In an attempt to prevent injury, most of us stretch before exercise and some (though not as many) stretch afterwards.

But if we’re doing fairly steady activities (jogging, cycling or walking) then while stretching will improve our flexibility there is not much evidence it will prevent muscle injury. In these cases the advice is to warm up with five minutes of light, gradual exercise instead.

The reason for this is that it’s important to increase the blood flow to the muscles and heart steadily, sudden changes will cause muscles to fatigue more quickly.

Stretching is more effective if you’re doing the type of exercise where you land constantly and forcefully on your feet, or stop and start quickly (sprinting, tennis, weight-lifting, or competitive team sports like football). The more flexible the muscles are the more impact they’ll be able to sustain.
But everyone benefits from stretching after a workout. During exercise lactic acid builds up in your muscles. This can lead to soreness and fatigue.

Stretching after a workout helps to eliminate the lactic acid and restore the muscle to its relaxed state, preventing the fatigue. By Sarah Davey