Tying Up in Horses: Causes, Prevention & Nutritional Support for Recurrent Exertional Rhabdomyolysis (RER)

What is Tying Up in Horses?

“Tying up” (also known as azoturia or Monday morning sickness) is a common term used to describe recurrent exertional rhabdomyolysis (RER), a painful muscle condition in which muscle damage occurs during or after exercise.

RER is one of the most recognised muscle disorders in performance horses and can range from mild stiffness through to severe episodes requiring veterinary intervention.
The word rhabdomyolysis comes from Greek meaning breakdown of skeletal muscle fibres.

Which Horses Are Most at Risk?

Tying up is most commonly seen in:
•    Young Thoroughbreds in training 
•    Highly excitable horses 
•    Horses in intense work programmes 
•    Horses with inconsistent exercise routines 
•    Horses exposed to stress or limited turnout

Research suggests both genetic and environmental factors contribute to risk.
The condition is believed to involve abnormal calcium regulation within muscle cells, disrupting normal contraction and relaxation and leading to muscle damage.

Signs of Tying Up in Horses

Clinical signs may include:
•    Stiffness and reluctance to move 
•    Firm, painful muscles (particularly over the hindquarters) 
•    Sweating 
•    Elevated heart rate 
•    Rapid breathing 
•    Muscle tremors

In severe cases, horses may pass dark brown (“coffee-coloured”) urine, caused by the release of muscle pigments into circulation.
If tying up is suspected, veterinary advice should be sought immediately.

Blood testing typically reveals elevated:
•    Creatine kinase (CK) 
•    Aspartate aminotransferase (AST) 
both markers of muscle damage.

Nutrition for Horses Prone to Tying Up

Nutrition plays an important role in reducing the risk and supporting recovery.
Feeding strategies typically focus on:

Reducing Dietary Starch

High-starch diets have historically been associated with tying up. Many trainers therefore opt for lower starch, higher fibre feeding programmes to provide controlled energy release.

Providing Quality Protein

Contrary to traditional thinking, reducing protein is not necessarily beneficial.
Protein provides the essential building blocks required for muscle repair following an episode. Horses should receive adequate levels of high-quality protein and essential amino acids, particularly:
•    Lysine 
•    Methionine 
•    Threonine

Branched-chain amino acids (BCAAs – leucine, isoleucine and valine) may also support muscle metabolism and recovery.

Using Fat as an Alternative Energy Source

Dietary fat supplies concentrated calories and provides “cool” energy that supports body condition without increasing excitability.

Supporting Muscle Function

Antioxidants including vitamin E and selenium help protect muscle cells from oxidative stress, while maintaining electrolyte balance supports hydration and normal muscle contraction.

For horses under increased muscular challenge, targeted nutritional support may also be beneficial. Plusvital Ty-Relief is formulated with citric acid, tri-sodium citrate, branched-chain amino acids (BCAAs) and vitamin E to support muscle function and recovery.

Importantly, although RER involves abnormal calcium handling within muscle cells, simply feeding additional calcium is not considered an effective preventative strategy.

Alongside nutrition, consistent exercise, regular turnout and avoiding abrupt changes in workload remain central to long-term management.

Written by Rebecca Watson, AFN Registered Nutritionist (RNutr)

Rebecca Watson is Chief Technical Officer at Plusvital and specializes in equine nutrition, performance, and health.

Last Reviewed: May 2026