Equine Exercise Physiology Research

While the term sports medicine is of modern extraction, exercise physiology in humans and horses has been around for more than 100 years. During the decline of the use of work horses in the early to mid-20th Century, not much research was done. With the emergence and gaining popularity of horses as sport, recreation, and companion animals has come a resurgence of research into their abilities--and disabilities--as athletes. Since the 1970s, the amount of research on training, nutrition, medications, and problems of the various body parts and systems has grown tremendously around the world.

Because of this growing body of research, the people involved convened for the first time as a group in 1982 in Oxford, England. The International Conference on Equine Exercise Physiology (ICEEP) was formed, and it was determined that the researchers should meet every four years to share their expanding findings. Information on ICEEP can be found online at www.ICEEP.org.

Following are synopses of some of the presentations from the 2002 ICEEP conference held in Lexington, Ky., Sept. 22-26.

Training Older Horses

The purpose of this Rutgers study was to test the hypotheses that aging would result in a decline in maximal heart rate (HRmax) and maximal aerobic capacity (VO2max), and that those effects would be reversible with training. Researchers used 18 healthy, unfit Standardbred mares in three age groups: Young (about seven years old); middle-aged (about 15 years old); and old (about 27 years old). Various parameters were measured before and after the mares were trained. The training regimen was exercise three days/week for eight weeks, then four days/week from weeks nine to 12 at submaximal intensity (about 60% HRmax) for about 30 minutes/day.

The researchers said it is well documented that maximal oxygen uptake (VO2max) decreases with age in humans and horses. "Studies of man, horses, and other species have demonstrated that the decline in aerobic capacity is the result of aging-induced decreases in both peripheral and central mechanisms affecting the ability to transport and utilize oxygen," they noted. "Human studies have suggested that, centrally, this age-related decrease in VO2max is due to a decline in maximal cardiac output. In man, this decline in maximal cardiac output appears to be the result of both a decline in factors affecting stroke volume and a decrease in maximal heart rate."

The researchers also noted that recent studies have documented that unfit older horses cannot regulate their body temperature as well as unfit younger horses.

In conclusion, the researchers said, this study documents that as in man, the horse experiences an age-related reduction in maximal heart rate that appears to influence cardiac output and maximal aerobic capacity. Also, the effects of age appear to be substantially greater in horses over 20 years old. Training doesn't appear to reverse the decline in maximal heart rate, but does partially reverse the age-related decrease in maximal aerobic capacity. This appears to be the result of improvements in stroke volume and, presumably, cardiac output and improved peripheral mechanisms.

"Studies have shown that more than 15% of the equine population in the U.S. is over 20 years of age, with many of those horses participating in athletic activities," noted the researchers. "Recently published papers have examined the effects of age on the physiology of the horse and data have generated a new understanding of how management practices can be adapted to meet the needs of the older animal. More work is needed to understand the mechanisms behind those changes so as better to formulate training regimens for the older active horse."

Breed and Exercise Capacity

Some breeds of horses have been selected by humans as best for certain athletic efforts. For example, many people think Arabians are best for endurance competitions (low-intensity exercise), and Thoroughbreds best for shorter distances at greater speeds (high-intensity exercise). Researchers at Kentucky Equine Research and the Equine Studies Group of the Waltham Centre for Pet Nutrition in England wanted to find out if there was a quantifiable difference between the exercise capacity of Thoroughbreds and Arabians.

In this study, selected measures of exercise capacity and metabolism in a small group of Thoroughbred and Arabian horses of similar age, recent training background, and diet were evaluated, noted the researchers. They hypothesized that the facility of Thoroughbred horses for high-intensity exercise would be reflected in greater aerobic and anaerobic capacities compared to Arabian horses. Given that Arabian horses have a higher proportion of types I and IIa (oxidative) muscle fibers than Thoroughbreds (according to previous research), it was also hypothesized that the respiratory exchange ratio (ability to use aerobic energy) would be lower in the Arabian horses during low-intensity exercise, reflecting a greater use of fat for energy.

