The Lactate Threshold Heart Rate (LTHR) is one of the most critical metrics for endurance runners. It represents the specific heart rate intensity where lactate (a byproduct of glucose metabolism) begins to accumulate in the bloodstream faster than your body can flush it out.
Below this threshold, you run aerobically, meaning you can sustain the effort for a long time. Above this threshold, you enter an anaerobic state; your muscles begin to burn, breathing becomes labored, and fatigue sets in rapidly. Knowing your LTHR allows you to set precise training zones, ensuring that your easy runs are truly easy and your hard workouts are effective.
How to Calculate Your LTHR: The 30-Minute Field Test
While you can get tested in a sports lab, the most reliable DIY method for runners is the 30-minute Time Trial developed by elite coach Joe Friel. Here is how to perform it properly:
Warm-up: Jog easily for 10–15 minutes. Perform a few strides (short accelerations) to get your heart rate up.
The Time Trial: Run for 30 minutes all-out. This should be a race-pace effort that you can sustain for exactly 30 minutes—no longer. It should be consistent; do not start too fast and fade.
The Data: At 10 minutes into the run, hit the "Lap" button on your heart rate monitor. You want to capture the average heart rate for the final 20 minutes of the run.
The Calculation: Your average heart rate for those last 20 minutes is a very close approximation of your LTHR.
Enter that average number into the calculator above to see your customized training zones.
Understanding Your Running Zones
Training by zones is more effective than training by pace because it accounts for heat, terrain, and fatigue. Here is what the zones calculated above mean for your training:
Zone 1 (Recovery): Very easy effort. Used for warm-ups, cool-downs, and recovery runs. You should be able to hold a full conversation.
Zone 2 (Aerobic): The "bread and butter" of endurance training. This builds mitochondrial density and capillary networks. It feels sustainable but requires focus to keep from going too hard.
Zone 3 (Tempo): Often called "grey zone" training. It is harder than easy running but not hard enough to elicit maximum adaptation. Use sparingly or for specific marathon-pace work.
Zone 4 (Sub-Threshold): Just below your red line. This raises your threshold, allowing you to run faster for longer without fatigue.
Zone 5a/b/c (Anaerobic): High-intensity intervals. These zones improve your VO2 max and speed but require significant recovery time.
Why Not Use 220 Minus Age?
The old formula of "220 minus your age" estimates your Maximum Heart Rate, not your Lactate Threshold. Furthermore, it is a generic statistical average that can be off by as much as 10-20 beats per minute for individuals. Basing training zones on an incorrect Max HR can lead to overtraining or undertraining. The LTHR field test is performance-based, making it a much more accurate reflection of your current fitness physiology.