Apple Watch reports different calorie burn for same exercise if tracked as an outdoor walk v. outdoor cycle

Calorie estimates are based on many factors, not just heart rate. Walking a certain distance requires far more energy than bicycling that same distance. I suggest doing some research into the different energy (and thus calorie) levels required for various activities. This information is widely available on the Internet. If one size fit all you wouldn't need different categories in the exercise app. All you would need is one - "being alive".

Even low levels of engagement of multiple muscle groups burns calories that may exceed more aggressive use of a single muscle group.

Some movements are much less mechanically efficient and require more power for the same result (distance)

Heart rate is mediated relatively directly by oxygen needs by muscles.

Consumption of calories by the body is mediated by other highly complex factors that may not overlap heart rate but are not directly measured by the watch

Because they cannot measure everything that goes into energy use by the body, Apple calorie estimates are best guesses based on calculations that factor many variables based on research.

https://support.apple.com/en-us/HT207941

mfhlaw wrote:

For anyone else who might weigh in, what I am saying is that the watch measures my exertion by monitoring my heart rate and calculating calories burned. It doesn’t otherwise know how hard I am exerting myself. All it knows is my heart rate. If the Watch records the same heart rate over time whether the exercise is tracked as a walk or a bike ride, why would the total calorie burn be different?

Calorie burn is not as simple as "higher heart rate=more calories". When you tell the watch which form of exercise you're doing, it factors that into the algorithm. Also included in that algorithm is your heart rate, your age and your weight.

Either you didn't read my post or didn't understand it. Either way you are now on your own.

Glad that made sense to you and was helpful.