Tying Up

Horse Feed & Supplements to Support Tying Up

What is tying up?

Tying up, or exertional rhabdomyolysis (ER), is an exercise-induced syndrome of muscle pain and cramping. Classic signs of ER include muscle pain and stiffness, excessive sweating, and reluctance to move. Researchers have uncovered several different causes for ER, including nutrition, over-exertion and genetics.

The two main chronic forms are recurrent exertional rhabdomyolysis (RER) and equine polysaccharide storage myopathy (EPSM). Horses may experience a single, acute episode of ER although they likely do not have the same muscle defects as RER or EPSM.

• RER is thought to be caused by abnormal regulation of calcium within the cell. Calcium is critical for normal muscle contraction and relaxation. Young horses in training, fillies, and horses with nervous temperaments appear to be most affected by RER.
• EPSM (also called PSSM) is often seen in stock-type horses (Quarter Horses), Warmbloods (Hanoverians and Westphalians), and some draft breeds (Percherons and Belgians). It is the result of a defect in the metabolism of glycogen (stored form of glucose) in the muscles.

Sugar stored in the muscle is known as glycogen. Normally, glycogen is broken down by an enzyme known as amylase and used as energy for performance. Horses with EPSM cannot metabolize glycogen normally. The muscle therefore can’t use glycogen for energy, and glycogen stores build up abnormally. Symptoms include those associated with ER, but may include exercise intolerance, muscle stiffness, back pain, shifting lameness, muscle atrophy, and a camped-out stance.

Dietary Goals for EPSM or RER horses

Both hot and humid conditions and over-exertion can cause acute sporadic ER episodes. In the cases of dehydration, an electrolyte, along with adequate water, can be used to replenish electrolytes lost in sweat.

Dietary management includes keeping starch and sugar in the total diet to less than 20% for RER horses¹, 10-12% or less for PSSM horses^2,3. Exercise should be introduced slowly, with changes and increases in intensity made gradually. Stress in the environment should be kept to a minimum.

1. Valberg, S. Recurrent exertional rhabdomyolysis. Online at: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/recurrent-exertional-rhabdomyolysis
2. Valberg, S. Type 1 Polysaccharide Storage Myopathy. Online at: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/type-1-polysaccharide-storage-myopathy
3. Valberg, S. Type 2 Polysaccharide Storage Myopathy. Online at: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/type-2-polysaccharide-storage-myopathy

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