The Top Line is the phrase used in the world of horse training to denote  the chain of musculature along the top of its neck (from the base of the skull), right along its back, and on round its hind quarters and down the backs of its hind legs to the hocks, the horse’s equivalent of ankles. The established dressage training traditions of many centuries indicate that the top line musculature needs to be firmly, evenly, and elastically toned throughout its entire length so that the horse’s neck and back act like a bridge to help support the weight of the saddle and rider. Of course a bridge is not in some ways the best analogy as a bridge is a relatively rigid structure. It has some give to accommodate different levels of load, and different environmental conditions such as temperature and wind,  but the ratio of stability to movement is high. A horse on the other hand has to have a considerable amount of mobility as well as stability so its back can swing with every stride and turn yet have enough stability to support the rider. Hence a term used in one famous book of equine training, “elastic bracing” but not rigid bracing, a phraseology that Walter Carrington was fond of using in a human context.

I find it very helpful to use an equivalent phrase in AT teaching, referring to the Back Line in us humans as our version of the horse’s top line. The back line is the entire chain or sheet of musculature and fascia (myofascia) from the occiput down the back of the neck, down the back, and on down the back of the legs to the heels. There are arguments to be made that it should also take in the undersides of the feet, but I prefer to keep it simple. It also needs to be differentiated from the line of extensor musculature which switches to the front of the body at the level of the thighs to include the quadriceps as extensors of the knee joints.

In a horse, to attain a good level of elastic bracing in movement as well as at rest, the entire top line needs to be evenly spread and toned throughout all the movements of classical dressage, not bunched up and contracted in some places and overstretched in others, and and the movements themselves help to develop this. I therefore find it very educational to view the particular movements and activities FM chose to focus on within this context. Just look at the simple movement of going from standing to sitting. If the head and torso go forward while the hips go back and the knees go forward and away from each other and the heels stay down, the back line will be evenly spread and toned from occiput to sit bones, round the gluteals, along the back of the thighs to the back of the knees, and right down to the heels. Conversely, any of the typical manifestations of misuse – pulling the head back, pulling in and narrowing the back, pulling the knees/thighs together, pulling up on the heels, will all contract and distort the back line. The same applies to the movement from sitting to standing. The exact same applies to the movements of going in and out of monkey. Coming up out of monkey deserves particular attention as in the last stages of coming from semi-flexed to fully vertical it’s so easy for the straightening of the legs to pull the lower back and neck forward, again distorting the back line. And a sustained monkey is itself a wonderful exercise for toning and spreading the entire back line. The purpose of the antagonistic pulls, head forward, hips back, knees forward and away, and heels down, can be seen as activating the even spread and tone of the whole back line.

Going up on the toes is another practice FM seems to have liked. In the typical manifestation of misuse the head is pulled back and down and the back pulled forward and narrowed, again contracting and distorting the back line. But if the back can stay back and the head forward then it’s almost as if the heels are drawn up from the ground by the stretch of the back line reaching its elastic limit.

Even in rotation movements, the directions of head forward and up, back back, and heels down ensure the back line is spread during the rotation. Try putting your arms out to the sides and rotating to one side and then the other with good direction, using the antagonistic pulls, and you’ll feel the effect right through your back line. Better still, put your hands on someone else when they perform this manoeuvre well and you’ll feel it clearly in them. Similarly, try sitting and leaning forward with one arm raised. Then turn slowly to the side opposite the raised arm. If you can keep the line from head through back, round the sit bones to the back of the knees to the heels, you will feel that back line spread and toned from head to heels.

Finally, hands on the back of the chair adds a spread of the back line through the shoulders and upper limbs as the length from head to heels is maintained.

All in all, the concept of the back line has a lot of explanatory power in understanding the practices FM chose to focus on.

See for example: 

https://www.manolomendezdressage.com/wp/wp-content/uploads/What-the-Topline-Says-about-Horse-and-Rider-by-Manolo-Mendez.pdf

But a google search on “the top line in a horse” will give links to many articles and YouTube videos.