Pectoral Musculature of the Equine

Anatomy


The pectoral muscle group of the horse is comprised of four separate muscles: transverse pectoral, ascending pectoral, descending pectoral and subclavius. These muscles are commonly categorised into superficial pectoral (transverse and descending), deep pectoral (ascending) and subclavius. The pectoral mm.* function together to adduct the forelimb and suspend the trunk between the two thoracic limbs. Individually, each muscle has their own function.

*mm. = muscle group.

Transverse Pectoral (part of superficial pectoral)

Transverse Pectoral

~ lies deep to the superficial pectoral muscles ~

~ wide and thin structure ~


Origin: Costal cartilages 1-6 and adjacent sternum.

Insertion: Forearm fascia.

Function: Connection between thoracic limb and trunk in addition to adduction, protraction and retraction of the thoracic limb.

Innervation: Cranial and caudal pectoral nerves.

Recognising sensitivity: Resistance to lift through withers (check spine for primary issues), hypertonicity leading to adaptation of a more base narrow posture. Sensitivity can be unilateral, demonstrated through how abduction of the associated forelimb during lateral movements being limited.

Descending Pectoral (part of superficial pectoral)

Descending Pectoral

~ slightly overlaps the cranial edge of the transverse pectoral ~

~ there is a groove between the descending pectoral muscle and brachiocephalic muscle that contains the cephalic vein; this area must avoid pressure ~

Origin: Manubrium of the sternum.

Insertion: Deltoid tuberosity and crest of the humerus.

Function: Connection between thoracic limb and trunk in addition to adduction, protraction and retraction of the thoracic limb.

Innervation: Cranial and caudal pectoral nerves.

Recognising sensitivity: Objection to palpation along the sternum; scapula does not naturally rotate forwards so the thoracic limb is held closer to the midline; tender to palpation; sensitive to girth; hypertonicity (tight cords) or lumpiness in the muscle.

Subclavius

Subclavius muscle

~ a prism structure ~

Origin: Sternum and costal cartilages (1-4)

Insertion: Blends with the supraspinatus muscle and shoulder fascia.

Function: Suspension of the trunk – assists the serratus ventralis when hoof is on the ground. It rotates the thoracic limb in a paramedian plane, leading to the movement of the scapula cranially and humerus caudally.

Innervation: Cranial pectoral nerve.

Ascending Pectoral (aka. deep pectoral)

Ascending Pectoral

~ the largest of the pectoral muscles and the one in contact with the girth ~

~ fan/triangular shaped ~

Origin: Sternum, tunica flava abdominis and distally on ribs 4-9.

Insertion: Major and minor tubercles of the humerus and the tendon of origin of the coracobrachialis muscle.

Function: Suspension of the trunk between the thoracic limbs, forelimb retraction and stabilisation of the glenohumeral joint; reinforces the action of the latissimus dorsi.

Innervation: Cranial and caudal pectoral nerves.

Recognising sensitivity: Rapid jerking of the back upwards with a light to medium palpation; objection and struggle to perform a forelimb protraction stretch; shortening of stride; poor synchronicity; decreased stamina; shortening of cranial phase of step cycle; placement of the limb medially during the weight bearing phase of step cycle; check latissimus dorsi and saddle fit.


Problems


Problems associated with the pectoral mm. begin to arise when any of the muscles in the group lose the ability to work effectively and correctly. This may be a result of injury, compensatory locomotive patterns, sensitivity, hypertonic muscle state to name a few. 

Dysfunction of the pectoral mm. will impede their functionality. Therefore, protraction, retraction, dorsal and ventral excursion will be limited. Suspension and shock absorbing capabilities associated with the trunk are limited; the trunk can lower, preventing engagement during locomotion… especially with the additional weight of the rider. Decreased shock-absorbing and suspension functionalities can cause chronic and destructive effects to the forelimbs ie. arthritic changes.

Abscesses of the Pectoral Muscles 

Causes of pectoral mm. dysfunction


Incorrect Equitation

  1. Horse is ridden “upside down” – hollowed back, contracted epaxial musculature, disengaged hypaxial musculature, head and neck position is contracted and behind the vertical or above the bit.
  2. Trunk is encouraged into a lower position between the thoracic limbs.
  3. Poor epaxial engagement (hollowing & tightening) and restricted head position/nose poking out prevent raising of the trunk back to a neutral position.
  4. Strain on the lumbosacral junction can lead to the development of secondary dysfunction, initially presenting as poor hindlimb engagement.
  5. This sequence is P A T H O L O G I C A L – the development of a disease.

FURTHER READING: What the Topline says about Horse and Rider, Manolo Mendez 

Poor Saddle Fit/Quality/Positioning…

  1. Poor placement of the saddle on the horse — the girth does not sit in the natural girth groove, saddle positioned too far forward over the scapula.

Poor Girth

  1. The length of the girth — a girth of incorrect length can cause pressure points.
  2. The width of the girth — a girth that is too narrow can place increasing amounts of pressure on a concentrated area, eventually having a negative effect on ascending pectoral muscle health and thus functionality.

~ does the horse object to the girth being tightened? Saddling up? ~

Discipline

  1. Disciplines that involve forelimb loading exercises (ie. jumping, high speed locomotion over varying terrains, barrel racing) can place additional stress on the pectoral mm.
  2. Horses at particular risk in this instance are those that are overexerted and/or underconditioned.

An Overlooked Primary Issue

  1. Pectoral mm. dysfunction may occur as a result of a primary issue; making pectoral muscle injury secondary and compensatory.
  2. Distal limb/C7-T1 pathology can lead to tension of the pectoral mm, for example.

Treatment of the pectoral mm.

Equinology – Targeting the Pectoral Muscles

Petrissage Massage Techniques (video embedded below)


A video of a horse being treated with pectoral, trapezius and triceps muscular soreness.

Conditioning the pectoral mm.


Incline and decline work — decline work utilises the pectoral mm. as brakes; can also be replicated in transitions.

Hill work – limited to walk and trot to begin with to have maximal effectiveness.

Isometric exercises

Shifting the weight gently from forelimbs to hindlimbs…

Varying terrains and surfaces for proprioceptive mechanism activation and thus neuromuscular feedback.

Pole work

Lateral exercises

Hydrotherapy treadmill

References:

Wyche, S. (2016) The Horse’s Muscles in Motion

Budras, K. (2012) Anatomy of the Horse

Pattillo, D. (online) Targetting the Pectoral Muscles: Part One

One thought on “Pectoral Musculature of the Equine

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: