Knee injuries, particularly a tearing of the anterior cruciate ligament (or ACL) that stabilises the knee joint, are the most common injuries among sporty types. Those who play tennis, squash, football or go skiing are most at risk, but anyone who takes part in an activity that involves twisting and turning - including dance and aerobics - is vulnerable to the agonising pop and tear of the ACL.

Physiotherapists say that the knee is vulnerable to any sharp changes in direction simply because it is designed to operate as a hinge, not to rotate. Active women are up to eight times more likely to suffer knee problems, partly because hormones such as oestrogen weaken ligaments at different phases of the menstrual cycle, and partly because the wider angle of their hips puts added stress on the knees.

But there are routes to prevention for all. Building the strength of muscles supporting the knee helps to protect it, as do exercises that develop other body parts. Knee workouts should include movements across all three planes: front and back, side-to-side and rotational. Here's how:

One-legged hopping: The American Council on Exercise says that hops are a great way to help the shoulders and abdominals to control what happens at the knees. Stand on one leg and hop over an imaginary straight line ten times. Change feet and repeat.

Squats: Stand with feet shoulder-width apart. Lower your body by bending your knees, with your hips moving back as if you are sitting in a chair. Keep the weight directly over your heels. Bend your knees to about 45 degrees, pause, then slowly return to the starting position.

Two-legged jumps: Jump on the balls of your feet (both feet together) right to left over an imaginary line. Jump around to face the opposite way and then jump left to right. Repeat ten times.

Sideways step-ups: Stand to the side of a step or small platform and step up and down on to it ten times on each leg.

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