Respiratory frequency and tidal volume during exercise: differential control and unbalanced interdependence

Abstract

Differentiating between respiratory frequency (fR) and tidal volume (V T) may improve our understanding of exercise hyperpnoea because fR and V T seem to be regulated by different inputs. We designed a series of exercise manipulations to improve our understanding of how fR and V T are regulated during exercise. Twelve cyclists performed an incremental test and three randomized experimental sessions in separate visits. In two of the three experimental visits, participants performed a moderate‐intensity sinusoidal test followed, after recovery, by a moderate‐to‐severe‐intensity sinusoidal test. These two visits differed in the period of the sinusoid (2 min vs. 8 min). In the third experimental visit, participants performed a trapezoidal test where the workload was self‐paced in order to match a predefined trapezoidal template of rating of perceived exertion (RPE). The results collectively reveal that fR changes more with RPE than with workload, gas exchange, V T or the amount of muscle activation. However, fR dissociates from RPE during moderate exercise. Both V T and minute ventilation (V˙E) showed a similar time course and a large correlation with V˙CO2in all the tests. Nevertheless, V˙CO2 was associated more with V˙E than with V T because V T seems to adjust continuously on the basis of fR levels to match V˙E with V˙CO2. The present findings provide novel insight into the differential control of fR and V T – and their unbalanced interdependence – during exercise. The emerging conceptual framework is expected to guide future research on the mechanisms underlying the long‐debated issue of exercise hyperpnoea.

A new model of ventilatory control during exercise

“…Conversely, what is not so well known is that V T is continuously adjusted on the basis of f R levels to match alveolar ventilation with metabolic requirements. This is evident from our data showing that the close association observed between and carbon dioxide output is mediated by V T but not f R (Nicolò et al., ). Therefore, the ‘unbalanced interdependence’ feature of our model suggests that V T changes continuously on the basis of f R levels, while f R is affected by V T to a more limited extent.…”

“…The ‘differential control’ feature suggests that f R is mainly regulated by fast inputs, but not metabolic inputs (Nicolò et al., ,b, ). Conversely, V T is substantially regulated by metabolic inputs.…”

“…From a functional perspective, this model suggests that the ventilatory system serves diverse functions via two different operating components. Respiratory frequency, the behavioural component, responds to a variety of non‐metabolic stressors, whereas V T , the metabolic component, is adjusted to meet metabolic requirements in any conditions (Nicolò, Girardi, & Sacchetti, , ).…”

“…Beyond measuring RPE, we proposed an original experimental design consisting of the manipulation of different exercise protocols, thus providing valuable insight into the differential regulation of f R and V T . Drawing on the results of these two studies, a third study involving a series of exercise manipulations provided a new framework for understanding how f R and V T are regulated during exercise (Nicolò et al., ).…”

“…However, indirect evidence should be supported by direct evidence obtained by more mechanistic studies (Zenon et al., ). Valuable insight into ventilatory control may come from the manipulation of exercise protocols as experimental interventions and from the measurement of RPE as a marker of central command (Nicolò et al., , ). Moreover, further knowledge on the control of ventilation can be obtained by switching our focus of attention from the control of to the control of f R and V T .…”