Equine adductor muscle injuries

Both human and equine athletes are subject to leg muscle injuries. The hamstrings (biceps femoris, semitendinosus and semimembranosus) are most commonly affected, but the adductor or groin muscles (adductor, gracilis, sartorius, medial aspect of the semimembranosus) are also frequently damaged. These injuries can vary from mild and quickly repairable to severe and potentially career-ending.

Human athletes can generally self-diagnose groin injuries because of the location of pain and the specific movement limitations following muscle damage. Groin or adductor muscle injuries in horses can be much more difficult to diagnose. These muscle units are not easily manipulated, and many horses resent deep palpation in the groin area under normal circumstances. Examination of this area can be even more problematic when a horse is tender and painful following injury. Additionally, low-level sprains with microscopic damage to muscle fibers may cause little to no lameness when examined with a standard diagnostic approach. These factors lead to the underdiagnosis of adductor muscle injury in horses. Better, more specific techniques are needed for evaluation and diagnosis of these problems.

Muscle function and risk of injury

The groin is defined as the area where the upper thigh and the medial pelvis meet. Muscles in this area originate on the medial surface of the pelvis, ischium, pelvic symphysis or prepubic tendon and insert on the caudal to medial femur and medial patellar ligaments. The main function of these muscles is to adduct, or to pull the limb toward the midline. During normal forward motion, these muscles counteract the lateral hip and thigh muscles and pull the swinging limb toward the midline to maintain balance.

Activities that require extensive work on the side of a hill or gully, often required for endurance competitions, can cause excessive stress on the adductor muscles of the uphill leg, increasing the risk of injury. These muscles are also important when rapidly starting or stopping motion and when executing sharp turns or quick changes in direction. Horses competing in dressage, jumping, cutting, reining and polo are at higher risk for these injuries because of the particular demands of their sports, but a large percentage of equine groin injuries occur when horses slip or splay out while on poor footing (slippery, wet grass, mud or ice) and overstretch their adductors.

Injury classification

Groin injuries can be classified based on their severity. The most common groin injury is a pull or strain. A grade one groin pull results in mild discomfort, and, generally, no lameness is observed. After injury, the horse may continue to work normally, and the discomfort may only be seen after the horse has stopped exercising and rested long enough for the muscle fibers to swell slightly. The area may or may not be tender on palpation, but some tightness can usually be felt. Since any resulting gait deficit is mild and possibly only noticed when turning or rapidly changing direction, grade one pulls are usually not diagnosed. These horses may show decreased performance, but the exact cause is often missed.

With grade two groin strains, horses will usually experience a sharp pain or discomfort in the inner thigh (as per human athletes). The affected muscles will begin tightening, but significant shortening and spasm of the muscles may take 12 to 24 hours to develop; swelling or bruising takes a few days to appear. In horses, this level of strain is associated with moderate discomfort, and, while walking may appear unaffected, running and turning are uncomfortable. Since the groin muscles are in spasm, excessive inward movement of the limb occurs, resulting in slowing on the advancement phase of the stride and shortening of the crossover step when turning.

Grade three strains occur at play, in the pasture or while performing and cause noticeable pain. This pain is severe, and riders often feel this injury and detect a sudden decrease in willingness to move and the development of lameness. In these cases, heat and swelling of the affected muscles and pain or tenderness on palpation is present. A noticeable lump or gap may be felt in the muscle if tearing has occurred.

Diagnosis

If a grade one or two groin strain is suspected, it is sometimes difficult to confirm. The horse might demonstrate lameness if ridden at speed or in tight turns, but there is potential risk of further muscle damage with this type of diagnostic approach. The possible development of a grade three strain is not an acceptable trade off for the diagnosis of a grade two injury. One method of confirming adductor muscle injury is to specifically load those muscles and observe how the horse reacts. The unaffected leg should be evaluated first to serve as a control comparison.

To perform an adductor stress test, stand to the side of the horse's hip, and take a firm hold of the horse's tail with your hand (use your left hand if you are right-handed and vice versa). Then lift the hindlimb off the ground with your other hand and lightly flex it. Wrap the tail under the point of the hock (medially), and use it to pull the leg toward you (Photo 1). This action pulls the horse's pelvis toward the midline and forces the horse to temporarily load the muscles of the medial thigh of the supporting leg.

Photo 1: To begin performing an adductor stress test, take a firm hold of the horse's tail, lift the hindlimb off the ground, and lightly flex it. Next, wrap the tail under the point of the hock and use it to pull the leg toward you.

If this adductor stress test is done on both the normal and suspected affected limb, there should be a noticeable weakness or inability to maintain body position when the medial thigh of the injured leg is weighted. Grade two or three strains may cause the horse to buckle or almost collapse, so this should be done slowly and carefully. This test is best suited, however, for possible grade one or subtle cases in which it is difficult to pinpoint the cause of the horse's problem. More severe adductor problems will already be highly suspected because of the horse's motion and history.

Once the existence of a groin strain is confirmed, other diagnostic testing can help you determine the extent of the injury and plan treatment and rehabilitation programs. Ultrasonographic examination of the affected area will show muscle fiber swelling and, depending on the severity of the injury, tearing and fluid (blood, serum) within the muscle. Ultrasonography can also be used to monitor progress and rehabilitation of these injuries by measuring the affected area to gauge healing. Reduced lesion size and improvement in fiber pattern signal positive steps toward repair.

Photo 2: A thermogram of a strained right gracilis muscle. The horse is being viewed from behind, and the colors on the view correspond to the heat being produced by the muscle tissue. The color scale on the right goes from blue-black to white; as the scale goes toward white, the temperature gets hotter. The gracilis muscle in this horse shows more heat (is whiter) on the right. This horse had a grade two strain of this muscle area.

Thermography (infrared heat evaluation) is another valuable diagnostic tool for adductor muscle injuries in horses (Photos 2 and 3). This modality allows practitioners to observe even slight increases in temperature due to inflammation in the muscle and does not require contact with the horse — an advantage when trying to evaluate a painful inner thigh area on an unwilling patient. Thermography can also be used to monitor healing and can help determine a time sequence for return to competition. In subtle cases, thermography may be able to direct practitioners to a specific area so that guided ultrasonography can confirm a difficult diagnosis.

Photo 3: Another thermogram of a strained right gracilis muscle.

Treatment and prevention

Treatment of groin injuries involves rest and cold application in the initial stages. Anti-inflammatory medication is also advantageous. Returning to work and exercise too quickly after a groin strain can result in reinjury, and this pattern of incomplete healing and secondary damage is common in horses (and human athletes) that do not successfully return from adductor muscle injury.

The best treatment for these injuries, however, is prevention. Exercises designed to strengthen the adductors and to promote flexibility (serpentines, incline hill lounge work, lateral dressage movements for the hind end) can reduce the risk of later damage. Additionally, paying attention to footing is crucial in avoiding adductor muscle injuries.

Dr. Marcella is an equine practitioner in Canton, Ga.