There is a demon, acedia, and it will exert its power to weaken yours. The relationship is directly inverse. As it flexes, you dystrophy. As you flex, it weakens.
Exert yourself then unto that which is meaningful. For there is idle activity, and there is fruitful inactivity. Think of Martha, who runs about the house, who does not do what is better. She collapses at the end of the day and has little peace for her effort. Likewise there is the man who spends his time doing this or that activity, mindlessly applying his body to that which does not require real effort or pain. For it is often more painful to be still than it is to remain busy.
Exertion unto a purpose. Was it a feat of strength for Mary to sit at the feet of Jesus? Yes, it must have been. For there was work to be done, and the desire to impress her Lord burned within her. But she chose that which is better.
Desire burns within us, but we must choose what is better.