I am asked yet again how to find one's lactate threshold heart rate (LTHR) by doing a 30-minute time trial. I really don't understand what seems to be so difficult about this. All that's required is running (or riding) as hard as you can possibly go for 30 minutes ALL BY YOURSELF. It must be solo. Doing this as a part of a race or with training partners will change the outcome. Your number will be too high. If you want to do it with others or as a part of a race then you need to make it 60 minutes duration instead of 30.
Once you've captured the data in your device, download it to your software and find your average heart rate for the last 20 minutes. That's an approximation of your LTHR (also often referred to as anaerobic threshold or functional threshold heart rate). If you don't have software all you have to do is push the lap button 10 minutes into the test. That will then capture the last 20 minutes as a standalone "interval." Your average heart rate for that portion is close to your LTHR. Note that this DOES NOT mean that you go easy for 10 minutes and then turn it on with 20 minutes remaining. It's 30 minutes all out.
Do not watch your heart rate during the test. You're not trying to produce a given number. Do not be concerned with anything other than are you going as hard as you can go right now. If the answer is "yes" then you are doing the test right.