Do the Muscles of the Child and of the Elderly differ in this?

  I turned to Tom on the elliptical next to me. Maybe the muscle knows three conditions: contracted, expanded and relaxed, I said.
  I think of my grandson, he replied. He knows run, run, run and carry me.
  As I left the workout, I wondered if maybe Tom's answer didn't contain the reason as to why stiffness and lack of muscle use is associated with growing old. Run, run, run equates with expanded muscles and carry me is the equivalent of relaxed muscles. That leaves out contracted muscles. Does the child not have contracted muscles? Are contracted muscles the result of stress and pressure and wear,  things the child has yet to know?
   And, I wondered whether the old have relaxed muscles. If we picked one up while they slept, how fast would their limbs bend? Would their knees bend immediately and fully when we lifted them? Or, would there be some stiffness that would keep their legs from bending so quickly and completely?
  Does the child know contracted muscles at all? Does the elderly person know relaxed muscles? How often does an older person have expanded muscles?
  I wondered but what expanded muscles are not what we should seek, if we were searching for some fountain of youth extending our lives. Are expanded muscles, with them perhaps relaxing when we sleep or at other periods, to be associated with long life? I do not mean the expanded muscles of a weightlifter. Rather, I mean just muscles that have a little ballooning effect, just enough to make them productive.