Which exercise primarily targets the quadriceps?

The reason squats are considered to primarily target the quadriceps is that this exercise involves a significant knee extension movement, which is the primary action of the quadriceps muscles. When performing squats, the quadriceps are heavily engaged to control the movement as you lower your body and to extend your knees as you rise back to a standing position.

Though other exercises like deadlifts, leg curls, and lunges involve the quadriceps to some degree, they do not specifically emphasize the muscle group in the same way that squats do. Deadlifts primarily target the posterior chain, including the hamstrings and glutes. Leg curls are focused on isolating the hamstrings. Lunges also engage the quadriceps, but they recruit other muscle groups such as the glutes and hamstrings more significantly, varying the degree of quadriceps activation based on the lunge variation.

Therefore, squats are the most effective exercise for targeting the quadriceps directly due to the mechanics of the movement and the primary muscle action involved.