[Muhammad Iqbal Khan (OP)]

hello,
      My question is that why does a person bend when he has to raise heavy load on his back, what effect does the load has on center of gravity of a person.

[JimBob]

Welcome Muhammad, thanks for joining the forum!

The center of gravity of the body with a load on its back shifts to the rear of the person, causing the person to bend forward to get the new center of gravity over his legs.
 

[DoctorBeaver]

Welcome Muhammad.

Jim has explained it well. If you didn't lean forward when carrying a heavy load on your back, then you'd topple over backwards.

[lyner]

Stand with your back to a wall and with your heels touching the wall. Try to touch your  toes. You will fall over because you can't stick your backside out to keep your  centre of mass over your feet (something you do without thinking about it).

[syhprum]

I find that by lowering my body by bending my knees I can just about manage to touch my toes momentarily,
If I was sixty years younger and lithe and supple as I was then I could resist toppling over.