I believe any excercice given should be as near to the actual dance as possible. Stand against a door. With heels raised and knees straight. Lower through the feet, then the knees to about an angle of 45 degrees. Then raise the opposite way around. In the Modern Waltz you will have to do this at least 28 times in one minute if there are a few Chasses in there, there will be two in one Chasse. So the lowering and raising will be more like 32 in one minute dancing at 28 BPM. Go to it but not to hard at first. And remember to stretch before and after