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Total cardiac cost is a measurement of the area under the curve obtained by plotting the heart rate for each minute during work and recovery. In a constant environment it varies with the work accomplished in a given time and, for a constant work performance, it varies with the severity of the conditions. The work accomplished per beat has a reciprocal relation to total cardiac cost when work and recovery times are constant. Recovery cost and the average of three early recovery heart rates, P1, P2, and P3, vary linearly with total cardiac cost when work or environmental stress increases. In these experiments P1, P2, and P3 are the heart rates per minute recorded during the last 30 sec of the 1st, 2nd, and 3rd min, respectively, of the recovery period. These recovery pulse rates provide a simple measurement of the degree of cardiac strain induced by work in any environment. cardiac cost; recovery pulse rates; work rate; warm environment; human male; human female Submitted on July 13, 1961

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