New Lactate Threshold after interval training?

Question

I understand what lactate threshold is, and the "normal" procedure for guesstimating it (short of a lab trial with actual measurements) seems to be to run a private 20 minute race after warming up and taking the average of the HR at the start and end of that time, and the average pace.

I have been using a Garmin Fenix 6 watch for years (running and other sports), and am often running diverse training sessions - sprints, intervals, fartleks etc. in addition to long slow runs.

Now to the question: after some runs of sufficient length and difficulty, the watch offers a new estimate of my LTHR. For example, today I ran a VO2max training session (45 minutes of easy pace, with two 4 x (30sec max effort - 30sec low pace) intervals broken up by a 5 minute recovery period). After this session, the watch offered a new LTHR estimation, which was only slightly different from my previous one, so I have no immediate cause to doubt it.

Reading Garmin's info about their LTHR procedure it only mentions some relationship to breathing, HRV etc. - I guess the actual algorithm used is a well-kept secret.

Is it safe to assume that a sport watch has enough measurements to guess a relatively correct LTHR based on "whatever" training session? My run was nowhere near a smooth, steady pace, as the usual DIY method implies, but was very spiky, owing to the all out sprint intervals. Are there scientifically proven methods that do not require a steady race pace, or is it more likely that the watch "thought" it was a regular run for whatever reason, and took a wild guess?

Answer 1

To answer the primary question, the science generally says that you have to do the treadmill test in a lab to find your precise V02 Max. Your watch is using a lot of data to approximate it, but without actually measuring your oxygen consumption during exertion, it isn't possible to determine your V02 Max. I have a Garmin watch as well and the estimates from some of my training runs seem reasonable, but I have never done the treadmill tests to verify it. I found a local provider that will do the test for $200. You could check your local area if you are interested in doing the test to get your real V02 Max.

One of the most renown experts on this subject is Jack Daniels (the PhD, not the moonshiner). His book, 'Daniels' Running Formula', which is now in 4th edition, is one of the definitive works on training paces and training plans. He generally focuses on VDOT (Velocity at VO2 max) as the primary factor in determining race and training paces. His method of determining VDOT is basically just 'run a race as fast as you can'. He has done a lot of study to come to that conclusion and coached a lot of very successful runners over many years. You may find his VDOT calculator useful.

I think he focuses on VDOT because it is an easy, practical measure of where you are and then he uses that to help you build a training plan to move forward. Many other coaching systems are based on the same principles. So you can do the real V02 Max test and get a definitive result, which will tell you your theoretical maximum paces. It may be a little different from what your watch is telling you. You could also use VDOT and JD's running formulas as a practical method of determining training and race paces. The second approach is cheaper and easier and probably just as good if all you want to do is improve your time in your next race.