The LTHR is the point, above which, increased blood acidification occurs in your body. Crossing this threshold will make your endurance performance rapidly decrease. In a race, you might maintain that level for up to an hour, but at a certain point you will slow down. When the lactate clears from your system, you can pick up the pace again, but the point is to avoid slowing down altogether. (Confused about the acronyms? Take a loot at our article "What The Heck is FTP, TSS, IF, and LTHR?")
The estimation of Lactate Threshold Heart Rate (LTHR) can be quite a mixed bag of theories. You can do a search on the internet and see what we mean. The only true way to get an accurate and reliable LTHR reading is to do a lab test where your blood is drawn and the lactate is measured. Without lab testing, we are all just "Estimating" LTHR.
The Sufferfest Training System will estimate LTHR on the video "Rubber Glove". We use the formula provided by Apex Coaching which is your average 20 minute heart rate during your FTP test, minus 12 beats per minute. This isn't the same as everyone else. However, Apex coaching has proven success using this method since they train Olympic cyclists, have coached two athletes that have broken the World Hour Record, and have helped athletes with Stage wins at the Tour de France.
If you want more information about LTHR and our reason behind our decisions, please take a look at our blog article "Everything You Think You Know About LTHR is Wrong."