Circadian variation in blood pressure responses to muscular exercise

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Abstract
The physiological responses to strenuous steady-rate exercise are known to vary with time of day. Less well understood are the acute reactions to abrupt short-term all-out muscular efforts. This study examined circadian variation in blood pressure responses to short-term intense exercise. Twelve males participated in an experiment that involved performance of maximal muscular efforts at six times of the day. The time points were equally spaced throughout the day and for each subject the tests were conducted within a 2-week period. Systolic and diastolic blood pressures were measured pre-exercise using sphygmomanometry and body temperature was recorded with a rectal probe. Muscle function tests consisted of (1) three slow and three fast isokinetic movements, knee extensions performed under both concentric and eccentric conditions; (2) a 20s maximal isometric contraction, and (3) a 60s fatigue loading with concentric and eccentric contractions. Blood pressures were again measured immediately after the exercise bouts. Rectal temperature showed a significant circadian rhythm, the peak occurring at 18:18 hours according to cosinor analysis. No significant rhythm was observed in blood pressure pre-exercise (p>0·05). The systolic pressures post-exercise did not vary with the time of day. Circadian rhythms were found in diastolic pressure after the slow isokinetic movements, after the 20s isometric exercise and after the 60s fatigue test (p<0·05). The peak time varied between 00:37 and 02:05. Results suggest that blood pressure is most perturbed by acute exercise at night time. Caution is advocated in performing brief bouts of intense exercise at night or early morning.

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