The IT band is the cause of a common malady for runners known as IT Band Syndrome. The illiotibial band runs along the outside of your thigh, from the top of your hip (the iliac crest—that bony ridge) down to your knee. I'll go out on a limb and guess that yours is probably tight unless you've been working on it.
What does "working" on your IT band mean? Diligent stretching and breaking up the knots. Use a foam roller to break up any tight spots, but I'll warn you—if you haven't done this before, it's pretty uncomfortable. It might want to make you curl up and cry.
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How to Use a Foam Roller to Massage the IT Band
Lie down on one side, propping yourself up with one elbow, and position the foam roller just under your hip. Now, move your body forward so that the roller works itself down on the outside of your thigh; stop when you get to your knee.
Roll in a gradual, slow motion, and when you reach your knee, reverse directions back up to your hip. Adjust the amount of tension by applying more or less of your body weight on the roller. If you're new at this, your IT band will probably be tender, and you might not even need to apply much weight before you feel it.
When you come to a particularly sore spot, pause and hold it on the roller—this is called applying direct pressure. As you hold the roller on that spot, the pressure will help break up the knot. Only hold it there for about a minute, then do short rolls back and forth over the area to help further release the knot.
You may come across quite a few knots, and you won't be able to break all of them up in a single self-massage session. Think about foam rolling as maintenance, kind of like you would do for your car. You only want to target a particular muscle or tendon for up to 15 minutes at a time. The best way to go about this is to sneak in short sessions after your run, or while you're watching TV. Foam roll on a continual basis instead of ignoring it for a while then going crazy on the roller for an hour once a month.
Only roll to the point of discomfort—yes, it will be tender and sore, but you don't want to go to the point of unbearable pain because you'll just end up doing more damage than good.
After a few days and weeks of consistent rolling, you'll see results, and foam rolling across that IT band will become less of a torturous thought.