Heart rate, pace and power.

Your watch can show heart rate, pace and sometimes power. Coaches talk about FTP and zones. Here's what each one measures, and how they fit with running by feel.

The short version

During a run: go by feel (RPE and the talk test). Glance at pace only on flat, steady efforts where even splits are the point.
After a run: look at heart rate and pace to check yourself. Was the easy run really easy? Did a steady effort stay steady?
Every 6–8 weeks: a 20–30 minute time trial gives you a threshold pace, so your RPE 7 has a number to check against.

RPE: how hard it feels

Perceived effort takes in everything at once: pace, hills, heat, sleep, stress, how your legs feel today. Rating a whole session by effort tracks heart-rate-based measures of training load well across dozens of studies.

Its weak spot is the easy end. Many runners drift into "moderately hard" on days meant to be easy, without noticing. That's why the after-the-run check with heart rate is useful: it tells you whether your 4 really was a 4.

Heart rate: useful afterwards

Heart rate is a good record and a poor steering wheel:

It lags: it takes a minute or more to catch up with a change in effort, so it's no use for short repeats.
Wrist sensors get less accurate as you speed up, and can lock on to your cadence instead of your pulse.
It drifts up over a long run at the same effort, especially in the heat.
Zones are only as good as the maximum they're worked out from, and age formulas are often far off.

After the run, though, it's very useful. On an easy run it should stay low and flat. On a steady run, compare the first and second halves: if your pace held and heart rate rose only a little (around 5% or less), the effort was sustainable.

Pace: the outcome

Pace is what you're training to change, so it's the best measure of progress. As a target during a run it only works on flat ground in good conditions: up a hill or into a headwind, the same effort is slower. That's why steady-state sessions are best on the flat.

Power and FTP

FTP, functional threshold power, comes from cycling: the power you can hold for about an hour, measured by a power meter on the bike. Running has no direct equivalent built into most watches. The closest ideas are threshold pace and critical speed: the fastest pace you could hold steadily for somewhere between 30 and 60 minutes.

Running power pods estimate power from your movement. On flat ground they mostly restate pace: one study found the power at critical speed matched the pod's "critical power" almost exactly. Power can help on hills, but you don't need it to train well.

Finding your threshold pace

After a good warm-up, run 20–30 minutes as hard as you can hold evenly, on a flat route or a track. Your average pace is close to threshold pace, and it should have felt like a 7 rising to an 8 near the end. Repeat every 6–8 weeks, rested, and use it to check your tempo runs, not to chase during easy ones.

Not ready for a hard test? Skip it. RPE and the talk test do the job on their own.