Tying-up in horses is a colloquial term for Exertional Rhabdomyolysis, a condition involving exercise-related muscle cramping and damage.
Some horses experience a single episode of tying up whereas others experience recurrent tying-up. During an episode, the affected horse becomes stiff and reluctant to move. Your horse may only taking short, shuffled steps.
In severe cases, a horse displays signs of distress including pawing at the ground, excessive sweating, and quick, shallow breathing due to the pain associated with this condition.
Tying-up episodes should be taken seriously. If your horse is displaying signs, a veterinarian should be contacted immediately. The veterinarian can help identify whether this is a sporadic case or whether your horse is genetically susceptible to recurrent tying-up episodes.
Sporadic cases typically arise due to dietary imbalances, excessive electrolyte loss, or mismanaged exercise routines. Correcting these issues will help to prevent future episodes.
Nutrition and exercise management are important for horses that are genetically predisposed to tying-up. For example, horses with polysaccharide storage myopathy (PSSM) have issues with sugar storage in their muscles. They should be maintained on a low sugar and starch diet and given regular exercise to decrease the risk of tying up.
Horses with poor exercise performance may be experiencing subclinical exertional rhabdomyolysis without overt symptoms.
Types of Tying-Up
There are two main types of tying-up that are seen in horses: sporadic cases and recurrent cases. 
1) Sporadic cases
Sporadic cases are not related to an underlying genetic condition but are due to contemporaneous management or nutritional issues which can include:
- Lack of proper conditioning
- Heat exhaustion
- Electrolyte imbalances
- Vitamin and mineral imbalances
Dietary changes and adjustments to your horse’s exercise routine can help decrease risk of tying-up. In particular, the three following interventions can make a significant impact:
- Ensuring adequate electrolyte and water intake
- Ensure your horse is meeting vitamin E requirements
- Ensure your horse is meeting selenium requirements
Avoiding long rest periods before strenuous exercise can also decrease the risk.
2) Recurrent cases
Recurrent cases often have an underlying genetic abnormality which affects how the muscle cells function. The most common types are:
- Recurrent exertional rhabdomyolysis (RER)
- Polysaccharide storage myopathy (PSSM1 and PSSM2)
Fortunately, these horses also benefit from management practices to decrease the frequency of tying-up episodes. Changes to the diet and exercise program may be recommended by your nutritionist or veterinarian.
If your horse has experienced a tying-up episode, is genetically predisposed to the condition, or you if you suspect subclinical tying-up, submit your horse’s diet for a complementary diet analysis to identify potential risk factors and prevention strategies.
Prevalence and Risk Factors
Recurrent Exertional Rhabdomyolysis
Recurrent exertional rhabdomyolysis affects 5-7% of racing Thoroughbreds and Standardbreds. In a typical racing season, up to 17% of horses might not be able to compete again due to recurrent ER. 
Horses are thought to be predisposed to tying-up based on several risk factors: 
- Genetics: Recurrent ER in Thoroughbreds and Standardbreds may have been inadvertently selected for by breeding horses for faster race times 
- Sex: Female horses are more likely to be affected than males
- Age: Young horses are most susceptible to severe episodes
- Temperament: Horses with a nervous, excitable temperament are more likely to be affected than calm horses
- Lameness: Horses with any form of lameness are more susceptible to tying-up
- Diet: High grain diets can increase the risk
- Exercise routine: Horses that are rested for more than one day before strenuous exercise
PSSM or EPSM is an inherited condition that primarily affects Quarter Horses and related breeds such as Paints and Appaloosas. It can also affect draft horses and warmbloods.
In Quarter Horses, the prevalence has been estimated at 6-12% and may be higher in certain familial lines. 
Signs of Tying-up
Whether your horse is experiencing tying up for the first time or whether it’s a recurrent issue, the episodes can look the same.
If you’ve ever experienced cramping calf muscles after exercise, you have some idea of what this feels like. In horses, it affects much larger muscle groups, typically the hindquarters, and sometimes the shoulder and back muscles.
It can occur during or after exercise and lasts for several hours, causing significant pain and distress to the animal.
Common signs of tying-up include:
- Sudden reluctance to move with cramping over the hindquarters
- May take only short steps with hindlimbs
- Profuse sweating
- Muscle tremors
- Colic-like symptoms indicative of significant pain including:
- Rapid breathing and a fast heart rate
- Pawing at the ground
- Attempting to lay down
- Looking at their flanks
As the episode progresses, the horse may become severely dehydrated from profuse sweating. The horse may develop hyperthermia (overheating) if sweating is not sufficient to cool them down.
Horses may have dark urine and dark mucous membranes in the mouth as a result of dehydration.
Tying-up with reluctance to move and significant pain is a serious condition that needs veterinary intervention.
Several symptoms of ER could be confused with colic. To obtain proper diagnosis and treatment, it is important to know the horse’s feeding plan, exercise program and medical history.
Some horses may experience subclinical episodes with less overt signs. These horses may show poor exercise performance, painful muscles and not maintaining collection when riding.
Exercise Muscle Physiology
In order to understand this condition, it is important to know what normal exercise muscle function looks like in the horse.
Calcium is an important macromineral that functions as a regulator of muscle contraction and energy production within muscle cells.
