At least this is the impression I have. The advantage running has for itself is that it raises your heartrate a bit more. Therefore, studies show that elite runners tend to have a slightly stronger heart than elite swimmers. However, running takes a huge toll on your body. Granted swimming can be bad for shoulders too, but not as much as running is for knees, ankles and hips. Moreover, swimming gives you that upper body concentric strength, which is very useful for guard players. Swimming also teaches you how to control your breathing. Last but not least, running is something that will inevitably make you stiff (again, studies show that being stiff in the thighs was something common among elite runners), and that's not what you want for bjj, right? What's your opinion on that? What do you do for cardio?