Bergstrom, HC, Housh, TJ, Cochrane, KC, Jenkins, NDM, Buckner, SL, Goldsmith, JA, Zuniga, JM, Schmidt, RJ, Johnson, GO, and Cramer, JT. Application of the critical heart model to treadmill running. J Strength Cond Res 29 (8): 2237–2248, 2015—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 V [Combining Dot Above] O 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 V [Combining Dot Above] O 2 responses were recorded during a constant HR run at the CHR. Polynomial regression analyses were used to examine the patterns of responses for V [Combining Dot Above] O 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 V [Combining Dot Above] O 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.