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Why a Power Meter is so much Better than Heart Rate Alone
Legal disclaimer: Since we are not doctors, never follow anything based on health-related topics on this or related sites without first consulting with your doctor or other trusted health professional.
A power meter is a terrific help on any workout where the goal is a consistent output effort (watts), offering constant biofeedback— not too hard and not too easy for the goal. One can also observe heart rate drift (via dehydration) across time, elevation, temperature, etc). It is thus an indispensable tool for objective assessment of performance.
Heart rate data is subject to all sorts of variations
Heart rate by itself can be a wildly variable indicator—all of the following can cause very different heart rates for the same power output:
- Hydration: dehydration steadily increases heart rate (heart rate drift, e.g., a higher heart rate for the same power output).
- Temperature: the same power output at 68°F vs 90°F shows a substantially lower heart rate, since the heart is not also working hard to cool the body.
- Humidity: high humidity forces the body to work much harder to cool itself.
- Elevation: at some point, the same power output will require much more anaerobic power generation and yet the heart rate will not behave the same way.
- Cadence: a high cadence increases the heart rate and breathing rate.
- State of recovery: unrecovered, the heart rate will be high. Overtrained, the heart rate might not be willing to rise to normal levels.
If the conditions are always constant, then heart rate can be used as a reliable reference most of the time, but it’s just too unreliable to train seriously in varying conditions day over day (and even same day), and especially with long workouts, where dehydration plays more and more of a role.
As another example, climbing from a 40° morning at 4000' elevation to a pleasant 70°F at 10,000' to a cool 40°F at 14,452' at the same effort will produce quite different heart rates—yet the power is the same.