The rectus femoris muscle exhibits greater hypertrophic adaptation to isolated knee extension exercises than to compound movements involving hip and knee flexion due to biomechanical limitations in force production during multi-joint movements.

Study: Drop-Set Training Elicits Differential Increases in Non-Uniform Hypertrophy of the Quadriceps in Leg Extension Exercise

This study showed that doing leg extensions (knee-only exercises) made the rectus femoris muscle grow more than other exercises, which supports the idea that isolated moves are better for this muscle than moves that also use the hip.

Study: Comparison of Muscle Hypertrophy and Strength Adaptations Induced by Back Squat and Leg Extension Resistance Exercises.

The study found that doing leg extensions (kicking your leg out while sitting) made the front thigh muscle grow more than doing squats, because squats involve the hip too and don’t work that muscle as hard.

Study: Hypertrophic Effects of Single- versus Multi-Joint Exercise: A Direct Comparison Between Knee Extension and Leg Press.

The study found that doing leg extensions (knee-only moves) made the rectus femoris muscle grow much more than doing leg presses (which also use the hips), because the hip movement in leg presses doesn’t let the rectus femoris work as hard.