Question: I swim 4-5 times per week, which is great cardiovascular exercise, but is it important to do weight bearing exercise, i.e. running/jogging or power walking? Is the fitness required for swimming different from that required for running? Whenever I try and run a couple of kilometers it nearly kills me, yet I can swim laps for an hour!

Answer: The type of cardiovascular exercise you choose should be something you enjoy; that way you will be motivated to stick with it. So if you enjoy swimming, by all means, continue to do it. In terms of pure aerobic conditioning for your heart and your health, swimming is right up there with the best of them. Like cross country skiing, swimming is one of the few forms of aerobic exercise that engages your entire body; arms, legs, torso, everything!

From a body-building, body-sculpting, fat-burning point of view, most fitness competitors and bodybuilders, myself included, prefer the weight bearing types of exercise. I can't quote any scientific studies here, but it's my feeling that swimming doesn't have the impact on your physique that power walking, jogging, cycling, stairclimber or elliptical machines have.

I was just watching the Sydney Olympics on TV yesterday, and I noticed that the physiques of the swimmers were dramatically different than the physiques of the track and field athletes. The swimmers were all very fit-looking, but none of them had the rock-hard leanness and muscularity of a Michael Johnson or a Marion Jones. Why is this? Scientifically speaking, I don't know; I've heard all kinds of theories; one is that the cold water makes you retain a thin layer of insulating body fat (kind of like sea-going mammals such as seals have). I believe it has to do with the fact that running is a weight bearing activity and weight bearing exercise simply affects the muscles differently. When you're floating in the water, the muscles just don't contract the way they do under the force of gravity.

You said that "whenever I try and run a couple of kilometers it nearly kills me, yet I can swim laps for an hour!" This is because of the rule of training specificity. The principle of training specificity says that the conditioning effect of any exercise is specific to that particular type of exercise. In other words, if you swim a lot, you get proficient at swimming, but if won't make you a better runner. Likewise, running a lot won't make you a good swimmer. Either type of exercise will make you more fit, but that fitness won't necessarily carry over to another form of activity.

If you want to know what type of exercise will produce the best results for you in terms of developing your body, I would suggest doing an experiment: Switch from only swimming to a weight bearing exercise such as walking, jogging, elliptical machine, or stairclimber exclusively for 3-4 weeks and see if it makes a difference. Then try a combination of both swimming and weight bearing exercise for 3-4 weeks and see how that works. Based on this little experiment, you should be able to figure out what exercise or what combination of exercises produces the best results for you.

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