Is it more effective to establish and adjust training zones based on power output, heart rate, or perceived exertion, and should this approach change as an athlete progresses from a beginner to a more advanced rider? Some argue that power output is the most reliable metric, while others claim that heart rate is a better indicator of an athletes true physiological state. Meanwhile, proponents of perceived exertion argue that it allows for a more nuanced and individualized approach to training. What are the benefits and drawbacks of each approach, and how do they impact the effectiveness of a training program over time? Should athletes focus on a single metric or use a combination of all three to inform their training decisions?