Origin: The spine of the scapula.
Insertion: into the deltoid tuberosity of the humerus.
Function: The deltoid is mainly responsible for flexion of the shoulder joint and can also aid in abduction of the shoulder. This makes it an important contributor to stride length.
Soreness: If there is tightness or discomfort in this area, the horse may have a shortened stride. The horse may also move away from palpation over the shoulder or stomp the forelimb.
Massage can relieve tension and discomfort in this muscle, whilst stretching exercises such as lifting the forelimb forward and extending the shoulder can help to stretch the deltoid muscle as well as other shoulder and forelimb muscles on a more regular basis. Good flexibility at the shoulder is important for jumpers to achieve good scope over the fence.
Horses do not have a collar bone and so the subclavius muscle, as well as the pectoral and serratus ventralis muscle, make up the thoracic sling.
Origin: The manubrium of the sternum
Inserts: into the fascia over the supraspinatus muscle.
Function: This muscle is involved in suspension and stabilisation of the trunk.
Soreness: Palpation along the front aspect of the scapula will elicit a stomping response from the horse or they will move away from you if there is tension or discomfort in this area. Tension in this muscle can result in shortening of the muscle, where the horse will struggle advancing the limb (shortened stride). The shoulder may become restricted, stiff and may appear more upright on the affected side. Exercises and disciplines which involve a lot of loading onto the forelimb will exacerbate the discomfort in this area and issues will become more noticeable.
The supraspinous and infraspinatus muscles are the main stabilisers of the scapula and the shoulder joint.
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.
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!
The serratus ventralis muscle can be split into two sections, the cervical and the thoracic.
Origin: from the scapula cartilage.
Insertion: into the last 4-5 cervical vertebrae.
Function: This portion of the muscle is responsible for the backwards and forwards movement of the scapula and therefore moving the limb.
Origin: Underside of the scapula and the intercostal fascia.
Insertion: Across the 1st to the 9th ribs.
Function: This portion of the muscle is part of the thoracic sling which acts to suspend the horses chest between the forelimb and is important in the moment of suspension in the horses natural forelimb movement and lifts the ribcage.
Soreness: The main causes of tightness in this muscle can arise during jumping from the concussive forces of landing and poorly fitting saddles which can restrict shoulder movement. A horse that is tight in this muscle may have a shortened stride, reduced scapula movement, girthing issues, difficulty turning on the forehand, and potentially poor respiration due to the location of the thoracic section where tension may reduce ribcage expansion.
Chin to fetlock carrot stretches can help to stretch the serratus muscles and increase flexibility in the neck and the shoulder blade.
The RCL is located at the poll of the horses head.
Origin: The occiput
Insertion: Into the first two cervical vertebrae, the atlas and axis.
Function: When this muscle lengthens bilaterally the head will drop into flexion. When one side contracts independently the head will incline to one side.
Soreness: Soreness in this muscle can display itself as difficulty to halter or bridle, head tilting and/or head shying. The head piece of the bridle runs directly over the RCL. As the horse drops its head down into a flexed position, the headpiece becomes significantly tighter, increasing the pressure on the RCL and the horse may become sore at the poll.
When the horse is standing in a relaxed position, you should be able to fit two fingers underneath the headpiece to reduce the pressure when asked to work in a contact during exercise.
The sternomandibular muscle, also known as the sternocephalicus, is a long, slim neck muscle.
Origin: The manubrium of the sternum
Insertion: Either side of the mandible (lower jaw).
Function: This muscle aids in opening the mouth, stabilising the jaw, moving the head downwards and lateral flexion of the head and neck.
Soreness: This muscle can be overdeveloped in horses with a high head carriage and tightness is typically palpated at the origin. This muscle can be largely affected by the use of a haynet. When a horse eats from a haynet, the consistent tugging motion can lead to tightness in the neck muscles as the muscles compensate for the action. This can lead to restricted range of motion in the upper neck. Feeding from the floor and frequent carrot stretches can help to alleviate the tension in upper neck.