In this episode of the Critical Oxygen podcast, Dave Schell and I discuss the merits and limitations of internal and external variables in endurance training. We explore metrics such as heart rate, lactate, ventilation variables, SMO2, speed, and power. We emphasize the importance of measuring both internal and external variables to understand your physiology and performance. We also discuss the role of perceived exertion and the need for calibration between subjective and objective measurements. Overall, we highlight the value of monitoring and tracking variables to optimize training and improve performance.