Sensation of breathlessness and respiratory oxygen cost during cycle exercise with and without conscious entrainment of the breathing rhythm

Abstract

The conscious entrainment of respiratory rhythm to exercise rhythm (ENT) has been hypothesized to alleviate breathing discomfort and reduce the oxygen (O₂) cost of ventilation with a resulting decrease in total O₂ uptake (V˙O₂) during rhythmic exercise. This hypothesis has been tested in the study reported here. Eight female subjects performed cycle exercise at 50 rpm under two work load conditions of 40% and 60% of maximal V˙O₂. During a 30-min exercise period at each work load, each subject was asked to breathe under two conditions for 15 min each: (1) spontaneously (non-ENT run), and (2) deliberately entraining the breathing rhythm to the cycling rhythm at preferred coupling ratios of the two rhythms (ENT run). In the ENT run, most subjects chose a ratio of 1:2. In each run, pulmonary ventilation (V˙ _E), total V˙O₂ and the breathlessness sensation (BS) were measured at 4–5 min. BS was assessed according to a Borg category scale. The remaining 10 min of each 15-min run were allotted for measurement of the O₂ cost of ventilation (ΔV˙O₂/ΔV˙ _E), assessed by a hypercapnia-induced hyperventilation method in which the V˙O₂ of the respiratory muscles (V˙O_2RM) was calculated by multiplying ΔV˙O₂/ΔV˙ _E by the prevailing V˙ _E. On average, there were no significant differences in any of the variables, V˙O₂,ΔV˙O₂/ΔV˙ _E,V˙O_2RM and BS, between the non-ENT and ENT runs performed at any work load. However, there were wide variations among the subjects in the differences (Δ) between the two runs, and significant correlations were found between ΔV˙O₂ vs ΔV˙ _E, ΔV˙O₂ vs ΔV˙O_2RM, and Δ BS vs ΔO_2RM of individual subjects. These results indicate that reductions of the total V˙O₂ and BS with ENT could occur in subjects in whom the V˙O_2RM decreased during ENT.