HR zones vs pace zones — when to use which

It's the question that comes up most often when a runner graduates from "I go out and run" to "I train on a plan". Short answer: HR for day-to-day, pace only for quality sessions on controlled terrain. The long answer is interesting, and getting it right saves you a lot of botched sessions.

What each one measures

Heart rate measures internal effort. Your heart responds to EVERYTHING: terrain, heat, altitude, how you slept, what you ate, whether you're tired from yesterday. It's the only metric that reflects what your body is actually handling right now.

Pace measures external effort. It's what other people see (and your GPS): how fast you're moving across the ground. It doesn't care whether you've had three bad nights or whether the wind is in your face. It just shows minutes per mile.

Both are useful. They measure different things. Confusing them is the most common mistake.

When HR wins

Almost always. In order of importance:

Z2 days (most of your volume): you want your body to adapt to a specific aerobic stimulus. Control by pace and you'll push when you're tired and coast when you're fresh. Control by HR and you keep the same physiological stimulus every time.

Trail / mountain: pace swings 100% with the gradient. Going uphill at 8% grade at the same pace as flat puts you in Z4-Z5. By HR, everything self-regulates: speed drops but effort stays in Z2.

Heat or altitude: at 90°F or 10,000 ft, the same pace hammers you much faster. HR adjusts automatically.

Day after a hard session: if you slept poorly or your legs are heavy, your Z2 is 15-20 bpm higher than usual. By HR you respect it. By pace you slip into Z3 without noticing and you drag fatigue forward.

When pace wins

Well-defined quality sessions on controlled terrain. Concrete examples:

1000m repeats on a track: you want to hit a target pace. HR takes 60-90s to stabilize; in short intervals you use it as secondary reference.

Long run at marathon pace: you're simulating race day. You have to get used to the pace, not the effort.

20-40 min tempo on flat ground: you nail threshold pace and hang on.

In all of these, pace is the primary metric — but always glance at HR as a sanity check. If on rep 4 your HR is 12 bpm above normal at the same pace, something's off — back off or stop.

The classic mistake

"My Z2 is between 9:00 and 9:25/mi (5:30-5:50/km)" → you head up to altitude (12,000 ft) → that same "Z2 pace" puts you at 165 bpm. That's not Z2 — that's Z3-Z4. But because you clung to the pace number, you spent a whole week at the wrong intensity and you don't get why you're not improving.

Or the other way: you drop to sea level, it's 65°F, you slept well. Your "Z2 pace of 9:00/mi" today has you at 135 bpm when it should be 145-150. You're giving away effort and training too soft.

The simple rule

1. Z1, Z2 (recovery and base): ALWAYS by HR.

2. Z3, Z4 (threshold, tempo): by pace if it's flat and controlled, HR as a check.

3. Z5 (VO2max, short reps): by target pace, HR barely matters (it doesn't stabilize).

4. Trail / mountain: EVERYTHING by HR.

How to set your zones

HR: you need lactate threshold HR (LTHR) or recent max HR. Vetta computes the 5 Karvonen zones from your LTHR.

Pace: you need current VDOT or threshold pace. Daniels zones give you E (easy), M (marathon), T (tempo), I (interval), R (repetition).

If you have both, Vetta uses the one you pick as primary and the other as reference. If you don't know which to pick: HR by default.

When NOT to look at either

There's a third option: RPE (rating of perceived exertion 1-10). It's the better choice when:

Very hot days where HR goes nuts.

Treadmill that doesn't show real pace.

You're just starting and don't have reliable metrics yet.

Technical trail where you can't look at your watch.

Properly trained RPE is as precise as HR for experienced runners. It's underrated.

In Vetta

Your profile has a `zone_mode` field with two options: HR or pace. That choice tells the engine how to translate the philosophy's zones (Z1, Z2, Z3...) into concrete numbers in your sessions. For Veladero-style altitude rosters we recommend HR no question; for flat ground you can pick whichever you prefer.

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