Muscle Meat - basically this is any meat without bones. Boneless Chicken breasts, ground beef, turkey breast, beef or bison steaks, beef/pork heart (it really is a muscle and not an organ), pork butt, lamb, deer meat, fish fillets etc. Any meat without bones in it.
Our mainstay muscle meat is heart - beef and pork. I will also get ground turkey and ground beef if I can get a good price. During the fall I get leftover deer meat from our neighbors. I will try to get a little variety as well - sometimes goat meat or mutton on the leg (which I cut off the leg).
Here is a top and bottom view of a Beef Heart:
I do NOT remove the fat from the meats I feed because dogs need both protein AND fat in their diets. If my dogs need to lose weight I feed less FOOD, not less fat.