The hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system (SNS) play roles in the stress response and the production of glucocorticoids (cortisol) and epinephrine, respectively. Exercise is a form of stress, requiring greater energy supply in somatic tissues, and activates both the HPA axis and SNS. Glucocorticoids promote gluconeogenesis - glucose synthesis - in the liver, while epinephrine stimulates ATP production. Exercise is not associated with fear or threat like psychological stress, so there must be some underlying differences.