The conventional wisdom is that if you routinely do long distance running, it will break down muscle tissue, and the runner will lose both fat and muscle mass. However, if you sprint, that should help build muscle, especially if you aren't particularly muscular to start with (e.g., a distance runner who has never been more for weights). Sprinting may not be as anabolic as the weight room, but it is still anabolic, right? And assuming it is, where is the line between catabolic running (i.e., you lose muscle) and anabolic running (i.e., you gain muscle or at least won't lose any)? How far can one run without running becoming catabolic?

I "think" the line gets crossed when your diet is lacking and you're not training correctly. But if you eat right and train properly... ie... properly build an endurance base and take in nutrition correctly when performing activities, you should not lose muscle mass.

I'm putting "think" in quotes because I am not a physiologist but I do have anecdotal evidence to support my theory.

I too have muscles and train endurance. Nutrition is the difference I believe.

I also think, for me, any "perceieved" muscle loss is due to becoming leaner, and a slowing if not complete stop of muscle "growth" due to non use.

When the endurance trainign gets around 18-20 hours a week, maintaining muscle growth is just too much work. But I do not believe the muscle is lost.

UNLESS, like the Ironman said- nutrition suffers, lack of protein to burn in the endurance effort, will result in burning muscle.

that is my .02

FF

Id like to add to the original post, what if I was losing weight while building muscle? I have gone from 290 to 253 atm and have put on muscle and the amount I can lift has gone up. If I do alot of cardio, will it eat the muscle of my body or the fat first?

I'm going to use the "think" quotes too. My understanding is that when you lose weight its rarely pure fat. The goal (of course) is to have it be more fat than muscle that wastes away and the only way that can happen is through proper nutrition before, during and after workouts. Post workout nutrition is vital.