to get serious about the answer to your question, you need to look for heartrate, temperature, or saliva amount... any can show the signs of when you NEED to stop working, depending on the exercise and climate.
If you know about muscle exercise, you could change the work within periods of time to allow proper rest and rebuild time. A proper diet will help with the rebullding of their lungs and muscles.
Do not go where the path may lead.. Make your own path and leave a trail.
~Ralph Waldo Emerson~
Those that matter dont care, and those that care dont matter..!!
You're not beaten when you lose, you're beaten when you quit..
"He grew into a forty-eight pound, steel-muscled song of war, which ran, diminuendo, from a heavy chest and head to a neatly tapered tail."
*John Tantor Foote*