3 percent). In contrast, when the work of inhalation was increased by having the person breathe against a resistance, leg blood flow decreased (7 percent). The changes in leg blood flow were mediated by changes in the neural input to the blood vessels in the limbs—the blood vessels narrowed (vasoconstricted) when inspiratory work was increased, and they dilated when inspiratory work was reduced. , 2002). What’s a metaboreflex? When muscles work very hard, metabolic by-products, such as lactic acid, accumulate within them.

As a result, muscles fatigue more quickly, and exercise performance is impaired. Indeed, this is precisely what has been found; changing the work of breathing to hasten or delay the onset of the inspiratory muscle metaboreflex changes leg fatigue and exercise performance in a highly predictable way—that is, increasing inspiratory muscle work hastens metaboreflex activation, accelerates limb fatigue, and impairs performance (reducing inspiratory muscle work does the opposite). It is thought that this metaboreflex may be a protective mechanism that forces people to slow down or stop in order to prevent catastrophic levels of breathing fatigue.

During exercise, air must flow in and out of the lungs more quickly than at rest, and the breathing pump muscles must contract more forcefully to generate the required increase in volume and airflow rate. Like other muscles, the breathing pump muscles have a finite ability to generate force (which results in the generation of pressures within the thorax), and this limited ability is one of the limiting factors to breathing during exercise. The ability of the breathing muscles to generate the pressure differentials that make breathing happen (negative pressure for inhalation and positive pressure for exhalation) is influenced by the act of breathing itself.