Breathing Control During Exercise Ignores Muscle Size, Study Finds

Every time you stand up from a chair and start walking, your breathing adjusts within seconds — long before carbon dioxide levels in your blood have had time to change. This rapid fine-tuning of ventilation is one of the most elegant feats of human physiology, and it depends in part on a property called peripheral hypercapnic chemosensitivity, the speed and magnitude with which the carotid bodies respond to a transient rise in carbon dioxide. A new exploratory study published in Physiological Reports set out to determine whether the size of the muscles you recruit during exercise changes that sensitivity, and the answer turned out to be a surprising no.

Researchers at the University of Waterloo recruited twenty-two healthy young adults — eleven women and eleven men — who exercised more than three times per week. Each participant completed a single laboratory session involving three very different exercise tasks: rhythmic handgrip, which activates only the small muscles of the forearm; single-leg extension, which engages the quadriceps and surrounding muscles of one limb; and cycling, a whole-body activity recruiting large muscle groups in both legs. The team, led by Armando J. Aguero, Connor J. Doherty and Paolo B. Dominelli, hypothesized that chemosensitivity would climb in step with the amount of active muscle, so that cycling would produce the largest boost and handgrip the smallest.

To measure peripheral hypercapnic chemosensitivity, the researchers used a clever transient stimulus. On four occasions during each resting and exercising condition, participants unknowingly received two breaths of a gas mixture containing 10 percent carbon dioxide, delivered through a silent manual valve switched by investigators watching real-time ventilatory flow. Because the stimulus lasted only two breaths, it was far too brief to engage the brain’s central chemoreceptors, which respond to slower, sustained changes in carbon dioxide. Instead, the response was attributed to the peripheral chemoreceptors, chiefly the carotid bodies. The sensitivity was then calculated as the change in inspired ventilation divided by the change in end-tidal carbon dioxide pressure, expressed in liters per minute per millimeter of mercury.

The exercise intensities were deliberately kept low. Handgrip was performed against a 2.5-kilogram load at thirty contractions per minute with a one-second-on, one-second-off rhythm. Single-leg extension pushed against a constant five watts of resistance on a custom ergometer at the same cadence. Cycling was performed at forty watts at a self-selected cadence of roughly sixty to eighty revolutions per minute. These modest workloads were chosen to ensure that the metabolic byproducts of exercise would be minimal, reducing the contribution of the muscle metaboreflex — the reflex triggered by metabolite accumulation in working muscle — and allowing the team to focus on neural mechanisms such as central command and mechanoreceptor feedback.

The results confirmed half of the hypothesis. All three exercise modalities produced a significant increase in peripheral hypercapnic chemosensitivity from rest to exercise. Handgrip raised sensitivity by about 11.9 percent, single-leg extension by roughly 52.4 percent, and cycling by approximately 49.2 percent. Heart rate, ventilation, tidal volume and the ventilatory response to the carbon dioxide bolus all rose during the active conditions, with the largest changes seen in the leg exercises. But the critical comparison between single-leg extension and cycling revealed the surprise: despite cycling recruiting far more muscle mass, the two exercises produced statistically indistinguishable increases in chemosensitivity, both in absolute terms and as a percentage change from baseline.

This finding challenges a straightforward assumption about how the body scales its ventilatory response. The researchers had reasoned that larger active muscle mass would demand greater central command — the feedforward signal from the brain’s motor and autonomic centers that drives anticipatory increases in heart rate and ventilation before and during exercise. Central command is known to increase in proportion to both exercise intensity and the amount of muscle recruited, progressing from isolated small muscle groups to combined limb exercise. If central command were the sole driver of chemosensitization, cycling should have outpaced single-leg extension. It did not.

The study’s authors suggest instead that the key factor is the transition from rest to exercise itself, rather than the magnitude of the effort. This interpretation is consistent with their earlier work showing that chemosensitivity rises when cycling begins but plateaus with further increases in workload. Together, the two findings paint a picture in which the act of initiating movement — engaging central command and muscle mechanoreceptors — flips a switch that sensitizes the carotid bodies, but adding more muscle or more intensity does not turn that switch further. The similar responses during single-leg extension and cycling imply that once a threshold of neural activation is crossed, recruiting additional muscle mass adds little to the chemosensory gain.

Mechanoreceptor feedback may also play a role. Type III muscle afferents respond to mechanical stretch and contraction and can augment sympathetic outflow and ventilatory drive early in exercise. Because single-leg extension and cycling stimulate more mechanoreceptors than handgrip, the intermediate rise in chemosensitivity seen in the leg exercises is most consistent with concurrent signaling from both central command and mechanoreceptor feedback. The authors caution, however, that voluntary exercise inherently involves both mechanisms, and their design cannot fully separate the contributions of each. Type IV chemoreceptive afferents, which respond to metabolic stress, are unlikely culprits given the low exercise intensities used and prior evidence that metaboreceptor feedback does not increase chemosensitivity beyond the onset of exercise.

Several methodological considerations shaped the interpretation. Resting chemosensitivity was higher when participants were seated on the cycle ergometer than during handgrip, which the authors attribute to an anticipatory response driven by central command before exercise begins — a phenomenon they had observed previously. Pre-stimulus end-tidal carbon dioxide also differed across conditions: it rose during the leg exercises, creating a mildly hypercapnic environment that would amplify the ventilatory response, but fell during handgrip, which should have blunted the response. Notably, handgrip still produced a significant increase in chemosensitivity despite this hypocapnic baseline, demonstrating that the exercise-related sensitization was robust enough to overcome an attenuating condition. The authors also note that end-tidal carbon dioxide serves as a surrogate for arterial carbon dioxide, a substitution that is reasonable at low exercise intensities where the gradient between the two is minimal.

The broader implications reach into exercise physiology, respiratory medicine and perhaps athletic performance. Understanding how the carotid bodies are sensitized during movement could inform approaches to conditions in which ventilatory control is disordered, and the finding that chemosensitization does not scale with muscle mass simplifies the search for the underlying neural signal. The authors propose that future studies use electrically evoked, non-volitional muscle contractions to activate muscle without engaging central command, or block muscle afferents with intrathecal fentanyl while preserving motor drive, to disentangle the two candidate mechanisms. For now, the message is clear: when it comes to sharpening the body’s carbon dioxide sensors, starting to move matters far more than how much muscle you move with.

Subject of Research: Effect of active muscle mass on peripheral hypercapnic chemosensitivity during exercise in healthy adults

Article Title: Greater muscle mass does not affect peripheral hypercapnic chemosensitivity during exercise in healthy adults

Article References: Aguero, A. J., Doherty, C. J., & Dominelli, P. B. (2026). Greater muscle mass does not affect peripheral hypercapnic chemosensitivity during exercise in healthy adults. Physiological Reports, 14(17), Article e71102. https://doi.org/10.14814/phy2.71102

Image Credits: AI Generated

DOI: 10.14814/phy2.71102

Keywords: peripheral hypercapnic chemosensitivity, carotid bodies, exercise physiology, central command, mechanoreceptor feedback, ventilatory control, carbon dioxide sensitivity, rhythmic handgrip, cycling, muscle afferents, Physiological Reports, respiratory physiology