All Tied Up?

By Catherine Rudenko, BSc. Eq. Science

Exertional rhabdomyolysis (ER), also known as azotoria, tying-up or set fast, is a process whereby the striated muscles, which connect to the bone and allow movement, suffer breakdown following exercise. Incidences of tying-up can range from mild muscular stiffness to the muscles becoming locked and the horse unable to move.

ER can be divided into two categories - Acute Exertional Rhabdomyolysis (AER) and Recurrent Exertional Rhabdomyolysis (RER). With AER, the horse in question would have a history of previously satisfactory performance, and is most commonly caused by strenuous exercise above the horse’s current level of training.

RER can be seen intermittently, sometimes after only mild exercise. Of these two conditions, RER is the most frustrating for any trainer and is an area in which more research is still needed to determine the exact causes.

Factors leading to tying-up in performance horses

AER and RER have slightly different effects on the body. More work is needed to fully understand what happens at a muscular level, particularly with RER. At the moment, the causes can be summarized for each as follows:

AER

The main cause is depletion of ATP (adenosine tri phosphate), the muscle’s energy source. ATP levels drop when there is marked lactic acid production following speed work. Levels also can drop if glycogen, another muscle fuel, is also depleted following endurance work.

Depleted ATP sources and a low muscle pH cause changes at a cellular level in the muscle. These changes include damage to cell membrane/cell structure, and damage to the myofibrils which are small fibres that make up muscle tissue.

RER

Main cause is less clear and damage can be caused even with light work. It appears to be an abnormality in how the cells regulate calcium, resulting in excess levels of calcium being excreted from the cell body. This causes contractions and muscle spasms.

RER is linked with several triggers including genetics, high starch diets, management and stress. Research from the USA shows RER to be more prevalent in mares, with up to 80% of 2 year old fillies being affected.

Nervous temperaments will increase prevalence by fivefold and a horse with an underlying lameness is four times as likely to suffer from RER.

Practical management of horses prone to tying-up

Dietary changes and specific training programs will help reduce episodes. There is no one magic cure, and best results are achieved when both dietary and management changes are used together.

Dietary changes

Low starch, high fibre and high fat diets are linked with lower incidences of RER. Lower starch diets provide the muscle with alternative sources of fuel and can also help from a stress point of view as such diets can lower excitability and nervousness.

On days of light work or rest, feed levels should be reduced, but not forage intake. This is to cater for possible carbohydrate storage issues in the muscle. Maintaining forage intake is important for stomach and gut health, and can also help reduce stress levels for stabled horses.

Vitamin E and selenium deficiency can cause AER, but most horses with RER do not show a deficiency. Vitamin E and selenium are natural antioxidants which are beneficial to muscle health and should be part of any good racing diet.

Daily electrolyte supplementation has been reported to lower incidences, although it is difficult to test for electrolyte imbalances and there is no concrete research available on this.

Supplementing with 30 - 45 grams sodium chloride (salt) has been successful in lowering incidences for some horses. Similar studies have shown that supplementing with 11 - 33grams of calcium daily may be of benefit.

Management changes

Some reports suggest lower incidence levels associated with daily work programs for horses with RER. A regular exercise program to steadily build fitness will be of benefit to any horse, particularly in avoiding AER. Turnout  to pasture will also be of benefit for stress reduction and increasing forage intake.

In Summary

Tying-up, whether AER or RER, is best managed by a combination of stress reduction, regular exercise, turnout and low starch, high-fibre diets.

For further information on tying up, contact the RED MILLS nutrition team.