Trainer measures your running fitness with VDOT, and uses it to set your training paces, predict race times and judge how realistic a goal is.
What VDOT is
VDOT is Jack Daniels' measure of running fitness. It's worked out from how fast you've run a given distance, using formulas he and Jimmy Gilbert fitted to the oxygen cost of running and to how long people can sustain a given effort. It behaves like a VO2max score (higher is fitter, and typical values fall between about 30 and 85), but it's based on performance rather than a lab test. So it also reflects your running economy and how well you can sustain effort.
Two runners with the same VDOT should race about the same times. A higher VDOT means faster paces in every training zone.
How Trainer estimates yours
Trainer looks at your outdoor runs from the last 12 weeks that were at least 3 km long. For each one, it works out a VDOT from the whole run and from your fastest 5 km within it (measured from GPS), and takes the highest. Treadmill runs are left out because their distances are often inaccurate.
Most training runs aren't all-out efforts, so this tends to underestimate your fitness. A recent race or time trial gives the most accurate number. Run one, import a newer export, and your paces update.
Race predictions
From your VDOT, Trainer predicts times for 5K, 10K, half marathon and marathon using Daniels' equivalent-performance tables. The predictions assume you've trained for the distance. A strong 5K runner with little endurance training won't hit their predicted marathon time, so treat longer-distance predictions as what your current speed could support with the right preparation.
Apple Watch VO2max
Your Apple Watch estimates VO2max from how your heart rate responds to effort during outdoor walks, runs and hikes. It's a different measurement from VDOT:
- VO2max is the size of your aerobic engine.
- VDOT is how fast you actually run, which also depends on running economy and how much of that engine you can sustain.
It's common for the Watch's number to be noticeably higher than your VDOT, especially when you haven't run any hard efforts recently. Trainer bases paces on VDOT, because it comes from your actual running.
How realistic is my goal?
When you set a target time, Trainer works out the VDOT that time needs and compares it with your current VDOT:
- Realistic: a gain of up to about 1 VDOT point per 4 weeks of training.
- Ambitious: up to about twice that.
- Unlikely: anything more. Trainer suggests a time a realistic gain would allow.
This is Trainer's own rule of thumb, not a research finding. Beginners and runners returning to training often improve faster, while experienced runners near their potential improve more slowly. Treat it as a guide, not a limit.
Sources
- Daniels J. Daniels' Running Formula. 3rd ed. Human Kinetics; 2014.