AdrenalineEpinephrine

Adrenaline, epinephrine in American usage, is released by the adrenal glands into the bloodstream in response to sympathetic nervous system activation. Where noradrenaline is mostly released locally by nerve endings, adrenaline is the circulating, whole body version of the same message.

Its effects arrive within seconds: heart rate and force of contraction rise, airways dilate, blood is redirected towards muscle, pupils widen, and glucose and fatty acids are released into circulation. It is the physiology of a fright, and of the last thirty seconds of a hard interval.

During exercise it rises with intensity and duration, mobilising fuel and contributing to the alertness of hard training. Competition adds a psychological layer on top, which is why people occasionally lift more or run faster on the day than they ever managed in training.

It also blunts pain perception, which is useful in an emergency and unhelpful in a competition, where an injury can go unnoticed until afterwards.

Clinically it is the treatment for anaphylaxis, given by injection, where its effects on airways and blood vessels are life saving. That is the one context where adrenaline is a medicine rather than a physiological response.

Chronically elevated sympathetic activity is a different matter from these acute surges, and shows up as a raised resting heart rate, suppressed heart rate variability, poor sleep and stalled progress.

In practice, there is nothing to optimise here. Pre-workout products that simply raise sympathetic activity buy alertness at the cost of sleep if taken late, and the useful management is of the chronic side rather than the acute.