Two thresholds, two questions
LT1 is where blood lactate first begins to rise above its easy-effort baseline. It's close to the first ventilatory threshold (VT1), where breathing starts to make conversation less comfortable. For a runner, the useful question is: how hard can I go before an easy run stops feeling easy?
LT2 is the harder boundary, near the highest effort at which your body can keep up with lactate production for an extended period. Different lab protocols draw that line differently, so don't expect every test to return the same number. The useful question here is: what pace is hard but sustainable, rather than a pace I'll blow up trying to hold? For more on the physiology, see Lactate Threshold: Your Hidden Redline.
Neither threshold is a magic heart-rate reading that holds true on every hill or hot day. Think of your test results as starting points for training, not a pair of hard borders painted on the road.
| What you want to find | Start with | Take away |
|---|---|---|
| LT1: your approximate easy-effort ceiling | A gradual talk test | The heart rate and pace near your last comfortably spoken stage |
| LT2: your approximate sustained hard effort | A 30-minute solo time trial | Average pace for 30 minutes; average heart rate for the final 20 minutes |
Do the tests on separate, rested days. Choose a flat route or track, avoid unusually hot weather, and note the conditions so you can make a fair comparison when you retest.
Test 1: find LT1 with your voice
The talk test costs nothing and gives you a useful check on whether a pace is truly comfortable. It estimates a breathing-related threshold, not a blood-lactate measurement; those markers tend to be related but aren't identical (Persinger et al., 2004; Cerezuela-Espejo et al., 2018).
- Warm up for 15 minutes at an unmistakably easy pace.
- Increase the effort in small steps, holding each one long enough for your breathing and heart rate to settle—about three minutes is a workable field-test interval. Don't turn the first step into a tempo run.
- Say the same short paragraph aloud near the end of each step. Pick a few sentences you can recite without rushing. Ask yourself whether you can say them comfortably, not whether you can force the words out.
- Stop increasing the pace when speech first feels noticeably uncomfortable. Record the heart rate and pace of the last comfortable step, and note the next step where that changed.
That transition gives you a rough LT1 range. If your last comfortable step averaged 142 bpm and conversation became strained at the next step, don't proclaim that LT1 is exactly 142. For easy running, start below that point and check that you can still speak normally. The talk test has been studied more extensively in clinical settings than in trained runners, and even a well-run test has room for individual variation (Reed & Pipe, 2014).
Test 2: find LT2 with a 30-minute run
On another day, run 30 minutes at the fastest pace you can sustain fairly evenly from start to finish. It should feel controlled at first and demanding by the end—not like a sprint followed by a long fade.
- Pick a flat, uninterrupted course and arrive rested. Warm up for 15–20 minutes.
- Start the 30-minute effort conservatively. If you can't imagine holding the opening pace for the full test, ease off now.
- Settle into the hardest pace you can maintain without fading. Finish strong if you have something left, but don't let a last-minute sprint stand in for the preceding 29 minutes.
- Record your average pace over the full 30 minutes as a practical LT2 pace estimate. If you wore a reliable heart-rate monitor, record your average heart rate over the final 20 minutes as a practical LT2 heart-rate estimate.
Why two different windows? Pace tells you what you sustained for the entire trial. Heart rate takes time to rise after you start, so the common coaching protocol excludes the opening 10 minutes from its heart-rate average. That final-20-minute rule is a coaching convention, not the exact heart-rate method validated in the study below.
In a study of 27 competitive runners and triathletes, a 30-minute time trial estimated the speed associated with a lab's fixed 4 mmol/L lactate marker better than several other field methods. The standard error was 0.21 m/s for speed and 8 bpm for heart rate (McGehee et al., 2005). That's encouraging, but it was one small study using a fixed lactate concentration—not a demonstration that your LT2 falls within a guaranteed number of seconds per kilometre or beats per minute of this test.
If you want another check
Critical speed is an option for runners comfortable with several maximal efforts. Run two or, preferably, three hard efforts of different durations on separate rested days, then use distance and time to estimate the speed separating sustainable from unsustainable work. It has a strong physiological rationale, but the result depends on the quality and range of your efforts; it isn't automatically interchangeable with a 30-minute time trial or a lab's lactate threshold (Nixon et al., 2021). If your critical speed comes out faster than your continuous threshold pace, don't treat the two numbers as a contradiction to be averaged away. They measure related boundaries using different protocols.
