How would energy usage vary if maximum exertion is reached?

Answer 1

I guess your talking about "substrate" utilization.

If one is working at maximum or near maximum intensity (exertion) then the body would most likely utilize the creatine-ATP system (Sprinting, Maximum Weight Lifting). I dont know if you're referring to calories burned but usually these types of exercise burn moderate amounts of calories while performing them. Research is out there is debating the "afterburn" of these types of exercises (calories burned after the exercise). Some say its significant, and others say its not. Try researching "EPOC" if your interested.

Answer 2

When maximum exertion is reached, energy usage increases as the body requires more fuel to power the intense physical activity. The muscles consume more energy to sustain the effort, leading to a higher metabolic rate and an increased demand for oxygen. This can result in a spike in calorie expenditure and overall energy consumption during and after the intense physical activity.