How to Learn Anatomy: Muscles

Site

The site refers to the anatomical location of the muscle. This provides context for which region it influences and which joints it may act upon.

Examples:

• The pectoralis major is a muscle of the anterior chest.
• The flexor carpi radialis lies in the anterior forearm.
• The gastrocnemius is located in the posterior leg.

Shape

Muscles exhibit a variety of shapes, which often reflect their function and fibre arrangement.

Common muscle shapes include:

• Fusiform – thick in the middle and tapered at both ends (e.g. biceps brachii)
• Strap – long and thin with parallel fibres (e.g. sternocleidomastoid, rectus abdominis)
• Triangular – broad origin with fibres converging on a narrow insertion (e.g. pectoralis major)
• Circular – fibres arranged in a ring, usually around an opening (e.g. orbicularis oculi)
• Unipennate – fibres insert obliquely on one side of a central tendon (e.g. flexor pollicis longus)
• Bipennate – fibres insert obliquely into both sides of a central tendon (e.g. rectus femoris)
• Multipennate – multiple feather-like fascicles converging obliquely on a central tendon (e.g. deltoid)

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Fig 2
Muscle classification by shape

Attachments

Each muscle has a proximal attachment (closer to the midline of the body) and a distal attachment (further away). Identifying these bony attachments allows the resulting movements at a joint to be predicted.

Sites of attachment are often used as landmarks in surgical and radiological procedures.

Examples:

• The pectoralis major originates from the clavicle, sternum, and upper costal cartilages. It inserts into the lateral lip of the intertubercular sulcus of the humerus.
• The biceps brachii  originates from the supraglenoid tubercle and coracoid process of the scapula. It inserts into the radial tuberosity.
• The sternocleidomastoid originates from the sternum and medial clavicle. It inserts into the mastoid process of the temporal bone.

Action

A muscle produces movement by acting across one or more joints.  The action can usually be deduced by considering its attachments and the orientation of its fibres – as these determine the direction and type of pull exerted on the joint.

Examples:

• The iliopsoas causes flexion at the hip joint.
• The quadriceps femoris extends the knee joint.
• The deltoid abducts at the shoulder joint.

Nerve Supply

Skeletal muscles are innervated by motor branches of peripheral nerves. Each muscle has a specific nerve supply, which facilitates voluntary contraction and coordination.

Example:

• The median nerve supplies many muscles of the anterior forearm.
• The axillary nerve supplies the deltoid.
• The femoral nerve supplies the quadriceps femoris.

Blood Supply

Muscles receive oxygenated blood via one or more arteries, supporting metabolism and function.

Example:

• The brachial artery supplies the biceps brachii.
• The profunda femoris artery supplies the adductor magnus.
• The posterior tibial artery supplies the gastrocnemius.