Limb Muscles - Horse Anatomy

Thoracic Limb

Extrinsic Musculature

These muscles are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs, as well as to stabilize the scapula.

Superficial Musculature

Trapezius

Innervatation: Accessory n.

Origin: Mid-dorsal raphe and supraspinous ligament

Insertion: Spine of the scapula

Body: Two parts, cervical and thoracic, separated by aponeurosis

Action: Raises the scapula against the trunk and swings cranially to advance the limb

Brachiocephalic

Innervation: Accessory n.

Two parts (cleidobrachial and cleidocephalic muscle) separated by the clavicle, where it exists

Origin: Mastoid process of temporal bone, nuchal crest, wing of atlas and the transverse processes of the 2nd-4th cervical vertebrae

Insertion: Deltoid tuberosity and crest of the humerus

Action: Advances the limb and extends the shoulder joint when limb is in motion. Draws the head and neck ventrally when the limb is fixed

Cleidomastoid

Innervatation: Accessory n. and Axillary n.

Origin: Mastoid process of the temporal bone and nuchal crest

Insertion: Deltoid tuberosity, crest of the humerus and fascia of the shoulder as the cleidobrachialis muscle

Action: If acting bilaterally, it draws the head and neck down and backwards. If the acting with one shoulder fixed, it draws the head, upper arm fascia and neck to one side.

Latissimus dorsi

Innervation:Local branch of brachial plexus

The broadest muscle of the back

Origin: Thoracolumbar fascia

Insertion: Teres tuberosity of the humerus

Action: Antagonist to the brachiocephalic m. It retracts the free limb and flexes shoulder joint. It also draws the trunk forward over the fixed limb.

Superficial Pectorals

Innervation: Brachial plexus

Two muscles, cranial (descending) and caudal (transverse)

Origin:

Cranial (descending) - Manubrium of the sternum

Caudal (transverse) - First six costal cartilages and ventral sternum

Insertion:

Cranial (descending) - The crest of the humerus, distal to the deltoid tuberosity

Caudal (transverse) - Covers the elbow joint to insert on the medial fascia of the forearm

Action: Adducts the forelimb, assist in protraction and retraction

The cranial (descending) pectoral muscle forms a visible prominence cranial to the sternum; extending between the manubrium and the humeral crest.

Deep Musculature

Deep Pectorals

Innervation: Cranial and caudal thoracic n.

Origin: Xiphoid and 4th costal cartilages of the sternum

Insertion: Two branches on the lesser and greater tubercles of the humerus, supraglenoid tubercle of scapula

Action:Draws the limb back, supports the trunk, moves the trunk cranially over an advanced limb and extends the shoulder.

Serratis ventralis

Divided into cranial (cervical) and caudal (thoracic) parts.

Innervation: Dorsal and ventral Cervical n., long thoracic n.

Origin: 1st-7th rib, transverse process of cervical vertebrae

Insertion: Medial scapula and scapular cartilage

Action: Supports the weight of the trunk. It is reinforced by strong fascia. The cervical portion can retract the limb and the caudal portion can advance the limb.

Rhomboids

Deep to the trapezius, forms the withers region. Divided into cervical and thoracic parts.

Innervation: Dorsal and ventral branches of Cervical n., Long thoracic n.

Origin:

Cervical - Nuchal ligament at the axis

Thoracic - Spinous processes of 2nd-6th cervical and 7th thoracic vertebrae

Insertion:

Cervical - Unites with trapezius to insert on scapular cartilage

Thoracic - Scapula

Action: Draws the limb forward and elevates the limb and neck. Fixes the scapula against the trunk.

Intrinsic Musculature

Muscles of the Shoulder

These muscles are grouped:

Lateral

Supraspinatus

Innervation:Suprascapular n. of the brachial plexus

Origin: Supraspinous fossa of the scapula

Insertion: Lesser and greater tubercles of the humerus

Action: Extends and braces the shoulder

Infraspinatus

Innervation:Suprascapular n. of the brachial plexus

Origin: Supraspinatous fossa and spine of the scapula

Insertion: Tendon of insertion bifurcates into a deep part and a superficial part.

Deep - Greater tubercle of the humerus

Superficial - Lateral humerus, distal to the greater tubercle

Action: Brace the shoulder, supporting flexion or extension.

Clinical significance: The bursa between the superficial tendon of the infraspinatus, and the lateral tubercle of the humerus can be the site of inflammation (bursitis).

