When people begin to exercise beyond the usual load, muscles will first make an emergency response and quickly provide energy by consuming part of muscle tissue. Then, the brain directs blood, with oxygen and nutrients, to quickly gather in muscle tissue, replenish nutrition for muscle, and release hormones to promote muscle tissue to increase toughness. After the exercise, the blood will continue to be in the muscle tissue, continue to provide nutrition for the muscle, and accelerate the muscle growth. This growth process will last for 48 hours. It is also the most primitive physiological response of human beings to the crisis.
Fitness training is to use this physiological reaction to repeatedly stimulate muscle growth every 48 hours. Weight loss classes also use this physiological reaction to continuously interrupt muscle growth and accelerate muscle consumption within 48 hours.