An important missing link that I see in training horses is a big gap people create between ground work and riding. Though trainers think their ground work prepare a horse for being ridden a lot of things left out and not taught to a horse from the ground which can prepare him to understand being ridden much more. One of the most invaluable work that I see missing is work in hand. This work can prepare a young horse to understand rein aids, to understand driving aids by the touch of the whip that lately can be used in the saddle to make a connection between it and a leg aid. No horse in the world will automatically know what leg aid means. They will ignore it if they are relaxed and lazy or be afraid of it and run from it if they are flighty and sensitive. The work in hand also teaches a horse movements that he will be performing lately under saddle such as halts, circles, leg-yields, shoulder-in, travers, renvers, half-pass, reinback, turn on the forehand and turn on the haunches.
A lot of regular ground work is aimed to make horse watch and move itself in accordance to handler's body. This is very important part in horse's education but it is not all. When a handler becomes a rider a horse doesn't see him/her any more only feel. There must be an education given to a horse that prepares him to understand how he should respond when being ridden. Work in hand is invaluable in that regard. It can teach a horse precise responses to rein aids and touches of the whip.

Lunging is very important part of a young horse education for many reasons. The obvious ones I will not list. However, lunging allows trainers to teach their horses voice commands for all the gaits. This is so useful when a young horse is being ridden. Telling a young horse to trot or canter is much more easier than kicking them into it. They stay calm, no need to run and they understand what a rider is asking because they have learned it already.
Driving or long lining helps to prepare a young horse to lead, to be in front and to be brave. I do a lot of trails in long lines with young horses. They become so familiar with all the trails, learn how to leave their buddies, be driven from behind and listen to rein aids. Sounds very helpful for future riding.

The previous work was concerned a lot with a horse's mind. It is very important that a horse accepts the handler, rider, etc. That a horse is calm and is OK with idea of being ridden. However, this is not the end of training, this is very much a beginning. In order for a horse to have a long and happy life, perform at his best and avoid injuries he needs to not only learn aids for stop, go and turn. But also be able to know how to balance and coordinate his actions with rider's weight on top of him. Develop his strength and agility to carry himself and his rider correctly and effortlessly. Gradually learning and also building strength and suppleness to execute more and more complicated movements or higher jumps.
It is pretty inconceivable to think of human children doing physical activities not appropriate for their age like lifting too heavy loads not to mention run with them and turn and stop quickly. Very easy to imagine how damaging this is to a child. However, for young horses this is a common sight. A young horse 2-3 months into saddle work is asked to stop from canter, spurred for not picking correct lead, pulled around to execute a tight turn, etc. Human athletes are carefully developed over many years of training to reach levels of effortless execution of difficult movements which require coordination, balance, suppleness and strength. Sadly, horses are often treated more like machines. They expected to do things well as soon as they learn the aids for it. For a young/green horse ANY maneuver under rider requires coordination, balance, suppleness and strength. A horse at liberty does not move the way it should move under rider. Horses are crooked, their trunks are twisted during turns, they lean in, their hind legs slide outward from under them. All of these natural ways for a horse to move need to be replaced with ability to stand upright and level during turns or straight movements, step under the body and under the rider's center of gravity with their hind legs, lift through the ribcage in order to prevent leaning into a turn, stay supple in their backs and engage their core to carry a weight of a rider with ease. Why we want that because it is very uncomfortable to ride horses when they move as they "see" it. A horse exactly like a human is an athlete and should be trained and developed as such not scared into execution before they are ready to do it. You cannot whip a gymnast in order to improve her split or a cartwheel.

Here is the link to Arroyo's 6th ride. First time off the lunge line, no fence around my arena - walk, trot, canter. Arroyo is 3 years old. That video was done 3 weeks ago. The other day we did our first short trail ride. All by ourselves, no other horse with us. He was calm and confident because he has done that trail so many times. The only new thing was he had to step over small logs with a rider on. :)
Arroyo under saddle

Happy riding...