Hypothetically, based on how Epsom salt baths are supposed to work, you should be able to use them preemptively (your body will just store the extra minerals).
So I took an Epsom salts bath last night. Today was CrossFit. I guess we will see if it helps! My muscles were definitely in a much better place than last week, but it’s hard to say whether that’s just me being stronger, a cumulative effect of the foam rolling, or what. When I told someone what we did in class today they said I’m going to be seriously sore for days, so …. we’ll see.
PS – I’m still really bothered that the ONLY research studies to support Epsom salts use are either non-published (which implies it didn’t pass peer review), or too small a group of subjects (only nine!) to really be a valid research population. Why hasn’t someone done a simple blind study where half the people take regular hot baths and half take Epsom salts baths? It wouldn’t bother me as much if Epsom salt usage was something humans have used for thousands of years and discovered through basic experimentation, like many herbs, but this started as a health fad in the 1600s! /rant
PPS – I got all excited telling my friend about all this research I did on Epsom salts, and only realized after that she thought I meant Epsom salts as a laxative! She must have thought I was very unhealthy when I told her that I bought Epsom salts to help me with CrossFit….
Open Notes and Ramblings 
As I remember from many, many years ago, we used to soak in Epsom salt baths to relieve the muscles after we did a lot of exercise. But I guess you’re probably sore from the cumulative exercises.
I took the Epsom salts bath last night just because I had some neck pain and I figured it couldn’t hurt (it seemed to do the trick – all better today). But hypothetically, it seems like taking Epsom baths as a regular thing will help your body during/after exercise – that it can be a regular thing, like eating properly, that supports fitness, rather than a post-exercise reactionary thing.
gotcha