How exercise affects circadian rhythms

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A new study suggests that an exercise program can influence a horse’s daily circadian rhythms enough to affect his athletic performance at particular times of day.

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Led by Barbara A. Murphy, BSc Eq, PhD, of University College--Dublin, the researchers examined the potential effects of an exercise regimen on the expression of a horse’s “core clock genes,” particularly those that influence muscle metabolism. “The core clock genes are very similar across all mammals and they are responsible for the 24-hour rhythms observed in almost every cell and organism in the body,” explains Murphy. “These clock genes make up what is known as an autoregulatory feedback loop, whereby the turning on and off of their genes and proteins forms a molecular clockwork that lasts 24 hours and is kept in synchrony with the environmental dawn and dusk through light signals that enter the eye. The muscle genes chosen in this study had previously been seen to undergo circadian (24 hour) oscillation in other species or were shown to have high importance for muscle metabolism in performance horses.”

For the study, researchers chose six healthy Thoroughbred mares who had not been on exercise programs. In the year prior to the study, the mares had been kept in a large pasture. When the research began, muscle biopsies were taken from each horse at four-hour intervals for 24 hours.

Researchers analyzed the samples to determine the horse’s natural circadian expression of muscle-specific clock genes. The horses were then put on an eight-week exercise regimen, which included 20-minute workouts conducted on an automatic walker at 10:30 a.m. each day. At the end of the study period, researchers again took biopsies from each horse every four hours over a 24-hour period. Gene expression in those samples was then compared to the patterns identified at the beginning of the study.

The data showed that prior to beginning the exercise program, the time-dependant muscle genes were expressed constantly across the 24-hour period.

“This fits with our understanding of horses maintained in a natural setting,” says Murphy. “They graze for up to 18 hours a day and are constantly moving throughout the day.”

At the end of the exercise pro- gram, however, a distinct pattern could be detected in muscle gene expression. “The shifts in the pattern of the genes matched their functions, so genes involved in regeneration and repair were not turned on at night, and genes whose protein products help protect muscles against stress were turned on just prior to the [10:30 a.m.] exercise. This is an anticipatory effect to provide optimal performance at the expected time of highest activity.”

Murphy adds these benefits would be most valuable to horses asked for short, intense exercise. “The shorter and faster the work, the more important it is that it be carried out at a time that matches the competition time,” she says. “Longer training, such as that for endurance work, will have a less important peak time for performance. It really affects the horses who only get out of their box for less than one hour a day; that hour becomes the time cue for muscle performance.” She concedes, however, timing exercise to competition times, particularly in the racing industry, “would be very hard to implement.”


Reference: “Exercise influences circadian gene expression in equine skeletal muscle,” The Veterinary Journal, July 2014


This article first appeared in EQUUS issue #445, October 2014. 

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