Imagine watching a fully-grown cat move... It is typically smooth, comfortable and lithe. There are no tensed muscles, no pumped up arms, raised 'gym shoulders'. The animal does not get out of breath.
A cat can go from complete passivity to combat readiness instantaneously. It does not tense muscles and prepare. It just moves. There is a sense of ease.
No struggling, grunting or groaning, no pain in the back or the knees. The body responds instantly to the dictates of the mind. A cat is spontaneous and free.
Young people are the same... Learn from this?