Application of the Critical Heart Model to Treadmill Running

The mathematical model used to estimate critical power has been applied to heart rate (HR) measurements during cycle ergometry to derive a fatigue threshold called the critical heart rate (CHR). This study had 2 purposes: (a) determine if the CHR model for cycle ergometry could be applied to treadmill running and (b) examine the times to exhaustion (T lim) and the Vo 2 responses during constant HR runs at the CHR. Thirteen runners (mean ± SD; age 23 ± 3 years) performed an incremental treadmill test to exhaustion. On separate days, 4 constant velocity runs to exhaustion were performed. The total number of heart beats (HB lim) for each velocity was calculated as the product of the average 5-second HR and T lim. The CHR was the slope coefficient of the HB lim vs. T lim relationship. The T lim and Vo 2 responses were recorded during a constant HR run at the CHR. Polynomial regression analyses were used to examine the patterns of responses for Vo 2 and velocity. The HB lim vs. T lim relationship (r 2 0.995-1.000) was described by the linear equation: HB lim a + CHR (T lim). The CHR (176 ± 7 b·min-1, 91 ± 3% HR peak) was maintained for 47.84 ± 11.04 minutes. There was no change in HR but quadratic decreases in velocity and Vo 2. These findings indicated that the CHR model for cycle ergometry was applicable to treadmill running and represented a sustainable (30-60 minutes) intensity but cannot be used to demarcate exercise intensity domains.