A person could live without some voluntary muscles, but you cannot live without any involuntary muscles.
The function of the involuntary muscle is to contract without concious control to preform bodily functions.
The functions of voluntary muscles are to move the body, to help hold the bones together, to help give the body it's shape, and to help give the body it's strength.
Muscle fiber is a cell in the muscle.
The muscular system interacts with the skeletal system to help the body move.
The muscular system interacts with the circulatory system to bring nutrients to the muscles and waste away.