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.