Funnily enough, I have the same problem with a horse I've been lent recently... He's quite short from the buttock down to the hock, so he's quite stiff when being asked to really engage his hind legs. In fact, he found it so hard to start with that we had some serious disagreements... rears... bucks...etc... until I figured what was going on. So I've started working him with my old riding instructor. It's really helped having someone on the ground. So, these are some of the things we've been doing:
- Initial warm-up. I start completely off his back, riding in two-point for walk-trot transitions. Because he's so stiff, it really helps to free up his back and get his hind-end really 'swinging'. I maintain a light contact, but don't ask for frame at this point. The trick is to let the horse find that impulsion and get really forward.
- Once he's moving forward and freely, we complete some circles, serpentines, bending and 'a few' steps of lateral work. This helps him to engage his quarters, move toward contact (pref long and low at this stage) and lift his frame. Once we have a correct frame, we move onto the next stage....
- Lots of trotting! Not sure if you experience the same issues in trot, but Oliver (the horse) would 'pull' himself along with his shoulders in all gaits. In actual fact he has a really naturally powerful trot. We work with transitioning within trot (extensions down the long side, then collections on the short side etc). By the end now, you can actually feel him 'reaching' to step under, and a really powerful propel forward.
Initially we'd leave it there... no getting to canter. We have just moved on to the canter in the last week... and this is where my errors come in.
- Stiffness in back. Although I was moving with the stride, Biz (my horse) is quite slight, so the movements never had to be very 'big'. On Oliver, who's a bigger striding horse, I was restricting his movement in my seat. My instructor has me make quite exaggerated 'hula-hoop' actions (best way to describe it) to ensure that I am really moving with him. The slightest sign of me getting stiff and he'll tighten up to a short, on-the-forehand canter. (Note: I also shorten my stirrups a hole to stay very 'light' in the saddle... just for now, but helps him and me!)
- Secondly. Due to the bucking (but still no excuse), I was riding him fractionally too short in his neck... which led to him being an inch or two above long & low, hollowing his back and dragging the hind end. I didn't even notice it until my instructor pointed it out!!
We've started to up the lateral work, which really helps with engaging the hind quarters... we're doing it slowly so he doesn't strain himself. Additionally, I'll canter alternate circles in the saddle, then in two-point, then in the saddle. Helps maintain the freeness of movement! All-in-all, after about 3 weeks of this, he's already improving (and for that count, me too).
Some horses are naturally built to have longer, freer gaits, so not all horses will be able to achieve the same level of extension/collection. However, I hope the above helps - we've tried it and it's starting to work for us. Best of luck.