Moderate exercise during pregnancy is good for mares, researchers have found.
Evidence gathered by the American researchers also suggest mares benefit from greater cardiovascular efficiency during pregnancy.
"Additionally, the lack of a change in foetal heart-rate suggests that the unborn foal is not stressed during moderate maternal exercise," they wrote in a research paper published in the journal, Comparative Exercise Physiology.
Researchers Robert Lehnhard, Helio Manso Filho, Robert Causey, Malcolm Watford and Kenneth McKeever set out to test two theories:
- That moderate exercise would alter maternal and foetal heart-rate in
the horse.
- That pregnancy would alter the heart-rate, plasma cortisol and plasma lactate response to moderate exercise in mares.
Six unfit, pregnant standardbreds, aged 6-19, underwent two incremental graded exercise tests.
The first was performed after about nine months of pregnant - roughly 80 per cent of full term.
The second test was about six months after the horses gave birth.
The exercise sessions comprised the mares running on a treadmill on a fixed 6 per cent incline, completing three 1-minute steps at velocities of 4, 6 and 7 metres per second.
The heart rate of the mares and the foetuses were measured before the exercise, at each speed, and at one-minutes intervals for five minutes after the exercise.
Plasma cortisol concentration was measured in samples collected before the exercise, immediately after it and after five minutes of recovery. Plasma lactate concentrations were measured at rest, at each speed, and five minutes after the exercise.
The mares were all on the same diet, vaccination, worming and farriery programmes, and had identical pasture and living conditions.
"Exercise caused no change in foetal heart rate and their values were near 100 beats per minutes or below," the researchers said.
Maternal heart rate and plasma lactate levels were actually higher among the mares in the exercise test after they had given birth.
"Interestingly, during pregnancy, plasma cortisol concentration was significantly lower at rest compared with post-[pregnancy]," they said.
Furthermore, while there was a normal exercise-induced increase in plasma cortisol concentration during the post-pregnancy exercise, there was no similar exercise-induced increase in plasma cortisol concentration during pregnancy.
"The results obtained from the foetal heart-rate measurements made in this experiment demonstrated that one acute bout of moderately intense exercise does appear to induce foetal cardiovascular stress during the late gestational period," they noted.
"However, one should note that, because of noise on the electrocardiogram signal, there were no recordings of foetal heart rate reported during exercise, and we cannot say for certain that there was no effect on foetal heart-rate during the [exercise].
"However, the immediate post-exercise recording was done early enough during recovery to reflect the effect of the [exercise] on foetal heart rate.
"Interestingly," the authors noted, "pregnancy appeared to affect the mares' heart-rate response to exercise, suggesting a pregnancy-induced difference in their cardiovascular physiology."
The lower heart rate found during exercise while pregnant may be explained by a greater circulating blood volume compared with non-pregnant animals, they said.
"Foals from the pregnant mares used in this experiment were born healthy, with normal body weights and without complications associated with delivery," they said.
The results suggest that pregnant mares should be able to perform limited moderate exercise without any major deleterious effects on their unborn foals or themselves during late gestation, they said.
The research was sponsored by the New Jersey State Initiative on Equine and New Jersey Agricultural Experiment Station Project. Researcher Manso Filho was supported by a fellowship from CNPq/Brazil and University Federal Rural of Pernambuco, Brazil.
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