UPDATE (January 2020): Using an archery arm guard worked for a while, but then we noticed a good bit of noise from the guard rubbing against the guitar. It didn’t seem to matter how much we padded it, it made a creaking sound which was unacceptable. As it turns out, someone else had thought of this and made an arm guard that does not make noise: we found the Luva Arm Guard for Guitarists [Armauflage fur Gitarristen] to be a perfect solution. It is more expensive than the archery arm guard (about $35 plus shipping), but works well. We recommend it. You might consider ordering a size smaller if you intend to wear it under your shirt sleeves, otherwise it can fit over a shirt with long sleeves. It is made in Brazil by Matepis Produtos Musicais (email firstname.lastname@example.org) or order from Switzerland: guitarwebshop.com/LUVA-Unterarmauflage-fuer-Gitarristen which was a little less expensive.
ORIGINAL POST (2019): The problem is that pressing the right forearm into the edge of the guitar can cause injury and exacerbate repetitive overuse of the forearm muscles. In answer to this problem there have been a couple different products created and marketed. They each have their own limitations. These products also have an eye to protecting the guitar’s finish. I have rediscovered a new use for an older product that is readily available, cheaper than the other options, provides superior protection for the arm, and also can protect the guitar’s finish.
One of the devices marketed to deal with this problem is the guitar arm rest. Some guitars even have them built on. It provides a rounded surface to rest the arm against rather than the sharp edge of the instrument. While I will admit that this is an improvement, it still means that you will be putting pressure on your forearm in the same vulnerable, possibly already injured, spot. You will be putting that pressure on a rounded ½ inch edge rather than a pointy edge. These items seem to affix to the guitar using either suction cups or a screw/grip mechanism. I have never had good luck with suction cups-they always come loose. The screw/grip mechanism on the one I looked at could not accommodate a guitar less than 3 ½ inches deep. Because I play little 19th-century guitars, that would not work.
The other device I have seen is a padded sleeve to cushion your arm from the edge of the guitar. Again, while that is an improvement, it merely cushions the pressure a bit. One will still be putting pressure, though somewhat more diffusely, against the same vulnerable, possibly injured, area of the forearm.
I was sharing my thoughts on the shortcomings of these devices with my wife, Tamara. I told her that what I wanted was something on my forearm with boning to spread the pressure over a much greater area, rather than just have it against the same spot, albeit more diffusely. (Boning is the technique of sewing a series of stiff rods into a garment to provide support or stiffness. Historically, the rod was a piece of whale bone, but it could be metal or plastic as well.) She asked why I didn’t just use my old archery forearm guard? It has boning and you can wear it under your long sleeve shirt. This was a great idea, I thought. I dug out my old arm guard, put it on, pulled the long sleeve shirt over it, and started to play.
The difference was immediately obvious. The boning distributes the pressure over a much greater area, reducing injury. Whereas a sleeve or arm rest would essentially distribute the pressure over an area ½ inch by the width of one’s arm, the boning distributes the pressure over a rectangle that is 2 ½ inches by 6 ½ inches, and runs the length of one’s forearm. This result in a huge pressure reduction on the forearm. By having the sleeve of your shirt over it, the top of the guitar is also protected from the guard. Another advantage to the use of a boned archery arm guard is that they are readily available and cheap. The hunting outfitter Cabelas has them for under $9.00 and they are available in black. Here is a link: https://www.cabelas.com/product/hunting/archery/releases-release-aids/arm-guards-finger-tabs-gloves/pc/104791680/c/104693580/sc/104529780/i/103864680/omp-strap-arm-gurad/2180762.uts?slotId=10. Be sure to get one with boning, many do not have it and, unfortunately, the product descriptions do not mention boning as present or absent in the design. You have to look at the picture to see if it is boned like the one pictured in this post or on the included link.
As a guitarist who loves music and instruments that have not changed essentially in a couple hundred years, I find it congruous that the old technology of the boned archery arm guard can be re-purposed to relieve a problem of which we’re now becoming aware. I also find it heartening that this old solution is readily available for a fraction of the cost of the newer devices. I make no claim of being the first to think of this. I have just come upon it recently, seen good possibilities, and have tried to spread the word. Good luck.