No one seems to have a clear definition of what it means training “cardio”. In fact this definition should not be applied to a specific training. But if you want to call something “cardio”, you should be referring to a training in order to improve or increase the capacity of your cardiovascular system. And a major component would be the improvement of their VO2max.
In one of his articles, Andrew Read, makes a review on the book The Cardio Code by Kenneth Jay.
In this book, Jay says that many people think that anything that increases the heart rate should be good for the cardiovascular system. But there are differences between what happens when you lift weights and what happens when you run.
Did you know that your heart aches different reactions during the concentric and eccentric contractions? And some changes that come from weight lifting are not good for the heart in the long run?
Strength training thickens the walls of the heart, making it less flexible. As another muscle, some degree of flexibility is actually a good thing. But when you start to work harder, your heart can accommodate more blood. In lifters, the heart is rigid with thicker walls, which restricts the blood supply. This takes time and patience to overcome, and it is one reason why so many athletes of this type have difficulty in cardiovascular training.
Anyway, the author of the book does not condemn the weight lifting training. But if an individual seeks to improve his cardiovascular fitness, it should look for activities such as running, cycling, as a strategy for this purpose.
The main message would be: Use the right tool for the job. For strength, lift weights. If you want the ability to work – use a combination of strength, more endurance – then circuit training is a great idea. But, to increase VO2 max, it is best to think of making some traditional training!