Breathing, leg-muscle activity and trunk stabilization became more tightly synchronized as nine adults cycled toward exhaustion, offering researchers a new way to study fatigue as a whole-body process. Sergi Garcia-Retortillo and an international team describe the graded cycling experiment in Physiological Reports, where they propose that the relationships among physiological signals may reveal performance limits that separate measurements miss.
Nine young adults completed graded cycling tests while researchers recorded breathing and electrical activity from leg and trunk muscles. Cross-correlations increased as fatigue accumulated, suggesting that ventilation and muscle activation behave as an integrated network under rising metabolic demand.
Participants completed two graded cycling tests in which workload rose by 25 watts each minute. The team recorded ventilation alongside electrical activity from the vastus lateralis in the thigh and the erector spinae in the trunk, then examined how the changing signals correlated rather than treating each measurement as an isolated output.
The correlations strengthened as riders moved toward exhaustion. The result fits a network-physiology model in which respiratory drive, postural stabilization and force production become increasingly interdependent under metabolic stress. It does not show that the systems move in perfect lockstep, but it suggests that fatigue is accompanied by tighter coordination across them.
The experiment also raises questions about what athletes perceive as “running out of breath.” Near exhaustion, ventilatory demand is rising while locomotor and stabilizing muscles are recruiting more intensely. The nervous system must coordinate posture, force, gas exchange and the sensation of effort at the same time.
Stronger signal coupling could reflect that shared drive or a compensatory strategy as independent reserves shrink. The current data cannot distinguish cause from consequence, and the researchers did not test whether training one part of the network changes the others.
Still, the approach may eventually help explain why an athlete can have normal lung function yet experience breathing as a performance limiter. It could also be relevant to rehabilitation, where disease or deconditioning alters the relationship between respiratory effort and movement. Those applications remain prospective, but they give the small laboratory finding a clear research path.
The sample is too small to prescribe a new training method, and synchronization is not proof that consciously changing the breath improves performance. The study is most useful as a mechanistic window into why breathing becomes inseparable from pacing and muscular fatigue during hard exercise.
For performance science, the method is as important as the result. Conventional testing may report oxygen uptake, breathing rate or muscle activation separately. A network approach asks whether the timing and strength of the relationships among those signals reveal an approaching performance limit sooner than any single number.
The study’s nine-person sample makes it a starting point, not a training prescription. Its contribution is a method for studying fatigue as coordination among systems, which larger studies can now test across sports, ages and fitness levels.
The next step is replication in larger and more varied groups, including trained athletes and older adults. Researchers will also need to connect signal coupling with outcomes coaches recognize, such as pace loss, technique breakdown or recovery time. Until then, the study offers a new map of fatigue rather than a new breathing drill.
