Muscle Imbalance and Spasm in Horses

Introduction to muscle imbalance and spasm

The muscles of a horse’s body hold the backbone and limbs together in the correct position. Properly functioning, healthy muscles, therefore, are responsible for balanced posture and movements. Muscle imbalance is a common issue that affects the wellbeing of a horse. This causes deviation in posture and leads to sub-luxation of the skeletal joints, limiting joint movements.

It is important for the size and strength of the muscles on each side of the body to be symmetrical. If a muscle or group of muscles on one side of the body becomes weaker, stronger, looser, tighter, larger, or smaller than the corresponding muscle or muscles on the other side, this will cause muscle imbalance.

Muscle imbalance does not occur suddenly. It is a condition which often results from carrying out unbalanced exercises, exercising with improper form or bad posture, carrying out several daily life activities, and in very rare cases is congenital. Injuries, poor nutrition, and neurologic issues can also muscle imbalance in horses.

Muscle imbalance can lead to an unbalanced appearance, pain for the horse, limited movements, and instability. It also leads to an increased risk of injury to muscle, bone, joints, tendons and ligaments. Fatigue is the main cause of sports injuries in horses. This occurs when the horse moves in an unbalanced posture. Ensuring that muscles are balanced can relieve fatigue and prevent injuries.

Muscle spasm is a painful condition where a muscle or group of muscles contracts forcefully and involuntarily. When this condition occurs, the muscle can become unable to relax. Muscle spasm can occur in any muscle but most commonly occurs in the limbs and the muscles of the abdomen. A sustained muscle spasm is known as muscle cramps, and a condition where painful muscle cramps occur in the limbs is called charley horse. Muscle spasms in the horse’s back limbs are known as shivers. Equine shivering is characterised by periodic involuntary spasms of the muscles in the pelvic region, pelvic limbs, and tail. Sometimes forelimbs are also affected. Affected horses show stiffness, s sudden jerky extensor movements in the tail (tail elevation), and a tendency to move backwards.

There are many potential causes for muscle spasms in horses. Dehydration and electrolyte imbalance in the extracellular fluid is the most common cause. Excessive sweating causes dehydration and loss of electrolytes. Horses which have been intensely exercised in hot, humid weather conditions can suffer from a depleted water level in the body caused by sweating which takes place to help dissipate the body heat. Fluid loss occurs at a rate of up to 15-20L per hour when horses sweat. This can result in huge deficits in minerals like Sodium(Na), Potassium(K), Chlorine(Cl), Magnesium(Mg), and Calcium (Ca).

Otobius megnini ear tick infestation is another common cause of muscle spasms in horses.

Diagnosing muscle imbalance and spasms

In order to diagnose a muscle imbalance in a horse, a clinician will take into consideration information from the following sources:

  • The horse’s history
  • A physical examination
  • The horse’s biochemical profile (obtained via a serum enzyme test)
  • Urinalysis
  • Blood test
  • Electromyography (EMG)
  • Muscle biopsy
  • Genetic test

An informative history is required when assessing almost any disease in an animal. A history of any muscle problems such as cramping, pain and weakness, and the frequency of such problems is essential information. It is also useful for the clinician to know the animal’s exercise schedule, diet, vaccination and medication history, as well as all information relating to factors that are often precursors to muscle problems such as lameness, stress, and strenuous exercise carried out.   

There are a few characteristic clinical signs in horses suffering from muscle imbalance and spasm. These can be diagnosed via careful physical examination and palpation. Palpation of muscle mass provides important information about any muscle pain the animal is suffering, as well as swelling and muscle tone.

If a horse’s limbs are balanced, they will land in a perpendicular way as the horse moves. Observing the horse both while standing and while walking, therefore, can help identify any gait abnormalities which may indicate that the limbs are unbalanced. Shivering horses show tenseness and trembling of the hind limb, a sudden elevation of the tail, or a tendency to move backwards, picking up their feet. Muscle imbalance and spasms can very often result in lameness, and a lameness evaluation can be carried out by means of a flexion test. This test indicates if the animal is experiencing any muscle pain or problems with its joints.

Extremely fatiguing exercises such as endurance rides can cause increased creatine kinase(CK) level in the serum. The standard level of CK in serum is less than 1000U/L. A serum biochemical profile can determine the animal’s CK level. Measuring the levels of electrolytes and mineral concentration in the horse’s urine and blood can help to detect electrolyte imbalances in horses which are suffering from muscle spasms and cramping. For this diagnosis, urinalysis and blood tests can be carried out. Dehydration is the most common cause of muscle spasms and can be determined as part of these tests.

Electromyography (EMG) is a technique which detects muscle potential by placing a needle very close to the motor endplate. When muscle contract action potential occurs, this can be observed as a wave pattern in the EMG. If the horse is suffering from muscle spasms or cramp problems; this wave pattern will appear irregular.

Muscle biopsy is an important technique used to obtain diagnostic information regarding muscle problems. Genetic tests can also be used to help detect congenital myopathies in horses.

