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The purity of the relationship between effort under controlled conditions and fixed power allows changes in performance to be more confidently attributed to actual changes in training status. This purity results from the absence of otherwise confounding variables and extends to the reliable measurement of effort as well.

Fitness is the relationship between effort and power.

Endurance is fitness over time with the primary basis of stimulus and progression being workout duration. To achieve proper stimulus, duration must result in adequate challenge to the body, which is based on the state of endurance. As an indicator of endurance development, cardiac drift can be used to optimize workout duration. The rule of thumb is to target a cardiac drift between 5-10%. Below 5% and subsequent workout duration should be increased. Above 10% and duration should be decreased. The singular focus on endurance development during the base phase allows microcycle uniformity (i.e. the same workout can be repeated) which provides for multiple repeating tests of training status. This enables highly-tuned workout frequency as well as a balance fatigue accumulation/dissipation in training approaches employing forced progression.

It's worth noting that adequate stimulus and recovery is completely agnostic to / compatible with training methodology with respect to how time is prioritized among targeted energy systems (zone distribution), forced vs responsive progression, progression based on duration vs intensity, and so on. This is because insight into training status is not something to ignore. It does mean, however, that the insight provided can absolutely affect or call for modification to the approach taken. For example, why continue to work on endurance after it is adequately developed and the body is ready for higher intensity sooner than a generic plan calls for? Conversely, trade-off analysis may be in order if substantial progress is still being made by the time base training is scheduled to end. Also, the ability to monitor training may call for elimination of arbitrary variation within, and inconsistent progression between, workouts. Removal of arbitrary variation is all upside and also better enables real-time load optimization via adding, extending and splitting intervals as needed to both ensure adequate workout stimulus and preserve workout quality. And the most obvious impact to the current training approach is adjustment of intensity, duration, frequency, ramp rate and recovery week frequency. None of these invalidate the training approach, however, as they merely adjust for these highly-personal factors to increase the effectiveness of the employed approach.

During the Build phase, intensity becomes the primary basis of stimulus and progression. To continually challenge the body, working power must increase as fitness improves. The purity of the relationship between effort (and its measurement) under controlled conditions and fixed power makes heart a reliable indicator of fitness improvement. Additionally, like cardiac drift does for endurance development, maximum sustainable intensity (MSI) self-regulates both intensity and progression.

Fatigue is ever-present, however, and changes much more quickly than fitness does and even more so than endurance. Because of this, the accumulation/dissipation of fatigue is easy to identify when rapid decreases/increases in heart rate coincide with increases/decreases in cardiac drift. This is because fatigue can cause an abnormally low heart rate (AKA suppressed heart rate or heart rate suppression) which disproportionately impacts the earlier part of a workout because heart rate will often return to normal levels later in the workout. This results in both an abnormally low average work interval heart rate and an exaggerated cardiac drift. Conversely, heart rate will increase along with a decrease in cardiac drift as fatigue dissipates. This phenomenon is 100% reproducible by simply performing a difficult workout three days in a row which will produce increasing cardiac drift, followed by a return of each to normal levels after a recovery day.