Your ability to generate power is directly proportional to your ability to relax. – David Allen
Something most people don’t know about high performance motorcycles, or really any high performance vehicle, is that the brakes are as important as the engine.
Because races, besides drag races* I suppose, don’t happen purely in a straight line, the ability to slow down is as important as how fast you can go, and how quickly you can get going fast.
So it is with people. The ability to use your mental and physical (and emotional) faculties is directly related to how well you can quiet and slow your mind and how effectively you can rest and recover.
Your mind is of little use if you can’t discern your thoughts and think carefully about things that matter. Nor will you be able to make quick decisions unless you’ve practiced quietness and stillness and all-the-while focused on the information, knowledge and wisdom necessary for good decisions.
Your body will not function well without sufficient rest and recovery. Just pushing and pushing, especially with the addition of stimulative chemicals, will result in sub-optimal performance and lead to burn-out in short order.
So, whether you view power in terms of ability to do either mental or physical work doesn’t matter; both require intelligent and effective rest and recovery in order to allow you to good work day-in-and-day-out.
* Brakes, or some sort of slowing mechanism is important on drag race vehicles, as they generate immense speed quickly, but drag strips don’t have long run-out areas – that’s why really fast drag cars have parachutes which slow them down after getting up to ~300 mph in just a quarter mile.