# The science behind the balance What makes a run easy or hard? How much of each belongs in a week? Explore the ideas and research behind your training mix. Author: Zonometry Reviewed: 2026-09-21 Canonical: https://zonometry.com/science/ ## Contents - Start with the question you have: https://zonometry.com/science/#start-here - Three questions behind every percentage: https://zonometry.com/science/#three-questions - What the evidence actually says: https://zonometry.com/science/#evidence - A small vocabulary that prevents big misunderstandings: https://zonometry.com/science/#terms - See your training at a glance: https://zonometry.com/science/#app - Sources and further reading: https://zonometry.com/science/#sources ## Start with the question you have Training advice often begins with a percentage. We think it helps to start one step earlier: what are the zones, what are you counting, and what question are you trying to answer? Those choices determine whether two apparently different training mixes describe different training or simply different ways of measuring it. This library connects the physiology to the screen on your phone. Start with the guides to understand the zones, or explore the studies to see how different ways of training compare. You can follow the reading order below or jump straight to the question behind your latest run. 01 · Understand the modelWhat makes three zones different from five? Watch Seiler’s presentation, then explore the two physiological thresholds. (https://zonometry.com/science/seiler-training-zones/) 02 · Connect it to your watchHow do Apple’s zones become easy, moderate, and hard? A practical mapping, a worked heart-rate example, and the limits of automatic estimates. (https://zonometry.com/guides/apple-watch-heart-rate-zones/) 03 · Read your own trainingIs your mix telling the story you think it is? Compare sessions with minutes, and see why one interval day changes the picture. (https://zonometry.com/guides/training-distribution/) ## Three questions behind every percentage 1 ### Where are the boundaries? A numbered zone only means something once its definition is known. The research model uses two physiological thresholds. A watch may estimate boundaries from a different set of inputs. 2 ### What are you counting? Counting workouts and counting minutes can give you different percentages for the same week. A hard workout often includes plenty of easy time. 3 ### Over what period? A Monday interval session can dominate a new week. The same session becomes a smaller part of the mix after the rest of the week is recorded. A single day and a month answer different questions. These are also the questions to ask when comparing a study with an app. Matching the number “80” is less informative than matching the definitions that produced it. Our worked example (https://zonometry.com/guides/training-distribution/#same-week) shows identical training that looks like 80/20 by sessions and about 91/9 by time. ## What the evidence actually says Research gives polarized training a serious basis, but it does not establish one universally optimal percentage. A 2014 intervention found substantial improvements in a polarized group. A broader 2015 review described both polarized and pyramidal patterns among successful athletes. A 2024 meta-analysis found a small pooled advantage for VO₂peak, with no clear superiority on the other pooled performance outcomes it examined. Together, they suggest that polarized training can work well, but the best mix depends on the athlete and the goal. [1–3] (https://zonometry.com/science/#sources) 2014 · ExperimentFour training approaches 48 enrolled athletes. Nine weeks. Several laboratory outcomes. (https://zonometry.com/science/polarized-training-2014/)2015 · Focused reviewMany successful patterns Sport, season, and measurement method change the distribution. (https://zonometry.com/science/training-intensity-review-2015/)2024 · Meta-analysisA more qualified answer 17 studies. Different outcomes. Benefits that depend on the comparison. (https://zonometry.com/science/polarized-training-meta-analysis-2024/) Newer evidence keeps the question open. A 2025 individual-participant-data network meta-analysis reported no overall difference between polarized and pyramidal interventions for VO₂max or time-trial performance when classified using time in heart-rate zones. [4] (https://zonometry.com/science/#source-4) ## A small vocabulary that prevents big misunderstandings LT1 / VT1 The first lactate or ventilatory threshold: a physiological reference used to mark the upper boundary of the low-intensity region. The assessment method matters. LT2 / VT2 The second threshold used to distinguish the middle region from higher intensity. Lactate and ventilatory measurements are related approaches, not interchangeable numbers in every test. Polarized A distribution dominated by low intensity, with a smaller high-intensity component and relatively little in between. Pyramidal Most training is low intensity, less is moderate, and less again is high intensity. Time in zones Minutes assigned to each region. Warmups and recoveries can occupy different zones within one workout. Session classification Each workout receives a category, such as from its purpose or perceived effort. One hard session still counts once, however much easy time it contains. For more on these patterns, read the 2015 review (https://zonometry.com/science/training-intensity-review-2015/). ## See your training at a glance Zonometry brings your Apple Health workouts into one colourful training track. See how much of your week was easy, moderate or hard, then tap a run to explore its heart rate, effort, splits and route. Switch between minutes and workouts, set your targets, and make the zones your own. Connect Apple Health to get started with your watch’s zones, or enter personal boundaries from an exercise test. The Apple Watch guide (https://zonometry.com/guides/apple-watch-heart-rate-zones/) walks through the setup and shows how the numbers add up. ## Sources and further reading - Stöggl T, Sperlich B (2014). Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. (https://doi.org/10.3389/fphys.2014.00033) Frontiers in Physiology 5:33. Read our paper notes. (https://zonometry.com/science/polarized-training-2014/) - Stöggl TL, Sperlich B (2015). The training intensity distribution among well-trained and elite endurance athletes. (https://doi.org/10.3389/fphys.2015.00295) Frontiers in Physiology 6:295. - Silva Oliveira P, Boppre G, Fonseca H (2024). Comparison of Polarized Versus Other Types of Endurance Training Intensity Distribution on Athletes’ Endurance Performance: A Systematic Review with Meta-analysis. (https://doi.org/10.1007/s40279-024-02034-z) Sports Medicine 54:2071–2095. - Which Training Intensity Distribution Intervention will Produce the Greatest Improvements in Maximal Oxygen Uptake and Time-Trial Performance in Endurance Athletes? (https://pubmed.ncbi.nlm.nih.gov/39888556/) Sports Medicine (2025). Published abstract; individual-participant-data network meta-analysis. - Seiler S. Training Intensity Zones: general rules and importance of individual testing. (https://www.youtube.com/watch?v=NPwyk9B0j-s) Original presentation.