AeT (Aerobic Threshold) — Your Pacer Wiki

Definition

AeT — the Aerobic Threshold — is the boundary between fat-dominant and glucose-dependent metabolism. Below AeT, your body produces energy almost entirely from fat oxidation. Above it, lactate begins to rise steadily above resting levels, and recovery cost starts to climb.

In the physiology literature the same line goes by several names — VT1 (first ventilatory threshold), LT1 (first lactate threshold), Easy pace upper bound (Daniels), MAF heart rate (Maffetone), DFA alpha1 = 0.75 (HRV-based). They describe the same physiological crossover from slightly different angles.

Why it matters to runners

Time spent at or below AeT is where aerobic capacity develops most efficiently. Mitochondria multiply. Capillary density increases. The body's ability to burn fat at a given pace gets better. This is why experienced coaches send athletes out for long, quiet, unspectacular miles — those miles are the foundation that higher-intensity work is eventually placed on top of.

Push too far above AeT for too long, and the cost rises faster than the benefit. Runners who live between AeT and AnT — the "grey zone" — often feel productive but accumulate fatigue without building the engine. Polarized training distributes intensity to the extremes precisely to avoid this trap.

Your Pacer treats your AeT as a moving reference point. Base-phase sessions are prescribed at or below it. When your AeT rises at the same heart rate — when the pace you could run "easy" gets faster — it shows up explicitly in the post-workout letter.

How it's measured

Several methods give converging estimates:

Heart-rate drift test (Johnston / House): 30 minutes at a steady effort. Compare the average HR of the first 15 minutes to the second 15. Drift < 3.5% means the pace was at or below AeT. See the HR Drift Test page for the full protocol.
DFA alpha1 = 0.75: a real-time HRV-based detection, objective and repeatable when HRV is recorded clean.
Lab test: blood lactate or ventilatory gas analysis — the most precise, also the most logistical.
Age formulas (180 − age, etc.): useful only as a starting estimate. Individual variance is large. Your Pacer uses these for the very first week, then overwrites with measured data.

Whatever the method, the estimate is an estimate. Confidence improves with repeated measurements and the data your wearable provides over weeks.

Single tests lie; convergence over time doesn't

A single drift test can be fooled. A plateau that looks flat for 30 minutes can be the real AeT, or it can be an LT-zone plateau that has not yet started to drift — the tell-tale acceleration usually begins around minute 60 to 75, past the window of a standard drift test. Without subjective input (RPE, breathing rhythm) it is often impossible to tell which is which from heart rate alone.

Your Pacer treats AeT estimation as a converging signal rather than a single measurement. The week-over-week trajectory of heart rate at the same pace disambiguates more reliably than any single session. Stable or declining plateau heart rate across three or more sessions in the same pace band means recovery is keeping pace — the intensity is at or below AeT. Rising plateau heart rate across the same sessions means the body cannot fully recover between runs — the intensity is above AeT, in LT territory.

When the device-only signal remains ambiguous, Your Pacer occasionally asks a single subjective question in the weekly letter — a talk test, a breathing-cadence check, an RPE score. One targeted question every few weeks, not a quiz. The answer closes the loop that pure heart-rate data cannot.

Related terms

AnT (Anaerobic Threshold) — the upper boundary of the aerobic-dominant zone.
ADS (Aerobic Deficiency Syndrome) — when AeT sits too far below AnT.
HR Drift Test — the standard self-administered AeT estimation protocol.
HRV — day-over-day recovery signal that feeds the same convergence read.