Girth Strap Tightness
Every time you saddle a horse, you tighten a girth. But how tight should you make that girth? Just enough to keep the saddle on? With space to slip a hand under the girth? As tight as it will go? Furthermore, how well does a horse breathe with this tight band strapped around his lungs?
John Bowers, BVSc, MACVSc, and Ron F. Slocombe, BVSc, MS, PhD, Dipl. ACVP, Chair of Veterinary Pathology at the University of Melbourne, Victoria, Australia, recently investigated this question in a study involving eight ex-racehorses. They found that most grooms in the Australian racing industry tighten the girths to a point that could have a detrimental effect on their charges' performance.
Studies with humans have shown that tight strapping of the chest can relieve shortness of breath at rest in patients with chronic obstructive airway disease, such as emphysema. However, this strapping impaired patients' ability to exercise. No previous studies had been done with horses, but anecdotal evidence suggested that the same was true for horses--that a tight strap around the chest decreases performance.
Several racehorse trainers in Victoria believed this to be true because their animals had performed poorly after their saddle girths had been over-tightened just prior to racing.
To set a baseline for their study, Bowers and Slocombe measured typical girth tightness at several racing stables around Melbourne. They found that the usual tension applied to keep a saddle on a Thoroughbred racehorse is 13 kilograms (kg) (29 pounds).
Methods And Results
Ten former racehorses purchased from sale yards were used in the study: two Standardbred geldings, three Standardbred mares, three Thoroughbred geldings, and two Thoroughbred mares. Two of the Standardbred mares had to be removed from the study, one for a heart condition, the other for lameness. The ages of the horses varied, but all were older than racing age (two to six years). Each of the remaining eight horses was jogged on a treadmill four times, each time with a girth tightened to 5, 10, 15, or 20 kg (11, 22, 33, 44 pounds) of tension. Girth tightness was measured with a strain gauge transducer, a small black box about the size of a half-roll of Life Savers attached to a canvas girth.
"The kilogram is a unit of mass, but in the context we use it, it is the tension that develops by being pulled with the masses mentioned," explained Slocombe. "So, if you picked up a bucket containing five liters of water, the tension you feel on your arm is five kilograms of tension (plus the weight of the bucket); five kilograms is a bit loose for a saddle girth. With exercise and movement, the saddle tends to slip in some animals at five kilograms. Above 15 kg is very tight. It is difficult to get your fingers under the girth at 20 kg, and it would leave marks in the chest once the girth is removed. You do see this depression in some horses after racing, indicating that the girth has certainly been tight during racing."
To prevent physical conditioning between sessions, each session was at least a week apart for each horse. The horses were acclimatized to the treadmill, but not racing fit. Exercise tests were run beforehand to establish a velocity that would produce 80% of each horse's maximum heart rate.
During each treadmill run, measurements were taken at one-minute intervals of girth tension when the horse inhaled (T/inh), tension when the horse exhaled (T/exp), and of breathing frequency. Heart rate was monitored every two minutes. Time and distance were measured from when the treadmill reached the top speed for the run to when the horse fatigued and could no longer keep pace with the treadmill despite verbal encouragement.
This study found that a tight girth cost nearly 20% of a horse's performance on a long run.
The horses took an average of 16 minutes to get tired, or run to fatigue (16.1 ± 1.3 minutes) at a tension of 5 kg. With tighter girths, the horses tired out at about 13 minutes (13.9 ± 1.0 for 10 kg, 12.6 ± 0.8 for 15 kg, and 13.0 ± 0.7 for 20 kg). That constitutes a loss of three minutes out of 16, or 19%. Because horses were always exercised at their 80% heart rate maximum speed, distances dropped by the same proportions.
Run distance at 5 kg of tension on the girth was 6,624 ± 495 meters (4.12 ± 0.31 miles) prior to fatigue. With tighter girths, the horses ran an average of 5,495 meters (3.41 miles) (5,812 ± 459 for 10 kg, 5,268 ± 404 for 15 kg, and 5,405 ± 281 for 20 kg). That constitutes a loss of 1,129 meters (0.7 miles) out of 6,624 (4.1 miles), or 17%.
On average, for each kilogram of increase in girth tension over 5 kg, the horses fatigued an average of 81.3 meters (0.05 miles or 88.9 yards) sooner, and tired 12 seconds quicker. If you assume a direct relationship between girth tightness and run-to-fatigue times, then a horse running with a girth tightened to 13 kg (the average girth tightness in previous studies of Australian Thoroughbred racehorses) fatigues 650.1 meters (0.4 miles or 715.1 yards) earlier than a horse with a girth tightened to 5 kg, or takes 16 seconds longer to cover the same distance. Sixteen seconds is a long time in a race.
