Why Does My Knee Hurt On An Elliptical Machine?

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A client once asked me, “You know, I don’t get why my knee hurts when I’m using an elliptical machine. I thought those things were supposed to be low impact, right? So, what’s the deal?”

Clinicians and trainers praise elliptical machines for being a good alternative “low impact” exercise for people with joint pain.

But how low, is low?

I couldn’t find any research about the loads delivered by an elliptical machine. Maybe some one knows. If so, please send it along. In the meantime, here’s what I did to figure it out.

I took a Lafayette Digital Muscle Tester and placed it under my right foot while on a Precor EFX 546i elliptical machine. I had to place a high density foam block under my other foot to make the pressure fairly equal under each foot (The digital muscle tester is a load cell. It records pressure applied to it)  I then set the resistance on the elliptical machine to a “moderate” level – between 8 and 10- and then used the machine for approximately ten repetitions.

When I got off the device, I looked at the force recorded by the digital muscle tester. It was 140 lbs. The muscle tester records peak force so this represented the maximum force that went into my leg.

I then repeated the procedure but increased the resistance setting on the machine to 12. This time, the peak force was 185 lbs.

At the time of the test, I weighed 211 lbs. So, for me, this was 66% and 88% of body weight. The machine’s resistance is a function of your body weight. If you weigh 150 lbs and exercise at the same setting and speed as I did, the load going into your leg will be a percentage of your body weight (probably close to 66% and 88%).

Of course, there are a few problems with my study. There was a “n” of 1. I didn’t evaluate the relationship of load, resistance setting, and speed. The faster you move at any setting, the more resistance you get. And, I didn’t have a continuous load curve to assess where in the arc of motion the load was the greatest. Someone with all of the right tools in a well equipped biomechanics lab could do this though.

But, what I do have is better than guessing; better than assuming that because elliptical is low impact, it’s ok for people with joint disease or joint pain.

Why Your Knee May Hurt On A Low Impact Machine

Most physical ailments – musculoskeletal problems – come down to this principle: when physical demand exceeds physical capacity, symptoms follow.

Your ability to withstand a certain amount of force with good form is what I refer to as “load tolerance”. People who have knee pain usually have a load tolerance equal to less than their body weight. So, for example, if your knee hurts while walking or running, you may find that in a pool, where you effectively weight less from the buoyancy of the water, you may not hurt at all. This is because the load tolerance of your joint(s) is greater than the load your joints experience in the water.

An elliptical machine feels ok as long as your load tolerance is at least 66% body weight (and it could be an reasonable option while re-training a stubborn Runner’s Knee).

A Simple Test

If  your  load tolerance for a Single Leg Squat or Stair Step (the elliptical machine is close to these motions) is only 50% body weight, there’s a good chance that the elliptical machine will be too much load. This may explain why some people with knee pain, for example, tolerate the device and some don’t.

A simple test to help you determine if an elliptical might be a good choice is the Back Slider test in the video below. If you can perform this test, you will likely not have difficulties with an elliptical machine.

 

Have you had any problems using an elliptical machine? What did you do?


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photo credit: Roel Wijnants via photopin cc

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