According to Boundless.com, muscles are arranged in groupings of agonist, antagonist and synergists that produce and modulate movement. When a muscle contracts, it moves the bone and spreads the force of the contraction to avoid the tearing of fibers embedded in the periosteum of the bone known as tendons.
There are three unique kinds of muscles in the human body: skeletal muscle, smooth muscle and cardiac muscle. Skeletal muscles operate in pairs and can be contracted voluntarily through exercise or movement. Smooth muscles are found in the digestive system, blood vessels, bladder, airways and uterus. They can stretch and maintain tension for long periods of time and contract involuntary. Cardiac muscle is found only in the heart and is specifically designed for endurance and consistency. It is a twitch muscle only and contracts involuntary each time the heart beats. In order to move, muscles work in pairs known as synergist, agonist and antagonist muscles. Agonist muscles are typically associated with the movement itself and contract while another muscle relaxes. Synerist muscles act around a moveable joint to produce motion similar to agonist muscles. Lastly, antagonist muscles act as opposing muscles to agonists in order to return the limb to its original position.