The muscular system is the body's collection of muscles and their associated attachment structures. Its skeletal muscles move the skeleton, maintain posture, stabilise joints and produce heat. Understanding their regional groups, attachments and nerve supply explains how individual muscles work together to control movement.
What does skeletal muscle do?
Skeletal muscle consists of elongated cells called muscle fibres, bundled together within connective tissue that supports and binds them. These fibres contract, developing tension that pulls on their attachments. Most skeletal muscles attach to bones, but muscles of facial expression commonly attach to skin.
The main functions of skeletal muscle are:
- Movement: muscles pull across joints to move bones; other muscles move the skin, tongue or structures involved in swallowing.
- Posture: sustained muscle activity supports the head and trunk and maintains the position of the limbs.
- Joint stabilisation: muscle tension helps hold the bones of a joint in position during rest and movement.
- Heat production: contracting muscle releases heat as part of its activity.
Skeletal muscle is distinct from cardiac muscle, which forms the muscular wall of the heart, and smooth muscle, which occurs in structures such as blood vessel walls and the digestive tract. This article focuses on skeletal muscle and its gross anatomy: structures visible without a microscope.
How are muscles grouped by region and compartment?
Regional anatomy groups muscles by their location in the head and neck, trunk, upper limb or lower limb. Within a region, muscles may be described as superficial, meaning nearer the body surface, or deep, meaning farther from it. Location alone does not establish a muscle's action: its attachments and course across a joint are essential.
A muscle compartment is a group enclosed by fascia, a sheet of connective tissue, and often by intermuscular septa, partitions between neighbouring groups. Bones may also form part of its boundaries. Muscles within a compartment commonly share actions and nerve supply, although important exceptions occur.
Innervation means nerve supply; motor nerves carry signals that activate muscle fibres. Compartment patterns are useful starting points, but each muscle's innervation must still be learned individually. A regional group may contain muscles supplied by several different nerves.
Which muscles form the main head and neck groups?
Muscles of facial expression move the skin around the eyes, nose and mouth and receive motor supply from the facial nerve. Muscles of mastication, meaning chewing, move the lower jaw and receive motor supply from the mandibular division of the trigeminal nerve. These are separate groups despite their neighbouring positions.
The sternocleidomastoid turns the face towards the opposite side and tilts the neck towards the same side when acting alone. Its principal motor supply is the accessory nerve. The left trapezius acts on the left scapula, or shoulder blade; the paired trapezius muscles position the scapulae and also receive motor supply from the accessory nerve.
Deep neck muscles support and move the cervical spine, the part of the vertebral column within the neck. Muscles above and below the hyoid bone, a bone suspended in the neck, help position it during swallowing. Their nerve supplies differ, so neither group should be assigned a single nerve without qualification.
What are the main muscle groups of the trunk?
Intrinsic back muscles are muscles belonging to the back itself, rather than muscles that primarily move the upper limb. They extend and rotate the vertebral column and help maintain posture. Extension generally straightens a joint or bends the trunk backwards, while rotation turns a structure around its axis; most intrinsic back muscles receive motor supply from posterior rami, the backward-directed branches of spinal nerves. Some intertransversarii receive anterior rami.
The rectus abdominis muscle flexes the trunk, bending it forwards. Together with the other abdominal wall muscles, it compresses abdominal contents and supports the abdominal wall. Broad tendinous sheets from the abdominal muscles contribute to the linea alba, the connective tissue seam along the front midline of the abdomen.
The diaphragm separates the chest from the abdomen and is the principal muscle of inspiration, the drawing of air into the lungs. Its contraction lowers its dome and increases chest volume; its motor supply comes from the phrenic nerves. Intercostal muscles, situated between the ribs, move and support the chest wall and receive intercostal nerves.
What are the main upper and lower limb muscle groups?
Upper limb muscles position the shoulder blade and move the shoulder, elbow, wrist and fingers. The pectoralis major muscle brings the arm towards the body and rotates it inwards. The latissimus dorsi muscle also performs these actions and draws the arm backwards at the shoulder.
