Don't mind the automaton voice, the visuals and information is gold. Great explanation. 

What are muscle knots?

All around great video, I love these guys at SciShow almost as much as I like the It’s Okay To Be Smart channel.  

Myofascial Pain and Trigger Points

 This video is geared for people with anatomy backgrounds and is perhaps overly academic, but if you really want to get to the nitty girtty of it, this will get you there.