construction. Use it as one input rather than as ground truth.

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Can I train myself to feel less effort at the same intensity?

Partly, and it is not the same as becoming fitter. Familiarity, accurate expectation, better pacing models and reduced uncertainty all lower perceived effort at a given workload. Cooling and menthol lower perceived effort without changing thermal state. What no attentional strategy does is raise the physiological ceiling, so the sensible use is to remove unnecessary perceived cost rather than to try to abolish it.

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Does the heartbeat counting test tell me anything useful?

Less than it appears to. Performance on it is confounded by beliefs about resting heart rate and by time-estimation ability, and many participants estimate rather than detect. Heartbeat discrimination tasks are better but harder to run. For practical purposes, checking whether an athlete’s daily self-reports predict their measured output is more informative than any laboratory interoception task.

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Is body scanning or mindfulness practice worth an athlete’s time?

It has reasonable evidence for reducing anxiety and improving attentional control, and it improves detection of the specific signals it trains. It does not have good evidence for improving physiological capacity, and for athletes who already over-monitor symptoms it can be counterproductive. Treat it as a tool with specific indications rather than as a general upgrade.

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Why does the same run feel completely different on two days with the same pace and heart rate?

Because the feeling is constructed from many inputs and a prediction. Sleep, glycogen status, hydration, ambient temperature, illness, muscle soreness, emotional state, expectation about the session, and how much of it you believe remains all contribute. This is not noise to be eliminated; it is information. A day when everything is objectively normal and the effort feels unusually high is worth investigating.

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If effort is constructed, does that mean athletes can just push through?

No, and this is the most common misreading. The construction is built substantially from real afferent evidence about the state of the working muscle, and the regulatory loop involving group III and IV afferents exists to limit peripheral disturbance. Overriding it experimentally, by blocking those afferents, produces greater peripheral fatigue and impaired subsequent function. The sensation is a construction and it is also usually about something.

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What is the difference between interoception and proprioception?

Proprioception is the sense of body position and movement, carried by large myelinated afferents from muscle spindles, tendon organs and joint receptors to somatosensory cortex (Kandel et al., 2021). Interoception is the sense of internal physiological state, carried by small-diameter afferents and the vagus to the insula. They are separate pathways with separate cortical targets, and an athlete can be excellent at one and poor at the other. Proprioception is covered in article 5.30.

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Recommended videos

Each video below was chosen because it covers a specific part of this article in more depth than text alone allows.

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Related reading on FitXplor

• 5.3 Attention, Arousal, and Performing Under Pressure
• 5.26 The Limbic System: Amygdala, Hippocampus, and Emotional Learning
• 5.21 The Cerebral Cortex: Lobes, Layers, and Functional Organisation
• 1.7 Recovery Science and Adaptation
• 5.13 Stress, Cortisol, and Allostatic Load in Training

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References

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Barrett, L. F., & Simmons, W. K. (2015). Interoceptive predictions in the brain. Nature Reviews Neuroscience, 16(7), 419–429.

Chambers, E. S., Bridge, M. W., & Jones, D. A. (2009). Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. Journal of Physiology, 587(8), 1779–1794.

Craig, A. D. (2002). How do you feel? Interoception: the sense of the physiological condition of the body. Nature Reviews Neuroscience, 3(8), 655–666.

Craig, A. D. (2009). How do you feel — now? The anterior insula and human awareness. Nature Reviews Neuroscience, 10(1), 59–70.

Critchley, H. D., Wiens, S., Rotshtein, P., Öhman, A., & Dolan, R. J. (2004). Neural systems supporting interoceptive awareness. Nature Neuroscience, 7(2), 189–195.

Garfinkel, S. N., Seth, A. K., Barrett, A. B., Suzuki, K., & Critchley, H. D. (2015). Knowing your own heart: distinguishing interoceptive accuracy from interoceptive awareness. Biological Psychology, 104, 65–74.

Hilty, L., Langer, N., Pascual-Marqui, R., Boutellier, U., & Lutz, K. (2011). Fatigue-induced increase in intracortical communication between mid/anterior insular and motor cortex during cycling exercise. European Journal of Neuroscience, 34(12), 2035–2042.

Marcora, S. M. (2009). Perception of effort during exercise is independent of afferent feedback from skeletal muscles, heart, and lungs. Journal of Applied Physiology, 106(6), 2060–2062.

Mehling, W. E., Price, C., Daubenmier, J. J., Acree, M., Bartmess, E., & Stewart, A. (2012). The Multidimensional Assessment of Interoceptive Awareness (MAIA). PLoS ONE, 7(11), e48230.

Pageaux, B. (2016). Perception of effort in exercise science: definition, measurement and perspectives. European Journal of Sport Science, 16(8), 885–894.

Seth, A. K. (2013). Interoceptive inference, emotion, and the embodied self. Trends in Cognitive Sciences, 17(11), 565–573.

Stone, M. R., Thomas, K., Wilkinson, M., Jones, A. M., St Clair Gibson, A., & Thompson, K. G. (2012). Effects of deception on exercise performance: implications for determinants of fatigue in humans. Medicine & Science in Sports & Exercise, 44(3), 534–541.

Stevens, C. J., Thoseby, B., Sculley, D. V., Callister, R., Taylor, L., & Dascombe, B. J. (2016). Running performance and thermal sensation in the heat are improved with menthol mouth rinse but not ice slurry ingestion. Scandinavian Journal of Medicine & Science in Sports, 26(10), 1209–1216.

Saw, A. E., Main, L. C., & Gastin, P. B. (2016). Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures. British Journal of Sports Medicine, 50(5), 281–291.

Zamunér, A. R., Moreno, M. A., Camargo, T. M., Graetz, J. P., Rebelo, A. C., Tamburús, N. Y., & da Silva, E. (2011). Assessment of subjective perceived exertion at the anaerobic threshold with the Borg CR-10 scale. Journal of Sports Science & Medicine, 10(1), 130–136.

Kandel, E. R., Koester, J. D., Mack, S. H., & Siegelbaum, S. A. (Eds.). (2021). Principles of Neural Science (6th ed.). McGraw Hill.