Muscle Disorders & Muscle Health
Understanding your horse's muscles
Healthy muscles are essential for much more than athletic performance. They help your horse move, maintain posture, stabilise joints, support the back and carry a rider comfortably.
Muscle problems can range from relatively simple soreness or loss of condition to specific disorders affecting the way muscle cells produce and use energy.
You may notice that your horse feels different before there is anything obviously wrong. They may seem stiff, tire more quickly, lose muscle over the back or hindquarters, become reluctant to work, or simply not move as freely as usual.
Some muscle disorders can be painful and potentially serious, so a significant or sudden change should not simply be assumed to be a fitness or conditioning problem.
Terms you may hear from your vet or practitioner
Muscle conditions are described using several different terms, some of which can sound more complicated than they really are.
Myopathy
Myopathy simply means a disease or disorder affecting muscle.
It is a broad term rather than one particular diagnosis. If your vet describes a horse as having a myopathy, further investigation may be needed to determine what type of muscle problem is involved.
Tying-up
Tying-up is the familiar name for episodes in which muscles become painful, tight and damaged, often in association with exercise.
Your vet may call this exertional rhabdomyolysis.
"Rhabdomyolysis" means that muscle cells have been damaged and their contents are being released into the bloodstream.
An affected horse may become stiff and reluctant to move, sweat excessively, appear distressed or develop hard, painful muscles, particularly around the hindquarters.
This is not simply normal stiffness after exercise.
If you suspect that a horse is tying-up, stop exercise and contact your vet. Do not force the horse to keep walking.
Azoturia
You may still hear people use the older term azoturia for tying-up.
Historically it has also been called Monday morning disease, particularly in working horses that were rested while continuing to receive a high-energy ration.
We now know that several different conditions can cause episodes of exertional muscle damage, so "tying-up" describes what is happening rather than necessarily identifying the underlying cause.
PSSM
Polysaccharide Storage Myopathy (PSSM) is a muscle disorder involving the way energy is stored or used within muscle.
Owners may hear terms such as PSSM1 and PSSM2. These should not automatically be treated as interchangeable diagnoses.
PSSM1 is associated with a recognised genetic variant and can be tested for. The terminology surrounding other suspected muscle disorders has changed as veterinary understanding has developed, so interpretation of testing and diagnosis should be discussed with your vet.
Affected horses can show signs including stiffness, muscle pain, exercise intolerance or episodes of tying-up, although signs vary considerably between horses.
RER
Recurrent Exertional Rhabdomyolysis (RER) means repeated episodes of muscle damage associated with exercise.
It is particularly recognised in some performance horses.
Management can involve exercise routine, diet, turnout and reducing factors that contribute to excitement or stress, but an individual management programme should be developed with your vet and nutrition professional.
Muscle atrophy
Muscle atrophy simply means loss or wasting of muscle.
It describes what has happened rather than telling us why.
Muscle can be lost because a horse is not using part of its body normally, because of pain, reduced exercise, ageing, poor nutrition, nerve problems or an underlying medical condition.
Visible muscle loss therefore deserves investigation rather than simply trying to "build the muscle back up".
What might an owner notice?
Muscle problems do not always produce dramatic symptoms.
You might notice:
- unusual stiffness, particularly during or after exercise
- reluctance to move forward or exercise
- muscles that feel unusually firm, tight or painful
- excessive sweating for the amount of work being performed
- trembling or muscle fasciculations — small involuntary muscle movements
- reduced stamina or exercise tolerance
- difficulty engaging the hindquarters
- deterioration in performance
- reluctance to go up or down hills
- difficulty maintaining a normal posture
- loss of muscle over the topline, back or hindquarters
- uneven muscle development
- difficulty maintaining muscle despite apparently appropriate work and nutrition
- a change in behaviour when being groomed, saddled or worked.
None of these signs identifies a particular muscle disorder on its own. Similar changes can result from joint pain, back problems, lameness, neurological conditions, poor saddle fit, dental problems, inappropriate training and other health conditions.
Muscle loss is not always a muscle disease
This distinction is particularly important.
If a horse loses muscle over its back or hindquarters, the muscle itself may not be the original problem.
Pain somewhere else can change the way the horse moves and loads its body. If a muscle or group of muscles is used less, it can gradually become smaller and weaker.
For example, joint discomfort, foot pain or back pain may change movement sufficiently for secondary muscle loss to develop.
This is why simply feeding more protein or increasing exercise is not necessarily the answer to unexplained muscle loss.
The reason the horse is losing muscle needs to be considered first.
Muscle health, topline and conditioning
Building healthy muscle requires several things to work together.
Appropriate exercise
Muscle develops in response to appropriate work. Exercise needs to be progressive enough to stimulate adaptation while allowing sufficient recovery.
Trying to rebuild muscle too quickly can result in fatigue, soreness or injury.
For a horse returning from injury, illness or a prolonged period of rest, a structured rehabilitation programme may be more appropriate than simply increasing workload.
Adequate nutrition
Muscle cannot be built without the nutrients required to make it.
The overall diet needs to provide appropriate energy and sufficient good-quality protein containing essential amino acids.
Simply feeding large quantities of protein does not automatically produce more muscle.
