Physical Strain

Our 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.

Increasing physical strain is Fatigue.

Decreasing physical strain is Relaxation.