What is the best thing for a horse that is a little muscle sore?

Thank you

I'm not 100% sure what your options are...but some time off, or easing up on training always helps! If you have access to a vibe plate, those work well before riding too, to really warm them up out of a muscle soreness. Or a heated blanket can work also.

How often do you ride? do you currently use any linement? Please let us know more information so we may be able to help you better. 

I try to ride 4 to 5 times a week. I am spraying apple cider vinegar after riding. Lots of long trotting.

Lame Away company makes a body wash that works amazing for this purpose. They also make a spray. If its to cold or not able to wash use the spray. I always use the spray before a run on my horses backs etc.. 

What do you currently feed?

I would go with a low starch and sugar feed...  its been so benefical to all my horses!