> But nobody does cardio to build muscle. The heart is a muscle. ;-) Cardio certainly isn't the best way to bulk up muscles; ideally you want to combine cardi

> But nobody does cardio to build muscle. The heart is a muscle. ;-) Cardio certainly isn't the best way to bulk up muscles; ideally you want to combine cardio with strength training. Nonetheless, it does build some muscle, and there are various ways to add resistance to cardio, such as with machines, carrying weights, running uphill, etc. In any case, cardio does increase your endurance.