"Warming up and cooling down are good for your exercise performance — you’ll do better, faster, stronger — and for your heart since the increased work on the heart ‘steps up’ with exercise,” said Richard Stein, M.D., professor of cardiology in the Department of Medicine at New York University and co-director of Cardiology Consult Services.
“Stretching also makes many people feel better during and after exercise and in some people decreases muscle pain and stiffness.” When done properly, stretching activities increase flexibility.
A good warm-up before a workout dilates your blood vessels, ensuring that your muscles are well supplied with oxygen. It also raises your muscles’ temperature for optimal flexibility and efficiency. By slowly raising your heart rate, the warm-up also helps minimize stress on your heart.
“Warming up before any workout or sport is critical for preventing injury and prepping your body,” said Johnny Lee, M.D., director of the Asian Heart Initiative at the New York University Langone Medical Center and president of New York Heart Associates in New York City.
“Stretching allows for greater range of motion and eases the stress on the joints and tendons, which could potentially prevent injury. Warming up, such as low-heart rate cardio, prepares the circulatory and respiratory system for the upcoming ‘age- and type-appropriate target heart rate’ exercising, whether it’s endurance or sprint type of activities.”
The cool-down is just as critical. It keeps the blood flowing throughout the body. Stopping suddenly can cause light-headedness because your heart rate and blood pressure drop rapidly.
Before you exercise, think about warming up your muscles like you would warm up your car. It increases the temperature and flexibility of your muscles, and helps you be more efficient and safer during your workout. A warm-up before moderate- or vigorous-intensity aerobic activity allows a gradual increase in heart rate and breathing at the start of the activity.
Cooling down after a workout is as important as warming up. After physical activity, your heart is still beating faster than normal, your body temperature is higher and your blood vessels are dilated. This means if you stop too fast, you could pass out or feel sick. A cool-down after physical activity allows a gradual decrease at the end of the episode.
It’s good to stretch when you’re cooling down because your limbs, muscles and joints are still warm. Stretching can help reduce the buildup of lactic acid, which can lead to muscles cramping and stiffness.
So do your body a favor. Take time to gradually progress into your workout and cool down when you’re done being physically active.
ref: https://www.heart.org/en/healthy-living/fitness/fitness-basics/warm-up-cool-down