Tying up — the common term for exertional rhabdomyolysis (ER) — is a painful, sometimes dangerous condition where a horse’s muscles break down during or shortly after exercise. The horse becomes suddenly distressed, reluctant to move, and in severe cases cannot walk at all. It can look like colic from the outside, but the pain is in the muscles, not the gut. Knowing how to recognize it and what to do in the first minutes can make a significant difference in outcome.
What Is Tying Up?
Exertional rhabdomyolysis is the breakdown (lysis) of skeletal muscle tissue during or after exercise. When muscle fibers break down, they release myoglobin (the oxygen-carrying protein in muscle) into the bloodstream. Myoglobin is filtered through the kidneys, turning urine dark brown or red — a hallmark sign called myoglobinuria. In severe cases, the myoglobin load can damage the kidneys (acute renal failure), making prompt fluid therapy critical.
Tying up ranges from mild (post-exercise muscle soreness and stiffness that resolves with rest and anti-inflammatories) to life-threatening (complete inability to move, severe muscle swelling, kidney failure).
Signs and Symptoms
An episode typically begins during or immediately after exercise — often within minutes of stopping. Classic signs:
- Sudden reluctance or inability to move — the horse plants its feet and refuses to go forward
- Hard, cramped, painful hindquarter muscles — the gluteals and epaxial muscles (back) are often rock-hard and tender to the touch
- Sweating — disproportionate to the exercise intensity
- Rapid breathing and elevated heart rate
- Pain signs: pawing, looking at flanks (can mimic colic), anxious expression
- Dark urine — brown or coffee-colored urine is a serious sign of myoglobinuria; call the vet urgently
- Muscle trembling or shaking
- In severe cases: recumbency (horse goes down and cannot rise)
Types of Tying Up: Sporadic vs. Chronic/Recurrent
Two broad categories have different causes, management strategies, and prognoses:
Sporadic (Exertional) Rhabdomyolysis
Occurs in otherwise healthy horses as a one-off event, usually triggered by identifiable causes:
- Exercise beyond conditioning level: asking a horse to work harder or longer than its fitness level allows
- High-grain diet with reduced exercise (“Monday morning disease”): rest days with full grain rations cause glycogen accumulation; return to work triggers ER. Classic in working horses rested over a weekend on full feed
- Electrolyte imbalances: severe deficiencies in sodium, potassium, calcium, or magnesium impair muscle function
- Vitamin E and selenium deficiency: these antioxidants protect muscle membranes; deficiency increases ER susceptibility. Selenium-deficient soils are common in many regions
- Concurrent illness or fever: respiratory infection + exercise is a documented trigger
- Heat and humidity: exercise in high heat/humidity stress increases ER risk, especially with inadequate electrolyte replacement
Chronic (Recurrent) Exertional Rhabdomyolysis
Horses that tie up repeatedly despite appropriate management have an underlying muscle disorder. Two well-characterized genetic conditions:
Polysaccharide Storage Myopathy (PSSM / EPSM)
PSSM (also called EPSM — Equine Polysaccharide Storage Myopathy) is a genetic disorder of glycogen metabolism. The muscle accumulates abnormal polysaccharide, leading to ER with even modest exercise, and often also to muscle atrophy and poor topline. Two types:
- PSSM Type 1: caused by a mutation in the GYS1 gene (glycogen synthase); affects draft breeds, warmbloods, Quarter Horses, and many related breeds. Highly prevalent: 36% of draft horses in some studies (McCue et al., J Vet Intern Med, 2008). Diagnosed by genetic test (hair or blood) or muscle biopsy with PAS staining showing abnormal polysaccharide accumulation
- PSSM Type 2: muscle biopsy shows abnormal glycogen without the GYS1 mutation; genetic basis less well characterized. Diagnosed by biopsy only
Management of PSSM: low-starch, high-fat diet (dramatically reduce grain/starch; replace with fat such as rice bran, vegetable oil, or fat-supplemented feeds); daily turnout and regular exercise — consistent daily movement is as important as diet. Many PSSM horses become comfortable and workable with diet + management changes alone.
Recurrent Exertional Rhabdomyolysis (RER)
RER is primarily a disorder of intracellular calcium regulation in muscle, causing irregular muscle contractions. Predominantly affects Thoroughbreds, Standardbreds, and Arabian breeds. Mares are more commonly affected than geldings. Triggers include exercise intensity, excitement, and dietary starch. Confirmed by muscle biopsy (shows normal glycogen; distinguishes from PSSM). Management: controlled exercise routine; reduce starch in diet; in mares with cycle-linked episodes, progesterone supplementation or dantrolene sodium (a calcium channel stabilizer) before exercise may help. Work with a veterinary internist for recurrent cases.
