Do Horses Get Headaches? The Possible Cause from Muscular Fatigue

The equine head is a vast and complex structure made up of bone, nerves, soft tissue and circulatory systems. However, for the sake of specificity, this article plans to focus on muscular structures and its relation to the possibility of forming ‘equine headaches’.

As with any creature, it is evolution that dictates the anatomical composition of that animal. This is something that has noticeably influenced the muscular characteristics of the equine head and neck. An example of this being the temporalis muscle which functions in closing the jaw. The temporomandibular joint in the horse does not require a great deal of vertical opening for the purpose of eating, hunting or social communication. Therefore, the gravitational pull on the jaw is minimal, so little muscular effort is needed to close the jaw. For this reason, the temporalis muscle is weakly developed and comparatively smaller than other muscles surrounding the skull. However, when judged against the Masseter and Pterygoid muscles, these are highly developed due to the wide lateromedial range of temporomandibular joint motion associated with power stroke mastication.

The domestication of horses has led to alterations in their natural environment for the purpose of human interaction. For instance, the addition of metal bits placed in the mouth, leather straps around the head, and various training aids that have multiple attachments to the head. Furthermore, the application of pressure to the jaw, poll and nasal bridge from these apparatuses is not natural but is deemed necessary to train horses for human use. However, could this impact on the surrounding head muscles, therefore, facilitating tension in this region and causing equine headaches?

Tension-type headaches, commonly known as muscle contraction headaches, can occur in humans. The nociceptive system for pain perception is a complex topic on its own, however, current knowledge suggests that tension-type headaches may have a muscular origin (Millea et al, 2002) and that pericranial myofascial mechanisms may hold importance as to why these headaches occur (Chowdhury, 2012). Myofascial trigger points are abnormally firm bundles of contracted muscle tissue (Bowen et al, 2017), occurring from stress, muscle tension, overloading of the muscle or previous injury (Macgregor and von Schweinitz, 2006; Scanlon and Sanders, 2014; Waldman, 2013). These, then active, myofascial trigger points can result in muscular and referred pain (Travel and Simons, 1999).

Interestingly enough, human patients experiencing migraines and tension-type headaches have reported higher pain responses within the temporalis, frontalis and splenius muscles surrounding the head and neck (Leistad et al, 2005). The research to date is ever expanding and using human studies we can seek predictions for what may be present in the equine. Therefore, antagonistic pressure from the horse’s bit could apply excessive force on the temporalis muscle, a small and weakly developed muscle, to maintain a closed jaw. This could inherently overload an already weak muscle, resulting in fatigue/tension of the muscle and lead to the development of headaches as previously mentioned in Leistad et al, (2005) work.

To further support this notion of cranial muscle fatigue, a study by Bhatia et al (2007) reported increased electrography (EMG) activity of the Temporalis muscle in patients experiencing tension-type headaches. With this, they found that after intervention of muscle relaxants the time for onset of fatigue increased. Therefore, speculating that the muscle fatigue resulted in increased muscle activity, in the form of spasms, and causing tension-type headaches. Additional research, with larger subject groups, could be seen as beneficial to test the correlation between contraction headaches and the length of time before the onset of fatigue occurs.

The saying that ‘a bit is only as harsh as the hands that use it’ does appear to hold some validity. With values of between 50-80 Newtons (N) being applied to the horse’s mouth from rein pressure in dressage horses (Preuschoft et al., 1999; Clayton et al., 2005), compared to much lower Western reining horses of 20 N (Preuschoft et al., 1999). However, it may not be as simple to just ride with a lighter hand. In some sports, such as dressage, the horse is required to seek a constant contact on the bit, thus exerting a continues pressure on the mouth (Fédération Equestre Internationale, 2009). In addition to this, research has found that with compliant, stiff and inelastic reins there is always some level of pressure being applied to the mouth immaterial to rein length (Clayton et al, 2011). This is suggested to be as a result of equine biomechanical motion. The head and neck, being 10% representative of body mass (Buchner et al., 1997), is considered as a cantilevered beam that is supported by the cervicothoracic junction. This being a highly mobile anatomical region (Clayton et al., 2010; Clayton and Townsend, 1989) results in the head, withers and sacral tuberosity projecting upwards in the suspension phase of the trot and inherently shifting more pressure onto the mouth from the bit. Under the influence of gravitational/inertial forces, the horse then descends into the diagonal stance phase and the stride phases are repeated through the duration of trot. Therefore, it may not necessarily be a case of just riding with a ‘lighter hand’ if the horse’s natural locomotion generates additional pressure onto the bit.

The addition of leather straps around the horse’s face could also have implications to cause headaches. Research by McGreevey et al (2012) found that after just 5 minutes of wearing a crank noseband, the skin surface temperature reduced from 1.44 degrees to 0.35 degrees. This interference with local vascular perfusion (blood flow) is believed to increase the accumulation of Ca2+ and result in muscle tenderness due to muscles being deprived of the necessary circulation for function (Brandt et al., 2014; Waldman, 2013).

The take home message from this article seeks to encourage holistic assessments of horses and do no neglect the treatment and maintenance of the more minor muscle groups of the equine head and neck.

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