Isolation exercises targeting a single joint (e.g., leg extensions) produce greater hypertrophy in bi-articular muscles (e.g., rectus femoris) compared to multi-joint compound movements (e.g., squats) due to avoidance of active insufficiency.

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

The study found that doing leg extensions made the front thigh muscle (rectus femoris) grow more than doing squats, which supports the idea that isolating the muscle with one movement works better than using it in a big compound move.

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

The study found that doing leg extensions made the front thigh muscle (rectus femoris) grow more than doing squats, which supports the idea that isolated exercises are better for growing muscles that cross two joints.

Study: The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial

The study found that leg extensions made the front thigh muscle (rectus femoris) grow more evenly than squats did, which supports the idea that isolated exercises are better for growing muscles that cross two joints.

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 (moving just the knee) made the rectus femoris muscle grow much more than doing leg presses (which move both knee and hip), because leg presses don’t fully engage that muscle. So isolation exercises are better for growing this specific muscle.