Tying up in horses is a muscle disorder in which muscle cells break down during or shortly after exercise, causing stiffness, pain, and reluctance to move. The clinical term is exertional rhabdomyolysis, and it has been recognized for over a century under names like azoturia, Monday morning disease, and paralytic myoglobinuria. It can range from a mild episode that resolves with rest to a severe emergency that damages the kidneys.
What Happens Inside the Muscles
During a tying-up episode, skeletal muscle fibers, particularly the fast-twitch fibers used for bursts of speed and power, undergo acute damage and die. As these cells break apart, they release their contents into the bloodstream. Two proteins matter most here. Creatine kinase (CK) floods the blood and serves as a reliable marker of fresh muscle damage. Myoglobin, the protein that gives muscle its red color, also spills out and is filtered through the kidneys.
In moderate to severe episodes, significant amounts of myoglobin end up in the urine, turning it dark brown or coffee-colored. This is more than cosmetic. Free myoglobin can clog the tiny filtering tubes inside the kidneys, leading to tubular damage. In the worst cases, this blockage causes acute kidney failure, which is why severe tying up is treated as a veterinary emergency.
Signs You’ll See During an Episode
The hallmark sign is a horse that suddenly becomes stiff and reluctant to move, often within minutes of starting exercise or shortly after finishing. The large muscles over the hindquarters and back feel rock-hard to the touch. Affected horses typically sweat heavily, even out of proportion to the work they were doing. Their breathing rate climbs, and they may show obvious signs of pain: pawing, looking at their flanks, or even trying to lie down. In milder cases, you might notice only a short, stilted stride in the hind legs that could be mistaken for a lameness issue.
The dark or discolored urine, when it appears, is one of the most distinctive clues. Not every episode produces visible urine changes, though. Mild cases may only show up on a blood test, with CK and AST (another muscle enzyme) levels climbing above 1,000 U/L.
Sporadic vs. Chronic Tying Up
There is an important distinction between a horse that ties up once and a horse that does it repeatedly. A single episode, called sporadic exertional rhabdomyolysis, can happen to almost any horse under the right circumstances. Common triggers include exercising beyond current fitness, working hard after several days off (hence the old name “Monday morning disease”), electrolyte imbalances from heavy sweating, vitamin E or selenium deficiency, viral illness, or hormonal shifts. These cases are usually preventable once the trigger is identified and corrected.
Chronic or recurrent tying up is a different situation. These horses tie up repeatedly despite what seems like appropriate management. In most cases, an underlying genetic condition is driving the problem, and it requires a different approach to diagnosis and long-term care.
Genetic Conditions Behind Recurrent Episodes
Polysaccharide Storage Myopathy (PSSM)
PSSM causes an abnormal accumulation of glycogen (stored sugar) in the muscles. Type 1 PSSM is caused by a specific mutation in the GYS1 gene, which tells muscle cells to produce glycogen nonstop. It is an autosomal dominant trait, meaning a horse only needs one copy of the mutation from one parent to be affected. Quarter Horses, Drafts, Warmbloods, and related stock breeds are most commonly diagnosed. Type 2 PSSM also results in abnormal glycogen storage, but horses with this form do not carry the GYS1 mutation. The cause remains unknown, and there may actually be multiple causes grouped under the same label. Notably, a recent study found no significant link between the commercial genetic test variants marketed for PSSM2 (P2, P3, P4) and confirmed diagnoses based on muscle biopsy, so the reliability of those commercial tests is uncertain.
Recurrent Exertional Rhabdomyolysis (RER)
RER is most common in Thoroughbreds, Standardbreds, and Arabians. It appears to involve abnormal regulation of calcium inside muscle cells. Calcium is the trigger that tells a muscle fiber to contract, and when its regulation goes wrong, the fiber can contract too forcefully or fail to relax, leading to damage. Young fillies and horses with nervous temperaments seem especially prone. Hormonal factors and stress levels both play a role, and episodes often cluster around the start of training or during high-intensity work.
How Tying Up Is Diagnosed
A veterinarian will usually start with a blood draw during or shortly after an episode. Elevated CK confirms acute muscle damage, while elevated AST, which rises more slowly and stays elevated longer, helps establish the timeline. These blood values tell you an episode happened but not why.
For horses that tie up more than once, further testing helps identify the underlying cause. Genetic testing for PSSM1 is straightforward: your veterinarian collects whole blood or hair samples with roots attached and sends them to a genetic testing laboratory. The analysis detects the presence or absence of the GYS1 mutation, and results are definitive.
For suspected RER or PSSM2, a muscle biopsy is the diagnostic standard. Samples are taken from the hamstring muscles (the semimembranosus or semitendinosus), then stained and examined under a microscope. A trained specialist can distinguish the characteristic tissue changes of RER from normal muscle and from the glycogen accumulation seen in PSSM. Since PSSM2 lacks a reliable genetic marker, biopsy is currently the only confirmed way to diagnose it.
What to Do During an Episode
When a horse shows signs of tying up during exercise, stop all activity immediately. Do not force the horse to walk back to the barn if it is resisting movement, as continued muscle contraction can worsen the damage. If the horse is sweating heavily, gently rub it dry and blanket it if the weather is cold. Offer water, and call your veterinarian. In severe cases, a horse may need intravenous fluids to flush myoglobin through the kidneys before it can cause lasting damage.
Most mild episodes resolve within hours to a couple of days with rest, fluids, and anti-inflammatory support. Severe episodes with dark urine or a horse that refuses to stand may take weeks of recovery and carry a risk of permanent kidney or muscle damage.
Long-Term Management Through Diet and Exercise
For horses with PSSM, dietary changes are the cornerstone of management. The goal is to reduce the sugar and starch the muscles have to process while providing alternative energy from fat. Current recommendations call for less than 20% of total digestible energy from non-structural carbohydrates (sugars and starches) and 15 to 20% of energy from fat. In practice, this means replacing sweet feeds and grain-heavy diets with low-starch feeds, adding a fat source like rice bran or oil, and providing plenty of forage. Many PSSM horses improve dramatically on diet change alone.
Consistent daily exercise is equally important. Horses with PSSM and RER both do worse with irregular schedules, especially the pattern of several days off followed by hard work. Turnout as much as possible, ideally 24/7, helps keep muscles active at a low level. When stall rest is unavoidable, reducing the grain ration on rest days is a simple but effective precaution.
For RER horses, management also focuses on reducing stress and maintaining calm training environments. Since nervous temperament is a risk factor, anything that lowers anxiety, from consistent routines to compatible turnout companions, can reduce the frequency of episodes. Some RER horses benefit from specific supplements targeting electrolyte balance, though the right approach depends on the individual horse and should be guided by bloodwork.