While there is room for flexibility in a horse's warm-up routine, an absolute must for both the warm-up and cool-down is 10min of walk. Sound boring? Turn on the radio or listen to music on an ipod. Take this opportunity to stretch and warm up yourself while on the back of your horse (some rider stretching exercises were provided on the website linked in my last blog). Here are some exercises you can do before you throw your leg over your horse. The walk should be done on a loose rein whereby you allow your horse to stretch out and warm up his muscles without any demands.
After walking for an equal amount of time in both directions, you can pick up the trot. At this point the trot might still be stiff and your horse will likely be lazy, he won't be using himself properly. He should not be expected to. Same as with anything 'horse', all is initiated by the horse and is done on the horse's timeline. This is his chance to do his best impression of a giraffe and to trot around with his head and neck wherever the heck he wants to put them and his hind legs trailing several kilometers behind him. Stay off his back during this time ie, post your trot. Let him trot along the rail on a loose rein for a good 5min minimum before starting to ask for large patterns - ie, figure-8's that take up the length of the entire arena, etc. As you feel your horse start to warm up and soften, you can start to ask for smaller patterns. Smaller figure-8's, circles, serpentines, etc. These are more demanding exercises by virtue of their size so they should only be asked of the horse as the horse tells you he can do them, as his muscles become sufficiently warm to tackle increased engagement and greater demand. You should feel him soften, relax, become more flexible, and start to naturally engage from behind. At this point you have probably taken 20-25min to warm your horse up - he should feel pretty good beneath you.
One exercise my instructor recently showed me that I wanted to share was one that stretches out a horse's shoulders. Some horses in particular hold a lot of tension in their shoulders, but this exercise can benefit any horse. Essentially the exercise is a figure-8 smooshed up against the arena fence or wall:
Two oval loops flattened against the wall, with a change of direction between them. Ask your horse to maintain balance and to not drop his shoulders in the corners. He should also maintain gait and pace within that gait. As for pattern size, the smooshed-figure-8 can be as large or as small as your horse is capable. If he is unbalanced and struggles with the indicated smaller size, increase the size of the loops (ie, the broken line in the image above). Only ask this of your horse after he's been trotting his warm-up a minimum of 5min and once he is comfortable with smaller patterns.
Where you take the remaining 5-15min of your warm-up should depend specifically on your horse's needs. With some horses you might want to start at this point including exercises such as lateral work and transitions. I find leg yields and shoulder-ins to be particularly beneficial in suppling some horses. I often ask a horse to alternate bends down the centerline to supple their neck and barrel somewhat. Jane Savoie has an excellent poll suppling exercise she explains here:
You might also want to add some gallop or canter in your horse's warm-up routine. A good gallop really allows a horse to stretch out and can be greatly beneficial to any horse. In fact, allowing your horse to open up in a good gallop as a regular part of your weekly routine has a lot of both mental and physical benefits to your horse.
A proper warm-up should take a rider 25-35min minimum, longer if the horse is older or if the weather is particularly cold. Your cool-down should be similar and should serve to really stretch and supple your horse in addition to progressively cooling him down and allowing his breathing to slow. Halting and putting your horse up after a work-out while his muscles are still warm may cause his muscles to stiffen and seize, which is of course of detriment to your horse.
Your horse is an athlete as any other - make sure you condition and treat him like one!