Updated: Apr 2
A horse's tail is more than just a pretty accessory - it tells you a lot about how he's using his hind end.
If you observe horses in motion, you may notice some carry their tails to one side rather than in the center. This recruitment pattern is an easily identifiable sign of dysfunctional movement or use of the pelvis or hind limbs. It happens when a horse has some weakness, soreness, or injury and is overusing the muscles of the hamstrings on one side of the body moreso than the other. As "lameness is, by definition, assymetry" (Dr. Deb Bennett, PhD), the sooner we can identify an assymetrical use pattern and correct it, the greater chance we have of preventing bigger problems from developing.
Aside from observing the tail of a moving horse, moving your horse's tail gently from side to side while he’s standing is another way you can determine how symmetrically he's using his hind end, as the tension patterns in the fascia and muscles will be evident even with the horse at a halt. As you move your horse's tail, notice whether it moves as easily and as far to one side as it does to the other, or if it seems to have a preference - moving more comfortably in one direction, and/or having more range of motion. Your horse may also give you some feedback, such as annoyance as you move the tail in the direction it is less accustomed to, or relaxation, blowing, or licking as you move the tail in its preferred direction. Keep in mind that healthy tissue and joints move freely in all of their intended directions; any preference to one direction over another represents asymmetry, and therefore lameness.
Understanding the hamstring muscles can help clarify the importance and relevance of tail carriage. The hamstrings consist of 3 muscles: biceps femoris, semitendinosus, and semimembranosus. The biceps femoris has complex attachments to the spinous and transverse processes of the last 3 sacral vertebrae, the sacro-sciatic ligament, and the tail fascia, as well as to the ischial tuberosity (point of the buttock); it inserts onto the patella, femur, tibia, and calcaneus. The biceps femoris' primary action is to extend the hip and stifle, though the caudal aspect flexes the stifle (working in opposition to its cranial fibers), abducts the limb, and extends the hock. Semitendinosus attaches to the last 2 sacral vertebrae and first 2 caudal vertebrae, tail fascia, sacrosciatic ligament, and ischial tuberosity; it crosses the hip, stifle, and hock joints and inserts onto the tibia and calcaneus. The actions of semitendinosus are to extend the hip, stifle, and hocks when the limb is planted; if the limb is not in contact with the ground, this muscle flexes the stifle, retracts, and adducts the limb. Semimembranosus also attaches to the sacrosicatic ligament and ischial tuberosity like the other hamstring muscles, and to the coccyx like semitendonosus, but does not attach to the sacrum itself or the tail fascia; it inserts onto the femur and tibia, but does not attach to the hock. Semimembranosus extends the hip and stifle joints when the limb is planted; if the limb is not planted, it retracts, adducts, and rotates the limb inward.
So you can see that all 3 hamstring muscles have some common attachments and actions, as well as some differences. They work synergistically to extend the hip and stifle; those that attach to the tail fascia (biceps femoris and semitendinosis) also extend the hock. Yet, they work in opposition to one another with regards to abduction and adduction (biceps femoris abducts, while semitendinosus and semimembranosus adduct the limb). Understanding these muscle attachments and actions, we begin to recognize how asymmetrical use of the hamstrings will affect the tail position - especially when the horse is in motion.
As much as we humans like a simple, direct "this symptom equals that problem," it's unfortunately not that straightforward. Asymmetrical tail carriage indicates that there is a problem and shows us where the recruitment is happening but does not tell us where the actual problem is. The side of recruitment (i.e. the side the tail is pulled to) is not necessarily the side of the lameness, or vice versa - it varies on each horse, and depends a lot on what the particular injury is. For example, a horse with chronic sacroiliac joint dysfunction on the right carries his tail to the left: his left hamstrings are working harder in the "push" phase of each stride to propel the body forward; initially, he overuses the left hamstrings while limiting mobility on the injured side to avoid aggravating it - over time, the muscles on the injured side also become weaker, reinforcing the assymetry. Another horse with an injury to her lower left hind limb also carries her tail to the left: in this case, she's over-using her hamstrings on the side of the injury to concentrate movement in the upper limb while keeping the lower leg relatively still.
Having identified whether your horse's tail is balanced or asymmetrical is just the first step, but an important one to head off bigger problems down the road. Once the assymetry is recognized and understood, you may need the help of your veterinarian to determine which side the primary lameness is on and what can be done to help your horse achieve greater balance, symmetry, and harmony in his body.