The skeletal muscles of the body provide the force needed to move the bones of the skeleton in a large variety of actions. The muscles are primarily made of protein and arranged in a way that allows them to work in pairs. For example, when we bend our arms at the elbow to curl a dumbell the bicep acts as the agonist (main force providing muscle) and contracts to pull the radius and ulna of the forearm towards the humerus of the upper arm (this action is flexion). At the same time to allow this to happen the tricep at the back of the upper arm relaxes and lengthens to allow the arm to flex.

When we then straighten our arm (extension) the pair of muscles swap roles and the tricep becomes the agonist (making the arm extend) and the bicep becomes the antagonist or relaxing muscle.

Put your cursor directly over the ends of the lines to get the names of the muscles.