You see, the knee isn’t designed to twist, but it’s positioned between two areas that twist all the time – your hip and your foot. And if one of those areas stiffens up, then your knee is often forced to take up the slack. Over time, you’ll start to put pressure through your knees in places that aren’t designed for it, and eventually you notice pain and stiffness. Equally, if you have an accident (a fall when skiing, or you get tackled when your foot is planted), the knee can often get forcibly twisted, damaging the ligaments or cartilage – the anterior cruciate ligament, or ACL, is particularly prone to tearing when you twist your knee.