So, this is an older article about the benefits of MSM for sore-muscled horses. I found it quite interesting, as I've had my horse on MSM for about 2 weeks now and he's doing wonderful - he's going on long trail rides daily and his muscles are softer than they've been in years. Here's one of the most interesting bits I found in the article:
"The results showed that all of the horses receiving MSM had dramatic improvement in three ways. Thermography showed less inflammation and soreness, particularly through the back and hind end. **emphasis mine** (The change was faster and more dramatic for the horses on the higher dose.) Additionally, their serum chemistry demonstrated significant drops in two crucial parameters: AST (aspartate aminotransferase) and CK (creatine kinase), tests that indicate metabolites from muscle damage. Finally, all treated horses improved their average training time--group two (the lower dose) by two seconds, and group three by 2.62 seconds."
I had my boy on plain MSM when he was around 5 (before major symptoms) and he did well on it, but later on I tried a joint supplement with MSM and other things, including glucosamine, and he didn't do well on it at all so I removed that supplement and didn't add back the MSM - lesson learned lol. I know that some horses can't handle MSM, but the article discusses how the main reason it helps could be that it's high in sulfur - perhaps a supplement with sulfur could potentially help horses with muscle issues that can't handle MSM?
One other thing to think about - some PSSM horses don't show elevated AST and CK, could at least some of these horses be on MSM during the time of testing? Or possibly some other source of sulfur?
MSM Helps Sore Muscles on TheHorse.com
UPDATE OCTOBER, 2017:
I took Jax off of all of his supplements for a few weeks, then added them back slowly to test/reaffirm the need for each in early September. He did really well for a couple of weeks, but started having back pain again. I went back through my notes and decided to add MSM back, and after a few days his back pain is much better. So, twice after adding MSM I noticed a major difference in muscle pain/stiffness, so he'll be staying on MSM from now on.