Hamstrings: Anatomy, Biomechanics, Functions, and Training

What is the biomechanics of the hamstring?

The hamstrings are a crucial group of three posterior thigh muscles vital for both hip extension and knee flexion, playing a fundamental role in locomotion, athletic performance, and maintaining proper lower limb stability and posture through complex biomechanical actions.

Anatomy of the Hamstring Muscles

The term "hamstring" collectively refers to three distinct muscles located on the posterior aspect of the thigh, originating from the ischial tuberosity (a bony prominence of the pelvis) and inserting onto various points around the knee joint. Understanding their individual origins and insertions is key to comprehending their specific actions.

• Biceps Femoris:

  • Long Head: Originates from the ischial tuberosity.
  • Short Head: Originates from the linea aspera of the femur (mid-thigh).
  • Insertion: Both heads merge to insert primarily onto the head of the fibula and lateral condyle of the tibia.
  • Action: Primarily responsible for knee flexion and lateral rotation of the tibia (when the knee is flexed). The long head also assists in hip extension.

• Semitendinosus:

  • Origin: Ischial tuberosity.
  • Insertion: Medial aspect of the proximal tibia, part of the pes anserinus group.
  • Action: Primarily responsible for knee flexion, hip extension, and medial rotation of the tibia (when the knee is flexed).

• Semimembranosus:

  • Origin: Ischial tuberosity.
  • Insertion: Posterior aspect of the medial condyle of the tibia.
  • Action: Primarily responsible for knee flexion, hip extension, and medial rotation of the tibia (when the knee is flexed). Due to its deeper and broader insertion, it has a significant role in knee stability.

All three muscles (long head of biceps femoris, semitendinosus, semimembranosus) are bi-articular, meaning they cross and act upon two joints: the hip and the knee. The short head of the biceps femoris is mono-articular, acting only on the knee.

Primary Biomechanical Functions

The dual-joint nature of the hamstrings allows them to perform diverse and critical movements:

• Knee Flexion: This is the most recognized action, pulling the lower leg towards the glutes. Examples include the concentric phase of a hamstring curl or the swing phase of walking.
• Hip Extension: Along with the gluteus maximus, the hamstrings are powerful hip extensors, moving the thigh backward relative to the pelvis. This is crucial for movements like standing up from a squat, deadlifts, or sprinting.
• Tibial Rotation:
  • Medial Rotation: The semitendinosus and semimembranosus internally rotate the tibia, particularly when the knee is flexed (e.g., controlling the final phase of knee flexion).
  • Lateral Rotation: The biceps femoris externally rotates the tibia, also when the knee is flexed. This rotational control is vital for pivoting and cutting movements.
• Pelvic Tilt: By extending the hip, the hamstrings also contribute to posterior pelvic tilt, which can influence spinal posture and lower back mechanics. Conversely, tight hamstrings can pull the pelvis into a posterior tilt, potentially flattening the lumbar curve.
• Deceleration: During dynamic movements, the hamstrings act eccentrically (lengthening under tension) to decelerate the lower leg during knee extension (e.g., the swing phase of running) and to control the forward lean of the trunk during hip flexion (e.g., the descent phase of a good morning exercise).

Hamstrings in Dynamic Movement

The hamstrings are integral to nearly all forms of human locomotion and athletic endeavors:

• Walking and Running Gait Cycle:
  • Swing Phase: As the leg swings forward, the hamstrings eccentrically contract to decelerate the forward momentum of the lower leg, preventing hyperextension of the knee.
  • Stance Phase: As the foot contacts the ground, the hamstrings concentrically contract to extend the hip, propelling the body forward.
• Jumping and Landing: They play a significant role in the propulsive force during the take-off phase of a jump (hip extension) and absorb impact forces during landing (eccentric hip extension and knee flexion).
• Squatting and Deadlifting: In these foundational strength movements, the hamstrings work synergistically with the glutes for hip extension and eccentrically to control the descent. Their contribution to knee flexion is also present, especially during the bottom portion of a deep squat.
• Sprinting: The hamstrings are highly active during the ground contact phase to extend the hip for propulsion and during the swing phase to powerfully flex the knee and then eccentrically control the knee extension as the leg reaches forward.
• Kicking: They contribute to both the powerful hip extension during the follow-through and the controlled knee flexion.

Synergists and Antagonists

Understanding the muscles that work with and against the hamstrings provides a more holistic view of lower limb biomechanics:

• Synergists:
  • Gluteus Maximus: The primary synergist for hip extension.
  • Gastrocnemius: Assists in knee flexion due to its bi-articular nature (crossing both the knee and ankle).
  • Adductor Magnus (posterior fibers): Can assist in hip extension.
• Antagonists:
  • Quadriceps Femoris: The primary antagonist for knee flexion (quads extend the knee) and hip extension (rectus femoris, a quad muscle, flexes the hip).
  • Iliopsoas: The primary antagonist for hip extension (flexes the hip).

Maintaining a balanced strength and flexibility relationship between the hamstrings and their antagonists, particularly the quadriceps, is crucial for injury prevention and optimal performance.

Common Biomechanical Considerations and Injuries

The complex actions and high demands placed on the hamstrings make them prone to certain injuries and biomechanical dysfunctions:

• Hamstring Strain/Tear: This is one of the most common muscle injuries in sports, particularly in activities involving high-speed running, sudden acceleration, or extreme stretching.
  • Mechanism: Often occurs during the late swing phase of sprinting (eccentric overload) or during powerful concentric contractions (e.g., sudden acceleration).
  • Risk Factors: Muscle imbalance (quads stronger than hamstrings), previous hamstring injury, poor flexibility, inadequate warm-up, fatigue, and poor running mechanics.
• Proximal Hamstring Tendinopathy: Inflammation or degeneration of the hamstring tendons at their origin (ischial tuberosity), often due to repetitive loading or compression, common in sitting athletes or those involved in hip-flexion-dominant activities.
• Role in ACL Injury Prevention: Strong and responsive hamstrings can help protect the anterior cruciate ligament (ACL) of the knee. As the hamstrings flex the knee and pull the tibia posteriorly, they counteract the anterior shear forces on the tibia that can stress the ACL, particularly during landing and cutting movements.
• Muscle Imbalance: An imbalance in strength or flexibility between the hamstrings and quadriceps can lead to patellofemoral pain, low back pain, or increased risk of injury.

Optimizing Hamstring Biomechanics Through Training

To enhance performance and reduce injury risk, training should target all aspects of hamstring function:

• Eccentric Training: Crucial for improving the hamstrings' ability to absorb force and decelerate movement, which is highly protective against strains. Exercises like Nordic hamstring curls, Romanian deadlifts (RDLs), and good mornings emphasize eccentric loading.
• Concentric Training: Essential for generating power and propulsion. Exercises like glute-ham raises, leg curls, and hip extension machines target concentric strength.
• Isometric Training: Holding contractions at specific joint angles can improve stability and strength at vulnerable positions.
• Balanced Training: Incorporate exercises that challenge both hip extension (e.g., RDLs, deadlifts, hip thrusts) and knee flexion (e.g., leg curls, Nordic curls) to ensure comprehensive development.
• Functional Movements: Integrate multi-joint exercises that mimic real-world movements and sports-specific actions to improve coordination and inter-muscular synergy. Examples include squats, lunges, and plyometrics.
• Flexibility and Mobility: Regular stretching and mobility work are important to maintain optimal range of motion and prevent tightness, which can hinder performance and increase injury risk.

By understanding the intricate biomechanics of the hamstrings, individuals can design more effective training programs, mitigate injury risks, and unlock their full athletic potential.