The quadriceps (as well as the glutes) will develop most from full depth in squatting and lunging movements, as they are responsible for knee extension. Do not worry about isolating the quadriceps; they are the biggest muscle group in the body followed by the glutes at number two, and they will develop very easily from these movements. Nothing is more unnecessary than leg extensions due to the fact that they have been linked to knee pain (such as patellar tendinitis), and research has shown they cause higher amounts of stress on the knee (Steinkam, et al, 1993). Furthermore, leg extensions are not applicable to everyday life or athletics. How many times do we need to extend our knee against opposing forces pushing from the anterior side of our lower leg? It is not a natural movement nor is it a natural way for the muscles controlling the knee to be trained. As stated before, the quadriceps will develop greatly from our squatting and lunging movements. The lower the hips go, with safe technique, the more the hip extensors (glutes and hamstrings) and knee extensors (quadriceps) will be worked (Delavier, 2001).
For coaching pointers, technique tips and video demonstration with squats and lunges, visit www.powerelevation.com. These are the best exercises for the quadriceps, glutes, and hamstrings, but perfect technique is necessary to stay safe.
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Delavier, F. (2001). Strength Training Anatomy. Champaign, IL: Human Kinetics.
Steinkam, L.A. Biomechanical Considerations in Patellofemoral Joint Rehabilitation. The American Journal of Sports Medicine. 21: 438-444.