Vivian Grisogono

As you lift the leg up, the front-thigh and hip flexor muscles shorten in a concentric contraction, and the same muscles pay out, in eccentric contraction, when you lower the leg down with control. The exercise trains co-ordination in these muscles, and protects the knee. Because the foot is held upwards, the anterior tibial muscles on the front of the shin are worked statically.
Position: Sit up, leaning slightly backwards, or lie on your back on a flat surface with your legs straight in front of you and slightly apart, feet turned slightly outwards, toes pointing up.
Movement: Lock one knee, and keeping your foot turned slightly outwards, lift the leg up as far as you comfortably can while keeping the knee locked absolutely straight. Hold for a count of 2, then slowly lower the leg and relax completely.
Repetitions and frequency: 3-10 times on each leg in turn, 1-3 times a day. If one leg is weaker, do more repetitions for this side.
Note: Avoid pain. Do not try to lift the leg too high. Do not do this exercise if you cannot lock your knee to straighten it completely.
After injury: A late-stage exercise following knee injuries, also useful after other leg injuries, especially to the calf muscles.