Muscle Disorders in Athletic Horses: Tying Up, Genetics and Management

By Dr Duncan Houston

“Tying up” is one of the most familiar terms in performance horse medicine, but it is also one of the most misunderstood.

A horse that ties up may become stiff, painful, sweaty, reluctant to move, or locked through the back and hindquarters during or after exercise. Bloodwork may show muscle enzyme leakage, and severe cases can cause dark urine, kidney stress, collapse, or recumbency.

The important point is this: tying up is a clinical sign, not one single disease.

Some horses tie up once because they were overworked, dehydrated, heat stressed, or exercised hard after time off. Others have inherited muscle disorders that require lifelong diet, exercise, breeding and management decisions. In 2025, genetic testing has made some diagnoses much clearer, but not every condition has a reliable DNA test. That distinction matters.

The goal is not to guess from breed, temperament, or one episode. The goal is to identify which muscle disorder your horse actually has, then manage that specific condition properly.

Quick Answer

Tying up, or exertional rhabdomyolysis, means painful muscle damage associated with exercise. It can be sporadic, triggered by workload, heat, dehydration or feed mismatch, or recurrent because of conditions such as PSSM1, PSSM2, recurrent exertional rhabdomyolysis, malignant hyperthermia or other inherited muscle disorders. PSSM1, HYPP, GBED, MYHM and malignant hyperthermia have genetic tests, but PSSM2 and myofibrillar myopathy do not currently have validated genetic tests and may require muscle biopsy or specialist evaluation. A horse that is stiff, sweating, reluctant to move, weak, recumbent, or passing dark urine should be treated as urgent. (Merck Veterinary Manual)

What Does “Tying Up” Actually Mean?

Tying up is the common name for exertional rhabdomyolysis, which means muscle cells are damaged during or after exercise.

When muscle fibres are damaged, they leak enzymes and pigments into the bloodstream. The main blood markers vets look at are:

Marker	Why it matters
CK	Rises quickly after muscle damage and helps assess recent injury
AST	Rises more slowly and can stay elevated longer
Myoglobin	Muscle pigment that can darken urine and stress the kidneys
Electrolytes	Abnormalities can worsen weakness, cramping or collapse

Common signs include:

• Stiffness during or after exercise

• Painful, firm back, loin, croup or hindquarter muscles

• Sweating

• Trembling

• Reluctance to move

• Short, stiff steps

• Anxiety or distress

• Increased heart rate

• Dark red, brown or coffee-coloured urine in severe cases

• Collapse or recumbency in extreme cases

Merck Veterinary Manual describes exertional rhabdomyolysis as muscle pain, stiffness, sweating, reluctance to move and increased muscle enzymes, with severe cases potentially causing myoglobinuria and renal compromise. (Merck Veterinary Manual)

When Is This an Emergency?

Tying up becomes an emergency when the horse is painful, weak, unable to move normally, or showing signs of severe muscle breakdown.

Call your vet immediately if your horse has:

• Sudden severe stiffness

• Refusal to move

• Heavy sweating

• Trembling or muscle fasciculations

• Painful hard muscles over the back, loin or hindquarters

• Dark brown, red or coffee-coloured urine

• Weakness or collapse

• Recumbency

• Rapid breathing

• Very high heart rate

• Signs after heat stress, dehydration or heavy exercise

• Repeated tying-up episodes

• Any episode in a horse with known PSSM, RER, HYPP, MYHM or malignant hyperthermia risk

Do not force the horse to keep walking to “loosen up.” During a true tying-up episode, more movement can worsen muscle damage. Severe rhabdomyolysis can stress the kidneys, especially if the horse is dehydrated. (Merck Veterinary Manual)

Not Every Tying-Up Horse Has the Same Disease

This is the part that matters clinically.

