Leg Extensions
The leg extension is the primary open-chain quadriceps isolation exercise. Seated on a leg extension machine with the ankle pad just above the foot, the exercise produces pure knee extension against resistance, isolating all four quadriceps heads (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) from any hip involvement. It provides the only direct rectus femoris stimulus in a shortened-hip-flexion configuration that squats and lunges cannot replicate.
Execution
- Sit on the machine with the back of the knees at the seat edge; adjust the ankle pad to sit just above the ankle
- The starting angle should be approximately 90° knee flexion (or the machine's maximum range)
- Extend the knees to full extension under controlled speed; hold briefly at the top
- Lower under control; do not let the weight drop through the eccentric
What the EMG Data Shows
Perry 2009 (isometric contractions):
| Muscle | Activation |
|---|---|
| Rectus femoris | 27.8% MVIC |
| Vastus lateralis | 25.9% MVIC |
| Vastus medialis | 22.9% MVIC |
These are isometric values, not dynamic full-ROM values. They indicate relatively balanced quad head activation during the leg extension pattern.
The Rectus Femoris Uniqueness
The rectus femoris is the only quadriceps head that also flexes the hip. In the seated position, the hip is flexed at ~90°, which places the rectus femoris in a shortened proximal configuration. This creates a stimulus that squats and lunges do not replicate: the rectus femoris must contract against a mechanically disadvantaged position at the hip.
Patellofemoral Considerations
Open-chain terminal knee extension (the last 30°) produces patellofemoral compressive forces. Trainees with active patellofemoral pain syndrome should limit range to 60–90° and avoid the last 30° until symptoms resolve. This restriction is not categorically necessary for healthy trainees.
For system-specific training applications, see each system's lens entry.