Leg Extension is an example of an open chain kenetic exercise (i.e isolated exercise) Not only is the Leg Extension not functional to every day life it can be potentially dangerous. –

Since the load is placed on the distal head of the Tibia, the line of load is perpendicular to line of muscle action across the knee joint. This pulls the patella up against the femur increasing patella-femoral joint compression forces (I.e draws the patella back onto the femur) –

As patella- femoral Pain (or anterior knee pain) is possibly the most common form of knee pain presented to Sports medicine professionals, this exercise is one to be avoided at a heavy load. –

Leg Extension has been shown to be less effective in stimulating the quadriceps when compared to the squat exercise according to EMG research. Exercises such as leg press, squat, deadlift and lunges are far more functional and effective in training the quadriceps than the leg extension