Build training zones around effort, not age

Good training zones are built around your effort thresholds, the points where your breathing deepens and the burn in your legs steps up. Those thresholds fit you better than zones set from a maximum heart rate that was only guessed from your age.

Why it works

Two effort thresholds mark where your body shifts gears: the ventilatory threshold (where breathing jumps) and the lactate threshold (where the burn sets in). At those points, your body changes how it burns fuel and how it clears acid. Train just below, at, or above a threshold, and your body gets used to a different demand each time.

The evidence

• Bishop, D.J., Beck, B., Biddle, S.J.H. et al. (2025). Physical Activity and Exercise Intensity Terminology: A Joint ACSM Expert Statement and ESSA Consensus Statement. Medicine and Science in Sports and Exercise.

  This is a joint expert and consensus statement, not an experiment. A panel from the American College of Sports Medicine and Exercise and Sport Science Australia noted that inconsistent intensity labels across studies and guidelines hold back research and make exercise advice harder to follow. To fix that, they propose one shared scale of 5 intensity levels: very low, low, moderate, high, and very high. The paper defines each level so researchers, clinicians, and coaches can describe exercise intensity the same way.

• Garber, C.E., Blissmer, B., Deschenes, M.R. et al. (2011). ACSM Position Stand: Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults. Medicine and Science in Sports and Exercise.

  This is an ACSM position stand that sets exercise recommendations for healthy adults, not a single experiment. For heart and lung fitness it recommends moderate-intensity cardio for at least 30 minutes a day on 5 or more days a week (150 or more minutes a week), or vigorous cardio for at least 20 minutes a day on 3 or more days a week (75 or more minutes a week), or a mix of the two. It also recommends resistance training for all major muscle groups and neuromotor exercise (balance and coordination work) on 2 to 3 days a week, plus flexibility work. The overall energy target is 500 to 1,000 MET-minutes a week.

• Seiler, S. (2010). Training Intensity and Duration Distribution in Endurance Athletes: What Is Best Practice?. International Journal of Sports Physiology and Performance.

  Reviewing training logs from nationally and internationally competitive endurance athletes, Seiler found their intensity distribution converges on roughly 80% of sessions at low intensity (below 2 mM blood lactate) and about 20% at high intensity, a pattern often called polarized or 80/20 training. The review concludes that about 2 high-intensity sessions per week are enough to drive performance gains in already well-trained athletes, and that piling on more hard sessions raises fatigue and injury risk without added benefit. This low-heavy, hard-but-limited mix outperforms spending large amounts of time at threshold (moderate) intensity.

Why we call confidence high

Two expert panels back this. A joint ACSM/ESSA statement (Bishop et al. 2025) and an ACSM position stand (Garber et al. 2011) both recommend threshold-based zone prescriptions. Seiler 2010 set the framework for how endurance athletes distribute their training across intensities.

Where it applies

Healthy adults