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.
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.