The public health benefits of increasing the frequency of moderately intense physical activity are widely promoted 1 and many previous epidemiological studies have shown that individuals who are physically inactive have higher blood pressure and are more likely to develop hypertension. 2-5 However, the use of subjective measures of physical activity has made it difficult to accurately quantify the relationship with blood pressure. 6 Therefore, it is uncertain whether benefits would result from

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... mall changes in the frequency of moderately intense activity leading to raised overall energy expenditure. Quantitating the relationship between physical activity and blood pressure presents many measurement problems, and few studies have been able to employ methods which are both objective and quantitative. 6 Physical activity is multi-dimensional and it is rare for the separate effects of its underlying dimensions to be measured, 7 a problem compounded by the fact that they are not truly independent. Even when the underlying dimension can be specified and measured precisely, the true exposure is not physical activity at one time, but the usual or habitual level, a problem analogous to that of usual blood pressure measurement. 8 Background Previous studies have demonstrated an association between physical inactivity and hypertension, but the methods used to assess activity have been subjective and imprecise. Recently methods have become available allowing measurement of energy expenditure in free-living populations. Our aim was to employ these methods to assess the independent association between energy expenditure, cardio-respiratory fitness and blood pressure. Methods In a cross-sectional study of 775 people (45-70 years) participating in a continuing population-based cohort study, energy expenditure was assessed by 4 days of heart rate monitoring with individual calibration of the relationship between heart rate and energy expenditure, a method validated against doubly-labelled water and whole body calorimetry. Cardio-respiratory fitness was assessed in a sub-maximal test. To adjust for measurement error in the assessment of usual energy expenditure and fitness, 190 subjects repeated both tests on three further occasions at 4-monthly intervals. Results A highly significant linear trend in blood pressure was found across quintiles of the physical activity level, the ratio of total energy expenditure to basal metabolic rate. The differences in the mean systolic/diastolic blood pressure between the top and bottom quintile was 6.3/4.4 mmHg in men and 10.7/5.9 mmHg in women. These effects were independent of obesity and cardio-respiratory fitness. Correction for measurement error suggests that the true underlying relationship between usual energy expenditure and blood pressure is stronger still. Conclusions These findings are compatible with a strong association between usual energy expenditure and blood pressure and support public health strategies aimed at increasing overall energy expenditure.