The Fine Print: Fear of Shear
Fitness For Consumption, Season 3, Episode 8
Dissecting the article that condemned leg extensions
If you’ve been attuned to conventional fitness wisdom, at some point you’ve been advised to avoid certain exercises, like leg extensions, at all costs. Why? Because of the noxious shear stress that may damage your knees. Do you too have the Fear of Shear?
Shear is a type of force that arises when adjacent structures move in opposing directions, and frankly, it occurs in most joint motion. So, if that’s the case, why does its presence ring so many alarm bells in the heads and hearts of physical therapists, trainers, doctors and fitness enthusiasts?
Well, it might have something to do with a paper published in the early 1990’s by members of the esteemed Mayo Clinic that found significantly higher levels of shear at the ACL in an open chain vs closed chain condition. Their unambiguous conclusion? Avoid open chain exercises, like leg extensions, if you are rehabbing an injured or newly reconstructed ACL. Of course, that translates into “avoid leg extensions at all costs, period.”
So, is this settled wisdom? Or…what can we learn when we put on our mental magnifying glasses and sift through The Fine Print?
In this episode we discuss:
- The terminal knee extension “paradox”
- Did the conditions set up by investigators favor a certain outcome?
- Did the subjects’ amount of practice time affect the outcomes?
- How significant, in terms of possible tissue failure, was the stress placed on the ACL in the open chain?
- What are the real life conditions that lead to ACL injuries? Do they happen in open or closed chain conditions?
Join us in The Fear of Shear, our third installment of our The Fine Print series, where we take a second look at research publications and expose some details easily missed at a first glance. Should you avoid open chain exercises? Well, you’ll have to listen for yourself and make an informed choice, but if you're avoiding knee extensions because you’ve got the Fear of Shear, perhaps, fear not.
Closed Chain Exercise* - An exercise in which the terminal end of the segment is fixed and restrained.
Open Chain Exercise* - An exercise in which the terminal end of the segment is free to move.
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