Lying Leg Curls
The lying leg curl is the standard prone isolation exercise for the hamstrings. Performed face-down on a leg curl machine, the knee flexion movement loads the biceps femoris, semitendinosus, and semimembranosus against pad resistance. The prone (hip neutral) position places all three hamstring heads in their natural mid-range length, producing a bell-shaped resistance curve with peak load at approximately 90° knee flexion.
Execution
- Adjust the leg curl machine pad so it sits just above the ankle and lie face-down
- Keep the hips flat on the pad throughout — do not allow the hips to lift as the legs curl up
- Curl the legs toward the glutes as far as the machine allows; hold briefly at the contracted position
- Lower under control through the full eccentric — do not let the weight drop
What the EMG Data Shows
Schoenfeld 2014 (n=13, prone):
| Muscle | Activation |
|---|---|
| Biceps femoris | 80% MVIC |
| Semitendinosus | 65% MVIC |
The biceps femoris generates significantly higher activation than the semitendinosus in the prone position. This biceps femoris dominance is a consistent finding in prone leg curl research.
Prone vs Seated: Hip Position Matters
The key mechanical difference between lying and seated leg curls is hip angle:
- Prone (lying): Hip at 0° — hamstrings in mid-range length
- Seated: Hip at ~90° flexion — hamstrings pre-stretched at proximal end
The seated position places the hamstrings in a more lengthened overall configuration. Both machines train knee flexion, but through different muscle length conditions — making them complementary rather than interchangeable.
Foot Positioning
Toe rotation affects medial vs lateral hamstring emphasis:
- Toes forward: Balanced biceps femoris and semitendinosus
- Toes in (internal rotation): Slightly greater biceps femoris emphasis
- Toes out (external rotation): Slightly greater semitendinosus/semimembranosus emphasis
For system-specific training applications, see each system's lens entry.