Cycling Fitness / Training Advice

"The only direct way to actually measure maximal oxygen consumption is to use open-circuit spirometry in a laboratory."

"It is well known that there is a linear relationship between oxygen consumption and running speed. The oxygen cost of running increases when running speed increases. At identical submaximal speeds, an endurance athlete runs at a lower
percentage of his or her VO2max than an untrained person, although both maintain similar VO2 [6].
Technology (such as GPS sensors and foot pods in wrist devices or mobile phones) enables reliable measurement of running speed along with heart rate (HR). Therefore, these parameters can be monitored continuously and automatically during each workout. Because VO2max is a key variable to fitness training that needs to be easily measurable without additional protocols, an automatic VO2max estimation method applicable for any uncontrolled workout has been developed by Firstbeat.
The method is based on the well-known connection between heart rate and the speed of the activity (e.g. running, walking).

The following calculation steps are used for VO2max estimation:
1) The personal background info (at least age) is logged
2) The person starts to exercise with a device that measures heart rate and speed
3) The collected data is segmented to different heart rate ranges and the reliability of different data segments is calculated
4) The most reliable data segments are used for estimating the person's aerobic fitness level (VO2max) by utilizing either linear or nonlinear dependency between the person's
heart rate and speed data.

https://www.firstbeat.com/app/uploads/2015/10/white_paper_vo2_estimation.pdf

"VO2 Max estimator using the formula developed by James D. George in "VO2max estimation from a submaximal 1-mile track jog for fit college-age individuals." The equation was developed by having subjects run easy miles, which for the research was restricted to a minimum mile time of 8 minutes for males, and 9 minutes for females. The VO2 max estimate is calculated from a sustainable pace, and the associated heart rate. The calculated VO2 will be valid once your heart rate has stabilized at the pace you are running. This calculation will not be accurate during workouts such as tempo runs or intervals where effort is high or pace frequently changes. This data field is valid for RUNNING ONLY.

NOTE: This data field uses both gender and weight from the User Profile, the user profile must be accurate to receive valid results. The calculation also uses current speed and heart rate, so a heart rate monitor is required."