Quadriceps

The quadriceps are four muscles:

(i) Rectus Femoris (ii) Vastus Medialis (iii) Vastus Lateralis (iv) Vastus Intermedius

Their function is knee-joint extension (the rectus femoris is also a hip flexor). In quadriceps training such as the seated leg extension, the working arc is about 90 degrees, from knee bent at 90° to straight (the knee is a hinge joint; its full flexion range is larger, roughly 133–142° in adults — see Knee Joint Range of Motion). The function of vastus lateralis and vastus medialis is to pull equally from each side; if one develops more, the kneecap moves toward it. The function of the patella (kneecap) is to increase force area and improve the mechanics of the knee joint. The medialis becomes strongest during the last 15 degrees of a knee-extension movement. Years of overuse can produce "Patellar Tracking Dysfunction (PTD)," in which the patella does not track straight when the knee flexes/extends (lateral tracking). The penniform quadriceps have a greater cross-section and produce more force.

Listed exercises: squats (back and front) — all 4 quads involved; leg extensions — all 4 quads involved; 45-degree leg press — little involvement of rectus femoris.

• 4 quadriceps: rectus femoris, vastus medialis, vastus lateralis, vastus intermedius.
• Function: knee extension (rectus femoris also flexes the hip).
• Leg-extension working arc: ~90° (hinge joint); full knee flexion is ~133–142° in adults.
• Vastus lateralis & medialis should pull equally; imbalance shifts the kneecap.
• Vastus medialis is strongest during the last 15° of knee extension.
• Patella increases force area and improves knee mechanics; PTD = lateral patellar mistracking.