“Tying up” can come from several different problems:

Condition	Typical horse type	Genetic test?	Main pattern
Sporadic exertional rhabdomyolysis	Any horse	No	One-off episode after overwork, heat, dehydration or feed mismatch
PSSM1	Quarter Horse-related breeds, drafts, some Warmbloods and others	Yes	Glycogen storage disorder, often triggered by rest plus starch
PSSM2	Light breeds, Warmbloods, Arabians, Morgans, Thoroughbreds, some Quarter Horses	No validated DNA test	Muscle pain, stiffness, poor performance, biopsy-based diagnosis
RER	Thoroughbreds, Standardbreds, Arabians, some sport horses	No routine genetic test	Fit, nervous horses with recurrent episodes linked to stress and training
HYPP	Quarter Horse-related breeds, Impressive bloodlines	Yes	Tremors, weakness, collapse, potassium-sensitive episodes
GBED	Quarter Horse-related breeds	Yes	Fatal foal disease, breeding concern
MYHM	Quarter Horse-related breeds	Yes	Immune-mediated muscle loss or non-exertional rhabdomyolysis
Malignant hyperthermia	Quarter Horse-related breeds, rare but serious	Yes	Hypermetabolic crisis with anaesthesia or severe stress

The mistake is treating all of these with the same diet, same supplement and same exercise plan. That is like treating every cough as asthma. Occasionally lucky, often wrong.

Sporadic Tying Up

Sporadic tying up usually happens when a horse without a chronic muscle disorder has a temporary trigger.

Common triggers include:

• Sudden hard work

• Exercise after time off

• Heat and humidity

• Dehydration

• Electrolyte loss

• Exhaustion

• Overtraining

• Viral illness or systemic stress

• High-energy feeding during reduced work

Most sporadic cases respond to rest, veterinary care, gradual return to work and correction of the trigger. Merck notes that sporadic cases usually have a temporary inciting cause and often respond to rest, gradual training increase and dietary adjustment. (Merck Veterinary Manual)

In practice, this is the horse that was fed like an athlete, rested like a couch ornament, then asked to perform like an Olympic finalist. Biology objects. Fair enough.

PSSM1: Polysaccharide Storage Myopathy Type 1

PSSM1 is a glycogen storage disease caused by a mutation in the GYS1 gene. Affected horses accumulate abnormal glycogen in muscle, which can lead to muscle pain, weakness, sweating, skin twitching, stiffness and reluctance to move. It is autosomal dominant, meaning one copy of the variant can be enough for a horse to be affected. (Veterinary Genetics Laboratory)

PSSM1 is seen most often in:

• Quarter Horses

• Paints

• Appaloosas

• Draft breeds

• Some Warmbloods

• Morgans and other breeds

Clinical signs can include:

• Tying up after light exercise

• Reluctance to move forward

• Sweating

• Muscle tremors

• Hindlimb stiffness

• Gait asymmetry

• Poor performance

• Stiffness after rest days

• In some draft horses, weakness or muscle loss

UC Davis notes that PSSM1 is caused by a known mutation and that most affected horses can be managed successfully through diet and exercise, although there is no cure. (Center for Equine Health)

Management of PSSM1

PSSM1 management usually focuses on:

• Low starch and sugar intake

• Forage-based diet

• Calories controlled to avoid obesity

• Fat added only when extra energy is needed

• Daily turnout

• Regular, consistent exercise

• Avoiding long rest periods

• Gradual return after episodes

• Vitamin and mineral balance

Merck notes that PSSM horses improve with dietary change plus gradual increases in daily exercise and turnout. (Merck Veterinary Manual)

The key line: PSSM1 is managed in the feed bin and the exercise schedule.

PSSM2 and Myofibrillar Myopathy

PSSM2 is more complicated.

PSSM2 is used for horses that have muscle disease and abnormal muscle biopsy findings but do not have the GYS1 mutation that causes PSSM1. UC Davis states that PSSM2 may have multiple causes and that there are currently no scientifically verified DNA tests for PSSM2. Muscle biopsy can be used for diagnosis. (Center for Equine Health)

Myofibrillar myopathy, or MFM, is a related but distinct muscle disorder described especially in some Warmbloods and Arabians. It may cause:

• Poor performance

• Stiffness

• Exercise intolerance

• Subtle muscle pain

• Reluctance to collect or engage

• Intermittent tying-up-like signs

• Normal or only mildly increased CK in some cases

This is where owners need to be careful. Commercial genetic panels have been marketed for PSSM2 and MFM, but peer-reviewed work has not validated those tests. A 2025 review states that there are currently no validated genetic tests for PSSM2 or MFM and that muscle biopsies are still required for diagnosis. (eScholarship)

So if someone tells you a horse “has PSSM2” based only on a commercial hair test, slow down. The result may not mean what the marketing says it means.

