Breathing in Synchrony

Have you ever noticed that when you are riding at anything faster than a walk or trot, the horse’s breathing sounds like it is happening in perfect harmony with the gait?

Well, it is.

When you train a horse for athletic performance, you are trying to improve the overall aerobic fitness, just as human athletes do. Increasing fitness shows several improvements: blood volume increases, circulation improves, the heart is able to pump more blood with less effort, and overall, the amount of oxygen that the heart and lungs deliver to working muscles and tissues per minute increases.

Take a racehorse as an extreme example: the lungs can respond to an enormous demand for oxygen to fuel muscle activity as the work effort increases. At a walk, a horse consumes only a few litres of oxygen a minute (normal resting respiratory rate in horses is about 8-15 breaths per minute), yet at a gallop oxygen consumption can be as high as 60 litres a minute.

As workload increases, the heart rate increases from a resting rate of about 28-40 beats per minute, up to a maximum of 220-240 beats per minute at maximum work effort. If the heart pumps about 1 litre of blood at every beat, the sprinting racehorse can circulate up to 240 litres of blood around the body every minute. That’s a 44 gallon drum full of blood - every minute!
The lungs have to keep up with this demand for oxygen through huge extremes of demand in all working horses.

But this ability to increase oxygen flow to the body has its limits, as there is a limit to how fast a galloping horse can breathe. All the training possible won’t be able to alter this respiratory rate limit.

Horses are different to humans with regard to how the front legs work: horses have no collarbone, so the movement of the forelegs is linked directly to the ribs and spine movement by the bulk of muscles seen in this region on the horse. So, breathing in the horse is linked closely to the front leg action.

As the front legs strike the ground at a canter or gallop, the compression transmitted through the legs forces the ribs upward, and this acts to force air out of the lungs.

At the same time, the horse is dropping its head and neck, and this acts to draw the ribs backward, increasing the compression on the lungs.

Simultaneously, as the horse’s forelegs touch the ground, the front part of the body is decelerating a little, and this forces the organs in the abdomen (which are attached indirectly to the diaphragm - the division between the chest and the abdomen), to move forward, further squeezing the lungs and forcing air out.

As the head and neck then raise, and the load is lifted from the forelegs, the ribs and sternum are pulled forward and down, and the front of the body accelerates, thus forcing the abdominal contents towards the rump, just like a huge piston. All this causes the lungs to expand, thus filling them with air.

The piston-like action of the abdominal contents moving forward and then back again, plus the effect of the head and neck dropping and then lifting again during a stride, are both fully linked to the stride when the horse is cantering or galloping, so the horse’s breathing is actually fully synchronised to its gait.

Horses take exactly one breath per stride. Breathing in occurs as the front legs lift off the ground, and breathing out occurs when the front legs strike the ground. Simple, isn’t it?

The link between the stride and the breathing rate occurs in all four legged mammals. All take one breath per stride (even kangaroos take one breath per hop).

In most cases of physical activity in horses this simple mechanism not only reduces the physical work a horse must do to breathe, thus saving energy, it keeps oxygen supply matched almost perfectly to oxygen demand in all but extreme cases. Speed is a function of the length of the stride x the number of strides in a given time, just as lung output is a function of the depth of each breath x the number of breaths in a given time. The 1 : 1 ratio between stride frequency and breath frequency allows the two to keep pace almost automatically.

But - there is a limit to how much a horse can increase speed by increasing the number of strides per minute. When horses are working at their near maximum, they appear to increase speed by taking longer strides. But there comes a time when the time available for each breath is too short at very high speeds, and the horse will eventually run out of breath (or, more correctly, run out of oxygen). At this stage the horse tires, and can fatigue if it continues.

You have all seen tired horses during racing, when they often appear to alter their gait in an attempt to obtain more air through the lungs. They often tend to have exaggerated up and down bobbing of the head and neck, in an effort to pump more air through the lungs, just as humans tend to get a little “wobbly” when they fatigue. At the same time, horses tend to try to prolong the time they are actually in the air, with no legs touching the ground, as this increases the time available for them to take a full breath, instead of increasing the frequency of strides. This reduces stride frequency and thus speed. Both of these things may help the horse to get more air, but they invariable reduce speed as a consequence. Doesn’t this make sense when you think of the saying “hitting the wall”?

The coupling of breathing rate with gait does not occur at a trot, pace or walk - you may have noticed that horses at a walk or trot generally carry their head and neck level, without bobbing up and down, so the pendulum effect to help pump air at these slower gaits does not occur.

Next time you are riding at a canter or faster, listen to the many sounds the horse makes as it breathes, and you will soon realise that these sounds are all carefully correlated to the steps the horse takes as it hits the ground.