If you have no idea how to take care of your legs, here's a quick exercise that should get the job done without you having to worry about which way is the fastest way to train legs.
It's simply going straight down as fast as you possibly can. Take two or three squats legs in your hands and sit cross-legged on the ground.
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Hold your hands with a slightly more than 45-degree angle to each other at roughly the 90-degree point. As you pull the legs in a smooth, steady motion, you are applying equal and opposite forces to each pair of muscle fibers. This can feel strange at first, but each fiber is holding onto the opposing fiber by a kind of rubber band; the more you keep pulling, the weaker the rubber band gets and the easier it is to move each fiber. Then slowly start working the other leg. Continue to pull the first leg in a smooth, steady motion while you pull each other's limbs in a similar motion. As you slowly increase the speed with which you pull the first leg, you will notice that the fibers in your other leg are beginning to tire.
Keep pulling the leg until it is done growing or for about twenty seconds after that; then slowly let go and switch legs. This is called bridging the second leg and is a common training method for young horses. The horse's legs are much broader than most riders realize. Since they are used by the horse's musculature to support him in the movement of his legs, it would take an extraordinary athlete to balance himself on his horse's legs. Most riders who do a lot of horseback riding find it pretty easy but the jumping parts take some practice.