Maximum Heart rate
This is the average maximum number of times your heart should beat per minute during exercise.
Maximum heart rate multiplied by .70 = 70% of your maximum heart rate and it is the minimum
number of heartbeats per minute that the watch needs before it will track Vo2Max and record a
reading.
This hearbeat measurement must be kept up for at least 20 mins and must be constant.
Apple quotes the figure to measure Vo2Max from 70% up to 85% of your maximum heart rate so I
used the lower measurement as a bare minimum in which to get a reading.
Also the watch needs a fair bit of previous workout data before it will record a VO2max
reading,I've never been able to find out how much data that should be but it seems that the watch
needs this data as some sort of comparison or calibration data.
Only three workout types can produce an estimated Vo2max on Apple Watch: Outdoor Walk,
Outdoor Run and Hiking.