As discussion over the effects of riding horses' head and neck positions continues, Dutch scientists have determined that all non-natural head and neck positions, including those similar to the modern-day competitive dressage position, affect breathing, and hyperflexion ("rollkur") causes the greatest effect.
Artificial positions--head and neck positions created through tack, pressure, and training--cause a "dynamic obstruction" of the airways, making it more difficult for horses to inhale, said Inge Wijnberg, DVM, PhD, assistant professor of equine internal medicine at the Faculty of Veterinary Medicine at Utrecht University, in the Netherlands.
Like a cricked garden hose, the trachea (or windpipe) probably becomes kinked in these artificial positions. Meanwhile, surrounding tissues, especially the neck muscles, might push and bulge into the trachea causing further constriction. The result is the same as with the cricked garden hose: increased pressure and reduced flow.
"This phenomenon would (also) explain the abnormal inspiratory (inhaling) respiratory sounds sometimes noticed when an extreme position ... (is) assumed," Wijnberg said.
To test this theory Wijnberg and colleagues investigated the effects of five different head and neck positions--including hyperflexion--on seven healthy Dutch Warmblood horses trained to a basic dressage level. Handlers longed the horses at the walk, trot, and canter, with fixed reins holding their heads in the required position for testing. The researchers tested each horse in every position and gait so the horses could be their own controls. They compared each horse's individual results in all positions.
Wijnberg inserted balloon catheters into the horses' esophagus to measure the pressure inside the trachea produced by the different positions. Although the esophagus is part of the digestive system, this elastic tube in the chest cavity can be used to register pressure differences created in the chest cavity during breathing.
The researchers also monitored breathing frequency, body temperature, heart rate, and the proportion of various gases (oxygen, nitrogen, carbon dioxide, etc.) present in the horses' bloodstream.
Wijnberg's team noted, above all, that the commonly recognized hyperflexion position caused the highest increase in pressure in the trachea, which increased even more at faster gaits. This suggests hampered airflow, she said.
This pressure increase, however, did not affect the amount of oxygen reaching the blood, Wijnberg said. "Compared to a natural head and neck position, there was no significant difference in arterial blood gas values during hyperflexion or any other artificial position," she said. This was probably because the exercise level was very low," she added.
"These new results ... are definitely indicative of a less than 'normal' situation with artificial positions," she said.
Further research is needed to determine what, if any, effects this increased pressure has on horses.
The study, "Effect of head and neck position on intrathoracic pressure and arterial blood gas values in Dutch Warmblood riding horses during moderate exercise," appeared in April 2012 in the American Journal of Veterinary Research. The abstract can be viewed online.
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