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

Abstract

Differentiating between respiratory frequency (f_R ) and tidal volume (V_T ) may improve our understanding of exercise hyperpnoea because f_R and V_T seem to be regulated by different inputs. We designed a series of exercise manipulations to improve our understanding of how f_R 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 f_R changes more with RPE than with workload, gas exchange, V_T or the amount of muscle activation. However, f_R dissociates from RPE during moderate exercise. Both V_T and minute ventilation ( ${\dot{V}}_E$ ) showed a similar time course and a large correlation with $\dot{V}{\text{CO}}_2$ in all the tests. Nevertheless, $\dot{V}{\text{CO}}_2$ was associated more with ${\dot{V}}_E$ than with V_T because V_T seems to adjust continuously on the basis of f_R levels to match ${\dot{V}}_E$ with $\dot{V}{\text{CO}}_2$ . The present findings provide novel insight into the differential control of f_R 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.

Keywords: Breathing control; exercise hyperpnoea; perceived exertion; sinusoidal exercise; ventilatory pattern.

Figure 1

Group mean response of mechanical, physiological and perceptual variables during the four sinusoidal tests. Left panels show variables during M_2 (solid line) and M‐S_2 (dotted line), while right panels show variables during M_8 (solid line) and M‐S_8 (dotted line). The Figure depicts filtered second‐by‐second data.

Figure 2

Group mean fitted sinusoidal responses for mechanical, physiological and perceptual variables as a function of the phase angle during the four sinusoidal tests. Workload is depicted in panel (A) for M_2 and M_8 (solid line) and for M‐S_2 and M‐S_8 (dashed line). For panels B–J, variables from the 2‐min and 8‐min sinusoidal tests are depicted in blue and red, respectively; the moderate tests and moderate‐to‐severe tests are depicted by solid and dashed lines, respectively.

Figure 3

Correlations between the phase lag of $\dot{V}{\text{CO}}_2$ and the phase lag of ${\dot{V}}_{\mathrm{E}}$ (A), V _T (B) and f_R (C) for M_2 (open triangles), M_8 (open circles), M‐S_2 (filled triangles) and M‐S_8 (filled circles). Correlations between the amplitude of $\dot{V}{\text{CO}}_2$ and the amplitude of ${\dot{V}}_{\mathrm{E}}$ (D), V _T (E) and f_R (F) for M_2 (open triangles), M_8 (open circles), M‐S_2 (filled triangles) and M‐S_8 (filled circles). For the correlations shown in panels D, E and F, the numbers within each panel depict different participants. These numbers help to highlight that when the amplitude of f_R is relatively low (1, 10, and 12), V _T is relatively high; when the amplitude of f_R is relatively high (2 and 9), V _T is relatively low. The reciprocal changes between V _T and f_R determine a stronger correlation between the amplitude of $\dot{V}{\text{CO}}_2$ and that of ${\dot{V}}_{\mathrm{E}}$ compared to the correlations between the amplitude of $\dot{V}{\text{CO}}_2$ and those of V _T and f_R.

Figure 4

In the left panels, group mean response of second‐by‐second data for workload (A), RMS (C) and HR (E) during the entire trapezoidal test. The RPE required by the test is depicted in dashed lines. In the right panels, 60‐sec average values of workload (B), RMS (D) and HR (F) for the first (filled circles) and the third (open circles) trapezoidal bouts. When a significant bout × time interaction was found, * depicts significant simple main effect of bout (P < 0.05).

Figure 5

In the left panels, group mean response of second‐by‐second data for ${\dot{V}}_{\mathrm{E}}$ (A), f_R (C), V _T (E), $\dot{V}{\text{CO}}_2$ (G)_, $\dot{V}{\mathrm{O}}_2$ (I) and P_{ETCO 2} (K) during the entire trapezoidal test. The RPE required by the test is depicted in dashed lines. In the right panels, 60‐sec average values of ${\dot{V}}_{\mathrm{E}}$ (B), f_R (D), V _T (F), $\dot{V}{\text{CO}}_2$ (H), $\dot{V}{\mathrm{O}}_2$ (J) and P_{ETCO 2} (L) for the first (filled circles) and the third (open circles) trapezoidal bouts. When a significant bout × time interaction was found, * depicts significant simple main effect of bout (P < 0.05).