Roughly, yes, if you use a chest strap. During a workout that gets steadily harder, an HRV measure called DFA α1 DFA α1 The short-term scaling exponent of detrended fluctuation analysis. Near 1 the beats have memory at every scale; near 0.5 they behave like noise. α2 is the same slope over longer stretches. More about DFA α1 falls from about 1.0 at easy effort towards 0.5 at hard effort. The heart rate where it crosses 0.75 has matched the first ventilatory (aerobic) threshold within about 2 beats a minute on average (Rogers and colleagues, 2021).
What DFA α1 measures
Most HRV measures ask how much the gaps between beats vary. Detrended fluctuation analysis (DFA) asks how they vary: whether a long gap tends to be followed by more long gaps, in a pattern that repeats across short and long stretches. α1 is the short-range version, over windows of about 4 to 16 beats (Quigley and colleagues, 2024).
- An α1 near 1.0 means the gaps are correlated in a fractal way, typical at rest and very easy effort.
- An α1 near 0.5 means they behave like random noise, with no memory from one beat to the next.
Switch the explorer between its presets to see both ends of that scale, and how the fitted slope changes:
Does the rhythm remember?
Five minutes of the teaching recording
- α1 (4–16 beats)
- 0.49
- α2 (16–64 beats)
- 0.65
The rested recording is synthetic: a clean breathing wave every four beats with random noise between beats, so over 4 to 16 beats it looks almost like noise (α1 0.49), and the slower waves give 16 to 64 beats some memory (α2 0.65). Real recordings at rest usually give an α1 of about 1 or higher.
The controls work when JavaScript is on.
A log–log chart of fluctuation against box size from 4 to 64 beats, with dots for each box size and two fitted lines: one over 4 to 16 beats for α1, one over 16 to 64 beats for α2. Readouts give α1 and α2, followed by a sentence explaining them. Four buttons choose the rested recording, white noise, 1/f noise or the recording with ectopic beats; a slider mixes white noise into the chosen series.
During exercise, the vagal brake lets go as effort rises. Michael and colleagues (2017) note that RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD drops below 5 ms by moderate effort, which leaves classic HRV measures with little to report. α1 keeps changing across the whole range, which is what makes it useful for intensity.
What the studies found
Rogers and colleagues (2021) tested 15 men on an incremental treadmill run with a research ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG at 1,000 Hz and a gas analyser. α1 was worked out over 2-minute windows, updated every 5 seconds. They took 0.75, halfway between 1.0 and 0.5, as the marker:
| Mean difference from the gas threshold | Agreement | |
|---|---|---|
| Heart rate at α1 = 0.75 | 1.9 beats a minute higher (154 vs 152 on average) | Individual differences from about 8 below to 12 above; r = 0.97 |
| Oxygen uptake at α1 = 0.75 | 0.33 ml/kg/min higher | r = 0.99 |
Rogers and Gronwald (2022) reviewed the work since: 0.75 linked with the aerobic threshold in recreational runners, cardiac patients and elite triathletes, and a further fall to about 0.5 went with the second, anaerobic threshold.
The limits
- Bad beats. Rogers and colleagues excluded 2 of 17 men because of frequent extra beats from the upper chambers. Rogers and Gronwald report negligible bias in α1 with 1 to 3% of beats corrected and larger bias at 6% with one correction method, although the threshold heart rate moved by no more than 1 beat a minute.
- Sensors. The first study used a 1,000 Hz ECG. A Polar H7 strap gave thresholds about 4 beats a minute lower than an ECG. Optical wrist sensors have not been shown to be accurate enough.
- Who was tested. The 2021 study had no women. Results come from incremental ramps, not steady workouts.
- Individual spread. An average difference of 2 beats a minute hides individual differences of up to 12.
How to try it
- Wear a chest strap that reports beat-to-beat intervals.
- Do an incremental test: start easy and raise the pace or power every few minutes until hard.
- Use an app that shows α1 live or analyses the file afterwards, and note the heart rate where α1 crosses 0.75.
- Repeat on another day, and check the number against how the effort feels and how your breathing changes.
This site’s HRV calculator cannot do step 3. It gives one α1 for the whole file you load. A ramp test needs α1 worked out over short moving windows, such as the 2-minute windows in the study above, and plotted against heart rate, so use an app built for that.
If you have known heart rhythm problems or frequent palpitations, α1 is likely to be unreliable, and you should talk to your doctor before maximal testing. For the bigger picture of HRV in training, see HRV for training and recovery.