The supraspinous and infraspinatus muscles are the main stabilisers of the scapula and the shoulder joint.

Supraspinatus

Origin: The superior aspect of the scapula

Inserts: into the humerus.

Function: Extends the shoulder joint and works as an antagonist to the deltoid muscle.

Infraspinatus

Origin: The infrascapula fossa

Inserts: into the humerus.

Function: Involved in rotating the forelimb laterally and abducting the forelimb.

Soreness: Atrophy of both of these muscles is visible as the scapula spine becomes more prominent. Weakness or tension in these muscles can cause instability of the shoulder and result in the horse struggling the advance the limb and therefore a shortened stride. The horse may also “drop their shoulder” on a circle.

Massage can help to relax, relieve pain and discomfort in these muscles and therefore improve the functionality of these muscles which in turn would improve performance and comfort during exercise. 

The trapezius muscle covers a vast area around the withers of the neck and over the top portion of the shoulder.

Origin: The nuchal ligament, supraspinous ligament of the 2-10 cervical vertebrae and thoracic vertebrae

Inserts: into the scapula spine.

Function: This muscle is responsible for elevation of the shoulder and cranial-caudal motion (moving the shoulder forward and backwards). Under bilateral concentric contraction the muscle causes slight neck extension. The trapezius muscle works as an antagonist to the brachiocephalic muscle.

Soreness: The thoracic section of the trapezius muscle lies directly under the gullet of the saddle, leaving it vulnerable to increased pressure and damage to the muscle with poorly fitting saddles. Atrophy of this section of the muscle is much more common than hypertrophy due to the restriction from the saddle. If the muscle is tight, the horse will lack flexibility in the neck and the shoulder, leading to a shortened stride. 

​The rhomboid is a deep muscle located along the topline of the horses neck.

Origin: The occiput, nuchal ligament and the supraspinous ligament from C2 to approximately T10

Inserts: Into the medial surface of the scapula cartilage.

Function: This muscle works with the trapezius muscle to elevate the shoulder and is responsible for protraction and retraction. It also helps to stabilise the shoulder and when the stance phase it aids with neck extension. Under concentric contraction, the rhomboid incline the base of the neck downwards and assume a more U shaped position, resulting in a poor head and neck carriage. Hypertrophy of the rhomboid muscle can appear in some horses if the horse is ridden in a hyperflexed position and if the horse is heavy on the contact.

Soreness: If this muscle is tight, the horse may struggle to stretch and extend the neck fully, which may be noticeable when practicing free walk and long and low exercises. As the rhomboideus muscle lies under the scapula, if the saddle is fitted too tightly or too forward, this may result in contraction of the rhomboid muscle to alleviate the discomfort by pulling the scapula closer to the body.

Lateral neck extension and chin to chest exercises help to stretch this muscle. 

The splenius muscle is one of the main neck extensors, that works with several other muscles such as the trapezius, rhomboid and serratus ventralis.

Origin: The fascia of the withers, the nuchal ligament and under the scapula

Inserts: Into the occipital bone and temporal bone in the head and the transverse processes of the 3-5 cervical vertebrae.

Function: When this muscle lengthens, the neck can drop and extend into a flexed position. This muscle is predominantly made of type I muscle fibres, which can contract for long periods of time but at a low level. It is responsible for postural control, such as holding the horses head and neck in a contact. This means that it is very important to allow the horse to stretch and round the neck into a soft contact, rather than being fixed into a contact, to avoid soreness and tightness developing in the muscle. The muscle can also elevate the head, produce lateral flexion of the head and neck and provide stabilisation of the spine and balance for forward movement. Under bilateral concentric contraction the splenius is able to lift and extend the head.

Soreness: When this muscle is sore, the horse may be inconsistent in the contact and struggle with lateral flexion on circles and whilst performing shoulder in movements.

A variety of carrot stretch exercises can really help to stretch this muscle including chin to chest, neck extension, lateral neck flexion and chin to point of hip stretches.

The function of a muscle can be categorised based on the density of type I and type II muscle fibres.

Type I muscle fibres are slow-twitch oxidative muscles, which means that they contract at a low level for longer periods of time and are resistant to fatigue. Muscles which are predominantly type I fibres are responsible for postural type work and are involved in maintaining position rather than actual movement.

Type II muscle fibres are fast-twitch glycolytic muscles, which means that they contract at a high level for shorter periods of time and are more likely to fatigue. Muscles which are predominately type II fibres are involved in producing movement for locomotion. Training and breed can influence the distribution of the muscle fibre types. 

The best way to look at the distribution of muscles fibres is visually. You can see that a major proportion of locomotor muscles are located in the hindquarters, which is why horses are known as “rear wheel drive” animals. The muscles in the hindquarters drive the horse forward under powerful contraction. It is important to remember that these muscles are much more prone to fatigue and soreness in the muscle can lead to a lack of impulsion and strength, which may cause issues for all disciplines such as difficulty performing collective and technical movements (pirouette, passage, piaffe), difficulty taking off before a jump or refusal, struggling with uphill work, inconsistencies working in a contact, hollowing of the back.

​You can see that the splenius and the rhomboid muscles mainly comprise of type I muscle fibres and are therefore predominantly postural muscles. They aid in maintaining the position of the head and neck during exercise and are under much strain when a horse is asked to work consistently in a contact. Soreness and trigger points are very common in these muscles especially in the dressage horse. Giving your horse several breaks during your training session and allowing the horse to stretch into long and low contact will help to relax and prevent overworking these muscles. As a tight muscle cannot strengthen!