How do muscles move bones?
When a muscle acts to move a bone, it's point of origin (which is usually the end of the muscle closest to the axial skeleton)is held fairly stable so that the contraction of the muscle pulls the bone to which it inserts into towards it, in a lever-like action across the joint, which acts as a pivot.
Every muscle works with another muscle (or several others). The acting muscle is known as the agonist, and the 'partner' muscles are called the antagonists. The antagonist's action is the opposite of the agonist's action, and helps to control the movement and prevent injury.
In reality muscles usually work in groups rather than individually, to increase stability and control of movement. For example the deltoid and supraspinatus work together to abduct the arm by contracting over the shoulder joint to pull on the humerus, as below :
As another example, the brachioradialis and the biceps brachii work together across the elbow joint to flex the arm :
Reference: Clinically Oriented Anatomy, Moore
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