I've noticed at times when I am doing a strength training routine that my muscles start to burn, but it's more of a "cool" or "cold" sensation throughout the muscle group I'm working. It doesn't hurt, and it happens more often during upper body strength training routines. I can't quite find the words to describe how it feels exactly, but I'm hoping someone in this community knows what I am talking about and what it means. I am genuinely curious! Thanks in advance.