Tying Up and Muscle Fatigue:
Research Continues, But The Worst Dangers May Be Hard
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
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.
Right Answer, Wrong Horse?
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
All Revved Up, No Place To Go
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.
The Hidden Dangers
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.
The Skeletal Connection
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
Circulation And Breathing
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.
Solutions, Old And New
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
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
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
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.