Heart Rate, Pace & Effort: How to Gauge Intensity

The three lenses

Pace is the easiest number to read, but the same pace is a different effort on different ground: uphill costs far more energy than level ground, and downhill costs less. Heart rate reflects what your body is doing, but it lags at the start of a run and drifts upward over a long run, by more in heat and with dehydration, and, a 2015 review notes, heat raises heart rate out of proportion to the work being done¹. Perceived effort, including the talk test, is always available.

Read pace, heart rate and feel together. When they agree, you know the intensity. When they disagree, that gap is information, and heat and dehydration are two common reasons a heart rate is high for the pace.

The heart-rate zones

Runsense sets your zones as percentages of your lactate-threshold heart rate (LTHR), using Joe Friel's running table. Your LTHR is lower than your maximum heart rate, so these percentages run higher than the maximum-heart-rate versions of five-zone models you may have seen. The guide on heart rate zones works the beats per minute through for an example LTHR.

• Zone 1, very easy: below 85% of LTHR.
• Zone 2, easy: 85 to 89% of LTHR.
• Zone 3, moderate: 90 to 94% of LTHR.
• Zone 4, hard: 95 to 99% of LTHR.
• Zone 5, maximum effort: 100% of LTHR and above. Your LTHR sits at the boundary between zones 4 and 5.

Why LTHR and not a formula

Age formulas for maximum heart rate fit a group better than a person. In a 2013 study of 762 adults the typical error was 12.4 beats a minute for 220 minus age and 11.4 for 208 minus 0.7 times age². In a 2018 study of 180 recreational marathon runners both formulas overestimated the measured maximum by about 5 beats in women, while 220 minus age underestimated it by about 3 beats in men³. LTHR comes from an effort you ran, not a formula.

You don't have to guess at it: Runsense can prescribe a guided lactate-threshold test and read your LTHR straight from the effort, then build your zones on it (see the guide on testing your LTHR).

The pace zones

The same five zones can be expressed as paces, anchored on your threshold pace: the pace you can sustain at lactate-threshold effort on flat ground. Pace zones read the opposite way from heart rate. Zone 5 is at threshold pace or faster, zone 4 runs from threshold pace to 6% longer per kilometer, zone 3 from 6 to 14% longer, zone 2 from 14 to 29% longer, and zone 1 is slower than that.

You never enter a threshold pace, and Runsense does not ask for one, because a guessed number would mis-anchor every zone. Runsense works it out from a guided threshold test or from a past run that was a threshold test in all but name, and only from a stretch on essentially flat ground. Every stored value carries where it came from and how much confidence it deserves. Pace numbers reach a prescription only when confidence in your threshold pace is high, they are stated as flat-ground paces, and they sit beside the zone and the feel and never replace them. Below high confidence the prescription keeps the zone and the feel and leaves the pace numbers out.

Why most of it should be easy

Keep easy days in zones 1 and 2, and if your easy runs fail the talk test, slow down. Easy running is what lets the hard days be hard. The guide on polarized training covers how much of your running should be easy, and the guide on the talk test covers how to check it.

Reading your own data

Look for patterns over several sessions and not a single run:

• Cardiac drift: a 2001 review found that on a long, steady run heart rate creeps up at a constant pace, starting after 10 to 20 minutes⁴. A 2015 review lists hyperthermia, dehydration, intensity and air temperature as changing its size¹, so when drift is larger than usual on a similar run, heat and fluids are the places to look. See the guide on heart rate decoupling.
• Pace depends on the ground and the weather. Read it against the hills and the heat before judging a run too fast or too slow.
• Cadence varies between runners. Watch your own trend and do not chase someone else's number; the guide on running cadence covers it.

When the numbers and the feel disagree

Do not simply trust the watch: ask why. Heat is one reason heart rate runs high for the pace, as a 2015 review notes¹, and one data point is not a trend. Treat the data as input to your judgment and not a replacement for it.

In a race, effort guards the pace. Marathon pace should feel controlled early, and if it already feels hard at mile 5, the pace is too aggressive. The guide on race-day pacing covers even and negative splits and the wall.