called the origin of a muscle. The insertion of a muscle is the part that attaches to the bone being pulled. The agonist muscle is the primary mover of a specific action of a joint. It can be assisted by a synergist, which is a “helper muscle.”. A synergist can also be a “fixator muscle” that stabilizes the bone that is the attachment for the agonist’s origin. An antagonist muscle opposes the action of the agonist. Antagonists help maintain a limb’s position and control rapid movement of an agonist muscle. Some examples of agonist/antagonist pairs in the human body: •
Biceps brachii versus triceps brachii—these are two muscles in the upper arm. The biceps flexes the forearm, while the triceps extends the forearm.
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Hamstrings versus quadriceps—these are muscles of the thigh. The hamstrings flex the leg, while the quadriceps muscles extend the leg.
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Flexor digitorum superficialis and the flexor digitorum profundus versus the extensor digitorum muscles—the first two muscles flex the digits (fingers) and the hand at the wrist, while the extensor digitorum extends the fingers and hand at the wrist. These muscles are located in the forearm.
Some muscles do not pull against the skeleton to allow for skeletal movements. An example of this is the muscles that produce facial expressions. These muscles insert and have their origins in the skin so that muscles change the shape of the face and facilitates facial expressions.
FASCICLE ARRANGEMENTS A bundle of muscle fibers covered by the perimysium is called a fascicle. The arrangement of the fascicles determines how a muscle looks and acts. These are some common fascicle arrangements: •
Parallel—the fascicles are arranged in a parallel fashion, which is the case in most skeletal muscles. Some form a round mass that tapers at both ends into tendons but they are basically parallel fascicles.
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