When it comes to stress, exercise and sheer terror are one and the same.
An adaptive and vitally important characteristic of the nervous system is its ability to increase arterial pressure almost instantaneously. This can take place in times of good stress (exercise, getting out of the way of an incoming bus), but also in times of bad stress (loss, grief, calamity, adversity, job strain).
During dangerous situations (real or perceived as they may be), arterial pressure rises to as high as twice its normal value within a few seconds. This spontaneous alarm reaction triggers a dramatic increase of arterial pressure that can immediately supply blood to any or all muscles of the body needed to respond. This translates into an enormously increased ability to fight against or to flee from the cause of danger.
The exact same increase in arterial pressure occurs during sports activities or exercise. During heavy exercise, a greatly increased blood flow is required for the muscles to function. Part of the increase results from local vasodilatation of the muscle veins and arteries, which is caused by increased metabolism of the muscle cells. The additional increase results from the elevation of arterial pressure caused by the nervous system during exercise. In most heavy exercise, arterial pressure can rise by about 30 to 40 percent, which increases blood flow to muscles and other body structures by twice its normal level.
One system, multiple applications
How can the same effect take place during a voluntary activity such as sport or exercise and during an involuntary reaction such as the alarm response? This is because the increase in arterial pressure during exercise and alarm results mainly from the activity of the brain stem, the oldest and more primordial part of the human brain. The brain stem does not know, and we might say does not care, what triggers the demand for additional resources. All it knows, with or without higher brain interventions (such as a decision to be afraid of something, or a decision to exercise), is that more blood is needed immediately to fulfill physical demands that may already be occurring (in the case of exercise or a real and impending threat) or that may be presumed to occur (in the case of perceived danger in a situation).
When the motor areas of the brain become activated by a decision to exercise, most of the reticular activating system of the brain stem is also mobilized. This activation includes greatly increased stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor center of the brain stem. Thus, the increase in arterial pressure permits to keep pace with the increase in muscle activity.
In the same way, a similar rise in pressure occurs during stressful situations. The need to prepare for a fight or to enable a swift departure from the scene of danger (the fight or flight alarm response), mobilize the reticular activating system and the vasomotor center of the brain stem.
The only real difference in these situations is the involvement of the individual’s pre-frontal cortex, the higher functioning areas of the brain. It is indeed a significant survival factor that no such involvement is needed when a split-second mobilization is required.