A common gym exercise is the knee extension machine, where the person sits on the machine and straightens the knee against a pad which is connected via cams to the weights. Although it seems like a good exercise it is very unnatural in function as this movement does not really occur in normal life, and the stresses are placed strongly on the area of the patellar tendon. This exercise may be useful in the early stages but for further strengthening more functional movements are required.
Squatting related movements such as gym ball squats, squatting, one leg bends and leg presses are more functional exercises and can be appropriately progressed. Forces are transmitted through the hips and the knees at normal angles and the muscles around these joints co-contract to achieve the raising or lowering of the body weight. Weights can be progressed steadily as these muscles are very strong and may need significant intensity of stimulation to become stronger.
Final strengthening needs to include co-ordination, balance and joint position sense so the physiotherapist will include active and dynamic exercises involving control of the knee joint in challenging manoeuvres. The physio then progresses the patient to sport specific exercises as fitness for an activity is closely related to performing the specific movements required by that activity.