I generally split the use of your legs aids into five different categories. There are three different positions for your leg: at the girth, slightly in front of the girth, and slightly behind the girth. There are also three ways you can squeeze your horse: your outside leg, your inside leg, and with both legs. The position of your legs and which leg or legs you squeeze with makes up the full combination of leg aids you can give your horse.
Both legs at the girth, both squeeze. When you first learn to ride one of the most important things to learn is how to halt your horse. Halting your horse is your first introduction to transitions. As you become more comfortable with starting and stopping your horse you will move on more advanced movements such as half halts and finally collection.
The leg aids for bending depend on if you want to affect the horse’s shoulders or haunch and in which direction.
Shoulder In: Outside leg slightly in front of the girth, inside leg at the girth. Outside leg squeezes.
Shoulder Out: Inside leg slightly in front of the girth, outside leg at the girth. Inside leg squeezes.
Haunch In: Outside leg slightly behind the girth, inside leg at the girth. Outside leg squeezes.
Haunch Out: Inside leg slightly behind the girth, outside leg at the girth. Inside leg squeezes.
By positioning your leg either in front of or behind the girth you can change the direction of the bend as well as which part of the horse you want to bend. A leg in front of the girth asks your horse to move it’s shoulders while a leg behind the girth asks your horse to move it’s haunch.
Outside leg behind the girth, inside leg at the girth, both legs squeeze. Impulse is how much force your horse uses to push off from the ground. Impulse isn’t speed — your horse can be going very slowly and have a lot of impulse. Think of your horse as a spring, a horse with lots of impulse would shoot up in the air if you pressed down on the spring and let go. A horse that lacks impulse would only bounce a little bit before falling over.
When you ask your horse to pick up a specific lead you need to give it a different leg aid than you would with a normal transition. This is so your horse knows which leg you want it to lead with.
Canter: Outside leg slightly behind the girth, inside leg in slightly front of the girth. Step your weight into your inside leg and press with your outside leg.
Counter Canter: Inside leg slightly behind the girth, outside leg slightly in front of the girth. Step your weight into your outside leg and press with your inside leg.
Removing pressure from both legs and then re-applying the pressure one or more times. Sometimes it’s important to correct your horse’s mistakes or bad behavior. When this happens you tap-tap-tap your horse’s side with both legs. Corrections should always be given with the personality and behavior of the horse in mind and with the least amount of correction required for the particular horse. For instance you wouldn’t give a forceful correction to a hot horse where a light squeeze-squeeze-squeeze would suffice. In the same vein you may need to give a lazy, laid back horse more of a nudge to get it’s attention.