Understanding tight Brachiocephalic Muscles and what causes an "Upside-down" neck in so many horses is crucial to helping horses regain awareness and function in their thoracic sling and topline muscles.

Function of the Equine Brachiocephalicus Muscles

The normal function of the brachiocephalicus (which includes the cleidocephalicus and cleidobrachialis muscles, working together with the omotransversarius muscles) is to flex the head and neck laterally (right or left) and/or to protract the forelimb and extend the shoulder (bring the front leg forward) when the head is 'fixed.'

Fixed does not necessarily describe a problem, but only describes the position of the head relative to the movement of the forelimb. The horses head can be 'fixed' by his or her attention being on something out in front or on the horizon, by the natural dynamic posture of a horse in motion, by unconscious OR conscious contractions of the strong spinal extensor muscles (ie. as part of the startle reflex, or a horse hollowing their back to avoid a pinching saddle), or by a rider holding a strong contact on one or both reins, etc. - all of which cause the horse to raise it's head and often brace several different neck and back muscles to maintain that position.

The brachiocephalic muscles do not assist in raising the head or neck, (because they are flexor muscles below, or ventral to, the spine, and muscles can't push, they can only pull). The reason they are so often tight, bulging or over-developed in a horse with high-headed posture (or conformation) is a combination of the following:

1. raising the head stretches the underneck muscles and repeatedly triggers them to contract via the stretch reflexes, and also prevents them from performing functionally by holding them in a lengthened position 24/7, resulting in 'stretch weakness'

2. regardless of head position, the horse will continue to use these muscles to the best of their ability to protract their front limbs during movement

3. When a horse head is 'fixed' in spinal extension by unconsciously habituated contractions of the dorsal neck and back muscles (the Green Light Posture), or by a strong bridle contact, and then the rider/trainer or bodyworker asks the horse to lower their head, they do so by recruiting their ventral neck muscles to pull the head and neck back down against the mechanical forces and contractions of their own topline muscles!

Muscle Development vs. Muscle Tension

To be effective at helping horses, we need to differentiate between muscle development and muscle tension. Development is when the muscle fibers grow bigger/stronger, and hypertrophy is excessive muscle development. Tension is muscular activity aka shortening/contracting of the muscle fibers which pulls on it's attachment points (origin/insertion and fascia), produces the force on the skeleton and myofascial tissues to create movement (one muscle group working at a time) or stabilization/brace (contractions in both opposing muscles or groups at the same time).

Muscle development happens gradually over time in response to work/stress and recovery/regrowth. Developed muscles can appear/feel soft, medium or hard, depending on the level of contraction they are currently producing - at rest this level is called 'resting muscle tonus' or baseline tension.

Tension levels can change quickly, either temporarily in response to reflexes, manual interventions, (bodywork or adjustments) OR decision-making (voluntary movement and inhibition), or semi-permanently when the horses brain learns or un-learns compensation patterns.

A muscle can be developed in response to 'correct work' or also in response to needing to counterbalance chronic tension in the opposing muscle group - which is what is happening to most high-headed and hollow-backed horses. When the head is up, the back is extended (hollowed) because the back and lifting neck muscles are tight - this means that when they go to use the brachiocephalic muscles to flex laterally or to protract their front legs, they have to work extra-hard to do their job, because they have to power through the resistance coming from the muscles that hold the head up and hollow the back, preventing the horse from moving easily or 'correctly'.

Equine Green Light Posture

If a horses ‘regular’ posture has her standing with a high head (and a hollow back always goes along with this) then it's likely that in addition to any possible breed tendency to stand and move this way she also has some habituated muscle tension (aka contractions) that are holding her there, unconsciously.

Various parts of the central nervous system can initiate or communicate instructions to the muscle tissue to contract and produce tension, but there is only ONE part of the brain that can change the resting level of muscle tone - muscles cannot act on their own, they ONLY respond to instructions from the CNS.

Motor Control and the Brain

To put it in rather simplified terms, the neural impulses that do voluntary movement come from the top of the brain; impulses that create chronic tension come from the brain stem; and impulses that control simple involuntary reflexes frequently come from the spinal cord. There are of course more complex reflexes that do involve parts of the brain, but I won't get into them here.

The point is, regardless of the level of muscle development, if the muscles feel hard even when your horse is just standing there doing nothing ('at rest') then they are contracting too much and too frequently - this is elevated resting muscle tone aka chronic tension. Since this kind of tension is learned, habituated and maintained 24/7 by the horses BRAIN in response to stress, repetitive motion, compensating for an injury or other trauma, balancing in a trailer on long hauls, ill fitting tack, etc. - it generally responds to manual releasing, medication, strength training or stretching with only temporary changes.

It CAN be reversed with a natural motor-sensory re-learning process, and this is what we do with Equine Hanna Somatics® (EHS) - the only method I have yet found that can actually cause the horse to change their own brain-stem mediated motor-output that controls chronic tension levels, and dial them back down to a more healthy neutral resting muscle tone aka 'normal baseline' tension level.

I believe that this kind of brain-level motor-intervention is the future of equine bodywork - and it's already here!

RESOURCES:

Pasquini, C. (1991). Atlas of Equine Anatomy (3rd ed.). Sudz Publishing.

Felten, D. L., M Kerry O'banion, Mary Summo Maida, & Netter, F. H. (2022). Netter’s atlas of neuroscience (4th ed.). Elsevier.

Anatomy of the Horse by Klaus-Dieter Budras, 6th Edition - download it FREE here: https://www.vet-ebooks.com/anatomy-of-the-horse-6th-edition/