Q: How can swimming use up muscle energy instead of just burning calories and fat?

A: All activities, not just swimming, burn some amount of muscle and some amount of fat for energy. A successful fat-loss program combines a good workout plan and a good diet plan to influence your body to use up as little muscle and as much fat as possible for energy. There's a number of discussions of this phenomenon and what it means for an average person trying to lose weight in the http://www.mikesfitness.com/content/burning-calories-muscle-cardio , http://www.mikesfitness.com/content/cardio-does-not-destroy-muscle-when-... , http://www.mikesfitness.com/content/weight-training-better-than-cardio , http://www.mikesfitness.com/content/lance-armstrong-calories-metabolism

Please do not take all this info not as a dig at swimming, which is a great form of cardio. This is just the way our bodies are -- they don't only burn fat for energy (as many people are misled into thinking) but also burn muscle.