Leg Curls: Lying vs. Seated, Hamstring Activation, and Benefits

What is the Difference Between Lying Leg Curls and Seated Leg Curls?

While both lying and seated leg curls effectively target the hamstrings, their primary distinction lies in the hip joint angle during the exercise, which significantly alters the hamstrings' length-tension relationship and can emphasize different hamstring heads due to their biarticular nature.

Introduction to Hamstring Training

The hamstrings are a powerful muscle group on the posterior aspect of the thigh, crucial for locomotion, athletic performance, and knee joint stability. Comprising three primary muscles—the semitendinosus, semimembranosus, and biceps femoris (long and short heads)—they primarily function to flex the knee and extend the hip. Leg curl variations are cornerstone exercises for isolating and strengthening the hamstrings, yet subtle differences in execution, particularly regarding body position, can lead to distinct physiological effects and training outcomes. Understanding these nuances is key to optimizing your hamstring development.

Anatomy of the Hamstrings

To appreciate the differences between leg curl variations, a brief review of hamstring anatomy is essential:

• Semitendinosus: Originates from the ischial tuberosity (pelvis) and inserts onto the medial tibia. It is a biarticular muscle, meaning it crosses both the hip and knee joints.
• Semimembranosus: Originates from the ischial tuberosity and inserts onto the medial tibia (deeper than semitendinosus). Also a biarticular muscle.
• Biceps Femoris (Long Head): Originates from the ischial tuberosity and inserts onto the head of the fibula and lateral tibia. A biarticular muscle.
• Biceps Femoris (Short Head): Originates from the linea aspera of the femur and inserts onto the head of the fibula. This is the only hamstring muscle that is monoarticular, crossing only the knee joint.

The biarticular nature of the semitendinosus, semimembranosus, and biceps femoris long head means their ability to generate force is influenced by the angle of both the hip and knee joints simultaneously.

The Lying Leg Curl

The lying leg curl is typically performed on a prone machine, where the individual lies face down with their hips extended, and the pads rest against the back of their ankles.

• Description: The user lies prone, extending their hips, and curls their lower legs towards their glutes against resistance.
• Hip Angle: In the lying leg curl, the hips are in a relatively extended position (close to 180 degrees or neutral).
• Muscle Activation & Biomechanics:
  • Shorter Muscle Length at Hip: With the hips extended, the hamstrings are already shortened at the hip joint.
  • Active Insufficiency: As the knee flexes, the hamstrings shorten further. This extended hip position can lead to a phenomenon known as active insufficiency, where a biarticular muscle (like the hamstrings) becomes so shortened across both joints that it loses its ability to generate maximal force.
  • Emphasis on Biceps Femoris: Research and anecdotal evidence suggest that the lying leg curl, due to the hip extension, may place a slightly greater emphasis on the biceps femoris long head, particularly towards the end range of knee flexion. The biceps femoris short head, being monoarticular, is also heavily recruited as it only crosses the knee.
  • Peak Contraction: The lying leg curl provides a strong peak contraction feeling in the hamstrings, especially at the top of the movement.
• Benefits:
  • Strong Peak Contraction: Excellent for developing the "peak" or "ball" of the hamstring.
  • Comfortable for Some: May be more comfortable for individuals with lower back issues compared to seated variations, as the spine is typically in a neutral position.
  • Recruitment of Biceps Femoris: Potentially greater activation of the lateral hamstring head.
• Considerations:
  • Reduced Stretch: The extended hip position means the hamstrings are not stretched as much at the hip joint, potentially limiting the eccentric (lengthening) stimulus.
  • Active Insufficiency: Can limit the overall force production potential due to the shortened state across both joints.

The Seated Leg Curl

The seated leg curl is performed on a machine where the individual sits upright, typically with a backrest, and the pads rest against the front of their thighs and the back of their ankles.

• Description: The user sits upright, often with their hips flexed (e.g., 90 degrees or less), and curls their lower legs towards their glutes against resistance.
• Hip Angle: In the seated leg curl, the hips are in a relatively flexed position (e.g., 90 degrees or more, depending on the machine design).
• Muscle Activation & Biomechanics:
  • Lengthened Muscle Length at Hip: With the hips flexed, the hamstrings are stretched or lengthened at the hip joint.
  • Improved Length-Tension Relationship: This lengthened position at the hip allows the hamstrings, particularly the biarticular heads (semitendinosus, semimembranosus, biceps femoris long head), to operate at a more advantageous position on their length-tension curve. This means they are better positioned to generate strong force throughout the full range of knee flexion.
  • Emphasis on Medial Hamstrings: Some research suggests that the seated leg curl, due to the hip flexion and the resulting stretch, may place a slightly greater emphasis on the semitendinosus and semimembranosus (medial hamstrings).
  • Greater Eccentric Load: The lengthened state of the hamstrings at the start of the movement allows for a greater eccentric stretch under load, which is highly beneficial for muscle hypertrophy and strength development.
• Benefits:
  • Enhanced Eccentric Loading: Superior for stimulating muscle growth due to the greater stretch on the hamstrings.
  • Improved Force Production: The advantageous length-tension relationship allows for greater force output throughout the full range of motion.
  • Potential for Medial Hamstring Emphasis: May better target the semitendinosus and semimembranosus.
  • Reduced Active Insufficiency: Less likely to encounter active insufficiency, allowing for more consistent tension.
• Considerations:
  • Lower Back Stress: For individuals with pre-existing lower back issues, maintaining an upright posture with hip flexion and then flexing the knees against resistance can sometimes place stress on the lumbar spine if core stability is lacking or form is compromised.
  • Less "Peak" Contraction Feel: While effective, some individuals may not feel the same "squeeze" at the top as with the lying curl due to the different length-tension relationship.

Key Differences Summarized

Which One Should You Do?

Both lying and seated leg curls are excellent exercises for hamstring development, and neither is inherently "superior" to the other. Their differences make them complementary, offering unique benefits:

• For Overall Hamstring Development and Hypertrophy: Incorporating both variations into your routine is ideal. The seated leg curl provides a fantastic eccentric stimulus due to the lengthened position, which is crucial for muscle growth. The lying leg curl offers a strong peak contraction and can help address the active insufficiency aspect of hamstring function.
• For Athletes: Varying your hamstring training with both positions can enhance strength and resilience across different joint angles, translating to better performance and injury prevention.
• For Addressing Weaknesses: If you feel your hamstrings lack development in a stretched position, prioritize the seated leg curl. If you struggle with a strong contraction or feel a lack of "fullness," the lying leg curl can be beneficial.
• Considering Comfort/Injury History: If you have lower back sensitivity, the lying leg curl might be a safer starting point. If you have knee issues that are aggravated by extreme flexion under load, adjust the range of motion for either.

Conclusion

The distinction between lying and seated leg curls is a prime example of how small changes in body position can profoundly impact muscle mechanics and training outcomes. By manipulating the hip joint angle, these two exercises alter the hamstrings' length-tension relationship, potentially emphasizing different hamstring heads and providing distinct stimuli for strength and hypertrophy. As an expert fitness educator, the recommendation is clear: embrace the variety. Incorporating both lying and seated leg curls into your training regimen will provide a comprehensive stimulus for robust, resilient, and fully developed hamstrings, ensuring you reap the maximum benefits from your efforts.