When the muscle is relaxed, calcium is stored within specific compartments in the cell called the sarcoplasmic reticulum. When the nervous system tells muscles to contract, this calcium is released to bind to muscle fibres and cause them to contract.
Calcium also stimulates energy production within the cell to support muscle contraction. In order for the muscle to relax, calcium needs to be moved back into the specialized compartments. This whole cycle repeats itself every time the muscle contracts and relaxes. 
What Happens When a Horse Ties Up?
Although it is not fully known what causes ER, most recent research suggests it is due to abnormal regulation of calcium within muscle cells.
Pumping calcium back into the storage compartments requires cellular energy. If this is depleted during intense exercise, too much calcium will remain outside these compartments.
This excess calcium build-up prevents muscles from relaxing and keeps them in the contracted state. This uses up more cellular energy and sets off a vicious cycle.
High levels of calcium outside the specialized compartments can cause damage to cell membranes resulting in muscle cell death (apoptosis). The contents of muscle cells can enter the bloodstream, which puts strain on the kidneys and detoxification pathways. In severe cases, this can cause acute kidney failure and can be fatal.
Horses with genetic abnormalities that predispose them to recurrent ER may be less efficient at pumping calcium back into the sarcoplasmic reticulum. They may also have excessive release of calcium into the cells.
The precise reason for this calcium dysregulation is different for different genetic variants. 
Your veterinarian will make a diagnosis based on the horse showing muscle stiffness and pain after exercise.
A blood test can reveal the extent of muscle damage by measuring levels of muscle enzymes in blood, including: 
- Creatine kinase (CK)
- Aspartate transaminase (AST)
- Lactate dehydrogenase (LDH)
Depending on when the blood sample is taken in relation to the tying-up event, the levels of these enzymes will vary. Levels of CK and LDH peak earlier than AST and are cleared faster from the blood than AST.
In severe cases, blood tests might also show electrolyte imbalances due to excessive sweating. This can include low levels of sodium, chloride, calcium, potassium and phosphorus in the blood.
Urine tests can also be done to measure the levels of the muscle protein myoglobin.
Controlled Exercise Tests
If you suspect your horse might have subclinical tying-up episodes that are affecting its performance, your vet might choose to do a controlled exercise test. This typically involves 15 minutes of easy exercise followed by a blood test 4-6 hours later.
Horses with subclinical rhabdomyolysis that don’t show severe symptoms can still show elevated CK levels indicating muscle damage.
Genetic tests can be done to identify and properly classify PSSM1/2 or other genetic conditions that predispose horses to tying-up. Muscle biopsies may also be taken to assess the extent of abnormalities in muscle cell structure and composition.
Horses with PSSM have excessive and abnormal glucose (sugar) storage in their skeletal muscles which affects how the muscle functions. This is often diagnosed by muscle biopsies which reveal abnormal polysaccharide accumulations that are less easily broken down by enzymes. 
Treatment & Recovery Management
In the acute state, the veterinarian will focus on keeping your horse comfortable by cooling them down, relieving anxiety and easing muscle pain.
This usually involves treatment with injectable tranquilizers for sedation and analgesics for pain management.
It is also critical to replace the fluids and electrolytes lost by excessive sweating.
For horses with mild to moderate cases, free-choice electrolytes and water may be sufficient but others might require electrolyte delivery by a nasogastric tube. In severe cases, intravenous electrolyte and fluid replacement is often required.
Following a tying-up episode, your horse should be carefully managed as they recover. Some management strategies during the first few days include: 
- Keeping the horse on stall rest for the first 24 hours
- Limiting turnout to small paddocks with minimal stimulation
- Handwalking can be done if they are willing but should not exceed 5-10 minutes at a time
Your veterinarian may decide to redo the blood test in a few days to assess whether the enzyme levels have returned to normal, at which point training can slowly resume with a consistent exercise schedule.
Adopting appropriate prevention strategies will depend on the underlying cause of tying-up. All horses are susceptible to sporadic cases of tying-up that are not linked to genetic abnormalities.
However, Thoroughbred and Standardbred fillies with a nervous disposition and horses with a known familial history of tying-up should be carefully managed.
Following some general principles can help decrease the risk of tying-up and prevent future episodes. 
1) Feed a low-starch, high-fat diet
Whenever possible, additional caloric needs should be met by feeds that are low in non-structural carbohydrates (NSC). Ideally, less than 20% of daily calories would come from NSC (starch + ethanol-soluble sugars).
Exercising horses can adapt to diets that are high in fat by providing oils as a major energy source. Heavily exercising horses could reach up to 25% of their daily caloric needs met by fat.
Oils high in omega-3 fatty acids have additional anti-inflammatory benefits that support exercise recovery.
For nervous and anxious horses, replacing starch with fat in the diet can have a calming effect and decrease their risk of tying-up.
2) Provide adequate vitamin E and selenium
It is especially important to ensure that exercising horses are meeting their requirement. Exercise and catabolism of nutrients for energy create excessive free radicals (oxidants) that can cause damage to cells if they are not neutralized by antioxidants.
The National Research Council’s Nutrient Requirements for Horses has stated the horse’s daily requirement for vitamin E is 500 – 1000 IU. For selenium, the daily recommendation is 1 mg for a 500 kg horse.
However, higher levels