American Journal of Clinical Nutrition, Vol 24, 1139-1147, Copyright © 1971 by The American Society for Clinical Nutrition, Inc.

Net heart rate as a substitute for respiratory calorimetry

¹ From the Ergonomics Section of The Operations Management Laboratory, Graduate School of Business Administration, University of California at Los Angeles

If one wishes to estimate the rate of energy expenditure during some kind of activity without employing direct or respiratory calorimetry, currently there would appear to be only two choices, regression equations based on heart rate and the combination respirometer-diary technique (20). The most promising heart-rate method is the development of a single set of regression equations that are based on net heart rate to minimize intersubject differences. Each regression equation in the set would be applicable to a given category of activities, which have similar circulatory patterns and hence are characterized by a similar relationship between the net heart rate and net rate of energy expenditure. Such a set of standard regression equations could be used in either of the following two manners. First, if all of the subject's activities were such that a single standard regression equation would suffice, only the net heart rate would need to be monitored during the study. Second, if the subject's activities were such that regression equations for more than one category were required, a diary would be needed to record the time periods, within which each category of activity was performed, in addition to the continuous monitoring of the heart rate. For example, if the subject were engaged in activities, some of which were performed while seated and some while walking, it would be necessary to record those periods in which the subject was seated and those in which he was walking. From such a diary, the proper regression equation for estimating the rate of energy expenditure could be applied to each period depending upon whether it was one of sitting or walking. It appears that only a few categories (e.g., standing, sitting, bending over, and walking) would be required to represent the groupings of activities that are characterized by particular circulatory patterns, leading to unique relationships between the heart rate and rate of energy expenditure. Thus, even if a diary is needed, only a few categories of activities and their associated regression equations would have to be used.

Compared with the respirometer-diary method, the heart-rate method has the disadvantage of requiring the continuous monitoring of heart rate. It is well established, however, that present-day miniaturized telemetry systems are reliable and do not interfere significantly with the subject in his performance of activities. The regression method has three advantages over the respirometer-diary method. First, a standard set of regression equations can be developed. This eliminates the kind of calibration that is required for the respirometer-diary method every time a new activity or new subject is studied. Second, whenever a single regression equation will suffice for all activities performed during the period of the study, no diary needs to be maintained. Finally, the estimate of energy expenditure is based upon measurement, albeit of the heart rate, during the actual performance of the activities. If the subject alters his intensity of performance, this alteration is reflected in changes in the heart rate. In the respirometer-diary method, a single estimate of rate of energy expenditure for each principal activity is used which, although intended to be representative, is determined outside the period of activity actually under study.

If one is going to employ the heart-rate method, the studies reviewed indicate that the key trade-off that the researcher must make is between the level of aggregation, in terms of subjects or tasks, or both, and the error of estimation. In terms of subjects, the choice is between separate regression equations for each subject and a common regression equation for all subjects. The cost of aggregating subjects is an increase in the standard deviation, which represents the residual variation about the regression equation based upon the net heart rate, of about 0.10 kcal/min. In terms of tasks, the choice is between separate regression equations for each task and a common regression equation for a group of tasks that are characterized by similar circulatory patterns. The cost of aggregating tasks is an increase in the standard deviation, which represents the residual variation about the regression equation based upon the net heart rate, of about 0.12 kcal/min.

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