thanks ill try the band workout
another theory
BTW...this is in the Starting Strength book:
"A different problem often encountered in more advanced trainees is the tendency to let the knees slide forward at the bottom. It is usually a problem developed over time, and is a rather embedded movement pattern that can be hard to fix if you let it go uncorrected too long. If your knees move forward at the bottom of the squat, you have relaxed your hamstrings, because hamstrings pull the knee back. They insert on the tibia and provide posterior tension, which should increase with the depth of the squat as the other attachment point on the pelvis tilts away. If this tension is insufficient to keep the knees from sliding forward as the bottom of the squat is approached, something is wrong. And when knees move forward at the bottom, tension is put on the hip flexors as they insert on the ASIS, the anterior superior iliac spine, or point of the hip. The muscles in question - the rectus femoris, the sartorius, and the tensor fascia latae - cross both the hip and the knee joints, and therefore produce movement around both joints (fig. 2-49). In the squat, their knee extensor function (the distal function) is our concern, since active hip flexion does not occur. At the bottom of the squat, these muscles act with the other knee extensors in the quadriceps group to straighten the knee. All the muscles in the group are under tension, but only the three hip flexors cross the hip to attach at the ASIS. Now, if at the bottom of the squat the knee should be allowed to move forward, tension is increased on these muscles and their attachment at the hip as the knee angle becomes more acute (fig. 2-50). The ASIS is pulled on very hard by these muscles at their attachment, and a marvelous dose of the weirdest tendinitis you have ever seen can be the result. This condition is thankfully rare, and some people squat this way for years without trouble, but if it develops it takes many weeks to heal
