body consists of number of muscles, which are contracting
& expanding all the time. Muscles need energy
i.e. Oxygen & Glucose for doing the work of contraction
and expansion. Every muscle has certain capacity to
work. Muscles cannot work if the work exceeds this
limit. The stage is called 'fatigue'.
Muscle can suffer from fatigue if the strain is more
than its capacity. Then it needs to relax (full stop
to work!). It needs sufficient rest to recharge itself
to work again.
physical strain is Fatigue.
physical strain is Relaxation.