Medial

Supscapularis

Innervation: Subscapular n. from the brachial plexus

Origin: Deep surface of the scapula

Insertion: Lesser tubercle of the humerus

Action: Braces medial shoulder joint. Primarily acts as an extensor of the shoulder, but can contribute to maintaining flecion.

Coracobrachialis

Innervation: Musculocutaneous n. of the brachial plexus

Origin: Coracoid process of scapula

Insertion: Proximal shaft of the humerus

Action: Fixator

Caudal (Flexors)

Deltoid

Innervation: Axillary n. of the brachial plexus

Origin: Caudal border and spine of the scapula. One head in the horse, compared with two in species with an acromion (site of second origin.

Insertion: Deltoid tuberosity of humerus

Action: Flexes shoulder

In the horse, the aponeurosis if the deltoid is partially fused to the infraspinatus.

Teres Major

Innervation: Axillary n. of the brachial plexus

Origin: Dorsal part of the caudal scapula

Insertion: Teres major tuberosity midway down the humerus

Action: Flexes the shoulder, supports adduction.

Teres Minor

Innervation: Axillary n. of the brachial plexus

Origin: Distal third of scapula, deep to the deltoid

Insertion: Teres minor tuberosity midway down the humerus

Action: Flexes the shoulder

NB// There are no defined extensors of the shoulder. Those involved (brachiocephalic m., biceps brachii, supraspinatus, and ascending pectorals) have other, more primary roles.

Muscles of the Elbow

Extensors

Triceps brachii

Innervation: Radial n. from the brachial plexus

It has three heads:

Origin:

Long head: Caudal margin of the scapula

Lateral and Medial heads: Shaft of the humerus (lateral and medial aspects, respectively)

Insertion: Olecranon, proteced by the tricipital bursa against the bone, and subcutaneous bursa against the skin

Action: Extends and stabilizes the elbow

Tensor fasciae antebrachii

Innervation: Radial n. from the brachial plexus

Overlies the triceps extending from the scapula to the olecranon

Origin: Caudal border of the scapula

Insertion:Antebrachial fascia

Action: Tenses antibrachial fascia and extends the elbow

Anconeus

Innervation: Radial n. from the brachial plexus

Origin: Distal humerus and olecranon fossa,deep to the triceps

Insertion: Lateral olecranon, joining the lateral head of the triceps

Action: Extends the elbow

Flexors

Biceps brachii

Innervation: Musculocutaneous n. from the brachial plexus

Origin: Supraglenoid tubercle of the scapula

Insertion: Distally, the muscle divides into a medial and lateral portion.

Medial - Medial tuberosity on the proximal radius

Lateral - Proximal radius and ulna

It runs through the intertubercular groove of the humerus facilitated by the bicipital bursa.

Action: Flexes the elbow, extends and stabilises the shoulder during the stance phase

Brachialis

Innervation: Musculocutaneous n. from the brachial plexus

Origin: Proximocaudal humerus

Insertion: Spirals to insert next to biceps on the medial radius

Action: Flexes the elbow

Muscles of the Carpal and Digital Joints

The horse, as well as other domestic mammals, have a limited range of motion in the carpal joints compared with humans; therefore these muscles act as either extensors or flexors.

Extensors

The craniolateral position on the forearm, almost all originate from the lateral epicondyle of the humerus. All innervated by the radial n. from the brachial plexus.

Extensor carpi radialis

The most medial

Insertion: Metacarpal tuberosity of the proximal third metacarpal (cannon) bone. Uniquely in the horse, the lacertus fibrosus (detachment of the biceps brachii) joins this muscle prior to its insertion.

Action: Extends and fixes the carpus, flexes the elbow

Extensor carpi obliquus (aka abductor pollicis longus)

Origin: Cranial radius

Insertion: Head of second metacarpal (medial splint) bone

Common Digital Extensor

Deep to the extensor carpi radialis

Insertion: Extensor process of the distal phalanx via the common digital extensor tendon

Action: Extends carpus and digit

Lateral Digital Extensor

Caudal to the common digital extensor muscle on the lateral antebrachium

Insertion: Dorsoproximal phalanx via the lateral digital extensor tendon

Action: Extends the carpus and metacarpophalangeal (fetlock) joint

Flexors

Caudal position on the forearm, originate from the caaudal medial epicondyle of the humerus. All innervated by the median or ulnar n. of the brachial plexus.

Flexor carpi radialis

The most medial, inserts on the proximal part of metacarpal II (medial splint)

Flexor carpi ulnaris

The most lateral, inserts on the accessory carpal bone.

