This horse actually has a naturally nice topline (note he is not ewe-necked on top), yet his underline is extremely developed. Also note his overall body attitude - his head is up, ears are back, and he is standing square with a wide rear (where all his power is for takeoff) stance for better push-off. His entire body language screams out: I am ready to explode!
For a horse to have a successful, long, sound career, however, we want to work their muscles to their best benefit. That means developing the muscles appropriate to keeping them sound and easing the weight of a rider - topline and abdominal muscles. A horse that is working in a relaxed manner - raised neck at its base, tracking underneath with a bent, shock-absorbing hind leg, raised back, etc, is going to develop different muscles than a horse that is tracking about with a tense jaw, whose tension travels all the way through the neck and down the back. They're going to develop a stronger topline, abdomen muscles (which help raise and support the back), etc.
Where our horsemanship comes in to play then, is in developing a calm, relaxed horse. If every time we work with our horse we encourage it to be reactive and flighty - to have a tense body - we're going to continue building the wrong muscles to best help our horse work under-saddle. If instead every time we work with our horse he is calm and relaxed with a supple jaw and back, we develop a horse with the correct muscling to best suit the work we're asking of him. This is not something that can be done through force - see-sawing on the reins, pushing the horse into contact, etc. Relaxation does not come through force, it comes naturally - with a relaxed horse! So the key is to always have the goal of beginning and ending on a relaxed note with your horse. Pretty soon, what happens last starts to happen first.
What riders typically do, however, when they experience roadblocks with their horse, is they start to throw in mechanical contraptions in an effort to force their horse into a particular frame. Except, by forcing the horse, the "frame" is full of tension and is without relaxation - which really defeats the whole purpose. If instead of focusing solely on the physical aspect, riders also included addressing the emotional side of things, they would have a better chance of achieving that strove-for "frame" they so desperately seek. A horse's mind controls his body. Therefore, by addressing mental and emotional relaxation, we achieve physical relaxation. With that starts to come the appropriate muscle development and the correct movement we seek from the horse.
Most people don't want to hear it, but our horses really are a product of us - our handling, our management, our riding. We have the power to either balance our horses' emotional states (or keep it balanced), or to create an emotional wreck of a horse. Coincidentally, a horse's emotional state plays a huge part on his physical state - his muscling. So, if how we work determines the physical and emotional state of our horses, the key is to work with them in such a way that the horse is relaxed and in tune with us - in partnership.
....horses should be trained in such a way that they not only love their riders, but look forward to the time they are with them. ~ Xenophon, 350 B.C.
Does your horse love you and look forward to your time together? If you take all the ropes off holding him to you, does he remain with you, working in partnership, or does he take off as fast as he can?
Correct neck muscling is particularly crucial to a horse sustaining the weight of a rider because otherwise the horse relies on a tense back to hold the rider, which can be detrimental to their development and long-term soundness. More here. Working a horse back to front, building on the dressage training scale, will build the appropriate muscling so your horse may effectively carry your weight. This is where the mental and emotional state of a horse and your partnership with that horse comes into play, because a horse's mental and emotional state reflects in its body. A relaxed mind is required for a relaxed body, which enables the rider to develop rhythm and suppleness - the foundation of your dressage training scale. If you are interested in the concepts of true versus false collection (which play a large role in muscle development), take a look at Sustainable Dressage - even if you are not a dressage rider you can find it very helpful. Dressage can be of great benefit to any horse and rider, regardless of intended discipline.