DFA α1 can provide a second, chest-strap-based estimate around LT1 during a gradual ramp. A fixed value of 0.75 has shown promising group-level agreement with lab thresholds, but group-level agreement does not make it an exact personal cutoff. One runner study found wide individual limits of agreement despite a small average difference (Van Hooren et al., 2023); another study found poor repeatability at the first threshold (Sempere-Ruiz et al., 2024). It needs clean beat-to-beat data from a suitable chest strap. Use it to cross-check the talk test, not to overrule a clearly easy or clearly hard run. Our DFA α1 evidence review explains where the method helps and where it falls short.
If you have access to a lab, ask which threshold definition and protocol it uses. Fixed values such as 2 or 4 mmol/L aren't universal physiological switches, and changing stage length or detection method can change the answer (Faude et al., 2009). Compare later tests made the same way.
Turn the results into training decisions
Picture a runner whose talk test shifts from comfortable to strained somewhere around 140–148 bpm, and whose 30-minute trial averages 5:00/km with 165 bpm over its final 20 minutes. Those are useful observations, not a licence to call 144 bpm and 5:00/km perfect physiological lines.
On easy days, use the LT1 result as an approximate heart-rate ceiling, with conversation and perceived effort as checks. If the weather is hot, you're tired, or the route climbs, slow the pace to keep the effort easy. Don't try to recreate the pace from your LT1 ramp regardless of conditions.
For sustained threshold work, start from your LT2 pace on similar terrain and in similar weather. Heart rate is useful context, but it rises slowly at the start of a repetition and can drift during a long one. On hills or in heat, back off by feel rather than insisting on the flat-course test pace. That's the distinction behind training LT1 by heart rate and LT2 by pace.
When setting zones, enter tested threshold heart rates in the Heart Rate Zone Calculator using its threshold-based method, rather than substituting an age-predicted maximum. If you want race-derived workout paces, the Training Pace Calculator uses a recent race result; it does not take your LT2 pace as its input. Compare its suggested threshold pace with your trial, and treat disagreement as a reason to look at the inputs, not to average unlike measurements. The three-zone physiology model (below LT1, between LT1 and LT2, above LT2) also isn't the same numbering scheme as the calculator's five training zones.
Know when to retest—and when not to worry
Write down your protocol, route or treadmill settings, weather, average pace and heart rate, and how rested you felt. Repeat under comparable conditions after a meaningful training block—roughly six to eight weeks is a practical schedule, not a scientifically precise expiry date. Retest after a long break if your old numbers plainly no longer match how you run.
Don't overhaul your zones because one test differs slightly from the last. Field tests have measurement error, and sleep, heat, fatigue, pacing and heart-rate drift all add noise. Look for a repeatable shift across comparable runs. A pace that feels clearly easier at the same heart rate over several weeks tells a better story than one unusually good Tuesday.
The goal isn't to identify a perfect LT1 or LT2 down to the beat. It's to stop guessing whether today's run is easy, learn what sustained hard work feels like, and give your training a set of numbers you can check against reality.
References
- Persinger R et al. (2004). Consistency of the talk test for exercise prescription. Med Sci Sports Exerc.
- Cerezuela-Espejo V et al. (2018). The relationship between lactate and ventilatory thresholds in runners: validity and reliability. Front Physiol.
- Reed JL, Pipe AL (2014). The talk test: a useful tool for prescribing and monitoring exercise intensity. Curr Opin Cardiol.
- McGehee JC, Tanner CJ, Houmard JA (2005). A comparison of methods for estimating the lactate threshold. J Strength Cond Res.
- Nixon RJ et al. (2021). Steady-state VO₂ above MLSS: evidence that critical speed better represents maximal metabolic steady state in well-trained runners. Eur J Appl Physiol.
- Van Hooren B et al. (2023, online). Correlation properties of heart rate variability to assess the first ventilatory threshold and fatigue in runners. J Sports Sci.
- Sempere-Ruiz N et al. (2024). Reliability and validity of a non-linear index of heart rate variability to determine intensity thresholds. Front Physiol.
- Faude O, Kindermann W, Meyer T (2009). Lactate threshold concepts: how valid are they? Sports Med.
- Wingo JE et al. (2020). Cardiovascular drift and maximal oxygen uptake during running and cycling in the heat. Med Sci Sports Exerc.
This article is for general education and isn't medical advice. If you're injured or managing a health condition, clear new training with your clinician.
