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How do antagonistic muscles move a jointed skeleton?

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Least tern, Sternula antillarum, flying with wings raised and its yellow-tipped black bill pointed forward. — Locomotion These birds largely reside in coastal areas of North America, although some populations live in the continent's interior along large rivers.

In a jointed skeleton, movement is achieved through a lever system where rigid skeletal elements are connected by flexible joints. Muscles span these joints, attaching to different elements at two specific sites: the origin (the more stable attachment) and the insertion (the site that moves).1

Because muscles can only generate force by contracting, they typically function in antagonistic pairs to produce movement in opposite directions. When one muscle in the pair contracts, it pulls on the skeletal element at its insertion, creating movement. To reverse this action or return the joint to its original position, the antagonistic muscle contracts while the first muscle relaxes.12

For example, in the human upper arm, the biceps and triceps act as an antagonistic pair to bend and straighten the lower arm, respectively. This arrangement allows for precise control over complex movements, as muscles can bend or rotate skeletal elements depending on their specific placement and the joint's degrees of freedom. This principle of antagonistic movement applies to both vertebrates, which have internal skeletons, and arthropods, which possess external skeletons.12