Knee Meets Seated Leg Extensions: Friend or Foe?

Posted: March 15, 2015 by TRU in Random, Rehab, Uncategorized
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leg ext2

In this week’s installment I’ve decided to discuss an exercise that gets a lot of negative attention: the seated leg extension machine. Seated leg extensions have been a mainstay in many bodybuilders leg training tool chests, and for good reason—they do a good job emphasizing the quadriceps. That being said, the knee stresses created by seated leg extensions have resulted in many physical therapists and strength coaches advising against this exercise. So what does the evidence have to say and does the leg extension machine belong in the dumpster or in your leg training routine? 

One Knee, Two joints:

The human knee is actually made up of two distinct joints: the tibiofemoral and patellofemoral joints, with each having a unique structure and function. This is important because what’s good (or bad) for one joint isn’t necessarily so for the other. As an example, a barbell squat may produce high compressive forces on the tibiofemoral joint, but less so for the patellofemoral joint. So someone with arthritis in their tibiofemoral joint may not tolerate squatting while someone with patellofemoral arthritis may not have a problem with squatting. Each of these types of arthritis are ambiguously called knee arthritis, and without knowing which of the knee joints is most involved, knowing which exercises will be most harmful or helpful isn’t clear.

Knee

what’s bad (or good) for one knee joint isn’t necessarily good for the other.

Knee Research

The current body of evidence is largely based on mathematical models that estimate joint forces at the knee—since in vivo (inside the knee) measurements are not so easy to do. A major limitation of this is that each calculation is dependent on making certain assumptions which result in significant variability depending on the model used. Thus, the estimated knee forces for something like jogging can range from Dr. Jekyll to Mr. Hyde…which isn’t all that helpful when it comes to making recommendations for knee health.

Scope

From an observational standpoint, people whose knees are exposed to repeated high joint forces overtime may be more likely to enjoy a future of degenerative knees. What complicates this however, is that we know some forces are good for the knees, and that intermittent compression can actually help with things like improved blood flow to the meniscus. So before we can answer whether the leg extension is detrimental or not, it would be very nice to know…

At what point does joint stress accelerate degenerative changes in the knee joint AND does the leg extension exercise meet this criteria?

Stay tuned for part II to find out,

TRU

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