Lots to reply to...
1 - Few saddles place you directly over the horse's center of gravity. A jump saddle or a racing saddle, ridden in a forward seat, probably would. Almost every other saddle will place you behind the horse's center of gravity.
I pulled this from the Internet for discussion, and I have no idea who the rider is:
At a rest, the horse's center of gravity is roughly at the front top of his boots. With motion, it will move forward. Is his weight forward or aft of the horse's center of gravity?
Aft. And that is good in a western saddle. There is a reason that my western saddle is 27 inches long, but my Aussie saddle is 21 inches long. And there is a reason why the tree extends further back with a western saddle. Western saddles were designed in part for roping. When you rope a steer, and the weight hits the horn, you need all the weight to the rear that you can get - I'm guessing, since I don't work cattle. But all that weight hitting the horn needs SOMETHING for balance...
I can't find my book right now, but Littauer discusses being aft of the horse's CG as adding a safety factor for some things, such as unexpected stops. It also tends to be easier on the rider, again according to Littauer. That would help explain why stock saddles tend to be built that way.
2 - Western riders DO expect their horses to be balanced, but that is NOT the same thing as riding with a collected gait. They definitely need to be able to shift their balance toward the rear, because that is critical in pivoting or fast turns - which is needed for working stock, or barrel racing, or reining, or most any other western competition.
Remember - no race horse ever won the Kentucky Derby by being unbalanced, OR by being collected. Balance does NOT = collection.
3 - Strung out. "I wonder if the idea of "heavy on the forehand" might not be better understood, for the purposes of this discussion, as shorthand for an inverted topline, disengaged hindquarters, and "strung out" posture, rather than as an absolute distribution of weigh..."
In my first post on this thread, I made that distinction. Post #2:
"There is a difference between extended and strung out. My Arabian mare does both. When she gets strung out, I milk the reins a bit with her shoulder movement and shift my weight further back, and she usually responds with smaller steps. If I work at it a little, I can usually get her down to a slow jog, which she is actually very good at."
A horse can get strung out either western or English. In either case, the answer is to gently restrain the front end while keeping the rear end moving, until the rear and front are working together again.
4 - "We do not automatically adjust ourselves to the burden of added weight, at least, not always in the most biomechanically constructive way. Most everyone knows that heavy weights should be lifted from the knees, but does everybody do that in practice?"
The first time you do it, you will often do it wrong. But if you are lifting heavy weights (sacks of grain or dumbbells) very often, you will either learn to do it right or give up due to pain.
I used to own a green, 750 lb Arabian. In riding her out, my 200 lbs of man & tack was a challenge for her. I could feel her adjust her balance, trying to make it work. And we went from very short rides to much longer ones as she got used to it. I also have a 650 lb mustang pony. My 200 lbs of person and tack is a challenge for him, and he adjusts to make it work. I can feel him carrying me differently than my 900 lb mare. But he knows how to do it, and has proven it by galloping as nauseum with me on his back.
5 - "It's good riding, regardless of discipline, not tack style that determines how heavy or light a horse is on his forehand"
Not entirely, in my experience. A horse can get strung out with either tack, but the further aft weight of a western saddle tends to discourage it. When my horses get strung out, shifting my weight to the rear seems to help settle them.
6 - "The portions of the bolded quotations rather confuse me... how can the stride be shortened, yet the distance traveled each stride increase?"
Good question. I do not know. Here is a more complete quote, but it confuses me too. However, please note what percentage of weight is carried on the front or rear, and how that can change without the rider trying to do anything:
How Much Weight Can Your Horse Safely Carry?
"Because a trotting horse looks like he is using his diagonal feet in perfect tandem, it might seem as if the reaction forces would be evenly distributed across the two legs that support him at each phase of the stride. But in fact, there are significant differences in the amount of forces borne by the front and rear legs. On a level surface the forelimbs consistently supported 57 percent of the forces while the hind limbs supported 43 percent. Going uphill, this pattern of distribution shifts, with 52 percent supported by the forelimbs while the hind limbs took on 48 percent. Time of contact also varied. At higher speeds, the two feet were on the ground about the same amount of time, but at slower speeds, the hind limbs tended to spend less time on the ground--an observation that had never been made before in quadrupeds, according to Wickler. For the front limbs, time of contact didn't change significantly whether on the level or on the incline, but the hind limbs tended to be in contact with the ground longer when going uphill.
To study the biomechanical effects of loads, the Cal State researchers trotted five Arabians at a consistent speed on a treadmill under three different conditions: on the level with no load, on a 10 percent incline with no load, and on the level while carrying a saddle and weights that totaled about 19 percent of their body mass. To record the motion and speed of the horses' foot movements, an accelerometer was attached to the right hind hoof, and the sessions were recorded with a high-speed video camera.
Carrying a load caused the horses to leave their feet on the ground an average of 7.7 percent longer than they did while trotting unburdened. On the level, the addition of a load caused the swing phase of the stride to become 3 percent shorter, but going uphill this phase of stride lasted 6 percent longer.
In short, explains Wickler, carrying a load causes a horse to shorten his stride, leave his feet on the ground longer and increase the distance his body travels (the "step length") with each stride. All of these gait adjustments work together to reduce the forces placed on the legs with each step. "Forces are damaging," says Wickler, "so keeping the foot on the ground reduces peak forces and reduces that potential for injury."