With weights on ankles, lie on your side and do leg lifts. Of course you need to do this on BOTH sides! Also do BOTH variations of leg lifts. One of which, you may not know about. That is bend your top leg forward out of the way and with the weight on the lower leg do lifts with IT. Also, lie flat on your belly and do lifts that way, too.
These types of leg lifts will tartget the muscle groups that you need to be able to position your leg properly to give the aide properly as well.
Start off with as many reps as you can comfortably do and add five more. When it gets easy to do that, add five more, that add ten more, keep adding ten more until you have the results you desire.
As you add reps you may or may not desire to add more weight.
The more reps you do the better off you will be. It’s the reps that count anyway. The weight is just an added benefit.
AND one answerer mentioned riding the three point. EXTREMELY GOOD idea!!!!!!!!!!!!!!!!!!!!!!!!!!!

of couse! Its good that at least you are trying to build up your muscles. The way i built up my muscles was doing english riding, especially the posting really helps build muscles. Also holding the three point position at say, a canter, is also great. But the best way to build up riding muscles is by riding since when you do volley ball, you arent using the muscles you use for riding but instead,the outer muscles thats required for running. good luck!!

there is a program called air alert u can buy that..or u can basically jump like a kangaroo up and down 25 times each 5 days a week that will increase your vertical leap and strengthen your calves

Jogging with weights may help a bit, as well as doing squats.
It doesn’t sound like it would be safe to do on this horse, but on a horse that’s easier to control, try riding without using the stirrups, and posting.

Stairs are great. Swimming is also very good, it will help all of your muscles. One of those things that you squeeze with your legs( can’t think of what it is called). If you can jog in the sand, that will work great.

i ride myself, and what i do to build up leg muscle is work the whole summer with out stirrups. (i ride English) if you ride western, then you could go to a gym and work your legs there. good luck!

yes… there are also some books on this at Borders or Barnes and Noble… you can check those out.