Physiological Responses at the Critical Heart Rate During Treadmill Running

The purposes of this study were to: 1) determine if the critical heart rate (CHR) model for cycle ergometry can be applied to treadmill running; and 2) examine the times to exhaustion (Tlim) as well as the VO2, ratings of perceived exertion (RPE), electromyographic amplitude (EMG AMP) and mean power frequency (MPF) responses during constant heart rate (HR) runs at CHR-5 b·min-1 (CHR-5), CHR, and CHR+5 b·min-1 (CHR+5). Thirteen runners performed an incremental treadmill test to exhaustion. On separate days, 4 constant velocity runs to exhaustion were performed. The total number of heart beats (HBlim) for each velocity was calculated as the product of the average 5 s HR and Tlim. The CHR was the slope coefficient of the HBlim versus Tlim relationship. The physiological responses were recorded during the constant HR runs. Polynomial regression analyses were used to examine the patterns of responses for all. The HBlim versus Tlim relationship (r2 = 0.995 – 1.000) was described by the linear equation: HBlim = a + CHR(Tlim). The CHR-5 (mean ± SD = 171 ± 8 b·min-1, 88 ± 3% HRpeak), CHR (175 ± 8 b·min-1, 91 ± 3% HRpeak), and CHR+5 (178 ± 6 b·min-1, 94 ± 3% HRpeak) were maintained for 56.97 ± 1.23, 48.37 ± 11.04, and 20.11 ± 16.08 min, respectively. There was no change in HR, quadratic decreases in velocity and VO2, and quadratic or linear increase in RPE during continuous runs at a constant HR. At CHR-5, EMG AMP decreased and EMG MPF increased. There was an increase in EMG AMP and no change in EMG MPF at CHR, while there was no change in EMG AMP and EMG MPF decreased at CHR+5. These findings indicated the CHR model for cycle ergometry was applicable to treadmill running, and, on average, HR values that were less than or equal to the CHR, represented sustainable (30 to 60 min) intensities, while CHR+5 did not. Adviser: Terry J. Hous