Whenever you exercise, be sure to stretch.
Stretch first after your warm-up, when your muscles aren't so tight, and again after the cool-down period. Stretching for five minutes after you warm up will improve your workout and may prevent injuries.
Why? Because repetitive exercise tends to reduce muscle flexibility. Also, tissues like muscle and skin lose elasticity with age. So if you increase the intensity or duration of your workouts, maintaining muscular flexibility in your lower legs, thighs, gluteals and back will become even more important.
Without proper stretching, your range of motion will become limited, which will adversely affect e.g. your running and swimming technique. For example, tightness in the upper shoulder area will reduce the effectiveness of the pull and glide part of your freestyle swim stroke, slowing you down. And tightness in the hamstrings can decrease your stride length in running, forcing you to expend more energy to make up the difference.
There are stretching techniques for almost every major muscle group. If you aren't familiar with different techniques, consult any sports physiologist or coach for advice. Below are some tips to help you stretch properly no matter what technique you use.
Never stretch cold muscles. Be sure to warm them up before stretching.
Hold each stretch for 30-60 seconds to give your muscle time to adapt to the stretch.
Never bounce in a stretching pose or force a muscle into a position that causes pain.
Relax and breathe deeply and slowly while holding each stretch position.
If you are stretching your arms, legs or sides, remember to stretch both sides.