Muscle Origin, Insertion and Action Explorer

What do muscle origin, insertion, and action mean?

In musculoskeletal anatomy, a muscle’s origin is the attachment that remains relatively fixed during contraction (typically proximal or medial), while its insertion is the movable attachment drawn towards the origin. A muscle’s action is the movement that pull produces at the joints it crosses, such as flexion, extension, abduction or rotation.

A muscle pulls on the bones at both its ends. Its origin is the attachment that usually stays fixed, often the end nearer the trunk, and its insertion the one that moves; contraction draws the insertion towards the origin, which is why every arrow on the model points from an insertion back to its origin. The roles are a convention about which end moves, not a property of the muscle: in a pull-up the hand and the humerus are fixed and latissimus dorsi pulls the trunk up to the arm, insertion fixed and origin moving.

The explorer has 29 muscles of the shoulder and arm, the jaw and the neck, with 110 attachment sites between them, described as Gray’s Anatomy (1918) describes them, in modern terms. 96 are painted on the bone, 6 are pinned because the muscle and the bone do not quite meet in the scan, and 8 are on structures the dataset does not segment, such as the ligamentum nuchae and the thoracolumbar fascia, which are named rather than drawn.

How the patches were measured

Every structure here is part of one model of one man, segmented from MRI and refined by medical illustrators, in one coordinate frame, so where a muscle meets a bone can be measured from the meshes rather than drawn. For each site, every vertex of the bone is measured against the muscle’s meshes, exactly, and a bone triangle is painted when all three of its corners lie within a threshold and its centre lies in a zone curated for that site. The threshold starts at 1 mm and steps up to 3 mm when a footprint comes to less than 25 mm², because the ends of tendons often stop short of bone in the scan; still under that, a site is shown as a pin.

The zone is there because contact is not attachment. Within 1 mm the deltoid touches the humerus over 10.8 cm², but only 7.1 cm² of that lies on the outer side of the shaft, at the deltoid tuberosity, centred 37 percent of the way down; the rest is where the muscle wraps over the greater tubercle on its way there. The card gives each site’s area, and pointing at the area shows the threshold it took.

The rotator cuff

Four muscles from the scapula hold the head of the humerus in its shallow socket. Supraspinatus, infraspinatus and teres minor insert in that order down the three facets of the greater tubercle, centred 3, 6 and 11 percent of the way down the humerus in this body, and subscapularis, from the front of the scapula, on the lesser tubercle, centred 7 percent down. Type cuff in the picker to list all four.

Which muscles attach to the humerus

Choose By bone and pick the humerus, or click it in the model: of the muscles here, 6 arise from it and 9 insert on it, each painted on its front and back and listed in the card. The shoulder muscles insert on its upper end and shaft, brachialis and two heads of triceps arise from its shaft, and brachioradialis and the forearm’s rotators from its lower end and epicondyles.

The muscles of mastication

Masseter runs from the zygomatic arch to the outer surface of the ramus and angle of the mandible, temporalis from the temporal fossa to the coronoid process, medial pterygoid from the lateral pterygoid plate, the palatine bone and the maxilla to the inner surface of the ramus and angle, and lateral pterygoid from the sphenoid to the neck of the condyle and the joint’s disc, which is not segmented, so that end is named rather than painted. The first three close the jaw and the lateral pterygoid pulls it forward and opens it. Type mastication in the picker to list all four; choosing one loads the skull in place of the arm.

Origin and insertion chart

Every muscle in the explorer, from the same records it paints, with its main action in the anatomical position and its nerve. Each name opens the muscle’s own page in muscle origins and insertions, with every attachment, its actions and its nerve roots.

What the model leaves out

• Fascia, ligaments and aponeuroses. They are not segmented, so attachments to them are named in the card but not drawn.
• Some tendon ends. A few tendons stop short of the bone in the scan, which is what the threshold ladder and the pins are for.
• Every bone of a run. Where a muscle arises from a run of ribs, cartilages or vertebrae, as latissimus dorsi does from the lower six thoracic spines, its footprint is measured on one of them, which the card names, and By bone lists the muscle on the rest.
• One body. The meshes are one adult man’s. The landmarks are much the same in everyone, though how many ribs or vertebrae a muscle reaches varies from person to person, and the sizes and angles here are his.
• The hip and leg, and the muscles of the wrist and hand. The explorer covers the shoulder and arm, the jaw and the sternocleidomastoid; the regional muscle explorers have the rest of the muscles in 3D.

Common mistakes

• Reading contact as attachment. A muscle lies on the bones it crosses: the deltoid on the greater tubercle, supraspinatus on the clavicle. Only where it ends on the bone is an attachment.
• Quoting actions outside the anatomical position. Actions are stated with the body standing, arms by the sides and palms forward. Away from it a muscle can change sides: the clavicular part of pectoralis major flexes the arm, while the sternocostal part extends it back down from flexion.

More bones and muscles

The bones themselves are in the upper limb skeleton, lower limb skeleton and full skeleton explorers, and every muscle mesh of the region in the upper limb, back and head and neck muscle explorers. How a muscle makes force is the muscle twitch and tetanus simulator’s subject, and which muscles a lift uses is the muscles worked explorer’s, with a page for each lift in muscles worked by exercise. The skin and muscles each spinal root supplies are set out root by root in dermatomes by nerve root.