The original quote was always do you best, so maybe I was wrong to say 100% effort when I meant do you best. At this point we're arguing semantics. Do your best, your best will look different depending on your goals.

There will be times when giving 100% is going to be stressful, you might burnout a bit, but if it's a consistent problem then you're giving over 100%. There are a lot of factors, as I said every situation is different and you want to do your best in that situation.

Let's take a runner for example. In two different races, they'll have two different approaches. In a sprint, it's all out for 10s and they are gassed at the end. If they were to run a marathon at that pace they'd never make it. They'd be too stressed and they'd burnout. Now, running a marathon in 4 hours is a good time, but definitely slower than a sprint. That doesn't mean that the runner didn't give it 100% though, they just changed their effort to meet their goal.

This quote is more about effort over outcome. Every situation is different, and every person is different. If you gave it 100% effort, that's what matters. You might not get the 100% outcome that you wanted, but you did your best and you'll do a lot better than if you gave 50% effort.