Forward motion is one of the most fundamental tenets of classical horsemanship. Anyone who has ever started colts knows this - you cannot teach them anything (stop, go, turn) until they are moving freely forward. As with all training problems if your horse is refusing to move forward you must first and foremost consider a physical cause. This could be an ill-fitting saddle, a painful bit, a sore joint or even a rib out of place that only hurts when you ask for a canter (right before you get bucked off!). With a horse that is not moving willingly forward, it could be any of these and dozens of other physical causes. Before you start it is best to have your horse assessed by a vet to rule out any issues.
Once a physical problem is ruled out then you can look to the horse’s training. First off, does he have any clue of what you are asking him to do? You’d be surprised how many horses are being ridden regularly but have very little actual training. When it comes to responding to the bit and cues, it has to be taught by someone. Often I see horses that are just simply untrained even though they are packing a rider down the trail with no problems.
The next thing to consider is whether a reluctance to move forward is a learned behaviour. In other words does the horse know to move off your leg but is ignoring your aid because it has worked so well for him in the past? It doesn’t take much of a perceived gain for a horse to learn that his refusal gets him what he wants - many insensitive horses will gladly endure the kicking and spurring from a rider if it means not having to lope circles or do any real work.
In some cases the problem is that the rider is giving the horse conflicting signals by pulling back on the reins when they want to go forward. If you are riding a lazy horse or one that is reluctant to move in certain situations (like when approaching a giant mud puddle!) and you pull back on the reins, the horse will choose to take that as permission to stop. Any backward pull on the reins inhibits forward motion; that is a fundamental truth that you should always remember. In some cases we do want to inhibit forward motion, like when asking the horse to stop or collect his frame, but he has to be moving freely forward first. In many cases riders are simply pulling on the reins, inhibiting forward motion when they don’t really mean to, like when the horse spooks, balks, backs or is just plain being lazy.
Remember that forward motion precedes all other training concerns and when you need the horse to move forward more you must reach forward with your hands. If you pull back on the reins when the horse is not moving freely forward or even just keep your hands in a neutral position, the horse is unlikely to move. Actively pulling back on the reins of a lazy horse or one that is not moving forward for any other reason will always make the problem worse.
The use of spurs is not a good choice on a lazy horse. Spurs are not an aid to make a slow horse faster; they are an advanced-level aid for riders to use on horses to motivate them to a higher level of performance and do more difficult manoeuvres. If the problem with your horse not moving forward is simply because he is lazy and has learned to ignore a polite request from your legs then he needs to be reminded to respond to a light leg. I prefer the use of the crop to reinforce your leg aid, rather than the spur, in the instance of the horse not moving forward off a light leg cue. A spur will often make the lazy horse even duller to your leg and make him sull up and balk even more. Kicking harder on a lazy horse also does not work well because the lazy horse will tolerate the pressure and just wait for you to quit kicking from exhaustion, which doesn’t take long!
In a horse that is trained but not responding, ask once lightly for forward movement with a shift of your weight forward and a gentle bump with your calves remembering to give with your hands. If he ignores your polite initial request immediately reinforce your leg cue with a sharp spank with the crop right where your leg cued him (make sure you keep reaching forward with your hands). If done right, this will undoubtedly send him expeditiously forward but do not contradict yourself as most riders do by then pulling back on the reins. Let him move and only stop him when he is voluntarily going forward without any ‘pedalling’ from you.
Ask your horse lightly to move off your legs again. Be prepared to reinforce with the crop if needed but always giving him the opportunity to respond to your light aids first. If your timing was good the first time, and you used adequate pressure to motivate the horse, the second time you ask with your legs he should step right off. You may need more than one spanking before the horse begins to take your leg cues seriously again but with good timing and the right skill level of the rider the horse will be moving freely forward in minutes. Again, this is a tip for refreshing a trained horse that has become unresponsive - you would train a young horse that didn’t know better a little differently.