Ulnaris lateralis

The most caudolateral

Insertion: Accessory carpal bone and the fourth metacarpal (lateral splint) bone.

Action: Flexes the carpus rather than extends it due to its insertion caudal to the carpal joint axis.

Superficial Digital Flexor

Inserts on the proximal extremity of middle phalanx and the distal extremity of proximal phalanx.

Deep Digital Flexor

Passes through the carpal canal before branching and continues to palmar distal phalanx.

Interosseus muscle/Suspensory ligament

This is covered in more detail in the Equine Tendons section.

Pelvic Limb

The muscles affecting the pelvic girdle and hip can be divided into two distinct groups; girdle muscles and rump muscles.

Girdle Muscles

The pelvic girdle muscles is made up of a number of distinct muscle groups.

Psoas Minor

The psoas minor muscle is marked by many tendinous intersections.

Iliac

The iliac muscle is a fleshy muscle that is cross-sectionally flat cranially and round caudally. It originates from two heads; a strong lateral head from the wing of the ilium and a small medial one from the ileal shaft. They enclose the greater psoas and will unite to form a common tendon for insertion.

Quadrate Lumbar

The quadrate lumbaris a thin, tendinous muscle whose origin is at the proximal end of the last rib and the transverse process of the cranial lumbar vertebrae. Its insertion is the transverse processes of the caudal lumbar vertebrae and the sacral wing.

Rump Muscles

The rump muscles are also made up of a number of different muscle groups and have considerably more groups than the girdle muscles.

Superficial Gluteal

The superficial gluteal originates in the gluteal fascia and unites with the tensor muscle fascia lata, passing over the greater trochanter and attaching onto the third trochanter which radiates into the femoral fascia. A synovial bursa exists between the tendinous insertion and the third trochanter.

Middle Gluteal

The middle gluteal originates from four separate areas; the 1st lumbar vertebrae, aponeurosis of the longissimus muscle, the sacrum and the broad sacrotuberous ligament. The short portion of this muscle inserts on the greater trochanter whilst the deep portion inserts on both the greater trochanter and intertrochanteric crest.

Tensor Muscle of the Fascia Lata

The tensor muscle of the fascia lata originates from the coxal tuberosity and joins the fascia lata and so indirectly attaches to the patella, lateral pattelar ligament and cranial border of the tibia. A caudodorsal detachment joins the superficial gluteal and so attaches to the greater trochanter.

Biceps Femoris

The biceps originate from the vertebral head which is a spinous processes of the sacral vertebrae, and also from the caudal border of the sacrotuberous ligament and the ischial tuberosity. The two bellies of the biceps then unite and redivide into three parts that form an extensive aponeurosis. The cranial branch of the aponeurosis inserts just distal to the third trochanter on the caudal aspect of the femur, to the patella and to the lateral patellar ligament. The middle branch inserts onto the crural fascia, the lateral patellar ligament and the cranial aspect of the tibia. The caudal branch radiates into the crural fascia and forms the tarsal tendon. This combines with the detachment from the semitendinous inserts on the calcaneus.

Semitendinosus

The semitendinous muscle originates from various points including; the pelvic head, the ventral aspect of ischial tuberosity, the sacrum, 1st caudal vertebrae and the sacrotuberous ligament. The two bellies of the semitendinous then unite and run as a flat tendon to the medial side of the leg. It then partially fans out into the crural fascia and partially inserts onto the cranial aspect of the tibia. The remaining tendon joins the tarsal tendon.

Semimembranosus

The semimembranous also originates from various points including the vertebral head (sacrotuberous ligament), 1st caudal vertebrae, pelvic head and the ventral aspect of ischial tuberosity. The two bellies of the muscle then unite and insert at three places; the medial femoral condyle via a short tendon, the medial collateral ligament of the femerotibial joint and to the medial condyle of the tibia via an aponeurosis.

Sartorius

The sartorius muscle originates from the iliac fascia and the tendon of the psoas minor. It inserts via the medial patellar ligament, crural fascia and the tibial tuberosity.

Gracilis

The gracilis originates from the accessory ligament of the femoral head.

Adductor Muscles

The equine adductor muscles are formed from a short adductor and a greater adductor. They both insert along the entire medial aspect of the femur from the lesser trochanter to the medial condyles and the medial collateral stifle ligament.

Internal Obturator

The internal obturator originates from the cranial and medial border of the obturator foramen, the pelvic symphysis and pelvic aspect of the body of the ilium. It inserts on the trochantic fossa having passed over the lesser sciatic notch.

Limb Muscles - Horse Anatomy Learning Resources
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