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.
The brachiocephalic muscle runs along the entirety of the neck
Origin: the mastoid processes of the temporal bone and the wing of the atlas
Insertion: the humerus at the deltoid tuberosity.
Function: The function of this muscle is to flex the neck and to move the shoulder forward. It is also involved in adduction of the limb, which essentially means that it allows the limb to move towards the other. This muscle is actively engaged when the horse is asked to work in a contact.
Soreness: Soreness in the muscle can result in restriction in the shoulder and the horse may struggle to flex on a circle, inconsistencies in the contact and the horses’ ability to leg yield may be reduced. Long-term under and overworking of this muscle can lead to a shortened stride cycle. Horses with poorly fitting saddles can develop soreness in this muscle at the base of the neck due to weakness developed as the shoulder is restricted. Rider asymmetry can also lead to tension and tenderness of the muscle.