By Barbara S. Veritas
What makes performance horses "tie up"? While the answers to this question are still being debated, horsemen and researchers have made a number of discoveries which can be useful to anyone dealing with this potentially debilitating disorder.
Most often, the culprit is identified as lactic acid, or lactate. Lactate is produced when not enough oxygen is available to help "burn" fuel for energy. Anaerobic metabolism, the kind used during high-intensity exercise, tends to produce large amounts of lactate.
In the classic model of tying up, too much lactate produces lactic acidosis, a lowering of the pH in the muscles and the body at large, which reduces the efficiency of metabolism. Fatigue sets in, and excess lactate makes it difficult for the muscles to relax following contraction. Finally, a large muscle group or groups "seize up", staying in a prolonged state of contraction. In full blown tying up, the horse can not move. In incidents short of tying up, the muscles become extremely sore and tight for prolonged periods of time.
In time, researchers found that many horses, especially fit, well-trained ones, could withstand lactate levels well above the amount found in horses that tied up. They discovered that it depends on the horse. Fit, well-trained and well-bred horses are better able to deal with higher levels of lactate in their systems. In general, though, if a horse develops a level of lactate significantly higher than he has learned to cope with, he will be more likely to tie up.
One constant in tight, sore muscles and tied up horses is the high concentration of calcium trapped within the main part of the muscle cells. In normal muscle function, calcium is cycled within the cell. To contract the muscle, calcium is released in response to electrical stimulus from the nervous system. To relax the muscle, the mitochondria (the energy-producing centers in the cells) return the calcium to the small bodies where it is stored. When the mitochondria lose their ability to recycle the calcium, it begins to build up, leading to constant contraction.
Dr. Stephanie Valberg of UC Davis refers to this as "mitochondrial exhaustion." The mitochondria are losing their ability to produce energy. As the mitochondria run out of steam, they are less able to recycle calcium; calcium builds up and the muscle loses its ability to relax. The very fit horse can produce quantities of lactate while still producing enough energy in enough muscle cells to keep cycling the calcium; he is able to avoid prolonged contraction of the entire muscle group.
Is it the high level of lactate that causes the mitochondria to fall down on the job? Or are high lactate levels just a symptom of muscle cells that are unable to produce enough energy to keep the calcium cycle going? The answer is probably somewhere in between. When there is a lot of lactate, the muscles and the blood become more acid. Too much acid will reduce the body's ability to work and produce energy. On the other hand, lactate is produced when muscle cells run out of the materials they need to produce energy. In this sense, excess lactate is a warning of inefficient metabolism.
Why do some horses tie up shortly after they begin exercising? When a horse is very excited or under stress, hormones are released that cause the nervous system to try to "fire" the muscle cells too frequently. The demand for oxygen jumps sharply and the breathing becomes heavy. Under this bombardment of stimulation, the cells rapidly shift to anaerobic metabolism. In short order the mitochondria become exhausted, calcium cycling shuts down and muscle groups begin to tie up. Lactate levels are usually very high in this type of incident.
There are other effects of muscle fatigue more common than full-blown tying up. Some are widely recognized by horsemen, while others are hidden and can create problems not usually thought of as "muscular." In addition to the cramps and muscle soreness every trainer deals with, muscle fatigue can affect the skeletal, circulatory and respiratory systems, too.
In the legs, the shift from muscular contraction to relaxation pulls and releases the tendons, controlling the movement of the bones. When muscles fatigue, the horse begins to lose control over his stride, increasing the danger that a tendon will be extended at the wrong time. The injuries that result range from bows to bone fractures. Even healthy, well-modeled bones can not take the strain of being hyperextended as the full force of the horse is applied.
The distribution of blood to tissues is largely controlled by muscular blood vessels called arterioles. The arterioles join the arteries to the capillary beds. The arterioles can contract to the point of completely shutting off blood flow to the capillaries. When fully relaxed they expand to several times their normal size, greatly increasing blood flow to specific areas. Fatigue in the muscles of the arterioles may be a factor in pulmonary hemorrhage and other circulatory problems.
Another "hidden" muscle subject to fatigue is the diaphragm. Full movement of the diaphragm is essential to good breathing and competitive performance. This muscle has such a high requirement for oxygen that it receives seven times the blood flow of the skeletal muscles. Fatigue here may be caused by high lactate within the diaphragm, high circulating levels of lactate produced by the skeletal muscles, or mitochondrial exhaustion. In any event, the result will be reduced intake of air and expiration of carbon dioxide, placing additional limits on performance.
What can be done to reduce lactate or increase the stamina of the mitochondria? Proper training can increase the horse's aerobic capacity. This will enable more of his muscle cells to produce energy with oxygen, and so reduce lactate build-up. Training can also improve a horse's ability to deal with lactate, producing more "buffering" agents. Proper warmups and warm-downs and reduction of feed (especially grain) on "off" days can help reduce the incidence of tying up. Even with these advantages, many horses still experience the problems of excess lactate and fatigue.
Several nutrients have been found to be helpful in improving muscle health and stamina. Vitamin E and selenium are essential to protect the integrity of muscle cells. Selenium is required for the formation of glutathione peroxidase, a key ingredient in energy production. Both vitamin E and selenium are important antioxidants, working to scavenge free radicals that damage muscle cells under stress. If these nutrients are deficient, supplementation may help to prevent tying up. While new research on vitamin E looks promising in other areas, so far it appears that additional amounts of E and selenium are unlikely to provide extra protection against tying up or muscle cramping.
DMG is N,N-Dimethylglycine, a metabolite that helps to push back the anaerobic threshold by improving oxygen uptake and transport within the cells. DMG has been shown to reduce the production of lactate in racehorses. It has been used for years with some success in managing chronic "tyers", and has been claimed by many horsemen to improve stamina and delay the onset of muscle fatigue. The limit to DMG's effectiveness seems to be that there's only so much oxygen to go around. The vast majority of the muscle cells are still working anaerobically during intensive exercise.
The most promising approach to getting a handle on tying up and muscle fatigue may be to deal directly with the needs of anaerobic metabolism. The biggest problem in "burning fuel" without oxygen is that a lot of it never gets to its destination. Glycogen and glucose, the main "fuels" for the muscle cells, are reduced to a substance called pyruvate. Pyruvate must then be converted in order to provide fuel for the Krebs (or citric acid) cycle. The Krebs cycle is where the energy of the cell, and the horse, is produced. It occurs within the mitochondria.
It is when pyruvate can not be converted that lactate is produced. The result is not only increasing levels of acid; energy production begins to decline as the mitochondria run out of usable fuel. This is the root cause of mitochondrial exhaustion. Supplements which would supply the nutrients needed to convert pyruvate could reduce lactate build up and improve the horse's ability to produce energy during anaerobic metabolism. Since many cells work anaerobically, even during moderate exercise, such a solution might have broad applications.
Improvements in our understanding of biochemistry, nutrition, and training and management routines are providing valuable tools. Successful trainers will use them all to keep their horses' muscles efficient when they're working - and relaxed when they're not.