Lower limb muscles support body weight and move the hip, knee, ankle and toes. The gluteus maximus extends the hip and rotates the thigh outwards. Hip abductor muscles move the thigh away from the body's midline and help keep the pelvis level when the opposite foot is lifted.
Flexion usually bends a joint, whereas extension straightens it. Adduction usually moves a part towards the body's midline, and abduction moves it away. For the fingers and toes, these movements are described relative to the axes of the middle finger and second toe, respectively. At the ankle joint, dorsiflexion lifts the foot towards the shin and plantarflexion points it away. Inversion turns the sole inwards and eversion turns it outwards; these movements occur mainly at the intertarsal joints.
The table summarises regional patterns. Nerve supplies apply to the named subgroups, not uniformly to every muscle in a region.
| Region | Main groups and shared actions | Principal motor nerve supply |
|---|---|---|
| Head and neck | Facial expression moves skin; mastication moves the jaw; superficial neck muscles position the head and shoulder blade | Facial nerve; mandibular division of trigeminal nerve; accessory nerve for sternocleidomastoid and trapezius |
| Trunk | Intrinsic back muscles move the spine; abdominal muscles support and compress the abdomen; respiratory muscles move the chest wall | Posterior rami for most intrinsic back muscles; thoracoabdominal nerves, subcostal nerves and branches of the first lumbar nerve for abdominal muscles; phrenic nerves for diaphragm; intercostal nerves for intercostals |
| Shoulder and arm | Shoulder muscles position the arm; front arm muscles mainly flex the elbow; back arm muscles extend it | Several shoulder nerves; musculocutaneous nerve for most front arm muscles; radial nerve for back arm muscles |
| Forearm and hand | Front forearm muscles mainly flex wrist and fingers; back forearm muscles mainly extend them; hand muscles control fine movement | Median and ulnar nerves for front forearm muscles; radial nerve and its branches for back forearm muscles; median and ulnar nerves within the hand |
| Gluteal region | Hip extension, abduction and rotation; pelvic stabilisation | Inferior gluteal nerve for gluteus maximus; superior gluteal nerve for the main hip abductors; separate branches for several deep rotators |
| Thigh | Front compartment mainly extends the knee; medial compartment mainly adducts the thigh; back compartment mainly extends the hip and flexes the knee | Femoral nerve; mainly obturator nerve; mainly the tibial division of the sciatic nerve, respectively |
| Leg | Front compartment dorsiflexes; lateral compartment everts; back compartment plantarflexes | Deep fibular, superficial fibular and tibial nerves, respectively |
| Foot | Muscles within the foot move the toes and support the arches | Mainly medial and lateral plantar nerves; deep fibular nerve for the short toe extensors on the top of the foot |
What are tendons and how do they attach muscles?
A tendon is a strong connective tissue structure that transmits muscle tension to an attachment site. Tendons are continuous with the connective tissue surrounding muscle fibres. Some muscles have long, cord-like tendons, while others attach through short tendons or fleshy attachments.
An aponeurosis is a broad, flattened tendinous sheet. A retinaculum is a thickened band of fascia that holds tendons close to underlying structures, especially near the wrist and ankle. Tendons and aponeuroses transmit muscle force, while retinacula hold tendons close to underlying structures and help guide them along their courses.
How do you learn a muscle's origin, insertion, action and innervation?
Use the same framework for each muscle:
- Origin: the attachment conventionally regarded as more fixed during the described movement.
- Insertion: the attachment conventionally regarded as more mobile.
- Action: the movement or stabilising effect produced by contraction.
- Innervation: the nerve supplying the muscle.
Origin and insertion are conventions, not permanently fixed roles. When the usual insertion is held steady, contraction can move the usual origin instead. Always relate the attachments to the joint crossed and the direction of the muscle's pull.
In the 3D model beside this article, select and isolate a structure, then rotate and zoom to examine its position relative to neighbouring structures. Connect that position with its attachments, action and nerve supply rather than learning its name alone.