Tying up is not a specific disorder with one specific cause. It is a collection of symptoms that may have several different causes. A hallmark of all types of tying up is that the symptoms are caused by exercise and become evident during exercise. The typical progression is that the horse begins to feel “rough”, with altered gaits, shortening of stride (especially behind), increased sweating.
The horse will slow down and eventually grind to a halt itself if not pulled up. The horse is obviously in pain, high pulse and respiratory rates and sweating. Muscles feel very hard to the touch, especially in The hindquarters and the pectorals.
Once pulled up, the horse is very reluctant to move, even walk. There is little, if any, interest in eating or drinking. When the horse urinates (which may be a while), urine may have a reddish brown discoloration from the muscle pigment, myoglobin. In some cases the urine may appear normal when the horse first starts to urinate, then turn dark by the end of the stream.
In addition to the classical picture of pain, distress, rock hard muscles and discolored urine (not always present), laboratory analysis confirms tying up by elevated muscle enzymes.
Horses that tie up repeatedly, usually starting at a young age, are likely to have a genetic cause. This blog will discuss causes of sporadic tying up with no underlying genetic cause.
Overexertion is an underestimated cause of sporadic tying up. Poorly conditioned horses that are overworked are most prone to this, as are horses in a conditioning program that fool their conditioners into thinking the are fitter than they are. During the process of training, muscles increase their stores of carbohydrate (in the form of glycogen), increase the density of capillaries supplying the muscle with blood, increase activity of enzyme systems involved in utilizing fat and carbohydrate as energy, and increase the number of mitochondria in the cells, the intracellular structures that use oxygen to burn carbohydrate and fat for the highest ATP/energy yield.
Several different things might contribute to this. If the muscle is overstretched, the muscle fibers and/or the connective tissue surrounding them may be damaged, leading to swelling and reduced blood flow. Horses unfit for the level of work they are doing exhaust their supplies of stored carbohydrate quicker, which can lead to an energy crisis. These horses also overheat more easily. The enzymes needed to utilize energy sources and regulate the contraction machinery as well as multiple cellular “pumps” that keep levels of key electrolytes at their proper levels are very sensitive to temperature changes.
Speaking of overheating, even slight dehydration has disastrous consequences on performance, most directly linked to dehydration. Heat is carried away from the hard working muscles by the blood that bathes them. The hot blood from the interior of the horse eventually makes its way to the vessels at the skin surface which dilate and release heat into the air. A dehydrated horse is like a car with not enough fluid in the radiator. The fluid in the tissues surrounding the cells, where the blood vessels travel, becomes depleted first, interfering with the exchange of heat between cells and the blood stream. Risk of tying up and isolated cramps is known to be higher when weather is hot, but overheating secondary to dehydration can occur at any time of the year.
Closely tied to both good hydration and normal muscle function is mineral status. Electrolytes are minerals present in the blood in their free and charged/ionized form. The most important for muscle function are sodium, chloride, potassium, calcium and magnesium. Sodium, chloride and potassium are responsible for holding water in the tissues and cells, also for nerve impulses to travel normally down the nerve axon and along the muscle sarcolemma and T tubule system. Calcium is needed for the release of acetylcholine from nerve ends and it is calcium that triggers muscle contraction. Magnesium regulates calcium release and is needed to store ATP which powers both contraction and reuptake of calcium.
Of all of these minerals, sodium is the one that is always deficient in the natural diet in the form of salt.
Since salt is sodium chloride, this typically takes care of any chloride shortage too. Hay contains abundant potassium. Most horses are more likely to get into trouble with too much calcium in their diet than too little. When calcium in the diet is high, it limits their ability to release the active, ionized calcium from bone when they need it.
Last but far from least is magnesium, the stepchild of minerals. I have seen more horses with muscle irritability, cramping, even tying up related to inadequate magnesium than all other dietary factors combined.
For an indepth description of the role of minerals in muscle function, see:
Horses in training are at particularly high risk of tying up when they are moving from moderate to heavy work levels. This would be defined as speed horses moving into the range of true speed/anaerobic work (heart rates over 160 to 180) and endurance horses moving to work of longer than 1 hour at a sustained strong trot or canter. Tying up is particularly likely to occur if they miss one or two days work, but actually drops if they miss 3 or more days work.
There are two forms of glycogen; proglycogen which is shorter and more quickly broken down and macroglycogen, a much larger final form of glycogen. Proglycogen predominates in the first few days after heavy exercise, being replaced by the mature macroglylcogen at around the 72 hour mark in horses. Since proglycogen is easier to break down, a high level may contribute to the tying up risk.
If your horse is tying up, you can get a free nutrition consultation at Mad Barn. You won’t be dealing with sales people and receptionists. The nutrition team includes vets, PhDs and MS degrees in nutrition. https://madbarn.com/nutrition-consultation/
Eleanor Kellon, VMD
Dr. K's Horse Sense 
Let me know how it goes!
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This concise article on tying up is incredibly informative. I want to know more. Do these horses require a different diet of feed? Racing Standardbreds are on a training plan of jogging 4 miles daily, except for Tuesday and Saturday when they train a mile in 2:20 or less (after warming up about 1.5 mi at a jog). Off Sunday.
Well presented, Dr. Kellon.
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A 2:20 mile is often the tipping point. You want to make sure they are consuming at least 2 ounces of salt a day. Try adding 5 grams of magnesium from magnesium oxide to the diet. If you have hard water, make it 10 grams. https://madbarn.com/product/magnesium-oxide/
If any of them have muscles that feel hard at rest (triceps and above the stifle), try them on acetyl-l-carnitine,10 grams a day https://madbarn.com/product/acetyl-l-carnitine/.
You could also try interval training three half miles 1:20, 1:15 and 1:10, jogging a mile slowly in between.
The final thing that’s often very helpful is 6 ounces of Karo syrup by dosing syringe or in a small amount of water as soon as they are done training, before cooling out, and give them their next grain meal no later than an hour after training. This gives a good supply of glucose to the muscle to replenish glycogen when it is most “hungry”
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Ok! I’m on it!
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