I know that tennis involves many small steps in different directions, and is quite different than running. However, what if court time is limited. Would running ( and say your kid is not on a soccer team) be a decent alternative? Would running put too much stress on the same muscles used for tennis and lead to quicker injuries? (I have heard that running the stairs is a very fast way to injury based on the USTA regional centers). Or would you suggest a different type of conditioning?