Same VO2 max. One lasted twice as long.

What the study found

Coyle and colleagues (1988, Journal of Applied Physiology) tested 14 trained cyclists with similar VO2 max and split them by lactate threshold: one group reached it at about 66% of VO2 max, the other at about 82%. Then everyone rode at 88% of their VO2 max until exhaustion.

The low threshold group lasted 29 minutes on average. The high threshold group lasted 61. Same engine size, very different endurance. The lactate threshold is the effort above which lactate starts piling up in the blood faster than the body clears it, and the higher it sits, the longer a hard pace stays sustainable.

The talk test: a field estimate

Quinn and Coons (2011, Journal of Sports Sciences) had 15 young adults read a short passage out loud while exercise got harder, and compared that with lab thresholds. The last stage where speech was still comfortable sat close to the lactate threshold.

You can use the same idea on a run. It will not give you a lab number, but it gives you a pace and a heart rate to start from.

1. Warm up for 10 minutes at an easy pace.
2. Speed up a little every 3 minutes.
3. At the end of each step, say one full sentence out loud.
4. The last pace where the sentence still comes out easy is close to your threshold. Note the pace and your heart rate there.

For any watch: find your threshold in past runs

Paste this into Claude or ChatGPT with your watch data. Asking for stretches where heart rate stays flat matters: late in a long run, heart rate drifts up at the same pace, and that drift would push the estimate too high.

Look at my runs from the last 8 weeks. Find the pace and heart rate where I spend time just above comfortable: steady runs where heart rate stays flat for 20 minutes or more without drifting up. Give me an estimated threshold heart rate and pace, tell me how confident you are with this much data, and suggest one threshold session for this week (for example 3 × 8 minutes at that heart rate with 2 minutes easy).

On Apple Watch: let telo read it and build the workout

With telo connected, the AI reads every run's duration, distance and average heart rate from Apple Health, and for the last 30 days it can open the heart rate series and the kilometer splits of a run to find steady stretches. It can also put the session on your Apple Watch as a planned workout.

Using telo, look at my runs from the last 8 weeks and estimate my threshold heart rate and pace. Then create a threshold workout on my Apple Watch: 10 min easy warm-up, 3 × 8 min at that heart rate with 2 min easy, 10 min cool-down.

What the test showed

On one account, telo returned 19 runs in 8 weeks, all recorded by an Apple Watch, with average heart rates from 126 to 151 bpm. Most were easy runs, so their averages say little about threshold on their own.

The useful one was an 81 minute run that got faster in the middle. Its heart rate series showed about 14 minutes steady between 160 and 167 bpm while the kilometers ran at 6:04 and 6:16 per km, after a stretch around 150 to 158 bpm at a slower pace. That puts a first estimate near 160 to 165 bpm and about 6:00 to 6:15 per km. Later in the same run heart rate stayed high while the pace slowed, which is drift, so the estimate is more likely a little high than low.

Where the data stops

The Coyle study had 14 trained male cyclists, seven per group; the talk test study had 15 young adults, and its protocol was reading a passage, not the run described here. The talk test is a field estimate, not a lab test, and it shifts with heat, fatigue and how hard you push the sentence.

Your watch stores minute by minute heart rate only for recent workouts; older runs keep a summary, so the AI can only find steady stretches in the last 30 days. Wrist heart rate can lag on fast changes. Retest every few weeks: the threshold moves with training. This is not medical advice.

Practical questions