Managing Horses Prone to Muscle Problems

-        Dr. Eleanor Host, Staff Veterinary Specialist for Uckele Health & Nutrition 


The more common muscle disorders that horses suffer from are caused by an inability to release stored muscle energy, which can cause of the painful experience commonly known as tying-up. 

Changing energy sources from carbohydrates high in starch or sugar to high-quality fats can greatly improve the degree and occurrences of tying-up, and provide an excellent alternative energy source for horses prone to these types of muscle disorders. Managing horses suffering from muscle disorders requires simple but thoughtful changes to their daily nutrition and activity levels.

The Details

EPSM = equine polysaccharide storage myopathy

PSSM = polysaccharide storage myopathy

EPSSM = equine polysaccharide storage myopathy

These are all the same condition.


By University of Minnesota diagnostic criteria:

  • Type 2 – associated with collections of abnormal glycogen in the muscle. Caused by a mutation in the GYS1 gene. Genetic testing is available.
  • Type 1 – associated with higher than normal amounts of glycogen in the muscle.
  • Type 2 can be diagnosed by biopsy or genetic testing.
  • Type 1 can only be diagnosed by biopsy.
  • EPSM is **not** synonymous with tying-up. There are many other causes of tying-up.



  • Type 2 very common in draft breeds originating from continental Europe such as Belgians, Percherons and Trekpaard and their crosses, but rare in Shires and Clydesdales, which originated in Britain and Scotland. Less common in Warmbloods. May also be found in Quarter Horses and QH related western breeds. Rare in Morgans.
  • Type 1 found in many breeds but again drafts, QH and their crosses predominate.
  • Rare in Thoroughbreds, Standardbreds, Arabians, Morgans and nondraft pony breeds.



  • Tying up, muscle spasms and elevated muscle enzymes in QH and light breeds.
  • Drafts and Warmbloods may show only muscle tenderness, reluctance to engage the hindquarters, back issues, gait abnormalities, muscle atrophy and weakness, often without elevated muscle enzymes.
  • Shivers is not caused by EPSM.



  • Tying-up or weakness with EPSM is an energy crisis in the muscle cell. Both muscle contraction and relaxation require energy.
  • Higher fat feeding can have beneficial effects by slowing glucose entry into glycogen.
  • It appears there is actually a shortage of glucose to burn in the muscles because:
  1. The EPSM muscle preferentially produces glycogen rather than burning glucose.
  2. The abnormal glycogen found in type 2 EPSM is not easily broken down by the cell.
  • EPSM muscle has decreased mitochondria (cellular structures that burn fat and glucose with oxygen and have a high energy yield) and higher levels of lactate indicating inefficient use of glucose.
  • All drafts, EPSM or not, and Quarter Horses with EPSM, are very insulin sensitive. However, this does not cause the EPSM. It is a reflection of their high need for glucose because of how inefficiently they utilize it.



Turnout, 24/7 if possible and as much formal exercise as possible is important to controlling EPSM. Exercise triggers the AMPK enzyme which stops glycogen production and directs glucose into energy pathways.


A forage based, low carbohydrate diet with combined sugar and starch of 12% or less is used to avoid surges of glucose being presented to the muscle and stimulating glycogen storage.


High fat feeding, minimum of 1 lb of fat for 1000 lbs of body weight (often much more), is a common treatment. Several months are required for maximum benefit. High fat feeding can train the muscle to use more fat to some extent. The major effect is likely through mechanisms that limit glucose uptake by muscle when there are high circulating levels of fat.


Acetyl-L-carnitine [ALCAR], a metabolite of the amino acid L-carnitine, is a naturally occurring compound in the muscle. Like exercise, ALCAR switches on the AMPK enzyme that directs glucose into energy rather than glycogen. The acetyl group can also be switched to produce acetyl-CoA, which favors burning of glucose/glycogen over fats. ALCAR typically produces obvious clinical benefit in less than a week, with improvements continuing as the horse becomes more fit and fine tuning of the diet improves muscular function. Dosage is 1 gram per 100 lbs of body weight. If supplementing with ALCAR, high fat should **not** be fed.


Several other basic nutrients impact muscle function, regardless of whether or not the horse has EPSM. Many horses with a diagnosis of “mild” type I EPSM by biopsy have responded to measures correcting intakes of these nutrients with no EPSM specific interventions. If the horse does have EPSM, these are issues that can interfere with a strong positive response to treatment. They are completely preventable, so no reason not to actively cover these bases.


  • - Salt [sodium chloride]. As little as 2% dehydration can have serious effects on muscle strength, power and endurance. Sodium is the major ion responsible for keeping water levels in the body at optimum. It is severely deficient in the equine diet. An average size horse requires 1 oz of salt daily in cold weather, 4+ in warm weather.

  • - Thyroid dysfunction has major impacts on skeletal muscle energy generation. Typical equine diets are borderline to clearly deficient in iodine. Avoid problems related to deficiency by providing at least the bare minimum of 3.5 mg for the average size horse.

  • - Magnesium deficiency causes muscle irritability and inflammation. If a forage analysis is available, adjust the calcium:magnesium ratio to between 2:1 and 1.5:1. If not, 1 gram per 100 lbs of body weight is a good approximation for most hays.

  • - Research has documented increased oxidative stress in muscle from EPSM horses, making intake of vitamin E and selenium even more important than normally. Usual starting dose for EPSM horses of average size is 2 mg/day of selenium and 5000 IU of vitamin E (in oil base, like Liquid E-50, for best absorption). Higher intakes of E can be of benefit to some horses.

  • - Another important antioxidant is glutathione. L-glutamine supplementation can support these levels and also doubles as an alternate energy source in muscle. Dosage is 1 gram per 100 lbs of body weight. Higher levels, up to 2 grams per 100 lbs, can be used for horses with ongoing issues of muscle pain.


When horses have issues with muscle atrophy or low energy levels for exercise, L-leucine supplementation can often help. This is the major amino acid in muscle tissue and has an anabolic effect. It can also serve as an alternate energy source. Give 2 grams per 100 lbs of body weight 30 minutes before and after exercise. When exercise is prolonged, repeat the dosing at hourly intervals.