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. TAKEAWAYS 1) Measuring both internal and external variables is important in understanding an athlete's physiology and performance. 2) Heart rate, power, and speed are essential metrics to monitor during training. 3) Internal variables provide insights into how the body is coping with exercise, while external variables indicate performance. 4) Calibration between subjective and objective measurements is crucial for accurate training. 5) Monitoring and tracking variables over time can help optimize training and improve performance. If you have a question or topic you want us to address, leave a comment! Follow Phil on Instagram - https://www.instagram.com/criticalo2 Follow Dave on Instagram - https://www.instagram.com/kaizenendurance/ OR YouTube - https://www.youtube.com/@UCT5Cy95BiLIDQh4KS5bohhQ Interested in Gravel or Mountain Bike Coach? Head to Dave's website - https://www.kaizenendurance.coach/ CHAPTERS 00:00 Introduction and Overview 10:25 The Importance of Internal Variables in Understanding Adaptation and Change 23:45 Conclusion and Takeaways Keywords internal variables, external variables, endurance training, metrics, heart rate, lactate, ventilation variables, SMO2, speed, power, perceived exertion, calibration, physiology, performance