While tight girths certainly affected performance, the relationship might not be quite as linear as the previous calculation implies. All horses dropped performance between 5 kg and 10 kg. Above 10 kg, some horses' times and distances dropped off steeply, others tapered off, and two varied up and down. At 20 kg, the average run time and run distance climbed back up slightly to values between those found at 10 kg and 15 kg. However, all three were well below the average run time and distance for five kilograms and substantially close to each other.
Slocombe hopes that this line of research will lead to improvement in the health, welfare, and understanding of respiratory conditions of elite equine athletes. As with most scientific studies, the girth tightness answers some questions, but poses other questions still to be answered. For example, why?
Bowers and Slocombe propose several anatomic reasons why a tight girth might cause a horse to tire faster. Perhaps the constriction decreases blood flow to the tissues directly under the girth, causing the respiratory muscles to tire faster. Or the respiratory muscles might operate less efficiently.
Another confusing result is the lack of difference between girths at 10 kg and those tightened to 20 kg. It is possible that girth tensions over 10 kg might displace soft tissues and fluid within the thoracic wall, minimizing the effect of a tighter girth. Or a horse might be forced to use alternate breathing strategies, such as greater use of the diaphragm, which might offset further chest constriction.
"Part of the problem is that we're working with small numbers of horses with a good deal of variation between them," Slocombe says. "Clearly some horses are much more susceptible to over-tightening than others. There is probably a wide range of individual tolerances to tight girths. Also, if you measure the same animal's respiratory mechanical properties five to 10 minutes apart, you get a 5-10% variability. It's a fairly insensitive way to evaluate what might be happening in the chest."
Another problem is that a small difference in performance might not be considered a valid scientific finding for a researcher who must rely on statistical significance of results. Such a small difference can be painfully obvious to the trainers, jockeys, and owners watching a race, however. "The difference between first and sixth in a race can be as little as three meters or so," Slocombe says, "but in research we typically work on a 5% probability error (allowing a 5% range in data for error and individual variation). Five percent of a 1,400-meter race is 70 meters."
The study cautions that these results are based on run-to-fatigue test--horses in a sprint race might face no decline in performance from an over-tightened girth. Furthermore, in applying the study outside racing, a reduced girth tension might not take into effect the need for saddle stability in the sudden moves involved in cutting or jumping.
Slocombe and Bowers also studied elastic vs. non-elastic girths, and found no significant differences. "An elastic girth tightened to 15 kilograms of tension is less constrictive, but at exhalation it compresses the chest more than an inelastic one," Slocombe says. "It's not a huge squeeze, but the horse never gets a break from his chest being compressed, which may be about as bad as having a non-elastic girth overly tight at inhalation. We were able to show that an elastic girth protects against the extremely high tensions generated when using inelastic girths when the horse breathes in. This may be beneficial for comfort, but did not translate into improved run times."
Currently, the are working on prototype "smart" girths that will have a visual indicator of excessive girth tightness, and/or will have elastic properties better suited to the range of tensions that should be applied. Similar concepts are being worked out on sports bras for female athletes.
What It Means To Your Horse
Should you loosen your girth? Slocombe says, "It is our view that girths are frequently over-tightened. Riders should be aware that there is an optimum tension, probably at least 8-10 kg, in order to prevent saddle slippage. But if it is much higher than that, it will begin to have a negative impact on horse performance. This varies between horses.
"We also know that it is very difficult for the rider to tell just how tight the girth has been cinched up. Therefore, we are in the process of developing an elasticized girth with an indicator system to tell exactly what tension has been reached. Prior to this being made available commercially, my best advice would be to use a fully elasticized girth and saddle the horse with the minimum tension to secure the saddle adequately.
"How can you tell if a loss in performance is due to too tight a saddle? I do not think you can, except to run the animal with a less severely tightened girth and see if run times improve."
Bowers, J.R.; Slocombe, R.F. Influence of girth strap tensions on athletic performance of racehorses. Equine Exercise Physiology 5, Equine Veterinary Journal Supplement, 30, 52-56, 1999.
About the Author
Katherine Walcott is a freelance writer living in the countryside near Birmingham, Al. She writes for anyone she can talk into paying her and rides whatever disciplines she can talk her horses into doing.
POLL: Rehabbing the Injured Horse