11.2 Naming Skeletal Muscles

Skeletal Muscle Nomenclature

Skeletal muscles are named using a system of Latin and Greek roots that encode information about each muscle's shape, size, location, and function. Learning to decode these names turns what seems like random memorization into a logical system where the name itself tells you what the muscle looks like, where it is, and what it does.

Criteria for Naming Skeletal Muscles

Muscles are named based on several key criteria. Most muscle names use one or two of these, and some combine three or more for a very precise description.

Shape describes what the muscle looks like, often comparing it to a geometric figure or object.

• Deltoid has a triangular shape, named after the Greek letter delta (Δ)
• Rhomboid is diamond-shaped, like a rhombus
• Trapezius resembles a trapezoid, a four-sided shape with one pair of parallel sides
• Serratus has a saw-toothed edge with finger-like projections (think "serrated")

Size compares a muscle's dimensions to other muscles in the same group or region.

• Maximus = largest in the group (gluteus maximus)
• Minimus = smallest in the group (gluteus minimus)
• Longus = longer than others in the group (adductor longus)
• Brevis = shorter than others in the group (extensor digitorum brevis)
• Major / Minor = larger or smaller of two similarly named muscles (pectoralis major vs. pectoralis minor)
• Latissimus = widest in a region (latissimus dorsi, the widest muscle of the back)

Location tells you where the muscle sits in the body.

• Pectoralis = chest region (pectoralis major and minor)
• Gluteus = buttocks (gluteus maximus, medius, minimus)
• Brachii = arm, specifically the upper arm (biceps brachii, triceps brachii)
• Femoris = thigh (rectus femoris, biceps femoris)
• Abdominis = abdomen (rectus abdominis, transversus abdominis)
• Dorsi = back (latissimus dorsi)

Function describes the primary movement the muscle performs.

• Flexor = bends a joint, decreasing the angle between bones (flexor carpi radialis)
• Extensor = straightens a joint, increasing the angle between bones (extensor digitorum)
• Adductor = pulls a limb toward the midline (adductor magnus)
• Abductor = moves a limb away from the midline (abductor pollicis longus)

Fiber Direction and Number of Origins

Beyond shape, size, location, and function, two additional naming criteria show up frequently.

Directional terms indicate how the muscle fibers are oriented.

• Rectus = fibers run straight, parallel to the midline (rectus abdominis)
• Transversus = fibers run horizontally across the body (transversus abdominis)
• Obliquus = fibers run at an angle
  • Obliquus externus (external oblique): fibers angle downward and medially
  • Obliquus internus (internal oblique): fibers angle upward and medially

These three muscles layer over each other in the abdominal wall, and their different fiber directions give the abdomen strength in multiple planes of movement.

Number of origins tells you how many heads (proximal attachment points) the muscle has.

• Biceps = two heads (biceps brachii, biceps femoris)
• Triceps = three heads (triceps brachii)
• Quadriceps = four heads (quadriceps femoris)

Applying the Naming Conventions

Most muscle names combine two or more of the criteria above. Once you can break a name into its parts, you can figure out a lot about a muscle you've never studied before.

• Serratus anterior = saw-toothed shape + located on the anterior (front) side of the thorax
• Adductor longus = performs adduction + longer than other adductors in the group
• Extensor digitorum longus = extends the digits + longer than the extensor digitorum brevis
• Flexor pollicis brevis = flexes the pollex (thumb) + shorter than the flexor pollicis longus
• Rectus femoris = straight fiber direction + located in the thigh (femoral region)
• Latissimus dorsi = widest muscle + located on the back

When you encounter a new muscle name on an exam, try breaking it into its root words. Even if you haven't memorized it, the name often gives you enough to make an educated answer.

Common Latin and Greek Roots Reference

Muscle Attachments and Functional Relationships

Understanding how muscles attach and interact is closely tied to naming, since terms like "biceps" and "triceps" directly reference attachment points.

Origin is the more stationary attachment point, typically proximal (closer to the trunk). Insertion is the more mobile attachment point, typically distal (farther from the trunk). When a muscle contracts, the insertion generally moves toward the origin.

Muscles also work in functional groups during any given movement:

• Agonist (prime mover): the muscle primarily responsible for the action. During elbow flexion, the biceps brachii is the agonist.
• Antagonist: the muscle that opposes the agonist. During elbow flexion, the triceps brachii is the antagonist. It relaxes to allow the movement and can contract to reverse it.
• Synergist: a muscle that assists the agonist, often by stabilizing a nearby joint so the prime mover can work more efficiently.

Two structural terms also come up in this context. A fascicle is a bundle of muscle fibers wrapped together by connective tissue, and the arrangement of fascicles within a muscle affects its strength and range of motion. A myofiber (muscle fiber) is a single muscle cell, the individual contractile unit of skeletal muscle tissue.