Your interval timer is counting the wrong minutes.
A regular interval timer counts down four minutes the second you press start — whether your heart rate is at 60% or 90%. So the minute or two it takes to get into zone gets counted as “interval,” and no two sessions deliver the same dose. A threshold-gated timer fixes it by changing what the clock counts.
The ramp-up problem
Say you press start and begin a four-minute VO₂max interval. It takes you 90 seconds of hard work to drive your heart rate above 85% of max. A normal timer has already been counting that whole time — so only 2½ of the 4 minuteswere actually at the target intensity. Next session you feel fresher and get there in 60 seconds; now it's 3 minutes. The workout looks identical on paper, but the real dose changed.
Multiply that across four intervals and weeks of training and you get the core failure: your sessions aren't comparable, so you can't tell whether you're getting fitter or just having a better day.
The fix: gate the clock on heart rate
A threshold-gated timer only counts the four minutes while you're above 85% of max heart rate. Ramp up at your own pace — the clock waits. Drop below mid-interval and it pauses until you climb back. The result: every interval is four full minutes in zone, and every session is the same dose.
In the chart below, only the shaded time — heart rate at or above the dashed 85% line — counts toward an interval.
Side by side
| Normal interval timer | Threshold-gated | |
|---|---|---|
| What starts the clock | The moment you press start | When your heart rate crosses 85% of max |
| Counts ramp-up as work | Yes — the first 1–2 min are often below zone | No — ramp-up doesn't count |
| If your HR drops mid-interval | Keeps counting anyway | Pauses until you climb back above 85% |
| Time truly in zone per interval | Varies every session | Always the full four minutes |
| Sessions comparable over time | No — the dose changes | Yes — identical dose every time |
| Recovery trend trustworthy | No — different dose each time | Yes — same dose makes it comparable |
Who actually needs this
Threshold gating matters most for longer heart-rate intervals — the Norwegian 4×4 and similar VO₂max work, where the whole point is sustained time at a target heart rate. For very short bursts like Tabata you never reach a heart-rate steady state, so gating adds little there.
If you're chasing a trackable VO₂max or watching your heart-rate recovery improve, the comparability that gating gives you is the difference between guessing and knowing.
An interval timer built around this
Stellar Intervals is a heart-rate interval timer built on exactly this idea. It reads your heart rate live at one sample per second from a Polar strap, a Garmin sensor, or AirPods Pro 3, runs the threshold-gated 4×4, and records the recovery after each interval — so the dose is identical every session and your progress is finally trackable.
Frequently asked questions
- What is a threshold-gated interval timer?
An interval timer whose clock only runs while your heart rate is above a set threshold — for the Norwegian 4×4, that's 85% of your maximum. If your heart rate drops below the threshold mid-interval, the timer pauses and resumes when you climb back, so each interval delivers a full dose of in-zone work rather than counting your ramp-up.
- What's wrong with a normal HIIT interval timer?
A normal timer starts its countdown the instant you press start, regardless of your heart rate. Since it takes most people one to two minutes of hard effort to reach VO₂max intensity, part of every 'interval' isn't actually at the target intensity — and how much varies day to day, so your sessions aren't comparable and your progress isn't trackable.
- Is there an interval timer that pauses below a heart rate?
Yes — that's exactly what a threshold-gated timer does. Stellar Intervals pauses the interval clock whenever your heart rate falls below 85% of max and resumes it when you go back above, guaranteeing four full minutes in zone per interval.
- Do I need a heart-rate monitor for this?
Yes — the timer is driven by your live heart rate, so you need a sensor: a Polar or Garmin chest strap, AirPods Pro 3 (iOS), or any standard Bluetooth heart-rate monitor.
- Does it matter for short intervals like Tabata?
Less so. For very short efforts (20–30 seconds) you never reach a heart-rate steady state, so heart-rate gating isn't meaningful. It matters most for longer VO₂max intervals like the Norwegian 4×4, where the goal is sustained time at a target heart rate.
Train at VO₂max.
A heart-rate interval timer that only counts the minutes that matter — above 85% of max. Same dose, every session.