When I first started running using Work Of Breathing as a reliable measure of effort was a common thing. Over the years I have carried that measure into most other activities.

The goal is to stay aerobic for all except emergencies or very short overload periods.

Easy Breathing: Mild effort, able to sustain for long periods,

Harder Breathing But Able To Carry On Conversation Without Gasping: Maximum sustainable effort. Still aerobic.

Heavy Breathing with Gasping to talk: Maximum effort for short periods. Borderline aerobic.

Heavy Breathing, Cannot Talk, Frequent Gasps: Anerobic, heavy stress on body. Not sustainable. High likelihood of damage to body.

Gasping To Breath: Anerobic, Not Sustainable, Hope the bear chasing you runs down very soon.

More reliable than any machine and handles the day to day body fitness.