Human adult muscles do not normally work at 100% of capability for at least 2 good reasons - in extreme circumstances, normal people have lifted cars off injured people, and performed such similar feats.
1) muscles keep something in reserve to extend endurance
2) if you have ever experienced a child - you will understand what I mean when I say 'GENTLY!'.... :-) we learn socially to restrain ourselves.
3) BTW, Adrenilin is a powerful muscle stimulant - fight or flight.