A voluntary muscles usually works across a joint. It is attached to both the bones by strong cords called tendons.
When the muscles contracts, usually just one bone moves.
For example when the biceps in the arm contracts, the radius moves but the scapula does not.
When a muscle contracts, usually just one bone moves. The other is stationary. The origin is where the muscle joins the stationary bone. The insertion is where it joins the moving bone. When a muscle contracts, the insertion moves towards the origin.
Tendons are the cords and straps that connect muscles to bones. At the bone, the fibres of the tendon are embedded in the periosteum of the bone. This anchors the tendon strongly and spreads the force of the contraction, so the tendon wonít tear away easily.
Muscles usually work in pairs or groups,† e.g. the biceps flexes the elbow and the triceps extends it.
This is called antagonistic muscle action. The working muscle is called the prime mover or agonist. (itís in agony!) The relaxing muscle is the antagonist. The other main pair of muscle that work together are the quadriceps and hamstrings.†
The prime mover is helped by other muscles called synergists. These contract at the same time as the prime mover. They hold the body in position so that the prime mover can work smoothly.
When muscles cause a limb to move through the joint's range of motion, they usually act in the following cooperating groups:
These muscles cause the movement to occur. They create the normal range of movement in a joint by contracting. Agonists are also referred to as prime movers since they are the muscles that are primarily responsible for generating the movement.
These muscles act in opposition to the movement generated by the agonists and are responsible for returning a limb to its initial position.
These muscles perform, or assist in performing, the same set of joint motion as the agonists. Synergists are sometimes referred to as neutralizers because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.
These muscles provide the necessary support to assist in holding the rest of the body in place while the movement occurs. Fixators are also sometimes called stabilizers.
This is a contraction in which no movement takes place, because the load on the muscle exceeds the tension generated by the contracting muscle. This occurs when a muscle attempts to push or pull an immovable object.
This is a contraction in which movement does take place, because the tension generated by the contracting muscle exceeds the load on the muscle. This occurs when you use your muscles to successfully push or pull an object.
Isotonic contractions are further divided into two types:
This is a contraction in which the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up.
This is a contraction in which the muscle increases in length (lengthens) as it resists a load, such as lowering a weight down in a slow, controlled fashion.
During a concentric contraction, the muscles that are shortening serve as the agonists and hence do all of the work. During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work).