The researchers used five Arabians and five Thoroughbreds which were on similar diets and exercise programs for two months before the study. Following a period during which horses were trained to exercise on a treadmill, horses completed three treadmill (three-degree incline) trials. Researchers looked at:

  • An incremental exercise test for determination of the maximal rate of oxygen consumption (VO2max) and several other indices of exercise capacity;
  • A single high-speed exercise test at 115% VO2max for estimation of maximal accumulated oxygen deficit, a measure of anaerobic capacity; and
  • A 90-minute test at 35% VO2max to assess the relative use of carbohydrates and fats during low-intensity exercise. During all exercise tests, oxygen consumption, carbon dioxide production, and respiratory exchange ratio were measured.

The main findings from the study, according to the researchers, were:

  • A higher VO2max, maximum oxygen pulse, and run time (time to fatigue) in the Thoroughbred compared to Arabian horses during the incremental exercise test;
  • An apparently greater anaerobic capacity in the Thoroughbred horses, as evidenced by higher values for maximal accumulated oxygen deficit during a single high-speed exercise test; and
  • A lower respiratory exchange ratio in the Arabians during a 90-minute test at 35% VO2max, indicating a greater use of fat for energy during low-intensity exercise.

"In conclusion, this study confirmed our hypothesis that Thoroughbreds had higher aerobic and anaerobic capacities when compared to the Arabian horses, which probably contributed to their superior high-intensity exercise performance," noted the researchers. "Conversely, the Arabians may be better adapted for endurance exercise as suggested by the greater use of fat as an energy source during the lower intensity work. These metabolic differences might reflect the known breed variation in muscle fiber types. Further studies are required, however, to determine the mechanisms underlying this apparent breed difference in energy metabolism during high- and low-intensity exercise."

Tarsal Joint Kinematics

In a "baseline" type of biomechanics study, researchers at the McPhail Equine Performance Center at Michigan State University evaluated the tarsal (hock) joint kinematics of four sound Quarter Horses with reflective triads (markers with three branches that help measure rotation) attached to transcutaneous (protruding through the skin) bone pins--the first study to use this technique for kinematics evaluation. This type of marker system was designed to allow visualization of the tarsal joint's movement in three dimensions, rather than the two-dimensional analysis of a side view that is normally seen.

The study found that the majority of tarsal joint motion occurs in the tarsocrural joint (between the tibia and the talus in the upper joint of the hock), but some other translations (sliding) and rotations, both during the swing and stance phases, occur elsewhere in this complex joint. In other words, as with most studies, much was learned but questions were raised that will lead to future research. Having an understanding of how a normal hock works will help researchers identify abnormal motion from poor conformation or lameness.

Naìve Foal Jumping Characteristics

Another biomechanics study, this one done at the University of Utrecht in the Netherlands, sought to quantify the kinematics of free jumping foals and establish whether these values could be used to predict future jumping performance. The study evaluated 41 Dutch Warmblood foals studied with video analysis of reflective markers.

The left lead for both front and hind limbs was most common in the approach stride, jump stride, and move-off stride. A rotary gallop (often called cross-cantering or counter-cantering) was very common during the jump stride and move-off stride.

Based on several measures of jumping technique, including the canter/gallop lead on the approaching stride, jump stride, and move-off stride, and height of fore and hindlimb clearance over the jump, the researchers concluded that the foals' jumping techniques were similar to those of adult horses. If these foals' individual patterns remained consistent with later patterns as trained adults, then those values found to coincide with desirable performance over fences could possibly be used to predict jumping ability in other young horses. Half of the foals are in jumping training, while the other half have remained at pasture. The final results of this study are expected to be released by the next ICEEP conference.

Predicting Dressage Ability

Since particular gait characteristics are required for a horse to be a successful dressage competitor, researchers at Ecole Nationale d'Equitation in France and at the University of Cordoba in Spain collaborated on a study that attempted to quantify the gait characteristics of horses suited for dressage. The gait patterns and conformation of German, French, and Spanish saddle horses were tested, with the finding that the German horses (Hanoverians, Oldenburgs, Westphalians) were better suited for the FEI definitions of dressage movements. (The Federation Equestre Internationale, FEI, is the international governing body for equestrian sport.) German and French horses correlated fairly closely with each other in several measured gait and conformation parameters, with the Spanish horses being substantially different from both.

The researchers suggested that the values obtained for the German horses could be used as a reference for dressage breeding and performance prospect evaluation.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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