How You Sit on the Horse
"Functional training" refers to training your body, or parts of your body, for activities you perform regularly. In order to functionally train for horseback riding, you must first observe what your body is doing while you ride. For instance, when you're sitting on the horse, your legs are spread, heels are down, and toes are slightly turned outward. Your legs shouldn't be tense or locked out, there should be a slight bend in your knees, and most of the pressure is directed toward the balls of your feet.
What Your Riding Stance Means
When you ride as described above, your legs, knees, and hips are absorbing pressure. Thus, it makes sense to develop these areas in your training. When you spread your legs as you ride, push down slightly on the stirrups and turn your toes slightly outward, your hips are extending, abducting and externally rotating.
Hip extension, abduction and external rotation exercises
1) Hip extension-Lying Hip Extension
2) Hip abduction-Side Lying Hip Abduction
3) Hip external rotation- Clamshell Exercise (Use band for added resistance)
Each of the above offers a link to a video for examples. There are many other exercises involving weight machines, resistance bands, etc. that can be of great value, as well. Calf raises (and especially lowering using a step), with or without bands help strengthen your legs for applying pressure on the stirrups. And, side-stepping with a squat movement with a resistance band is another great way to work on your hip abduction.