You set out running at a pace that’s brisk but well within your limits and find yourself struggling and out of breath.
You’re in decent shape, but you get short of breath climbing stairs.
In both cases you struggle not because you’re unfit, but because your body is unprepared for the exertion. Before you start exercising your system is getting just enough oxygen into your body for whatever you’ve been doing up to then – resting, or standing around, or having a gentle walk – and finds itself playing catch-up when your muscles suddenly start doing more work.
This can make a given level of exercise feel far harder than it actually is as your heart and lungs work overtime to pay off the “debt” of oxygen while still giving your muscles what they need for the current moment.
And this in turn can make you uncomfortable, put you off your stride, and make you start to doubt your fitness, or even whether you should be running at all. In a contact sport it could see you get knocked back, mentally rattled and a point or two behind, playing catch-up for the rest of the game. Oxygen debt becomes scoreboard debt.
You probably already know the answer to this: you can mostly eliminate the problem with a decent warm-up, including a burst of exercise at your planned level of intensity, shortly before starting.
It’s a metaphor of course.
Physically, intellectually, socially, artistically, it will be to your benefit if you know that this is going to happen (so that you don’t give in to doubt) and allow time for yourself and others to get your system up to speed and your head into the game.
*I use the term oxygen debt loosely here – oxygen deficit is probably more accurate. See here.