Different causes of myopathy

The word ‘myopathy’ is  defined as a disease of the skeletal muscles.  The major sign of this disease is muscle weakness which occurs due to dysfunction of muscle fibres. Other symptoms are muscle cramps, pain and stiffness. Myopathies can either be genetic or acquired. There are several myopathies that commonly occur in horses, all of which have several possible causes.

Types of Myopathies

  • Equine exertional rhabdomyolysis(ER)
  • Equine polysaccharide storage myopathy(PSSM)
  • Hyperkalemic periodic paralysis(HYPP)
  • Fibrotic myopathy
  • Inflammatory myopathy
  • Nutritional myopathy
  • Toxic myopathy
  • Atypical myopathy
  • Post anesthetic myopathy
  • Ischemic myopathy

Equine exertional rhabdomyolysis (ER) occurs due to abnormal Ca homeostasis within the muscle cells; causing muscle contraction to be intermittently disrupted, damaging the muscle tissue of the horse. The most common causes of this are excessive exercise and a high-grain diet.

Equine polysaccharide storage myopathy (PSSM) is a genetic disorder that occurs due to a dominantly inherited gene mutation.

Hyperkalemic periodic paralysis (HYPP) is another genetic condition that results from a point mutation in the Na channel gene in the skeletal muscle. When these abnormal Na channels are activated, they decrease Na-K exchange through the channel. This results in an increase in intracellular Na level and K level in the extracellular space, causing hyperkalemia. The most common cause of this myopathy is feed that is high in potassium, such as soya bean meal or oil, sugar molasses, beet molasses, alfalfa hay, brome hay, canola oil, grains such as corn, wheat, oats, and barley or beer pulp.

Fibrotic myopathy occurs when the inner thigh muscles are injured via exercise, trauma, or IM injection.

Inflammatory myopathy occurs via bacteria, viruses, and some parasites infecting the muscles.  Equine influenza 2 and Equine herpesvirus 1 are common viruses which cause this myopathy.
Sarcocystis fayeri, an infectious parasite, enters the horse via feed which is contaminated with canine feces.  Another infectious parasite, Anaplasma phagocytophilum, can enter a horse via a tick bite.

                       Bacteria such as Streptococcus equi equi and Clostridium spp. (C.septicum / C.perfringens/ C.chauvoei) can be a cause of inflammatory myopathy in horses. S.equi, Staphylococcus aureus, Corynebacterium pseudotuberculosis often cause muscle abscesses.

Nutritional myopathy in horses is associated with a deficiency of antioxidants like Selenium (Se) or vitamin E (Vit. E). A Vitamin E deficiency occurs when the horse eats low quality grass hay. This condition is common in foals up to 2 weeks old and young adult horses.

Toxic myopathy is caused by ingestion of poisonous portions (leaves/fruit/seeds) of certain plants like coyotillo (Karwinskia humboldtians), coffee sennas (Cassia occidentalis), white snakeroot (Eupatorium rugosum), and box elder seeds (Acer negundo). As well as certain plant toxins, ionophores are also toxic to horses. These are often used as a feed additive for poultry and other livestock, but should not be fed to horses. If a horse’s feed is accidentally contaminated with ionophores, toxic myopathy occurs. Horses are ten times more sensitive to ionophores than any other species.

Atypical myopathy is a fatal muscle disease caused by the ingestion of sycamore seeds, seedlings, and leaves.

Problems in the horse’s circulatory system cause post-anesthetic myopathy and ischemic myopathies.

How to treat muscle problems

As long as you get to the root cause of a horse’s muscle problems, treatment is easy. In mild cases, owners can manage these problems themselves, but in severe cases it is essential to seek help from a veterinarian.

Dehydration and electrolyte imbalance is the primary cause of most muscle disorders. Ionized Ca in the serum is an essential mineral which allows muscle contraction to take place. To prevent a hypo-calcemic condition, a sufficient amount of Ca, Mg, Na, K, and Cl, as well as essential minerals for healthy muscles should be provided as part of the horse’s diet. Changing the horse’s diet is a common treatment for several different muscle problems, however the exactt dietary change required varies according to the problem. A horse with nutritional myopathy, for instance, must be provided with sufficient dietary Se and Vitatim E.
          A diet containing fresh grasses is good for horses with HYPP as fresh grasses have a high water content, and eating them can also help to reduce K intake.

There are no antitoxins for muscle problems like atypical myopathy and toxic myopathy, however certain medications can help stop or reduce the absorption of toxins from the intestine.

Besides dietary changes, horses should be provided with freshwater or electrolyte supplemented water to help prevent dehydration. Anti-inflammatory drugs and antibiotics are the main treatments for inflammatory myopathy. Injectable methocarbamol, IV fluids and pain relievers can also be used to treat muscle disorders. If these treatments do not work sufficiently, surgery may be required.


Healthy muscles are essential for the wellbeing of a horse. Because horses are athletic animals, injuries and muscle disorders happen frequently. The common causes of these disorders are strenuous exercise, dehydration, electrolytic imbalance, intoxication, and genetic mutations. The best way to prevent these disorders is through providing sufficient dietary nutrients, however problems can often be treated with proper management and medication.

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