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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Anatomical terms of muscle | 2/3 | https://en.wikipedia.org/wiki/Anatomical_terms_of_muscle | reference | science, encyclopedia | 2026-05-05T07:13:35.226075+00:00 | kb-cron |
Synergist muscles also called fixators, act around a joint to help the action of an agonist muscle. Synergist muscles can also act to counter or neutralize the force of an agonist and are also known as neutralizers when they do this. As neutralizers they help to cancel out or neutralize extra motion produced from the agonists to ensure that the force generated works within the desired plane of motion. Muscle fibers can only contract up to 40% of their fully stretched length. Thus the short fibers of pennate muscles are more suitable where power rather than range of contraction is required. This limitation in the range of contraction affects all muscles, and those that act over several joints may be unable to shorten sufficiently to produce the full range of movement at all of them simultaneously (active insufficiency, e.g., the fingers cannot be fully flexed when the wrist is also flexed). Likewise, the opposing muscles may be unable to stretch sufficiently to allow such movement to take place (passive insufficiency). For both these reasons, it is often essential to use other synergists, in this type of action to fix certain of the joints so that others can be moved effectively, e.g., fixation of the wrist during full flexion of the fingers in clenching the fist. Synergists are muscles that facilitate the fixation action. There is an important difference between a helping synergist muscle and a true synergist muscle. A true synergist muscle is one that only neutralizes an undesired joint action, whereas a helping synergist is one that neutralizes an undesired action but also assists with the desired action.
=== Neutralizer action === A muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralizing action. A good famous example of this are the hamstrings; the semitendinosus and semimembranosus muscles perform knee flexion and knee internal rotation whereas the biceps femoris carries out knee flexion and knee external rotation. For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way.
==== Composite muscle ==== Composite or hybrid muscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers. For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated from a different nerve supply.
== Muscle naming ==
There are a number of terms used in the naming of muscles including those relating to size, shape, action, location, their orientation, and their number of heads.
By size brevis means short; longus means long; major means large; maximus means largest; minor means small, and minimus smallest. These terms are often used after the particular muscle such as gluteus maximus, and gluteus minimus. By shape deltoid means triangular; quadratus means having four sides; rhomboideus means having a rhomboid shape; teres means round or cylindrical, trapezius means having a trapezoid shape, rectus means straight. Examples are the pronator teres, the pronator quadratus and the rectus abdominis. By action abductor moving away from the midline; adductor moving towards the midline; depressor moving downwards; elevator moving upwards; flexor moving that decreases an angle; extensor moving that increase an angle or straightens; pronator moving to face down; supinator moving to face upwards; Internal rotator rotating towards the body; external rotator rotating away from the body.
== Form ==
=== Insertion and origin === The insertion and origin of a muscle are the two places where it is anchored, one at each end. The connective tissue of the attachment is called an enthesis.
==== Origin ==== The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than a muscle's insertion. For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso. This is the case when grabbing objects lighter than the body, as in the typical use of a lat pull down machine. This can be reversed however, such as in a chin up where the torso moves up to meet the arm. The head of a muscle, also called caput musculi is the part at the end of a muscle at its origin, where it attaches to a fixed bone. Some muscles such as the biceps have more than one head.
==== Insertion ==== The insertion of a muscle is the structure that it attaches to and tends to be moved by the contraction of the muscle. This may be a bone, a tendon or the subcutaneous dermal connective tissue. Insertions are usually connections of muscle via tendon to bone. The insertion is a bone that tends to be distal, have less mass, and greater motion than the origin during a contraction.
==== Intrinsic and extrinsic muscles ==== Intrinsic muscles have their origin in the part of the body that they act on, and are contained within that part. Extrinsic muscles have their origin outside of the part of the body that they act on. Examples are the intrinsic and extrinsic muscles of the tongue, and those of the hand.
=== Muscle fibres ===
Muscles may also be described by the direction that the muscle fibres run, in their muscle architecture.