When working out, sometimes you can feel your muscles being the limiting factor, sometimes your cardiovascular system.

As you get more fit, it oscillates which is the limiting factor.

If your muscles are the ones you can feel holding you back, it's your cardiovascular system that's supporting you.

As you push your muscles they get stronger, and they become the platform to then stretch your cardiovascular system, and it repeats.

Each steady foothold gives you the leverage to take the next step on the other side.