Endiku, I am going to have to disagree with you on that. Backing up puts the hind end into use, and collecting and lowering his head while backing up activates the neck/back muscles, and this will build muscle. Feeding on the ground stretches his topline, and ask a well informed vet/nutritionist or some other horse professional, and they will say certain feeds help build topline. Amino acids is the building blocks of muscle. I am not sure how beet pulp helps, but I know many people that say they have noticed a huge difference in their horses with feeding beet pulp.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown