Any horse I've had that is choppy/has sore feet/has had founder/is foundering/bad feet/ect, I've always worked around the knee. If you start scratching around your horses knees (especially behind the knee for those founder cases) you'll release muscles, restore blood flow/muscle movement, and ultimately have a horse with fine feet. You'll know that you've got the spot when a horse starts pawing when you scratch around, stomps and snorts. This may sound a little weird, but if you pick up any kind of anatomy book for horses and look at the let, all the muscles that control the foot attach in around the knee and its like having a tight muscle in your back. Those muscles then proceed to cut off blood flow with the veins that travel the same path up the leg, and you end up with all of these different symptoms. If you're willing to give a good scratch, you will see the horse's reaction.