Horses have many different facial behaviors, and it wasn’t so long ago that those expressions were a mystery to us humans. Now, we can interpret some of these facial behaviors.

The horse grimace scale has allowed researchers to see tension and pain by analyzing facial behaviors. The scale was designed to measure the amount of muscular tension held in six facial expressions visible by looking at the horse’s profile.

The six expressions are: stiffly backward ears, tightening around the eyes and tension above the eye area, obviously strained chewing muscles, tight mouth, pronounced chin, strained nostrils, and flattening of the profile. 

Each facial expression is scored using a scale of 0 to 2; the higher the number, the more tension is noted. A horse could have a total score of 12 over 12, indicating high levels of muscular tension across the face, and this likely means there is a higher degree of pain.

Using muscular tension to determine equine stress was first reported by researcher and Ph.D. Candidate Ellen Rankins from the Rutgers University Equine Science Center.

The masseter muscles, on each side of the horse’s face, are the primary muscles that open and close the horse’s lower jaw. By using surface electromyography, which is the process of using electrodes on the skin to determine a muscle’s electrical activity, the masseter muscle showed significant changes when the horse was under stress. 

In a program for veterans with post-traumatic stress syndrome (PTSD), eight geldings were tested to see if the program caused stress in the horses involved. We know that equine-assisted programs help humans, but this test addressed the concern about stress on horses.

Using the horse grimace scale, four horses were involved in the veterans’ program, and the other four were used as test controls and were not part of the actual veterans’ program. 

Photographs were taken of the horse’s faces before, during, and after for comparison. The study showed that using horses in equine-assisted programs did not cause stress in their lives.