While power-based training and dissecting every micro-watt in multiple ways appears to be the dominant "new wave," don't forget that there are other ways to monitor fatigue and predict performance that have been around for a long time and that can be much simpler, cheaper and potentially just as effective.
Cyclists are possibly the luckiest of modern athletes in terms of performance monitoring, in that we are the main sport in which we can record the actual power output of our efforts. This is due to the bicycle being a more stable platform compared to speed skating boots or running shoes. And the pedal stroke is relatively continuous, unlike more "cyclical" or "intermittent" efforts such as running, speed skating, or rowing. Because of this, we have been blessed with a plethora of options for power monitors.
With the popularization of power monitors have come a quantum advance in training methodology based on wattage output. Driving the advance has been Dr. Andy Coggan and TrainingPeaks' development of parameters such as Training Stress Scores (TSS), Acute Training Load (ATL), Chronic Training Load (CTL) and Training Stress Balance (TSB). The objective with these analyses is to determine whether we are well rested, undertrained, or overtrained.
There is no doubt that power-based training can be very "power"-ful and beneficial, but it cannot be denied that power monitors are still in their infancy in their adoption by the general peloton. This is due to the expense of the equipment and to their real or perceived complexity in use and analysis.
Which begs the question: Are there other ways to monitor performance?
Absolutely—and it's one that should be very close to all of our hearts—the HRM (heart rate monitor).
Sometimes perceived or advertised as a poorer cousin to the power monitor, the HRM is a powerful tool in its own right. Yes, heart rate response to exercise can be acutely affected by some factors not related to your true physiological state, such as caffeine and other stimulants. Other modulating factors, such as hydration status, fatigue, emotional stress, sleep quality, etc., have been some experts think that HRMs are unreliable because of the variability.
However, it can equally be argued that heart rate is valuable as a performance monitor specifically because it can be affected by your physiological state. Whether you have a power monitor or not, paying careful attention to your heart rate response to different efforts can give you good insight into your readiness to train or race.