Emergency: What to Do During an Episode
If your horse ties up during exercise:
- Stop immediately. Do not walk the horse forward to “walk it out” — this is the opposite of what’s needed. Forced movement with muscle breakdown actively increases muscle damage and myoglobin release
- Keep the horse still and calm. Stand with the horse, keep it quiet
- Call the vet. Any episode involving dark urine, recumbency, or severe distress is a veterinary emergency. Mild episodes warrant a same-day call; anything severe warrants an immediate call
- Keep the horse warm. Apply blankets to the hindquarters in cold weather — cold worsens muscle cramping. Do not hose with cold water
- Do not administer NSAIDs (bute, banamine) without veterinary guidance. NSAIDs are sometimes used for pain, but in severe rhabdomyolysis with myoglobinuria they can worsen renal damage by reducing renal blood flow. Your vet will decide this
- Do not feed grain. Water is fine if the horse will drink
- Transport carefully if needed: if the vet recommends transport to a clinic, do so in a well-bedded trailer; minimize further muscle stress
Veterinary Treatment
The vet’s priorities are:
- Blood work: CK (creatine kinase) and AST (aspartate aminotransferase) are the key muscle enzymes; peak CK >50,000 IU/L indicates severe rhabdomyolysis. BUN and creatinine assess kidney function
- Urinalysis: confirm myoglobinuria
- IV fluids: the primary treatment for moderate-to-severe ER. High fluid volumes flush myoglobin through the kidneys before it causes tubular damage
- Pain management: butorphanol or flunixin at low doses if the horse is in severe pain; conservative dosing to protect kidneys
- Muscle relaxants: acepromazine (a phenothiazine) is sometimes used — it reduces anxiety and has mild vasodilatory effects that may improve muscle circulation; use is controlled
- Selenium/vitamin E supplementation if deficiency is suspected
- Electrolyte correction: IV electrolytes as needed
Diagnosis of the Underlying Cause
After stabilization, workup for recurrent cases includes:
- Genetic testing for GYS1 mutation (PSSM Type 1) — done from mane hair or blood; widely available through veterinary and commercial labs
- Muscle biopsy — the definitive diagnostic for PSSM Type 2, RER, and other myopathies; taken from the semimembranosus or gluteus medius under sedation
- Selenium and vitamin E serum levels
- Diet history: starch and sugar content of current ration
- Thyroid panel: hypothyroidism is a rare contributor
Prevention
For Sporadic ER (no underlying condition)
- Reduce grain on rest days: cut concentrate rations by at least half on non-work days; eliminate them entirely on unexpected days off
- Regular conditioning: fit horses tied up far less often; build fitness gradually and maintain it consistently
- Electrolyte supplementation: provide electrolytes in feed or water year-round for horses in regular work; increase during heat, competition, or heavy sweating
- Vitamin E and selenium: have blood levels checked; supplement if deficient (selenium supplementation must be precise — toxicity is possible; do not exceed safe ranges without testing)
- Adequate warm-up: proper walk/trot warm-up before demanding work, especially in cold weather
For PSSM
- Low-starch, high-fat diet (target <10% non-structural carbohydrates in total diet)
- Daily turnout — no long stall rest periods
- Gradual return to exercise after any rest period
- Avoid alfalfa (higher in sugar than grass hay for some horses); test hay NSC if possible
For RER
- Consistent daily exercise routine — irregular schedules worsen RER
- Reduce grain/starch; replace calories with fat
- Reduce pre-exercise excitement; quiet, calm handling
- Dantrolene sodium (given orally 1–1.5 hours before exercise) may reduce ER episodes in confirmed RER horses — veterinary prescription
When to Call the Vet
- Immediately: horse is recumbent (down and unable to rise), has dark brown/red urine, or is in severe distress
- Same day: any episode of muscle cramping/stiffness after exercise, even if mild
- Scheduled visit: horse has tied up more than once; needs workup for underlying myopathy
Tying up is a genuine emergency when severe. Do not delay calling your veterinarian. This article is for educational purposes and does not constitute veterinary advice.
For guidance on recognizing signs of pain that owners often miss, see Signs of Pain That Owners Miss. For information on metabolic conditions related to muscle health, see our coverage of Equine Metabolic Syndrome. Quick equine health definitions at horse-info.org.
