A key way to reduce ACL stress and injury risk | Hamstrings and landing biomechanics

Hamstrings stiffness and landing biomechanics linked to anterior cruciate ligament loading (Blackburn et al; Journal of Athletic Training 2013)

This paper examined whether greater hamstring stiffness during landing resulted in less anterior (forward) shear force on the tibia (shin bone) and thus on the knee joint as a whole, specifically the anterior cruciate ligament (ACL).

Methods

36 participants (18 male and 18 female, average age = 23) were first measured for hamstring stiffness (via oscillatory technique) and categorized as either high or low hamstring stiffness.

Each participant then performed a double-legged jump landing task in which their lower body kinematics were recorded via a 3-dimension motion capture interfaced with a force landing plate.

The researchers identified peak knee-flexion and -valgus angles, vertical and posterior ground reaction forces, anterior tibial shear force, internal knee-extension and -varus moments, and knee-flexion angles during the landing task.

Results

The high hamstring stiffness group exhibited 1.1 times less peak anterior tibial shear force (the shin bone sliding forward, a key stressor on the ACL), 3.6 times less peak varus force (knee bowing outwards), and greater knee flexion (bending) at key moments.

Practical takeaways

Greater hamstring stiffness may result in better landing biomechanics and less overall injury risk, including less stress on the ACL. Therefore, exercises that increase hamstring musculotendinous stiffness (many examples but two key ones are deadlifts and nordic hamstring curls) are a key piece of any training program.