The horse also needs an appropriate exercise stimulus and must be physically comfortable enough to use its body correctly.
Comfort and movement
A horse experiencing pain may protect an uncomfortable area by changing the way it moves.
Over time, some muscles may become overworked while others are used less.
Maintaining muscle health therefore also means considering the feet, joints, back, saddle fit and the horse's overall way of moving.
Rest and recovery
Muscles adapt during recovery as well as during exercise.
Repeated strenuous work without sufficient recovery can contribute to fatigue and muscle soreness. Conditioning programmes should therefore balance exercise with appropriate rest.
Older horses and muscle loss
Older horses commonly lose muscle more readily, particularly along the topline.
Some of this can be associated with ageing, but noticeable muscle loss should not automatically be dismissed as "just getting old".
Dental problems, reduced food intake, inadequate dietary protein, PPID, chronic pain, reduced activity and other health problems may all contribute.
A horse that is progressively losing muscle despite apparently adequate feeding deserves further investigation.
When should I contact my vet?
Seek veterinary advice promptly if your horse develops sudden severe stiffness, painful or hard muscles, excessive sweating, trembling, marked reluctance to move or signs suggestive of tying-up.
Veterinary investigation may also be appropriate when there is:
- repeated muscle soreness
- recurrent tying-up
- unexplained muscle loss
- substantial asymmetry between muscle groups
- persistent exercise intolerance
- progressive weakness
- an unexplained deterioration in performance.
Blood tests may sometimes be used to measure substances such as CK (creatine kinase) and AST (aspartate aminotransferase).
These are enzymes that can increase in the bloodstream when muscle cells have been damaged. Your vet interprets the results alongside the horse's history, examination and, where appropriate, further testing.
Supporting healthy muscle
For horses without an acute muscle disorder, maintaining good muscle health is usually based on the whole horse rather than one supplement or one exercise.
Consider:
- an appropriate forage-based diet
- sufficient quality protein and essential amino acids
- balanced vitamins and minerals
- gradual, progressive conditioning
- regular turnout and movement where appropriate
- adequate recovery between harder exercise sessions
- maintaining healthy feet and comfortable movement
- appropriate saddle fit
- investigating persistent pain, stiffness or muscle loss rather than working through it.
For horses with a diagnosed muscle disorder, dietary and exercise recommendations can be considerably more specific. Follow the programme recommended by your vet and, where appropriate, an appropriately qualified equine nutrition professional.
The important thing to remember
Muscle is often where we see the effect of a problem, but it is not necessarily where the problem started.
A horse that becomes stiff, loses topline or develops uneven muscles may have a primary muscle disorder — but it may also be responding to pain, altered movement, reduced activity, nutritional deficiencies or another health condition.
Recognising the change and investigating why it is happening gives you a much better opportunity to support the horse appropriately.
Suggested graphics
A maximum of three graphics is recommended for this page.
Graphic 1 — Hero graphic: "Healthy Muscle, Healthy Movement"
Purpose: Introduce the relationship between muscle health, movement and the rest of the horse's body.
Visual: A clear side-on horse with the major superficial muscle groups subtly highlighted, particularly the neck, topline, back, abdominal area and hindquarters.
Standalone wording:
Healthy Muscle, Healthy Movement
Your horse's muscles help to:
- create movement
- stabilise joints
- support the back
- maintain posture
- carry and control the body
- support athletic performance.
Changes in muscle development, comfort or function can sometimes be an early indication that something else is wrong.
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Graphic 2 — "What does tying-up actually mean?"
Purpose: Translate terminology owners may hear into understandable language.
Visual: Simple three-stage illustration:
Exercise → Muscle cells become damaged → Pain, stiffness and muscle enzymes released into the bloodstream
Standalone wording:
Tying-up — what does it mean?
Your vet may call it exertional rhabdomyolysis.
It means that muscle cells have become damaged in association with exercise.
Signs can include:
- sudden stiffness
- painful or hard muscles
- sweating
- trembling
- reluctance to move.
Suspect tying-up? Stop exercise, do not force the horse to keep walking and contact your vet.
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Graphic 3 — "Muscle loss: look for the reason"
Purpose: Reinforce one of the most important educational messages on the page.
Visual: Horse silhouette in the centre with arrows leading towards it from several possible contributing factors.
Standalone wording:
Muscle Loss — Look for the Reason
Loss of muscle does not necessarily mean that the muscle itself is diseased.
Possible contributing factors include:
Pain or altered movement
Joint, foot or back discomfort can change how muscles are used.
Reduced exercise
Muscle that is used less gradually becomes smaller and weaker.
Nutrition
Insufficient energy, protein or essential nutrients can affect muscle maintenance.
Age and health
Ageing and some medical conditions can contribute to muscle loss.
Nerve problems
Damage affecting the nerves supplying a muscle can cause significant muscle wasting.
Persistent or unexplained muscle loss should be investigated rather than simply treated as a conditioning problem.
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Important information
This information is intended to help horse owners understand muscle health and terminology they may hear from veterinary surgeons and other equine professionals. It is not intended to diagnose a condition or replace veterinary advice.
Sudden severe stiffness, painful or hard muscles, excessive sweating, weakness or marked reluctance to move requires prompt veterinary advice.