Heart-Rate Zones
Heart-rate lag: why heart rate trails effort in intervals, and how to read it
Heart-rate lag is the delay between a change in effort and the change in heart rate that follows it. When an interval starts, power or pace jumps within a second, but heart rate needs tens of seconds to rise most of the way and keeps climbing for longer at hard intensities. In efforts of a minute or less, the number on the screen therefore describes the work that has just happened more than the work happening now. For short intervals, effort should be set by output and perceived exertion, and heart rate read as a record of the accumulated load across the session.
What sets the speed of the response
Heart rate is controlled by two branches of the autonomic nervous system. The parasympathetic (vagal) branch slows the heart and acts within a beat or two. The sympathetic branch speeds it up and strengthens each contraction, but builds more slowly.
A 2014 review by White and Raven in the Journal of Physiology, drawing mainly on human studies that blocked one branch at a time with drugs, describes the sequence. At the start of exercise, signals from the brain reset the blood-pressure reflex, and the heart is released from part of its vagal brake almost immediately. As the workload rises, sympathetic activity increases more gradually. In the data the review summarises, the two branches had roughly equal influence at about 140 beats per minute, with sympathetic control dominating above that. The authors stress that the exact crossover differs between people.
The practical consequence is a two-speed response: a quick initial jump, then a slower climb that matters more the harder the effort.
How long the rise takes
Researchers describe the rise with a time constant, the time needed to complete about 63% of the change from the starting heart rate to the eventual plateau. After three time constants, about 95% of the change is complete.
A 2017 study in Physiological Reports measured this in 32 young men cycling for five minutes at light workloads. At 100 W, the time constant was about 8 seconds in endurance-trained cyclists and about 27 seconds in untrained men, with wide individual variation. When the trained cyclists worked at 60% of their VO₂max, the maximum rate at which the body can take up and use oxygen, the time constant lengthened to about 22 seconds. These were light efforts in young men, so the values understate the delay towards a peak during hard intervals, where the slower sympathetic climb continues.
Illustration: one 20-second effort, two response speeds
The following calculation uses a simple exponential model and the two time constants measured at 100 W. It illustrates the mechanism and does not predict any individual's readings.
| Time into the effort | Share of the rise completed, time constant 8 s | Share completed, time constant 27 s |
|---|---|---|
| 10 s | 71% | 31% |
| 20 s | 92% | 52% |
| 30 s | 98% | 67% |
| 60 s | about 100% | 89% |
Take a member with a stored maximum of 185 beats per minute. In the five-zone system Pulz8 uses, Zone 4 starts at 80% of maximum (148) and Zone 5 at 90% (167). Suppose the member starts a 20-second effort at 130 and the effort would eventually push heart rate to 170 if sustained, a rise of 40 beats. With a time constant of 8 seconds, heart rate reaches about 167 by the end of the effort, just inside Zone 5. With 27 seconds, it reaches about 151, Zone 4. The work is the same, yet the screen shows two different colours.
This is why a tile on the live board can look a zone lower than the effort feels during short work, then turn a zone higher after the work has stopped.
Short intervals: heart rate stays high during rest
Lag also affects the rest periods. A 2025 retrospective analysis by Astorino and colleagues pooled seven laboratory studies in which 104 active adults cycled intervals of 30 to 60 seconds at 70–85% of their maximal workload, with 10 to 75 seconds of recovery. Mean heart rate was 159 beats per minute during the work and 160 during the recovery, a difference that was not statistically significant. Peak heart rate averaged about 91% of maximum. The authors attribute the high values during recovery partly to the delayed rise during each short interval, which carries into the following rest.
The analysis covers cycling in a laboratory and compares averages across whole work and rest periods, so it does not show the beat-by-beat shape. Its main lesson for the gym holds: in formats with short rests, heart rate barely falls between efforts, and a tile that stays orange or red during a 15-second pause does not mean the member is working through the rest.
Heart rate climbs from set to set
Within a session, heart rate tends to rise across repeated sets even when the work per set stays similar. In a 2013 study of a body-weight Tabata workout, 16 trained volunteers completed four 4-minute segments of 20 seconds of work and 10 seconds of rest. Heart rate averaged 86% of maximum, and heart rate, perceived exertion and blood lactate all increased stepwise from one segment to the next. The report is a short letter with a small sample, but the pattern matches other interval studies.
Valstad and colleagues (2018) had 12 trained students run either 4 × 4 minutes at 90–95% of peak heart rate or four sets of eight 20-second maximal sprints with 10-second rests. Mean and peak heart rate did not differ between the sessions, and heart rate rose with every set in both. The short format was run faster, about 3.5 against 2.95 m/s, and was rated easier for breathing. Heart rate alone could not distinguish two sessions with very different running speeds.
Reading zones by interval length
| Format | What heart rate shows | Better guide during the work |
|---|---|---|
| Efforts of 30 s or less | Mostly the previous efforts; the response to the latest one shows at its end and into the rest | Output (reps, watts, pace) and perceived exertion |
| Efforts of about 1 min | The second half of each effort and the trend across sets | Output and perceived exertion, with heart rate as a check |
| Efforts of 3–5 min | The last two to three minutes give a meaningful zone | Heart rate together with breathing and perceived exertion |
Sensor type adds a further delay. Optical wrist sensors can respond more slowly than chest straps to sudden changes, as the article on heart-rate sensor accuracy explains.
Key points
- Set short intervals by output and effort, not by a heart-rate target that cannot be reached in time.
- Read heart rate at the end of an effort and in the first seconds of rest, where the response to that effort usually appears.
- Ignore the first minute of longer intervals when judging the zone, and use the last minutes instead.
- Expect heart rate to stay high during short rests and to climb from set to set at the same output.
- Compare members by trend, not by colour, because the speed of the response differs between people.
Sources
- White DW, Raven PB. Autonomic neural control of heart rate during dynamic exercise: revisited. Journal of Physiology, 2014.
- Trounson KM, Roberts S, Balloch A, Warmington SA. Light exercise heart rate on-kinetics: a comparison of data fitted with sigmoidal and exponential functions and the impact of fitness and exercise intensity. Physiological Reports, 2017.
- Astorino TA, Bogdanis GC, Costa EC. Heart rate and oxygen uptake during recovery from high-intensity interval training: a retrospective analysis. International Journal of Environmental Research and Public Health, 2025.
- Emberts T, Porcari J, Doberstein S, Steffen J, Foster C. Exercise intensity and energy expenditure of a Tabata workout. Journal of Sports Science and Medicine, 2013.
- Valstad SA, von Heimburg E, Welde B, van den Tillaar R. Comparison of long and short high-intensity interval exercise bouts on running performance, physiological and perceptual responses. Sports Medicine International Open, 2018.
Heart rate is a training signal, not medical advice. Medication such as beta-blockers, heat, illness and fatigue all change how heart rate responds, so anyone with a heart condition should agree on intensity guidance with their doctor.