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Whenever I use my CycleOps magnetic trainer (like the one pictured below) my knees always end up hurting.

CycleOps Mag Trainer

I don't have this problem when riding out on the road. My cadence is the same for the most part. I usually keep it on a medium resistance and the bike is level. The trainer itself rides really smooth and feels pretty similar to riding on flat ground.

Is there a reason why the trainer would cause pain in my knees?

  • Your usual bike, same pedals, same shoes, cleats or not? – Carel Oct 13 '16 at 14:57
  • @Carel Same everything. – npsantini Oct 13 '16 at 14:57
  • Where in your knee does it hurt? – andy256 Oct 13 '16 at 15:32
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    @andy256 in the front. – npsantini Oct 13 '16 at 15:34
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    The answer is related to crank inertial load. The CIL on your trainer is different from the CIL while riding on the road, even though the bike and cleats and shoes and position are the same. I don't have time to give a full answer at the moment but I'll upvote an answer that does. [Edited to add: the same thing happens to me so I had to work out the answer a few years ago]. – R. Chung Oct 13 '16 at 15:45
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Thanks to @R.Chung I was able to do some research and figure out what my problem might be.

This article, Climbing vs time-trialling: same effort, different power output, explains in great detail the differences in inertial load.

To sum it up in relation to my problem:

When on the road where the elevation changes often I use slow twitch muscles and exert force longer during the pedal stroke and occasionally fast twitch muscles for stretches of flat ground.

On the trainer I use fast twitch muscles a lot more, because I only exert force for a very short period of time on only a small portion of my pedal stroke.

The muscles in my legs/knees are not used to having to fire that rapidly for such a sustained and extended period of time which is what I believe is causing me pain.

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    Pretty much it. Unless your trainer has a huge flywheel the CIL on a trainer is low. In order to keep power output the same as on the road, you either have to increase cadence or else "pull up" on the recovery side to raise average torque since you can't push down enough on the power side. That changes the coordination of the leg flexors and extensors. Some people are more sensitive to coordination changes than others (like, you and me) and the result can be achy knees, especially in the front of the knee. – R. Chung Oct 13 '16 at 16:33
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    As an aside, cadence tends to vary with CIL. When you climb, CIL is low and cadence tends to be low; when on the flat, CIL is high and cadence tends to be higher. On a trainer CIL is low but in order to get high power output we often end up trying to maintain high cadence, which is the opposite to how we generally respond outdoors on the road. See my answer to "optimal cadence" for examples, and that's why cadence on a trainer isn't necessarily indicative of cadence on the road. – R. Chung Oct 13 '16 at 16:47
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    @R.Chung Honestly, I think your answer on "What is the optimal cadence?" is probably the best written answer I've ever seen on the bicycles SE. It also helps to answer my question pretty well. – npsantini Oct 13 '16 at 16:58
  • Changes in muscle recruitment patterns make sense, my only comment is about your assertion of using only fast twitch. Fast twitch muscle fibers have very low endurance, unless you are riding for a very short amount of time, you are still primary powering your activity with slow twitch. You may recruit more fast twitch, but I can't see it being powered exclusively with fast twitch muscle fibers. – Rider_X Oct 13 '16 at 17:43
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    @andy256 You're right there are no muscles in the knee -- however, there are tendons that connect that patella to the quadriceps and to the shinbone. Although the muscles are above and below the knee, the tendons actually cover the patella. – R. Chung Oct 14 '16 at 2:34

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