As a strengthening exercise, I find riding forward and back (the "accordion"), in any gait, is hard to beat. I've seen Forrest's natural aptitude for lengthening at liberty and on the longe, along with requesting a few steps of it while stretching over his back. So it seemed all the prerequisites were in place to ask him for a focused attempt, each direction, on a 20 meter circle.
What my aim is for this exercise is for it A) to make sense to him B) that he remain relaxed the entire time and C) that, while I don't expect an upper-level frame, I'm able to maintain his balance while lengthening, without him diving onto his forehand.
After his routine warm-up to make sure his back is loose and all the spooking (hopefully) is out of his system, we pick up a nice, forward, rising trot down the long side.
As we begin our 20-meter circle, I give Forrest a half halt on the outside rein to tell him something's about to happen and to help him remain balanced as I close my legs and begin to build towards our lengthening.
This was a good effort and I find that Forrest is now right on my aids so I can bring him back by giving a half halt on the downbeat of my post, and simply weighting my heels. In this happy, 'forward thinking' trot, we change rein through the circle to track right
Back on the circle, I repeat the aids: a simple outside half halt, a closing of both legs, and Forrest responds by giving me another honest effort, which again, is easy to throttle back by simply sitting up straight, half halting on the downbeat of my post, and weighting my heels.
Forrest, being only four, is far too young for me to focus on trot lengthenings any longer than what we've just achieved. A few steps here and there are fine. I know they're in there and that he's capable of doing it, and that's where we'll leave it. There is no need to try to come pounding across a diagonal at this stage.
As an end note, a lovely by-product of riding the accordian exercise is that, if you have successfully engaged your horse's hind end, the halt that follows should be nice and square!