Recurrent Exertional Rhabdomyolysis

Recurrent exertional rhabdomyolysis, or RER, is a repeated tying-up syndrome often seen in fit, nervous or excitable performance horses.

It is classically associated with:

• Thoroughbreds

• Standardbreds

• Arabians

• Some racing or high-performance horses

University of Minnesota describes RER as a clinical syndrome involving painful muscle contractures with exercise and skeletal muscle fibre necrosis, and notes that 5% to 10% of Thoroughbreds may develop exertional rhabdomyolysis during a racing season. (College of Veterinary Medicine)

RER is often triggered by:

• Excitement

• Stress

• High-grain feeding

• Stall confinement

• Irregular exercise

• Sudden routine changes

• Training stress

• Nervous temperament

Unlike PSSM1, there is no simple commercial genetic test that confirms RER in routine practice. Research supports a heritable component, but environmental triggers strongly influence whether signs appear. (eScholarship)

Management of RER

RER management usually focuses on:

• Reducing starch

• Using fat as an alternative energy source where calories are needed

• Maintaining enough calories for athletic work

• Consistent daily exercise

• Reducing stress and excitement

• Regular turnout where practical

• Avoiding long stall rest

• Keeping feeding and training routines predictable

A high-performance RER horse may still need a lot of calories. The goal is not simply “less feed.” The goal is safer energy and calmer routine.

HYPP: Hyperkalemic Periodic Paralysis

HYPP is not classic tying up, but it is one of the most important inherited muscle disorders in Quarter Horse-related breeds.

HYPP is caused by a mutation in the SCN4A sodium channel gene. It is primarily found in descendants of the Quarter Horse sire Impressive. Affected horses can have episodes of muscle tremors, weakness, collapse and sometimes loud breathing from upper airway muscle paralysis. Severe attacks can cause sudden death. (Veterinary Genetics Laboratory)

HYPP signs include:

• Muscle twitching

• Trembling

• Weakness

• Collapse

• Noisy breathing

• Episodes linked to stress, fasting or high-potassium feed

• Sudden death in severe cases

UC Davis reports HYPP as autosomal dominant. Horses with one copy can show signs and pass the allele to 50% of offspring, while horses with two copies are usually more severely affected. (Veterinary Genetics Laboratory)

Management of HYPP

HYPP management is different from PSSM or RER.

It focuses on:

• Genetic testing

• Low-potassium diet where needed

• Avoiding high-potassium feeds such as alfalfa in susceptible horses

• Avoiding fasting followed by large meals

• Small frequent meals

• Consistent routine

• Stress reduction

• Medication in some cases

• Breeding decisions to avoid passing the variant on

Do not manage HYPP like PSSM. HYPP is a potassium and muscle membrane excitability problem, not a glycogen storage disorder.

GBED: Glycogen Branching Enzyme Deficiency

GBED is not usually a performance horse tying-up diagnosis. It is a fatal foal disease and a major breeding concern in Quarter Horse-related breeds.

Affected foals cannot correctly store glycogen in several organs, including skeletal muscle, heart, liver and brain. UC Davis describes GBED as a fatal disease of developing fetuses or newborn foals and an autosomal recessive disorder in Quarter Horses and related breeds. A genetic test is available to avoid producing affected foals. (Center for Equine Health)

GBED may cause:

• Late-term abortion

• Stillbirth

• Weak foals

• Low muscle tone

• Hypothermia

• Limb deformities

• Seizures

• Cardiac arrest

• Respiratory failure

• Death or euthanasia in affected foals

UC Davis VGL explains that carrier-to-carrier matings have a 25% chance of producing an affected foal. (Veterinary Genetics Laboratory)

The practical point: GBED is prevented through breeding decisions, not managed as an adult performance condition.

