The anaerobic threshold (AT) has been defined as the theoretical highest exercise level that can be maintained for prolonged periods. It is of practical importance to the competitive endurance athlete to measure progress and plan training programs. The primary objective of this study was to assess the reliability and validity of breakpoint in the respiratory rate (RR) during incremental exercise as a marker for the AT. Secondary objectives were 1) to assess the reliability of the ventilatory threshold (VE) and ventilatory equivalent (VE/VO₂) breakpoint, and 2) to assess differences in these 3 methods for their potential to measure change in fitness, as measured by standard error of measurement (SEM), coefficient of variability (CV), and correlation coefficient (R). Fifteen competitive male cyclists (5 category II, 6 category III, 1 category IV, 3 category V United States Cycling Federation) completed 2 maximal oxygen consumption tests within one week on an electronically braked cycle ergometer. A repeated measures Analysis of Variance using 2x3 design (test and methods) resulted in no significant differences (F = 0.02, p = 0.978), indicating that 1)all 3 methods are reproducible, and 2) RR, when compared to VE and VE/VO₂, is a valid method of assessing the anaerobic threshold. The lowest SEM, lowest CV and highest R were obtained with the VE method (SEM = 19.4 watts, CV = 6.7%, R = 0.872), compared to VE/VO₂ (SEM = 21.5 watts, CV = 7.4%, R=.811) and RR (SEM = 35.3 watts, CV = 12.2%, R = 0.800). From the results of this study, it is concluded that the RR method is a valid and reliable method for detecting AT. However, due to the relatively high SEM and CV, and low R, when compared to VE and VE/VO₂, its insensitivity to small changes seen in highly fit athletes would preclude its use in measuring changes in AT. It appears that either VE or VE/VO₂ would be appropriate for measuring AT changes in highly fit athletes.

• Respiratory rate is a valid and reliable marker of the anaerobic threshold.

• Due to a relatively high standard error of measurement and coefficient of variability for the respiratory rate method, use of ventilation (VE) and ventilatory equivalent for oxygen (VE/VO is preferred when assessing changes in anaerobic threshold.

• When assessing changes in maximal aerobic capacity, maximal watts has a lower standard error of measurement and coefficient of variability and is preferred over changes in maximal oxygen consumption.