I learned this technique from snowboarding offseason training, then i got a little creative with it. Supposed to tone and improve flexibility. It sounds silly, I know, but try it, let me know what you think. [Its a little harder than you'd assume.]
sitting down on an elevated surface, lift your legs out in front of you try writing the alphabet in the air using your ankles. then try it for your knees, by leaning back a bit and lifting your legs up enough to give them a complete range of motion. Then lean on your back, and try writing the alphabet by directing your legs with your hips.
I also do this with my wrists elbows and shoulders. [I use a weight for a little resistence.]