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.