The graph shows respiratory Flow along with the HR. FLOW is more reactive than HR in responding to work done. Furthermore, when the athlete reaches maximal pressure one experiences "Expiratory Flow Limitation" . The flow then starts to decrease when expiratory flow cannot be further increased by increasing expiratory muscles effort (i.e., by increasing pleural and alveolar pressure) because it is maximum at that tidal volume. The only way to increasing expiratory flow is to gradually increase operating lung volumes (moving towards the total lung capacity). Real-time feedback from expiratory flow can help an athlete to try and increase the expiratory flow if it starts to drop.
 Expiratory Flow Limitation Definition, Mechanisms, Methods, and Significance hindawi.com/journals/pm/2013/749860/
 The respiratory muscles during exercise