Doing squats and leg presses help (if you're not adverse to lifting weights......it's not some peoples bag). I have lifted weights for years but never really concentrated on my legs. I started a couple of months ago and have noticed a remarked improvement in my lower body stamina. I too suffered and still, to a lesser extent, do suffer from the "rubber leg syndrome". Squats especially, strengthen the muscles that are directly used in crouching and rising from the butterfly position. It is only logical that having more powerful quads and thigh muscles will result in an improved ability to preform explosive, powerful movements repeatedly and effectively. I am speaking from what I know and not what I think I know.