I used to get pretty intense tendonitis (AKA tennis elbow) on really bumpy rides as a result of absorbing a lot of the shock with my lower arms (I suspect). I really started concentrating on relaxing my upper body (as snapdragen alludes to). It seemed to take care of the worst of it.

On a related note, I learned back in my whitewater boating days to use tension in my favor. I forced myself to put all of my tension/stress/whatever in my abs and butt. It made me a much more relaxed boater overall; I lost the upper body tension that was messing with my ability to ride with the river instead of against it. The bonus is that it helped with core strength as well. Now whenever I'm stressed out and I notice it, I feel it in my abs (instead of my back/neck/shoulders/jaw...the usual places for most folks).

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