Warming up before you exercise helps to prevent injuries and lets you jump higher, run faster, lift heavier or throw further. Your warm-up should involve the same muscles and motions you plan to use in your sport. For example, before you start to run very fast, do a series of runs of gradually-increasing intensity to increase the circulation of blood to the muscles you will be using.
Muscles are made up of millions of individual fibers, just like a rope made from many threads. When you start to exercise at a very slow pace, you increase the blood flow to muscle fibers, increase their temperature, and bring in more oxygen, so the muscles are more pliable and resistant to injury. When you contract a muscle for the first time, you use less than one percent of your muscle fibers. The second time you bring in more fibers, and you keep on increasing the number of muscle fibers used in each contraction for several minutes of using that muscle. It’s called recruitment. When you are able to contract more muscle fibers, there is less force on each individual fiber to help protect them from injury. Usually you are warmed up when you start to sweat.
The same principle applies to your heart. Angina is a condition in which the blood vessels leading to the heart are partially blocked so the person has no pain at rest, but during exercise, the blocked arteries don't permit enough blood to get through to the heart muscles, causing pain. If people with angina exercise very slowly before they pick up the pace, they are able to exercise longer and more intensely before they felt heart pain. Always check with your doctor if you feel any heart pain during exercise.
Competitive athletes in sports requiring speed and endurance perform better after they warm up with increasing intensity. Warming up slowly does not increase the maximum amount of oxygen that you can bring to muscles that you need during competition. If you are a runner, skier, cyclist, or an athlete in any sport that requires endurance, warm up at a gradually increasing pace. Use a series of increasingly intense repetitions of 10 to 30 seconds duration, with short recoveries, until you are near your maximum pace. This type of warm-up increases endurance because intensity increases the maximum amount of oxygen that you can bring to your muscles, as you continue to compete, and lets your muscles contract with greater force as you begin to fatigue. You will then be able to bring in more oxygen to your muscles than you could have done without the intense warm-up.