MYHM: Myosin-Heavy Chain Myopathy

MYHM is another Quarter Horse-related muscle disease that owners should know about, especially because it can look dramatic and confusing.

UC Davis describes MYHM as a muscle disease in Quarter Horses and related breeds with two clinical presentations:

• Immune-mediated myositis, causing severe muscle atrophy after an immune trigger

• Non-exertional rhabdomyolysis, causing severe muscle pain and damage not associated with exercise (Veterinary Genetics Laboratory)

Signs may include:

• Sudden muscle loss

• Stiffness

• Weakness

• Fever or reduced appetite in some cases

• Dark urine

• Recumbency

• Very high muscle enzymes

• Episodes not clearly linked to exercise

A genetic test is available for the MYH1 variant associated with MYHM. Horses with two copies may be more severely affected, but penetrance is variable, which means not every horse with the variant shows the same severity. (Veterinary Genetics Laboratory)

This is one of the reasons “muscle disease” in Quarter Horses should not be reduced to just PSSM and HYPP.

Malignant Hyperthermia

Malignant hyperthermia, or MH, is rare but important.

It is associated with a mutation in the RYR1 gene and can cause a severe hypermetabolic crisis, especially with anaesthesia or major stress. UC Davis states that the RYR1 mutation can cause dangerously high spikes in body temperature and muscle breakdown in response to anaesthesia or stress and can be fatal. (Veterinary Genetics Laboratory)

Signs may include:

• Dangerous fever

• Muscle rigidity

• Rapid heart rate

• Rapid breathing

• Severe muscle breakdown

• Acidosis

• Collapse

• Death in severe cases

MH is not the most common cause of tying up, but it matters for Quarter Horse-related breeds and anaesthetic planning. If a horse has an MH variant, the vet needs to know before surgery, sedation or anaesthesia.

How Worried Should You Be?

Low Concern

This is more likely when:

• The horse had one mild episode after unusually hard work.

• Signs resolved quickly.

• There was no dark urine.

• The horse is bright, eating and moving comfortably now.

• Bloodwork showed only mild enzyme elevation.

• A clear trigger such as heat, dehydration or overwork was present.

Action: speak with your vet, reduce work temporarily, correct hydration and electrolytes, review feeding on rest days and return to exercise gradually.

Moderate Concern

This is more likely when:

• The horse has tied up more than once.

• Stiffness happens after normal work.

• The horse is on a high-starch diet.

• The horse is an easy keeper, stock breed, draft or Warmblood.

• The horse is a nervous Thoroughbred, Standardbred or Arabian in training.

• Episodes follow rest days.

• Muscle enzymes are clearly elevated.

Action: book a veterinary workup. Genetic testing, diet review, CK and AST monitoring, exercise review and possible muscle biopsy may be needed.

High Concern

This is more likely when:

• Episodes are frequent or worsening.

• The horse ties up with light exercise.

• Dark urine is present.

• The horse becomes very painful or reluctant to move.

• The horse has weakness, collapse or recumbency.

• Kidney values are abnormal.

• PSSM, RER, HYPP, MYHM or MH is suspected.

Action: stop training and get a diagnosis before continuing. This is no longer a simple feed tweak.

Critical

Treat this as an emergency if:

• The horse cannot move normally.

• The horse is recumbent.

• The horse has dark urine.

• Severe sweating, weakness or collapse occurs.

• Breathing is abnormal.

• The horse is dehydrated, overheated or systemically unwell.

• HYPP signs include collapse or noisy breathing.

• MH is suspected after anaesthesia or severe stress.

• A foal is weak, seizuring, hypothermic or unable to stand.

Action: call your vet immediately. This horse may need fluids, pain control, electrolyte correction, kidney monitoring, genetic testing or referral care.

How Vets Diagnose Muscle Disorders

Diagnosis usually depends on the pattern.

