Sure. Let me preface with it’s my belief that the post was hullaboo and unsubstantiated, and I think TTHW et all are peddlers of the worst order that prey on well-intentioned horse people who just want what’s best for Dobbin.
Chronic pain can cause a body to change in how it is muscled or appears over time. It can cause secondary issues too – the person whose hips hurt may find that years down the road their knees and back begin to hurt too, because of how they compensated for it - causing muscle loss in some areas and inflammation in others. The horse with navicular may lose topline because they won’t move out. A horse with kissing spine will sometimes have major muscle asymmetry on the shoulders and across the spine because they find it more difficult to engage one side. Myself, with an ACL and MCL reconstruction, have noticed that my posture has shifted to the left over the years, because of months of compensating and the subsequent loss of my hamstring muscle which was used in the grafting. My butt cheeks are two different sizes now 
I am mobile and fit, but it is a conscious effort to do exercises that continue to build up that muscle loss and subsequent weakness of my right side. A body worker would likely be able to tell I had work done to my right side because bodies are a response to their environment, and tell a story of the participant’s engagement in that environment.
When looking at horses, their topline does tell a story. It can tell a story of how they are used, how they are bred, how they are fed, and how they use their body. I will go against the grain and say the quintessential “TBs don’t have topline” isn’t true - TBs genetically have fantastic toplines. However, many TBs raced first and come with some baggage that can hinder developing a good topline down the years.
Some horses will lose muscle or definition in specific areas because of chronic pain. The general places to look vary from horse to horse and what physical complaints they have. Sometimes you can see it in the neck - the muscle closest to their shoulder can look over-developed or, confusingly, pinched. To give you a hard and fast specific: horses with symptomatic cervical arthritis up high tend to have overdeveloped trapezius muscles with the area in the middle of their neck (example, where you give an IM shot) looking pinched. Sometimes you see muscle atrophy or asymmetry up close to the poll, when compared from side to side. Some horses lose muscling along the flank, stifle, and up by the point of hip. If you see asymmetry here it can be anything from an old healed broken pelvis, to something like chronic stifle pain. Horses with stifle injuries tend to have a more “upswept” bottom of their belly and can show muscle loss along the flank down across their stifle. Some horses will have pinched looking muscles here with a depression between where their stifle and point of buttock are.
Horses in chronic pain can also develop defensive posture; they’ll stand under themselves with their hind hooves camped past the stifle line. This can change the muscling of their hindquarter and croup, making the topline look less developed while muscling around the hamstring seems fine. Sometimes this can cause hunters bumps to appear too - the SI gets strained from this abnormal posture.
The absence of muscling (atrophy) in some areas, and the over-muscling in another, is usually a clue of chronic compensation.
And then there is how they move. They might jerk their head up when asked for a transition. Some use their neck as a blatant rudder – they may hold their neck stiffly, or not engage with lifting their shoulders. They’ll toe-stub or drag their hind end. They might have range of motion deviations, bringing one limb towards the midline abnormally, or might travel with their head crooked to the side.
All of these have to be considered at once. You can’t just look at a photo and decide that horse is suffering from chronic pain. But you can take the clues all along the body, from how they stand, where their muscling is or isn’t, and how they move, to paint a picture of how they are or aren’t using their body appropriately.
