Electric current through the body causes muscles to contract strongly and involuntarily. It only takes a few milli amps to do this.
Depending upon where you contact the voltage and the return path will determine which muscles are affected.
An electric shock occurs when a person comes into contact with an electrical energy source. Electrical energy flows through a portion of the body causing a shock. Exposure to electrical energy may result in no injury at all or may result in devastating damage or death.