As you move around, they stretch to their normal length. Then when you start to exercise, your muscles stretch even more, to about 10 percent longer than resting length. This means you have a 20 percent change in muscle length from the time you get out of bed until your muscles are well warmed up.
According to basic laws of physics, muscles work more efficiently when they are longer; they can exert more force with less effort. This means, too, that longer muscles are much less prone to injury.
Make it a practice to warm up before a run or race. Pedal for a few minutes indoors on a stationary bike, or jump rope for a few turns before you head down the road. If you'd rather warm up on the run, begin with a walk or a slow jog and gradually move into your training pace.
Cooling down can also help you avoid injury. An easy jog after a hard workout or race has been shown to speed recovery by helping remove any lactic acid that may have accumulated. It also gently brings your muscles back to a resting state.
A good warmup and cooldown are especially important before and after a hard workout such as intervals or a race in which you push your muscles to their limits.
The extra time you spend warming up your muscles before a training run or race and cooling down afterward is worth the effort in improved efficiency and decreased likelihood of getting injured.