Working at speed calls for a healthy, well-trained heart. Andrea Oakes discovers how technology can help us track heart performance and detect potential problems

Sports such as point-to-pointing and eventing make exceptional demands on a horse’s heart. This finely tuned muscle will beat 220-240 times per minute at a fast gallop, each time pushing up to 280 litres of oxygenated blood around the body.

XLEquine vet Imogen Burrows explains that the well-trained heart is a highly effective pump, with the capacity to store a large volume of blood and the muscular ability to propel it around the body. Rather than beating faster, it works harder.

“Training results in heart muscle enlargement (hypertrophy), accommodating the need for greater energy and oxygenation of muscles by pumping blood more efficiently,” says Imogen. “The heart rate will then be lower at given exercise levels.

“The average horse has a maximum heart rate of around 240 beats per minute (bpm). Untrained, he might reach this rate at a speed of eight metres per second (m/s), but the speed at which he reaches maximum rate when race-fit may rise to 12m/s.”

The time it takes for the heart to return to normal after extreme exercise is another indication of fitness.

“During recovery, the heart rate drops rapidly over the first couple of minutes, before returning to a resting rate of between 28bpm and 40bpm,” says Imogen. “The fitter the horse, the shorter the recovery time.”

Sophisticated monitoring
Trainers and riders have traditionally relied on what they see and feel to gauge fitness, but girth heart monitors can now deliver instant and precise heart rate measurements during exercise and recovery.

“This is a very effective way to assess a horse’s progress,” says Imogen. “Resting and maximal heart rates are not altered by training, and so are not useful fitness indicators, but heart rates at set speeds and recovery rates can be used to guide workload.”

Real-time heart monitoring can also aid the development of aerobic and anaerobic capacity.

“Increasing duration of exercise (endurance) requires predominantly aerobic energy production, whereas sprinting relies mainly on anaerobic processes,” says Imogen.

“Point-to-pointing requires both pathways, with anaerobic metabolism contributing only around 30% of energy.

“Training programmes can be tailored using a percentage of maximal heart rates (HRmax). Aerobic training begins when the horse is working at 60-70% HRmax, with a heart rate of between 144bpm and 168bpm. Pushing this slightly to 180-192bpm stretches aerobic capacity and improves efficiency of fuel utilisation during exercise. Anaerobic metabolism occurs at even higher rates.

“The instant feedback provided by a girth monitor allows the rider to adjust pace as necessary to keep the horse working comfortably within the correct heart rate zone,” adds Imogen.

Losing power
Different girth heart monitors can also help to detect health problems. An unusually high heart rate during exercise can indicate pain or sickness, while reduced performance or delayed recovery might point to a cardiac issue.

“Heart problems fall into two categories: abnormalities of rhythm (arrhythmias) or disruption of blood flow (murmurs),” says Imogen. “Collapse or sudden death is rare, however. In the vast majority of cases the muscles will run out of energy and the horse’s ability to perform will be impaired.

“Cardiac output — the ‘engine’ that powers the equine athlete — is a function of heart rate (the number of beats per minute) and stroke volume (the amount of blood pumped per beat).

“Because a significant heart problem reduces stroke volume, heart rate must increase to maintain cardiac output. The horse will reach his maximum heart rate at a much lower speed, resulting in poor performance.

“Fitness itself does not predispose the heart to problems, yet cardiac issues are more likely to be detected in the elite athlete,” adds Imogen. “This is because — while most leisure horses will be working well within their cardiac limits — the performance horse is being asked to exert himself beyond these moderate levels.”

This article was first published in Horse & Hound magazine (27 November 2014)