Your LTHR can be measured in the lab by taking blood samples as exercise intensity increases. This is an intrusive test and not readily available to everyone. However, your LTHR can be estimated with a field test. You can test this value with swimming, biking or running.
The test consists of a warm up followed by the fastest pace that can be maintained for 30 minutes. This is a time trial and it is all out. These tests are very difficult and take a lot of mental and physical stamina. At 10 minutes into the 30 minute session hit the lap button on your heart rate monitor. Continue to push hard until 30 minutes has elapsed. At that point end the session. You should feel very relieved that the session is complete. Your estimated LTHR is your average heart rate for the last 20 minutes of the session.
You may need to do multiple LTHR tests (separated by several weeks) in order to get your best results. There is a skill to performing well at time trials. With this LTHR value you can determine your training zones. Although I have not done an all out 30 minute LTHR test I do have the value from the pump and run race. In that race I averaged 185 BPM. Using this value I get the following heart rate zones. The numbers in parenthesis are from the MHR (max heart rate) calculations yesterday (using the Karvonen method).
Zone 1: Overdistance, strength = (136 – 152) 139 - 157
75% - 85% LTHR
Zone 2: Endurance, strength = (153 – 160) 157 - 165
85% - 89% LTHR
Zone 3: Endurance, strength = (161 – 168) 166 - 174
90% - 94% LTHR
Zone 4: Intervals, hills, race/pace = (169 – 184) 175 - 184
95% - 99% LTHR
Zone 5: Racing, speed = (185 – 200) 185 - 196
100% - > 106% LTHR
These are different methods to determine heart rate training zones. As you can see, in the lower and upper heart rate zones the numbers are fairly close. However, zone 3 and zone 4 numbers diverge. For example, Zone 4 is where the LTHR is supposed to take place. There is an enormous difference between the lower ends of this zone (169 BPM compared to 175 BPM). For me, running at 169 BPH is much easier than running at 175 BPM.
I guess after all of the math you still need to use feel to determine where you should be training.
NOTE: One quick proviso – these zones might be a little high considering that this was a 20 minute all out effort and not the average of the last 20 minutes of a 30 minute all out effort.