Skeletal muscles are attached (with some exceptions, such as the muscles of the tongue and pharynx) to the skeleton by means of tendons, usually in pairs that pull in opposite directions, e.g., the biceps (flexor) and triceps (extensor) that move the forearm at the elbow. The means by which all types of muscles contract is thought to be generally the same, although muscles are classified as phasic, or fast twitch, and tonic, or slow twitch, to differentiate between the various lengths of time a muscle may require to move in response to stimulation. Striated muscle is usually considered phasic, while cardiac and smooth muscle are thought to be tonic.
Perhaps because its action is most varied, striated muscle has been studied most extensively. This type of muscle is composed of numerous cylindrically shaped bundles of cells, each enclosed in a sheath called the sarcolemma. Each muscle fiber contains several hundred to several thousand tightly packed strands called myofibrils that consist of alternating filaments of the protein substances actin and myosin. Actin and myosin interact before muscle contraction, forming the contractile material actomyosin.
The energy required for muscle contraction comes from the breakdown of adenosine triphosphate (ATP), a substance that is present in the cells and is formed during cellular respiration. A muscle fiber is stimulated to contract by electrical impulses from the nervous system. The point of contact between nerve and muscle is the neuromuscular junction, where the chemical substance acetylcholine is secreted, initiating the changes that cause the muscle to contract. During resting states, some of the fibers in the musculature are maintained in a state of partial contraction, known as muscle tone. This permits muscles to contract quickly when stimulated without having to overcome the inertia of total relaxation.
For an agonist to be effective as a mover in the skeletal system it must actually cross one or more structure(s) that can move. This is typically where the muscle crosses a joint by way of a connecting tendon. As the myofibrils of a muscle are excited into action and then contract, they will create tension and pull through the tendon and pulling the lever arm of bone on the opposite side of the joint closer to the muscles origin.