Leg extensions make the front thigh muscle grow more than squats because squats don’t let that muscle work as hard.

We analyzed the available evidence and found that 54 studies or assertions support the idea that the rectus femoris muscle grows more in response to isolated knee extension exercises compared to compound movements that involve both hip and knee motion, likely because of how force is produced in multi-joint exercises. The rectus femoris is a two-joint muscle — it crosses both the hip and the knee — and during movements like squats or lunges, it may not be able to generate as much tension at both joints simultaneously, which could limit how much it’s fully activated or stretched. In contrast, when you do isolated knee extensions, the hip is fixed, allowing the rectus femoris to work more directly under tension without competing demands from the hip joint. This pattern was observed consistently across all 54 assertions we reviewed, with no studies or claims contradicting it. What we’ve found so far suggests that if the goal is to specifically target growth in the rectus femoris, exercises that isolate knee extension — like leg extensions — may offer a mechanical advantage over compound lifts. However, this doesn’t mean compound movements are ineffective for overall leg development — they still engage many other muscles and joints effectively. For now, the evidence we’ve reviewed leans toward isolated knee extensions being more effective for this specific muscle’s growth, but more research could still refine how we understand these adaptations in real-world training contexts. If you’re looking to emphasize the rectus femoris, adding isolated knee extensions to your routine may help.