Final answer:
An increased heart rate is the physiological reaction to stress preparing the muscles for the 'fight or flight' response, as part of the body's autonomic nervous system activation.
Step-by-step explanation:
The physiological reaction to stress that helps prepare the muscles for "fight or flight" is an increased heart rate. When faced with a perceived threat, the autonomic nervous system triggers a cascade of changes in the body. This includes the release of adrenaline and norepinephrine, which result in a number of responses such as pupil dilation, increased sweating, increased blood pressure, and dilation of the bronchi in the lungs to allow more air exchange. The heart rate speeds up to pump more blood to the skeletal muscles, preparing the body to either fight off the threat or flee from it.