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.