• Abi Kroupa

The Amazing Biomechanics of a Horse's Respiratory System

The equine respiratory system is highly specialized and unique. Horses are obligate nasal breathers which means they only breathe through their nose. Even more impressive, during the canter or gallop, horses do not breathe by expanding and contracting their chests. All air movement comes from the movement of the diaphragm. One Stride = One Breath at the canter and gallop. The unique biomechanics of a horse's breathing is helpful for riders, owners, and trainers to understand and is an important consideration during the training and conditioning of our equine partners.



During exercise, the respiratory system's primary function involves supplying oxygen and removing carbon dioxide from the blood in the pulmonary capillaries. The horse's respiratory system also plays a vital role in regulating body temperature and PH balance.


The resistance to airflow in the airways affects the energy expended to drive respiration. About 90% of resistance during exercise is in the upper airways. The greatest breathing resistance occurs at the narrowest parts of the respiratory tract: the nostrils and the larynx. This resistance to airflow is reduced by flaring the nostrils and dilating the larynx.


Correlation Between Respiratory Rate and Heart Rate

Horses have a normal resting respiratory rate of 10-20 breaths per minute. At the walk and, to some extent, the trot, the horse's body selects an appropriate respiratory rate for their fitness and exercise intensity. Respiration rate may increase to 120 breaths per minute during exercise. However, at the canter and gallop, the respiratory system functions with what's called respiratory-locomotor coupling. This means the horse takes in one breath with each stride. When exercise ceases, the respiratory rate decreases and briefly settles in the range of 60-100 breaths per minute, with the horse breathing deeply until the oxygen debt is repaid. A fit horse's breathing rate should decline to under 50 breaths per minute within 10-15 minutes.


To determine the respiration rate, one can watch either the horse's flanks or nostrils. The abdomen rises and falls with each breath, which causes the sides to move in and out. The horse's nostrils flare and contract with each breath.


Normal adult horses have a heart rate of 30 to 40 beats per minute at rest. Horses strenuously exercising can reach heart rates up to 250 beats per minute. In comparison, a person's maximum heart rate is 220 minus the individual's age. The horse's heart rate should recover to below 70 beats per minute within 10 to 15 minutes. The horse's heart rate per minute should always exceed the respiration rate. If the respiration rate exceeds the heart rate, the horse has a serious problem. The horse must stop and rest until the condition corrects itself. If the condition does not correct itself, the horseman must call a veterinarian.


Respiratory Response to Exercise

At a constant stride, horses have a locomotor-respiratory breathing mechanism that settles into the stride rhythm, minimizing the energy used to breathe. Consider this locomotor-respiratory breathing like a piston within a cylinder of an engine. Respiration is driven by locomotor forces associated with weight-bearing on the front limbs and the pressure of the abdominal organs against the diaphragm. One significant contributor to this locomotor-respiratory "piston" breathing is the compression of the chest as the front legs bear weight. This is when the expiration phase of breathing occurs (the horse breaths out). This augments muscular action of the diaphragm through movements of the abdominal organs - the power behind the respiratory piston. In the canter and gallop, the two front legs are on the ground simultaneously, maximizing the effect of chest compression (or expiration of air).

In addition, a horse with a longer stride length has a slower stride rate than a horse traveling at the same speed with a shorter stride length. This means that the longer-striding horse can breathe more deeply. A short stride length will limit the time available for the inspiration breathing phase, reducing the oxygen volume intake.


Each time the horse's stride rate changes, it takes several strides before the locomotor-respiratory "piston" catches up with the new rhythm. During this time, the energetic cost of breathing increases. Therefore, it is energetically efficient for a horse to change speed by adjusting its stride length while maintaining the same rhythm. As riders, we can help the horse maintain this rhythm. Riders can also accidentally or purposefully change the horse's rhythm.


Conditioning

Training and Conditioning increase the elasticity of airways, the transfer of blood gases to and from the blood, and the integrity of lung and diaphragm tissue. However, the increase in oxygen uptake and decrease in respiratory rate seen with conditioning is primarily a result of strength and conditioning of the muscular and cardiovascular systems.


Other Fun Facts

  • Tightening a horse's girth too much will affect his performance - not because of constricting the chest and preventing the lungs from expanding, but because it decreases the effectiveness of the muscles around the front of the chest and shoulder that move the forelegs. Anatomical girths such as – The Total Saddle Fit Shoulder Relief Girth help prevent the saddle from interfering with the horse's shoulder and forelegs.


  • Horses hold their breath over jumps and do not breathe again until they land; they begin by exhaling or breathing out.

  • At rest, an average horse draws in approximately 1.3 gallons (5 liters) of air with each breath. At about 15 breaths per minute, that's nearly 20 gallons of air moving into and out of the horse's lungs every minute, easily managed by their 55-liter (or approximately 15-gallon capacity lungs). That said, horse air intake can drastically increase as much as 120 breaths per minute and doubles from1.5 gallons of air per breath to nearly 3 gallons per breath (10 liters). This is roughly 360 GALLONS OF AIR PER MINUTE filtered in your horse's lungs during strenuous exercise!


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