Exercise Intensity Zones

The zones 1-4 below illustrate how the body switches between the different mechanisms of ATP1 regeneration, based on exercise intensity and duration.

As exercise intensity increases, metabolic processes move from cardiovascular (aerobic) to biochemical (anaerobic).2 This is called the "Kindermann aerobic-anaerobic transition",3 and peaks during the 2nd intensity zone.

During this transition, the body goes from using mainly fats to restore ATP to using mainly carbohydtates. Because lactate is a by-product of carbohydtate metabolism, the more carbohydrate is used, the more lactate is produced. The way lactate is handled by the body is marked by two thresholds: aerobic — also called "Lactic 1" (L1) or "Ventilatory 1" (V1); and anaerobic — also called "Lactic 2" (L2) or "Ventilatory 2" (V2)4.

  1. Aerobic (AeT / L1 / V1): the point at which blood lactate levels start to rise. The aerobic threshold marks the beginning of Kindermann's "aerobic-anaerobic transition", which is effectively the 2nd intensity zone.
  2. Anaerobic (AT / L2 / V2): end of the "aerobic-anaerobic transition". This is the point of maximal lactate steady state, and represents the highest work rate that can be maintained for an extended period. Once you're past this threshold, lactate cannot be cleared from the blood at the rate it's produced, and fatigue sets in quickly.

Zones 1-3 are traditionally used in endurance training.

The last zone (X) is an exception to the rule. This is the zone of extreme efforts lasting up to 12 seconds. I am including it here to accommodate strength training at maximal and near-maximal efforts.

These 4 major "intensity buckets" are the most generic way of categorizing the distinct phases of an exerciser's physiological state. An additional advantage of such a broad scheme is that each zone can be unambiguously identified using subjective criteria (Rate of Perceived Exertion, or RPE) such as the "talk test".

Based on the specifics of one's sport, one may further subdivide these zones to achieve more specific and granular categorizations of effort intensity. For example, one could subdivide Zone 1 into "activation" and "fat max" zones.

—The Zones—

  1. 1

    Aerobic Oxidative. The zone of low-intensity, long-duration work. This is the only zone that can convert carbohydrate, fat, and protein stores into ATP. At the outset of physical activity in this zone, carbohydrate metabolism is activated; however the body soon switches to fat as the main fuel. As modest levels of carbohydrates are utilized, this is the low-lactate zone.

    This energy system is high-capacity, but slow-rate; thus it enables low power production.

    • energy production: aerobic
    • % MHR: 70
    • % VO2max: 50
    • maximum duration at full intensity: over 90 minutes
    • talk test: you can talk in full sentences.
  2. Aerobic Threshold


    Glycolytic (slow). Medium-intensity zone. Now you're working harder, so the body needs to produce energy faster than can be done aerobically. Hence, the body increases its utilization of carbohydrate in proportion to fats. This is the lactate accommodation zone, meaning lactate is cleared from the blood at the same rate it is produced (hence it is also called "steady-state" zone).

    This energy system has medium capacity, and creates energy at a medium rate (hence mid-range power production). It is also called slow glycolysis, and produces additional ATP from the recycling of pyruvic acid via the Krebs cycle (an aerobic process).

    • energy production: mixed aerobic and anaerobic
    • % MHR: 75-80
    • % VO2max: 60-65
    • maximum duration at full intensity: 15-90 seconds (the higher end)
    • talk test: you can say 4-5 words at a time.
  3. Anaerobic Threshold


    Glycolytic (fast). High-intensity zone. Now the intensity is so high that ATP is replenished through the body's carbohydrate stores to an overwhelming degree. When this zone is reached, blood lactate levels rise quickly — exceeding the clearance rate. This is fast glycolysis, and results in a rapid build-up of lactate. Hence the name: lactate accumulation zone.

    This energy system is relatively low-capacity, and produces energy at a fast rate. Power production: high.

    • energy production: mostly anaerobic
    • % MHR: 90-92
    • % VO2max: 80-85
    • maximum duration at full intensity: 15-90 seconds (the lower end)
    • talk test: you can say 1-2 words at a time.
  4. X

    Ultra-high intensity zone. Energy is created through the release of body's immediate store of ATP (the first 1 to 1.5 seconds of effort); and a re-combination of ADP with P (12 seconds of effort). Increase in heart rate will lag behind the effort. As this effort does not rely on carbohydrate, this is the alactic zone.

    This energy system is the smallest in capacity, but releases energy at an explosive rate, thus enabling extreme efforts.

    • energy production: anaerobic
    • % MHR: 95-100
    • % VO2max: 85+
    • maximum duration at full intensity: 12-15 seconds
    • talk test: cannot talk.


  1. Adenosine triphosphate on Wikipedia  

  2. Heart Rate Training, 2nd Edition. Roy T. Benson, Declan Connolly. Human Kinetics 2020. ISBN: 9781492590224. 

  3. Kindermann W, Simon G, Keul J (1979). The significance of the aerobic-anaerobic determination of work load intensities during endurance training.  

  4. K. Stephen Seiler, Glenn Øvrevik Kjerland (2004). Quantifying training intensity distribution in elite endurance athletes: is there evidence for an "optimal" distribution?