Double product response is accelerated above the blood lactate threshold.

Catecholamine concentrations are known to increase sharply in the blood at work rates above the blood lactate threshold (LT); thus, we hypothesized that the double product (DP, heart rate-systolic blood pressure product) may also abruptly increase at work rates above the LT. Ninety healthy students performed a stepwise incremental test on a cycle ergometer. The slopes of the two regression lines below and above the LT for the heart rate (HR), the indirectly measured systolic blood pressure (SBP), and the DP, respectively, were compared using VO2 as an independent variable. For all three parameters the slope of the regression line above the LT was significantly higher than that below the LT. Although either the same or a lower slope was found in the HR (N = 23) or SBP (N = 22) responses in some subjects above the LT as compared with that below the LT, a steeper DP slope above the LT was evident in all 90 subjects. The same results were also obtained during 4-min constant work rate exercise sessions at a variety of submaximal intensities performed in a random order in seven subjects. In addition, we established the DP break point (DPBP) determining protocol with a fairly good test-retest reliability (r = 0.951) and a correlation coefficient between the DPBP and the LT (r = 0.900). These results suggest that the DP increases more steeply above the LT, and, as a result, the DPBP is considered to be a valid and useful parameter as a marker of the LT.