Anatomy chapters
Muscles, tendons, ligaments and aponeuroses
The three types of muscle, how skeletal muscles are named and classified, and how tendons, ligaments and aponeuroses differ.
- Three muscle types: skeletal (voluntary, striated), cardiac (involuntary, striated) and smooth (involuntary, non-striated).
- Skeletal muscles attach to bone by tendons or flat sheets called aponeuroses.
- Ligaments join bone to bone; tendons join muscle to bone.
On this page
Overview
Muscle is tissue that contracts. There are three types: skeletal, cardiac and smooth.
Skeletal muscles move the skeleton. They attach to bone through tendons and aponeuroses. Bones are held to each other by ligaments.
Exams test the differences between these structures and the classification of skeletal muscle by fibre arrangement and action.
Types of muscle
| Feature | Skeletal | Cardiac | Smooth |
|---|---|---|---|
| Control | Voluntary | Involuntary | Involuntary |
| Striations | Present | Present | Absent |
| Cell shape | Long cylinders | Branched | Spindle-shaped |
| Nuclei | Many, at the edge | One or two, central | One, central |
| Special feature | Motor end plates | Intercalated discs | Gap junctions; slow, sustained contraction |
| Site | Limbs, trunk, face | Heart | Walls of gut, vessels, bladder, uterus |
Intercalated discs are found only in cardiac muscle. They join cells end to end and let the impulse spread.
The upper third of the oesophagus is skeletal muscle, the middle third is mixed and the lower third is smooth. Do not call all gut muscle smooth.
Parts of a skeletal muscle
Each muscle has a fleshy belly and two attachments.
- Origin: the attachment that usually stays fixed.
- Insertion: the attachment that usually moves.
The origin is usually proximal, but the roles can reverse. During a pull-up, latissimus dorsi pulls the trunk towards the fixed arm.
Classification by fibre arrangement
Parallel fibres
The fibres run along the length of the muscle. These muscles shorten a long way but are less powerful.
- Strap muscles: sartorius, the infrahyoid muscles.
- Fusiform: biceps brachii.
- Quadrilateral: pronator quadratus, thyrohyoid.
Oblique (pennate) fibres
The fibres join a tendon at an angle, like the barbs of a feather. Many more fibres fit in, so these muscles are strong but shorten less.
| Type | Arrangement | Example |
|---|---|---|
| Unipennate | Fibres on one side of the tendon | Flexor pollicis longus, extensor digitorum longus |
| Bipennate | Fibres on both sides | Rectus femoris, dorsal interossei |
| Multipennate | Several tendon septa | Deltoid (acromial part), subscapularis |
| Circumpennate | Fibres converge on a central tendon from all sides | Tibialis anterior |
Pennate muscles have more fibres per unit area, so they are more powerful. Parallel muscles give greater range of movement.
Other shapes
- Spiral or twisted: trapezius, latissimus dorsi.
- Sphincters: orbicularis oris, orbicularis oculi.
- Cruciate: fibres in crossing layers, as in sternocleidomastoid and masseter.
Classification by action
- Prime mover (agonist): the main muscle producing a movement, such as brachialis in elbow flexion.
- Antagonist: opposes the movement and relaxes to let it happen, such as triceps during elbow flexion.
- Synergist: steadies nearby joints so the prime mover works smoothly. The long flexors of the fingers need the wrist extensors as synergists to make a strong fist.
- Fixator: holds a proximal part steady, such as the muscles that fix the scapula while the arm moves.
A muscle has no fixed role. The same muscle can be a prime mover in one movement and a fixator in another.
Tendons
A tendon is a cord of dense, regular collagen tissue that joins muscle to bone. It is strong and does not stretch much.
Where tendons cross joints or pass under retinacula, they are wrapped in a synovial sheath that lets them glide. Examples are the flexor tendons at the wrist and in the fingers.
Aponeuroses
An aponeurosis is a flat, sheet-like tendon. It spreads the pull of a muscle over a wide area.
Examples include the bicipital aponeurosis, the palmar aponeurosis, the epicranial aponeurosis of the scalp, and the aponeuroses of the external oblique, internal oblique and transversus abdominis that form the rectus sheath.
Ligaments
A ligament is a band of dense collagen tissue that joins bone to bone. It stabilises a joint and stops abnormal movement.
Some ligaments are elastic, such as the ligamenta flava between the laminae of the vertebrae, which help the spine straighten after bending.
"Ligament" is also used for peritoneal folds (falciform ligament) and fetal remnants (ligamentum teres). These are not joint ligaments.
Bursae
A bursa is a closed sac lined by synovial membrane with a little fluid inside. It lies where a tendon or skin rubs over bone and reduces friction.
Examples are the subacromial bursa, prepatellar bursa and olecranon bursa.
Clinical relevance
Bursitis
How it happens: repeated pressure or friction inflames a bursa.
What you see: a soft, often tender swelling. Prepatellar bursitis ("housemaid's knee") follows kneeling; olecranon bursitis follows leaning on the elbow.
Tendon rupture
How it happens: sudden strong contraction against resistance, or a tendon weakened by age or steroid injection.
What you see: loss of the movement and a gap in the tendon. Rupture of the long head of biceps gives the "Popeye" bulge of the muscle belly.
Tenosynovitis
How it happens: inflammation or infection of a tendon sheath.
What you see: pain and swelling along the tendon. Infection of the flexor sheaths of the hand can spread rapidly and needs urgent treatment.
Sprain and strain
A sprain is an injury to a ligament. A strain is an injury to a muscle or tendon.
Sprain = ligament. Strain = muscle or tendon.
Questions ask you to match a muscle to its fibre type (unipennate, bipennate, multipennate), to compare power and range, and to distinguish tendon, ligament and aponeurosis. The three types of muscle and their microscopic features are also tested.
Quick revision
- Skeletal: voluntary, striated, multinucleate with peripheral nuclei.
- Cardiac: striated, branched, intercalated discs. Smooth: spindle cells, no striations.
- Pennate muscles: more power. Parallel muscles: more range.
- Unipennate: flexor pollicis longus. Bipennate: rectus femoris. Multipennate: deltoid.
- Tendon: muscle to bone. Ligament: bone to bone. Aponeurosis: flat tendon.
- Bursa reduces friction; inflamed bursa is bursitis.
Written by The House Job from standard anatomy references (Snell's Clinical Anatomy by Regions is the book named in the UHS PGET syllabus).
FAQ
What is the difference between a tendon and a ligament?
A tendon joins muscle to bone. A ligament joins bone to bone and stabilises a joint.
Which muscles are the most powerful for their size?
Multipennate muscles, because they pack the most fibres into a given area.
What is an aponeurosis?
A flat, sheet-like tendon, such as the palmar or bicipital aponeurosis.
Where are intercalated discs found?
Only in cardiac muscle.