In the early 1990’s there were several studies that looked at the shearing forces of the knee during open and closed kinetic chain exercises, and determined that there were higher stresses imposed on the knee during open chain conditions.

Open chain exercises are exercises that are performed where the hand or foot is free to move.  Closed chain exercises are physical exercises performed where the hand or foot is fixed, and cannot move. The leg extension movement is a classic open kinetic chain movement.

Shear stress arises from a force vector perpendicular to the surface.

Physicians, and trainers at the time felt that due to new research findings that rehabilitation of the knee following ACL reconstructive surgery would be better using functional closed chain exercises.  Up until this point most knees were rehabilitated in an open chained fashion.  The leg extension was the stall-work of training, and rehab, its value was suddenly decimated.

Shearing forces exist in all extensor movements of the knee, and is the principle force contributing to knee extension.  In other words, it is pretty tough to move around without shearing forces to the hinge joint between the tibia, and fibula.  The question of course is, how much stress is good, and how much before the sheer becomes strain?  This is always up to the one doing the rehabilitation.

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As it has turned out since the 1990’s the leg extension exercise is still the same great exercise it had always been with the exception of those few years where it got some bad publicity.

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