It could also be caused by a muscular imbalance. You may want to look at your leg strength in sections. Are your quads (front of leg) strong than your hams, are your hams stronger than quads. A decent ratio for non-injury is a one to one strength. You will normally find very explosive athletes to have a 1:1.5 ratio (quad to ham). On average or in the average athlete it is usually 1:0.75 or worse. This is when injuries occur most often. Your hams being equal or strong is usually better.