Dai Kawakita
Heart rate-synchronized breathing (HRSB) -- a technique in which the respiratory cycle is entrained to the cardiac cycle -- may optimize autonomic nervous function and ventilatory efficiency, potentially improving exercise tolerance. However, its effect on the anaerobic threshold (AT) remains unclear. This study examined the influence of HRSB on AT and related physiological indices during incremental cycle ergometry in healthy young adults. Three healthy male adults (mean age: 22.3 ± 1.2 years) underwent two cardiopulmonary exercise tests on separate days: Day 1 (normal breathing, control) and Day 2 (HRSB intervention). During HRSB, participants inhaled for 2 heartbeats and exhaled for 4 heartbeats (2:4 ratio) guided by auditory cardiac-cycle cues. Respiratory-cardiac entrainment rate, AT oxygen uptake (AT-VO₂), ventilatory equivalent for oxygen (VE/VO₂), and subjective rating of perceived exertion (RPE) were compared within individuals. Participants A and B demonstrated improvements in AT-VO₂ (+2.0 and +1.3 mL/min/kg, respectively), reductions in VE/VO₂ (25.0 to 22.1 and 24.3 to 21.8), and lower RPE scores (13 to 11 and 13 to 12). Participant C showed a marked decline in AT-VO₂ (-4.9 mL/min/kg) and a substantial increase in VE/VO₂ (16.0 to 25.7), indicating deteriorated ventilatory efficiency. HRSB may improve exercise tolerance and ventilatory efficiency in some individuals, likely through enhanced parasympathetic activity and baroreflex sensitivity. However, outcomes varied markedly between participants, suggesting that individual differences in autonomic response and respiratory control capacity significantly moderate the effect. Future studies with larger samples and individualized resonance-frequency assessment are warranted.