Squash is a very physically demanding as a sport, not too sure about rowing as i haven't done it but i play squash at quite a high level where i live. It requires massive amounts of endurance, and i've had games last 3 hours most of which you are sprinting corner to corner returning shots. I would definitely recommend it to anyone as long as you have a friend of similar skill level to play it with. It's a fun game, and i'm not sure where the guy got the information that its bad for your knees, as squash was rated one of the healthiest sports to play because of its endurance. Its great for building stamina as well as cutting excess fat. However i can imagine that rowing would be better for building core muscle