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 of 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 caudal 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 the second metacarpal(medial splint)bone

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

Overlies, and is partially fused with, the deep digital flexor muscle.

Insertion: Proximal extremity of middle phalanx and the distal extremity of proximal phalanx via the superficial digital flexor tendon.

Deep Digital Flexor

Deep to the superficial digital flexor muscle and flexor muscles of the carpus. Passes through the carpal canal before branching and continues to insert on the palmar distal phalanx via the deep digital flexor tendon.

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 muscules and rump muscles.

Girdle Muscles

The pelvic girdle musculature (sublumbar muscle) is made up of a number of distinct muscle groups. These arise from the lumbar vertebrae and insert on the pelvis or femur. These muscles function in dorsiflexion and ventriflexion of the spine. They also provide stabilisation to the vertebral column and pelvis.

Psoas Minor

Innervation:Intercostal n., Femoral n.

Origin: Last three thoracic vertebrae and 1st-4th lumbar vertebrae

Insertion: Ilium

Action: Fixator, flexes lumbar vertebral column

The muscle is marked by multiple tendinous intersections.

Iliac

The iliac muscle is a fleshy muscle that is cross-sectionally flat cranially and round caudally.

Innervation:Lumbar n., Genitofemoral n., Femoral n.

Origin: Two heads, a strong lateral head and small medial head. They enclose the greater psoas and will unite to form a common tendon for insertion.

Medial - Ileal shaft

Lateral - Wing of the ileum

Insertion: Lesser trochanter of the femur

Quadrate Lumbar

Thin, tendinous muscle

Innervation: Intercostal n., ventral branches of Lumbar n., Genitofemoral n., Femoral n.

Origin: Proximal end of last rib and transverse process of the cranial lumbar vertebrae

Insertion: Transverse processes of caudal lumbar vertebrae, wing of the sacrum, wing of the ilium

Action: Fixator of lumbar vertebral column

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. They extend between the ilium and the thigh. These muscles are particularly large in the horse, providing the power for forward locomotion.

Superficial Gluteal

Innervation: Caudal gluteal n.

Origin: Gluteal fascia

Insertion: Unites with the tensor fascia lata, passing over the greater trochanter and attaching on the third trochanter with radiations to the femoral fascia. A synovial bursa exists between the tendinous insertion and the third trochanter.

Action: Extends the hip, retracts the limb and supports outward rotation

Middle Gluteal

The largest of the gluteals, it lies deep to the superficial gluteal and provides the visible shape of the croup. A sheet of tendon divides the muscle into a deep (accessory gluteal muscle) and superficial part. It fuses caudally with the piriformis muscle.

Innervation: Caudal gluteal n.

Origin: 1st lumbar vertebra, aponeurosis of the longissimus muscle, sacrum, sacrotuberous ligament

Insertion:

Deep - Greater trochanter and intertrochanteric crest

Superficial - Greater trochanter

Action: Poweful hip extensor, retracts and abducts the limb. Transfers the power of the hindlimb to the trunk, allowing the horse to rear up.

Deep Gluteal

Deepest of the gluteals, directly overlying the coxofemoral (hip) joint

Innervation: Cranial gluteal n.

Origin: Lateral ilium

Insertion: Greater trochanter of the femur

Action: Supports the middle gluteal to abduct the limb

Piriformis

Fused to the middle gluteal muscle

Innervation: Cranial gluteal n.

Origin: Last sacral vertebra, sacrotuberous ligament

Insertion: Passes over the greater trochanter of the femur to insert via a seperate tendon to that of the middle gluteal on the caudal femur.

Action: Extends the hip and abducts the limb

Tensor Muscle of the Fascia Lata

Innervation: Cranial gluteal n.

Origin: Tuber coxae, extending distally on the cranial border of the quadriceps muscle

Insertion: Combines with the fascia lata and so indirectly attaches to the patella, lateral patellar ligament and cranial border of the tibia. A caudodorsal detachment joins the superficial gluteal and so attaches to the greater trochanter of the femur.

Action Tenses the fascia lata, indirectly flexing the hip and extending the stifle. Advances the limb during the swing phase of the stride.

Biceps Femoris

Largest and most lateral of the caudal thigh muscles. It lies very superficially, covered only by fascia and skin. The biceps are comprised of two parts, a strong cranial (vertebral head) and smaller caudal part (pelvic head).

