A horse’s body tends to add new muscle cells at its own pace, which is sometimes slower than horsemen would prefer. Even young, growing horses receiving plenty of exercise often need a little help to pack dense muscle onto their limber frames. And as horses mature—especially horses recovering from the trauma of injury, disease, or severe stress—growing new muscle becomes an uphill battle.
Feeding extra protein—even protein rich in essential amino acids—does little to correct the problem, since you can only push so many nutrients through a series of progressively constricted metabolic pathways. To continue to add muscle mass, a way has to be found to retool the metabolism to remain in, or return to, muscle-building mode.