Is cardio essential? In a nutshell, yes. Sorry.

Let's break it down:

Is cardio necessary for a powerlifter to lift a heavy barbell once?

No. But if he'd like to meet his grandkids – or least not be a wheezing sack of soft potatoes by age 40 – then yes.

Is cardio necessary to lose fat?

No. Does it make fat loss faster and easier to maintain? Yes. It also improves insulin sensitivity, which basically allows you to "use" carbs better for things like stimulating protein synthesis and restoring glycogen.

Is cardio necessary to build big muscles?

No. Can a little cardio help you become a better bodybuilder? Yes, via improved capillary density and venous return, higher work capacity, better/faster recovery, etc. And you won't gas out during long time under tension training, supersets, drop sets, or other tiring hypertrophy training methods.

Do you need a lot of cardio?

The good news is that it doesn't take much. For the slow stuff, try two sessions per week, 65-70% of max heart rate or 120-140 BPM, for 20 minutes. Throw in a CrossFit-style workout every once in a while. Shorter-duration HIIT and longer-duration cardio have unique heart health benefits, so you need a blend.

Do YOU Need Cardio?

Here's a popular test I came up with a while back:

Run one mile on the treadmill with the speed set at 6 miles per hour. Don't try to go faster; the idea is to be able to SUSTAIN this pace.

That'll take you exactly 10 minutes and it should be pretty easy. If it's not, you need cardio... or you're just too fat. Either way, you've learned something.

Here's another way to know. Let's say you're doing a classic drop set on the leg press: you do 8-10 reps and your partner takes off a couple of plates when you reach failure or close to it. After a couple of drops, what gives out first – your legs or your ability to breathe?

If your legs have a few more reps in them but you just can't suck in enough air, then you need some cardio.

Related:  The Best Cardio Machine in the Gym

Related:  The Best Cardio for Hardcore Lifters