Your vet may recommend:

• Full history

• Exercise and rest-day review

• Diet and forage review

• Blood CK and AST

• Kidney values

• Electrolytes

• Urinalysis for myoglobin

• Genetic testing for PSSM1, HYPP, GBED, MYHM or MH where appropriate

• Muscle biopsy for PSSM2, MFM or unclear cases

• Vitamin E and selenium assessment in selected horses

• Evaluation for lameness, back pain or other causes of poor performance

UC Davis states that PSSM1 has a DNA test, while PSSM2 does not currently have a scientifically verified DNA test and may require muscle biopsy. University of Minnesota similarly notes that muscle biopsies are required to test for PSSM2 when horses with muscle pain test negative for PSSM1. (Center for Equine Health)

The diagnosis matters because a HYPP horse, a PSSM1 horse, a nervous RER Thoroughbred and a MYHM horse do not need the same plan.

What To Do During a Tying-Up Episode

If your horse starts tying up:

1. Stop exercise immediately.

2. Call your vet.

3. Keep the horse calm.

4. Do not force walking.

5. Offer water if the horse is calm and able to drink.

6. Keep the horse warm if sweaty in cold weather.

7. Provide shade and cooling if heat stress is possible.

8. Do not give extra grain.

9. Do not give random supplements.

10. Monitor urine colour.

11. Wait for veterinary advice before giving medication.

Your vet may need to check muscle enzymes, hydration, electrolytes, kidney values and urine. NSAIDs may be used in some cases, but dehydration and kidney risk matter, so do not medicate blindly.

Long-Term Management by Condition

PSSM1

Best managed with:

• Low-starch diet

• Controlled calories

• Fat only if extra energy is needed

• Daily turnout

• Regular exercise

• Avoiding prolonged rest

• Genetic testing and breeding decisions

PSSM2 and MFM

Best managed with:

• Specialist diagnosis, often muscle biopsy

• Avoiding overinterpretation of unvalidated commercial DNA tests

• Diet and exercise plan based on clinical signs

• Rehab and fitness management

• Vitamin E assessment where relevant

• Rechecking for other causes of poor performance

RER

Best managed with:

• Lower-starch, higher-fat performance diet when calories are needed

• Stress reduction

• Consistent routine

• Regular exercise

• Turnout where practical

• Avoiding high-grain, high-excitement management

HYPP

Best managed with:

• Genetic testing

• Low-potassium diet in affected horses

• Avoiding fasting

• Small frequent meals

• Avoiding sudden feed changes

• Stress reduction

• Medication in selected cases

• Avoiding breeding affected horses

GBED

Best managed with:

• Genetic testing before breeding

• Avoiding carrier-to-carrier matings

• Testing Quarter Horse-related breeding stock

MYHM

Best managed with:

• Genetic testing

• Identifying immune or stress triggers where possible

• Prompt treatment during episodes

• Monitoring for muscle loss, dark urine and weakness

• Breeding advice

Malignant Hyperthermia

Best managed with:

• Genetic testing in at-risk horses

• Alerting vets before anaesthesia

• Avoiding known triggering anaesthetic plans where possible

• Emergency preparedness for hyperthermia and muscle breakdown

Common Mistakes Owners Make

Calling Every Episode PSSM

PSSM is common in some breeds, but RER, sporadic tying up, HYPP, MYHM, MH, heat illness, electrolyte loss and other diseases can all cause muscle signs.

Trusting Unvalidated Genetic Tests

PSSM1, HYPP, GBED, MYHM and MH have recognised genetic tests. PSSM2 and MFM do not currently have validated DNA tests. That distinction matters.

Feeding the Same Diet to Every Tying-Up Horse

A fat PSSM horse and a fit RER Thoroughbred do not need the same calorie plan.

Giving Full Grain on Rest Days

Rest plus full starch is a classic way to trigger trouble in susceptible horses.

Overusing Supplements

Vitamin E, selenium, magnesium and electrolytes may help when indicated, but random stacking can cause harm.

Ignoring Dark Urine

Dark urine can mean serious muscle breakdown and kidney risk.

Breeding Without Testing

GBED, HYPP, PSSM1, MYHM and MH all have breeding implications. Testing helps prevent avoidable disease.

Prevention

You cannot prevent every muscle disorder, but you can reduce risk and severity.

