The quadriceps is the name given to the group of four muscles located to the front of the upper leg which are responsible for the extension of the knee. By supplying resistance to the extension of the knee, the leg extension machine successfully targets all four quadriceps muscles.
Many athletes believe that by changing the direction the toes are pointing during the leg extension we can shift emphasis onto either the inner (mainly the vastus medialis), or the outer (vastus lateralis) quadriceps. To what extent this is true, if at all, is unknown to us at present, although it is a fairly wide spread belief.
Being an isolation movement, the leg extension is probably best viewed as a supplementary lower body exercise, with multi-joint movements such as the squat and deadlift considered the staple. A great use of the leg extension by bodybuilders is just prior to squats, enabling the bodybuilder to pre-exhaust the quadriceps before the squat to make the exercise more challenging for the target muscle.
The leg extension can also play a critical role for those who are rehabbing from a knee injury, allowing the athlete to slowly build up the quadriceps strength and muscle hypertrophy.