Fig. 2.

Schematic illustrating an adaptation of Wasserman’s classic “Gears” diagram. It demonstrates Wasserman’s conception of how the respiratory, cardiovascular and neuromuscular systems conflate to enable exercise to be sustained. O₂ flows from the atmosphere through the lungs, pulmonary and peripheral circulation to the muscle mitochondria where it is ultimately consumed. CO₂ produced by the contracting muscle flows along the same pathway in reverse. Muscle work leads to increased cardiac output and redistribution of blood flow, and increased ventilation in response to both the increased metabolism and evolution of CO₂ from the blood as the result of lactic acid buffering. The efficacy of these processes determines the ability to sustain muscular exercise. These concepts are reconsidered in this review within the context of critical power. This figure was created with BioRender.com and was exported under a paid subscription. $\dot{Q}$CO₂ cellular carbon dioxide production, $\dot{Q}$O₂$/\dot{V}$O₂ matching matching of oxygen delivery to local oxygen consumption, $\dot{Q}$ O₂ cellular oxygen consumption, $\dot{Q}/\dot{V}$ matching matching of ventilation to perfusion, $\dot{V}$_A alveolar ventilation, $\dot{V}\mathrm{CO}2$ Pulmonary carbon dioxide production, $\dot{V}$_D dead space ventilation, $\dot{V}$_E minute ventilation, $\dot{V}$O₂ pulmonary oxygen consumption.

Adapted from Wasserman et al. [156], with permission