I feel for the swing of the ribcage. Up as the ribcage swings toward the side you are posting with.
[QUOTE=airhorse;7143083]
I feel for the swing of the ribcage. Up as the ribcage swings toward the side you are posting with.[/QUOTE]
Thatās another good concept to understand about the biomechanical physiology of how a horses moves his body in as he trots.
Simple put, in order for a horse to move a hind leg forwards to take a step, the horseās barrel (ribcage) must swing to the opposite side of the forward moving hind leg, in order to make room for that forward moving leg to travel forwards.
The horseās pelvic angle, and lateral spinal curvature also flex as the horse takes each step, and all of these enable the horse to move efficiently in a biomechanical sense.
So as you have noticed, as you rise on the correct diagonal, the inside hind will be traveling forwards, and this will cause the horses ribcage to swing to the outside as you are rising out of the tack as you post.
Note that the same is true at the walk as well. As you feel your horses ribcage swing from side to side, one can always tell which hind leg is taking a step forwards, as the horses ribcage will always swing away from the side of the forward stepping hind leg.
Next if the student rider begins to time their leg aids along with the forward moving hind legs, theyāve begun the path to learning to ride with effective and independent aids.
But in order to apply those aids effectively and independently, The student rider must also be developing a centered and balanced core so their legs may remain free to move with independence (in other words if a rider is depending on their knees or legs for balance, those legs may not be fully free to give subtile timed aids to ask the horse to do something as the need arises).
Alterhorse, I love your reply, and the bio-mechanics of it. It makes so much sense, once you know how to ride and have developed āa feelā. With the human staple, I have resorted to making her look backwards over her inside shoulder. It seems to work!
Learning diagonals took me a looonnnggg time !! Finally when I got it, the concept just sort of clicked. I always was taught ārise and fall with the leg on the wallā.
Funny thing is my sis got this concept way before I did and she does not have a particular rhyme or science to it ā just feels natural ? 
Maybe have her posting for two ābeatsā, then sit a beat. A very hard & strenuous exercise but might help her understand the concept.
I believe it is sort like identifying the correct leads in canter, it comes with time (plus I always peer down at my horseās shoulder!) Some people are more visual (like me) and some are more keen to āfeelingā like my sister.
Great suggestions in this thread! Iāve used white-out (if people still know what that is) on the shoulders to help students who find it difficult to learn the diagonals.
I do want to mention one thing, though. Some commenters say that it doesnāt matter which diagonal you post on. While thatās true, a real danger comes from always posting on the same diagonal. Doing this repeatedly can actually make your horse crooked and unbalanced in his muscle development, which in turn can produce lameness, back pain and soreness. By changing your trot diagonal, you avoid putting continual stress on the same side of the horseās body. It is a way to help your horse in learn to activate both sides of his body equally.
I will have them compare shoulders, and decide which one is going ahead while they are going up. Then they can determine which diagonal they are on. For riders who canāt get it, Iāll let them try to tell me whether they are on the right or wrong diagonal. If they get it wrong, or cannot tell, I will tell them when they are on the right diagonal, have them look at it and memorize what it looks like, then try again. Eventually they will learn what it looks like. For some riders, it does NOT look like they are rising when the outside shoulder is going ahead, so encourage them to remember what it looks like TO THEM when they are correct.
I also have had good success with the tape on the shoulders. For some students, a long piece of tape along the ridge of the scapula is necessary. If the problem is dyslexia, what helped a student of mine was color coding I put one red and one green polo wrap on the horse, red and green tape on the shoulders and one red and one green glove on the student.
I also agree with teaching to feel the inside leg pushing off and saying āNowā to get the rhythm and develop awareness.
Finally, putting them on the lunge line and keeping the circle fairly small at first should really help with feeling the diagnol.
Zombie.
Darn it!
Have you had her ride bareback, or with an older āpancakeā flat saddle, such as a PDN? That way she can definitely feel the diagonals.
