The autonomic nervous system governs bodily functions that generally happen without conscious control — heart rate, blood vessel diameter, digestion — and during exercise, it makes the specific adjustments that let the cardiovascular system meet the muscles' rising demand for oxygen and fuel. What's easy to overlook is that this activation doesn't wait for exercise to actually begin: heart rate and other autonomic changes measurably rise in the moments just before physical activity starts, driven by anticipation rather than by any muscle actually working harder yet.
Anticipation triggers a real, measurable physiological response
This anticipatory rise, often called a pre-exercise or anticipatory response, is a genuine autonomic nervous system activation, not merely a subjective feeling of nervousness or excitement with no physical correlate — heart rate can rise measurably in the seconds before a runner's starting gun fires, driven by signals from the brain rather than from any actual increase in the muscles' current oxygen demand, since the muscles themselves haven't yet begun working any harder at that point. This anticipatory activation is generally understood as the nervous system preparing the cardiovascular system in advance for a demand it expects is about to arrive, rather than waiting to react only once that demand has already materialised.
The autonomic system has two branches pulling in opposite directions
The autonomic nervous system's exercise response, anticipatory or otherwise, works through two distinct branches that generally pull in opposite directions: the sympathetic branch, which raises heart rate, redirects blood flow toward working muscles and away from less immediately essential functions like digestion, and generally prepares the body for increased physical demand; and the parasympathetic branch, which does roughly the reverse, promoting a calmer, lower-heart-rate resting state. During anticipation and exercise, sympathetic activity generally increases while parasympathetic activity is generally suppressed, and the balance between the two branches shifts back the other way during recovery once the physical activity itself has ended, gradually returning heart rate and blood flow to their resting baseline.
What we're still unsure about
The existence of a genuine anticipatory autonomic response before exercise, and the broad sympathetic-versus-parasympathetic framework underlying it, are well established through extensive heart-rate and physiological monitoring research. What's less precisely settled is exactly how much of a given individual's specific anticipatory response is shaped by genuinely learned prior experience with a specific activity, as opposed to a more general, less experience-dependent anticipatory mechanism — trained athletes anticipating a familiar, well-rehearsed event often show a different, more calibrated anticipatory pattern than someone facing an unfamiliar physical challenge, and researchers are still working out exactly how much of that difference reflects learning specifically, versus other individual physiological differences that aren't primarily about prior experience at all.
This sits inside Exercise Physiology & the Autonomic Nervous System, one of seven topics in Physiology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.