South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Challenges of endurance exercise: Overview of endurance work,
failure to finish, and causes of fatalities
Schott HC*
Professor, Equine Internal Medicine
Department of Large Animal Clinical Sciences
D-202 Veterinary Medical Center
Michigan State University, East Lansing, MI 48824-1314
(517)-353-9710 [email protected]
Long distance exercise provides a number of challenges to competing athletes. Repeated
concussion leads to musculoskeletal strain and eventual injury resulting in lameness as the most
common performance-limiting problem of endurance athletes. Next, prolonged, low intensity
endurance exercise imposes a thermoregulatory challenge by the heat load produced by ongoing
metabolic activity of muscle. In both humans and horses, this challenge is largely met by heat
dissipation through evaporation of sweat. Evaporative cooling is a highly efficient means of heat
dissipation, except when athletes compete under conditions of high ambient heat and humidity. In
contrast, canine athletes do not sweat: evaporative cooling occurs via panting and they usually
perform prolonged exercise in cool ambient conditions (Figure 1). Differences in sweat
composition between humans and horses lead to a comparatively greater loss of body electrolytes
with each liter of sweat produced by athletic horses. This physiological difference may be
important in the apparently greater risk to equine endurance athletes for development of
“metabolic” problems, ranging from premature fatigue to multiple organ failure and death, termed
“exhausted horse syndrome”. A further risk factor for “metabolic failure” in competitive equine
athletes is that the rider externally imposes the level of work effort sustained during prolonged
competition.
Figure 1. Humans, dogs, and horses all compete in endurance exercise events during which deficits in body water and
electrolyte content may develop. However, mechanisms of thermoregulation and maintenance of body fluids and
effective circulating volume vary between these species. Humans and horses utilize evaporative cooling via sweating
while dogs thermoregulate via respiration (panting). Human sweat becomes hypotonic with endurance training while
horse sweat remains nearly isotonic due to species differences in sweat glands. In contrast, dogs primarily lose water
via evaporative cooling across the respiratory tract and in urine. Thus, the magnitude of electrolyte depletion that may
develop during prolonged endurance competition also varies with species.
Although the exhausted horse syndrome is, unfortunately, a widely recognized phenomenon at
endurance events, the pathophysiology of “metabolic failure” and the array of associated clinical
problems that may develop during or after exercise remains incompletely understood.
Traditionally, dehydration consequent to prolonged sweating has been implicated as the most
important risk factor for metabolic failure. However, recent reports of human endurance exercise,
71