You've heard that before exercising, it's a good idea to do some dynamic stretching that includes swinging your legs and arms to improve your range of motion. This not only benefits your muscles, says Thornton, but it's also a great way to loosen up your fascia. When your muscles stay tight, the fascia clings to them, and both can become rigid over time.
The fix: Take a brisk walk or jog lightly in place, and do some sports-specific moves (for example, leg swings for runners, or shoulder rolls for swimmers). Or try the lazy woman's warm-up: just start your workout, but stay at a slow, easy, I could-do-this-for-hours pace for about five minutes.