The IT Band is connective tissue that doesn’t forgive bad posture the way muscles do. There is endless amount of connective tissue in the body and the IT Band is a particularly dense portion of this material that serves to stabilize the knee in walking because a muscle that long wouldn’t be able to accomplish the task.
The negative byproduct of the need for connective tissue to do this is that is it much stronger and more rigid than muscle. When we let our thighs sink forward (everybody does it, just some less than others) our muscles suffer terribly. The glutes, hamstrings and piriformis all chronically shorten while the quadriceps lengthens and bears an inordinate amount of weight from the upper body.
The tough fibrous IT band does not alter its length or alignment when the pelvis and thighs shift down and forward—it stays right where it is and causes plenty of problems because it does. The standard function of the IT Band, and its relation the tensor fasciae latae, is to abduct and internally rotate the hip while stabilizing the knee. If it stays where it is when the thighs lean forward it is being pulled constantly into external rotation.
This is why so many people suffer with IT Band issues, especially when it comes to runners. We need to take our thighs back and get the legs under the pelvis and back in line with the IT Band.