Practical steps include:

• Match feed to actual workload

• Reduce starch in tying-up-prone horses

• Avoid full grain on rest days

• Maintain regular exercise

• Provide turnout where practical

• Use low-starch diets for PSSM horses

• Use higher-fat, lower-starch performance feeds for RER horses when calories are needed

• Maintain hydration and salt intake

• Use electrolytes for heavy sweating

• Test or calculate selenium before supplementing

• Consider vitamin E testing in horses with limited pasture

• Avoid long periods of stall rest in PSSM horses once comfortable

• Reduce stress in RER horses

• Test at-risk breeding stock

• Tell your vet about HYPP or MH status before sedation or anaesthesia

• Recheck CK and AST after significant episodes

The best prevention is boring consistency. Muscles love boring consistency. Horses love making that difficult. Naturally.

Myth vs Reality

Myth	Reality
Tying up is one disease.	It is a clinical sign with several possible causes.
All tying-up horses need high-fat diets.	Some do, especially PSSM and some RER horses, but calories must match body condition and workload.
PSSM2 has a reliable hair test.	There are currently no validated genetic tests for PSSM2 or MFM.
HYPP is just tying up.	HYPP is a potassium-sensitive sodium channel disorder that can cause tremors, weakness, collapse and sudden death.
GBED affects adult performance horses.	GBED is a fatal fetal or neonatal disease and mainly a breeding concern.
Supplements can replace diagnosis.	Supplements may support management, but diagnosis decides the plan.

FAQs About Muscle Disorders in Horses

What is the difference between tying up and PSSM?

Tying up is the clinical episode of muscle pain and breakdown. PSSM is one possible cause of recurrent tying up, especially in Quarter Horse-related breeds, drafts and some Warmbloods. PSSM1 has a genetic test, while PSSM2 does not currently have a validated DNA test. (Center for Equine Health)

Can a horse have tying up without a genetic disease?

Yes. Sporadic tying up can occur after overexertion, heat stress, dehydration, electrolyte loss, sudden workload change or feeding too much energy during reduced work.

What genetic tests should Quarter Horse-related breeds consider?

Depending on the horse and breeding plans, testing may include PSSM1, HYPP, GBED, MYHM, MH, HERDA and other breed-relevant conditions. UC Davis offers a Quarter Horse and related breeds disease panel that includes several of these tests. (Veterinary Genetics Laboratory)

Is PSSM2 diagnosed by hair testing?

Not reliably. Current peer-reviewed sources state that there are no validated genetic tests for PSSM2 or MFM, and muscle biopsy remains important for diagnosis in suitable cases. (eScholarship)

When should I call a vet for muscle stiffness?

Call immediately if the horse is painful, sweating, reluctant to move, weak, recumbent, breathing abnormally or passing dark urine. Recurrent mild episodes also deserve a veterinary workup because the underlying cause changes the management plan.

The Bottom Line

Muscle disorders in athletic horses are much more complex than one phrase like “tying up” can capture.

A horse may tie up once because of heat, dehydration, overwork or a feed mismatch. Another may have PSSM1. Another may have RER. A Quarter Horse may have HYPP, MYHM, MH or a breeding-related concern such as GBED. A Warmblood or Arabian with vague stiffness and poor performance may need investigation for PSSM2 or MFM, but not with an unvalidated shortcut test.

The safest approach is:

• Treat active painful episodes seriously.

• Check CK, AST, kidney values and urine when needed.

• Use genetic testing where it is valid.

• Do not overinterpret unvalidated tests.

• Match diet to the actual diagnosis.

• Keep exercise consistent.

• Adjust feed on rest days.

• Manage stress, hydration and electrolytes.

• Make breeding decisions based on real genetic risk.

The horse that ties up is not just “crampy.” The muscles are telling you there is a mismatch between genetics, diet, exercise, stress, hydration or disease.

The job is to find the mismatch and fix the system.

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If your horse has tied up, shows recurrent muscle stiffness, has confusing genetic test results, or you are unsure whether PSSM, RER, HYPP, MYHM, GBED or another muscle disorder is involved, ASK A VET™ can help you organise the signs, bloodwork, diet history, genetic results and the right questions to discuss with your treating veterinarian.