Innervation: Caudal gluteal n.

Origin:

Cranial: Spinous processes of the sacral vertebrae, caudal border of the sacrotuberous ligament

Caudal: Ischial tuberosity

Insertion: The two bellies then unite and redivide into three parts that form an extensive aponeurosis.

Cranial - Just distal to the third trochanter on the lateral aspect of the femur, patella, patellar ligament

Middle - Crural fascia, patella, lateral patellar ligament, cranial aspect of the tibia

Caudal - Radiates into the crural fascia and forms the 'tarsal tendon. This combines with the detachment from the semitendinous inserts on the calcaneus.

Action: Generally extends and abducts the limb. The cranial (vertebral) part extends the hip and stifle. The caudal (pelvic) part extends the hip but flexes the stifle. It also helps extend the tarsus through its attachment via the tarsal tendon on the calcaneus.

Semitendinosus

Large muscle, forming the contour of the caudal thigh. In horses, the muscle is comprised of two heads; the bellies of which then reunite and run as a flat tendon to the medial side of the leg.

Innervation: Caudal gluteal n.

Origin:

Pelvic head - Ventral aspect of the ischial tuberosity

Vertebral head - Spinous and transverse processes of the sacrum, 1st caudal vertebrae, sacrotuberous ligament

Insertion: The reunited tendon fans out into the crural fascia and partially inserts onto the cranial aspect of the tibia. The remaining tendon joins the tarsal tendon of the biceps muscle to insert on the calcaneal tuberosity.

Action: Extends the hip, stifle and tarsus during weightbearing causing propulsion. When the limb is non-weightbearing, it flexes the stifle, rotates the leg outwards and moves it backwards.

Semimembranosus

Most medial muscle in the 'hamstring' group, it unites with the semitendinosus to form the caudal contour of the thigh. In the horse it has two heads, a vertebral head and stronger pelvic head.

Innervation: Caudal gluteal n., Tibial n.

Origin:

Pelvic Head - Ventral aspect of the ischial tuberosity

Vertebral head - Sacrotuberous ligament, 1st caudal vertebra

Insertion: The two bellies of the muscle unite to 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.

Action: Extends the hop and stifle during weightbearing, facillitating propulsion. When non-weightbearing, adducts and retracts the limb.

Sartorius

Innervation: Femoral n.

Origin: Iliac fascia and tendon of the psoas minor muscle

Insertion: Unites with the medial patellar ligament and crural fascia to isnert on the tibial tuberosity

Action: Flexes thte hip, advances and adducts the limb. Its union with the fascia crural fascia and fascia of the stifle allos it to also extend the stifle.

Gracilis

Extensive sheet of muscle covering the caudal aspect of the medial thigh

Innervation: Obturator n.

Origin: Accesspry ligament of the femoral head

Insertion: Merges with the crural fascia via an aponeurosis to insert on the tibial crest

Action: Adducts the limb, can also move the rump sideways if the foot is on the ground. Aids extension of the stifle.

Adductor Muscles

In the horse there are two adductors, the cranial short adductor muscle and the caudal greater adductor muscle.

Innervation: Obturator n.

Origin: Ventral surface of the pelvis and apaneurosis of the gracillis muscle

Insertion: Entire medial aspect of the femur, from the lesser trochanter to the medial condyles, and the medial collateral stifle ligament.

Action: Adduct the limb, can also retract the limb and move the rump forward and sideways

Inner Pelvic Muscles

The inner pelvic muscles are small and lie close to the coxofemoral (hip) joint. They run between the pelvis and trochanteric fossa of the femur.

Internal Obturator

In the horse this muscle has two heads, a small and tendinous pubic head and a larger pelvic head.

Innervation: Sciatic n.

Origin:

Pubic head - Cranial and medial borders of the obturator foramen

Pelvic head - Pelvic symphysis and pelvic aspect of the body of the ilium

Insertion: Trochanteric fossa together with the gemellus muscle, having passed over the lesser sciatic notch

Action: Rotates the femur laterally, aids hip extension

External Obturator

Innervation: Obturator n.

Origin: Vental surface of pelvis, close to the obturator foramen

Insertion: Trochanteric fossa

Action: Supinates the femur and adducts the limb

Gemellus

Innervation:

Origin

Insertion

Action

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