I've heard in a few places that you shouldn't do too much interval training. Twice per week seemed to be a good recommendation. So if you run and cycle, should you limit interval training to once per week for each sport? I'm guessing the point is to give your cardiovascular system enough time to recover.

I don't think I have the willpower to do more than two interval days per week, so I'm just mostly curious if it would be a bad idea.

The length-tension relationships are all wrong if you expect direct transfer to running or vice-versa, particularly in the quadriceps, which could cause injury in some.

I wouldn't expect much muscular crossover, but I would expect some aerobic fitness crossover.

I was thinking more from the point of view of what would be too taxing a routine, since running & cycling intervals would both stress your cardiovascular system.

I also wonder what pro triathletes do, since they have to cut their workout into 3 different sports.

There will be an aerobic fitness crossover. Add swimming & you may seen a even greater crossover.

There are so many factors involved.

The definition of a taxing routine, from one person to the next fitness fanatic, is apples & oranges. Its about knowledge, research, application & listening to your body. Your body will know when its been taxed, trying to stay within the realms of not under/over training.

Are you looking to get into triathlons?.