The way the muscles of your horse are developed can give you big clues as to what areas are getting over-stressed and which muscles the horse might be avoiding using.
I have an equine client that is massive. He is half draft, half Dutch warmblood and is impressively huge. When I work on him, I work up a sweat trying to get through the density of his muscles. But a red flag has been waving since I first started working on him: his pectoral muscles have always felt flabby in comparison with the rest of him.
I always aim to massage this big guy to help him achieve better balance and shift weight off his front end. His front legs have been injury prone and his owner has found that keeping him on a 3 or 4 week body work schedule helps keep him free from injury. Keeping a horse balanced, both laterally and longitudinally, is one of the best insurance policies you can find. Noticing the weaker muscles on your horse can guide you towards a fitness and injury prevention program.