Whether you have been running for years or are just getting started, you have probably heard people talk about training zones. In this article I want to explain what training zones actually are, why they matter, and how we can use different tools to make sure we are training in the right zone for the desired outcome.
At its core, zone training is simply a way of structuring training intensity so that different physiological adaptations are targeted deliberately. Different zones stress different systems in the body, and over time this leads to improved endurance performance.
All zone-based training systems, and there are many, are anchored around three key metabolic markers that are closely linked to endurance performance. These are aerobic threshold, lactate threshold, and VO2 max. They are sometimes referred to by different names, which adds to the confusion, but the underlying physiology is the same.
Before diving into these markers, it helps to briefly understand the role of pyruvate.
Pyruvate is a byproduct of energy production from carbohydrate, specifically when glycogen is broken down to produce ATP. As exercise intensity increases, energy demand rises and more pyruvate is produced.
Pyruvate has two possible metabolic fates. It can be transported into the mitochondria, converted into acetyl-CoA, and used to produce additional energy via aerobic pathways. Alternatively, it can remain in the muscle cell cytosol and be converted into lactate.
When lactate accumulates faster than it can be cleared, energy production slows and we experience fatigue (Jornet et al., 2019). This balance between production and clearance underpins the thresholds discussed below.
Aerobic threshold represents the upper limit of predominantly aerobic metabolism. It is the exercise intensity at which lactate begins to rise above baseline levels, typically around 1 mmol/L.
This point also marks a gradual shift from energy being produced primarily from fat toward a greater reliance on carbohydrate. Exercise performed below aerobic threshold can be sustained for many hours. Once intensity rises above aerobic threshold but remains below lactate threshold, exercise duration becomes more limited, generally up to around an hour.
Aerobic threshold is best measured through laboratory testing, but it correlates well with practical markers such as heart rate, breathing pattern, pace, and perceived exertion. These markers are commonly used to guide training zones in real-world settings.
Lactate threshold is the intensity at which lactate production exceeds the body’s ability to clear it, leading to a rapid rise in blood lactate levels, typically to 4 mmol/L or higher.
At this point, carbohydrate becomes the dominant fuel source. Exercise above lactate threshold can only be sustained for short periods, usually several minutes.
As with aerobic threshold, lactate threshold is ideally identified in a laboratory, but it can be estimated using heart rate, pace, breathing, and perceived exertion markers that correlate well with this intensity.
VO2 max is defined as the maximum amount of oxygen per minute per kilogram of bodyweight that an individual can take up and use during intense exercise. It represents the upper limit of aerobic energy production.
VO2 max can be improved through training, but the absolute ceiling of an individual’s VO2 max is largely determined by genetics. Importantly, VO2 max in isolation has often been a poor predictor of endurance performance because many other factors influence race outcomes (Costill & Fox, 1969).
That said, VO2 max still matters. Because it represents the ceiling of aerobic capacity, a runner cannot sustain aerobic work above this level. In well-trained ultrarunners, lactate threshold pace can sit at around 95 percent of VO2 max pace. In this case, increasing VO2 max becomes important so that lactate threshold can continue to improve alongside it (Koop et al., 2021).
The goal of endurance training is to raise VO2 max toward your genetic ceiling while simultaneously pushing aerobic threshold and lactate threshold as close to VO2 max as possible. When this happens, you can sustain faster speeds with lower relative effort.
In elite endurance athletes, lactate threshold often sits within 5 percent of VO2 max, and aerobic threshold is within 10 percent of lactate threshold (Koop et al., 2021). This is why elite marathon runners can hold extremely fast paces for the full distance. That pace still sits within their aerobic capacity.
By contrast, less trained athletes show a much larger gap between aerobic threshold and lactate threshold, which limits sustainable performance.
Of the three metabolic markers, aerobic threshold and lactate threshold are the most trainable and can continue improving for many years with consistent training (Jornet et al., 2019). Aerobic capacity can continue to improve well into the early 40s, which helps explain why many ultrarunners peak later than athletes in shorter endurance events (Cejka et al., 2015).
Very simply:
Aerobic threshold improves through high volumes of running below aerobic threshold, which usually means a lot of genuinely easy running.
Lactate threshold improves through tempo and interval work performed at or just below lactate threshold, where the body is repeatedly exposed to elevated lactate and learns to use it more efficiently as fuel.
VO2 max improves through short, high-intensity intervals performed above lactate threshold, often referred to as speed or VO2 max sessions.
Most zone-based training systems are built around these principles and use heart rate, pace, or perceived exertion as proxies for the underlying metabolic markers.
While running, we cannot directly measure blood lactate, so we rely on correlates that track reasonably well with metabolic intensity. The three most commonly used are heart rate, pace, and rate of perceived exertion. Each has strengths and limitations.
The most accurate way to use heart rate is to establish zones via laboratory testing that identifies aerobic threshold, lactate threshold, and VO2 max heart rates. In practice, many athletes rely on formula-based heart rate zones derived from a percentage of maximum heart rate. These are general estimates and may not be precise for individuals.
Heart rate reflects total physiological stress, not just running intensity. Factors such as temperature, altitude, hydration, emotional state, fatigue, caffeine intake, and cardiac drift all influence heart rate independently of pace (Fowler, 1992).
For trail and ultrarunners who train across varied terrain and environmental conditions, heart rate can become less reliable as a real-time intensity guide. Wrist-based heart rate monitors can also be inaccurate by as much as 34 beats per minute, making a chest strap essential if heart rate is being used seriously (Gillinov et al., 2017).
Pace zones are ideally established in the lab but can also be estimated using a 30-minute maximal effort test. The average pace from this test correlates reasonably well with lactate threshold pace and can be used to derive training zones.
The main limitation of pace-based training is that pace does not account for changes in terrain, elevation, temperature, or fatigue. For trail runners especially, the same pace can represent very different physiological demands. An easy pace uphill is rarely easy.
Rate of perceived exertion, or RPE, reflects how hard an effort feels physically and mentally. The Borg scale, developed in the 1960s, assigns numerical values to perceived effort.
RPE-based zone training requires good body awareness and experience. Perception is subjective, and individuals differ in pain tolerance and effort interpretation. However, with practice, athletes can learn to correlate perceived exertion with physiological intensity quite accurately.
One major advantage of RPE is that it provides real-time feedback that integrates all stressors. For ultrarunners in particular, no metric is more immediately relevant than how an effort actually feels in the moment.
At Ascent Running, we recommend using heart rate, pace, and perceived exertion together rather than relying on a single metric. Learning how these markers relate to one another allows you to train more accurately across different conditions and ensure that workouts are delivering the intended physiological stimulus.
Over time, this integrated approach leads to better decision-making, better training quality, and more consistent progress.
Cejka, N. et al. (2015) “Performance and age of the fastest female and male 100-km ultramarathoners worldwide from 1960 to 2012,” Journal of Strength and Conditioning Research, 29(5), pp. 1180–1190. Available at: https://doi.org/10.1519/jsc.0000000000000370.
Costill, D.L. and Fox, E.L. (1969) “Energetics of marathon running,” Medicine and Science in Sports and Exercise, 1(2). Available at: https://doi.org/10.1249/00005768-196906000-00005.
Fowler, J.A. (1992) “Exercise physiology: Energy, nutrition and human performance,” Physiotherapy, 78(2), p. 148. Available at: https://doi.org/10.1016/s0031-9406(10)61985-2.
Gillinov, A.M. et al. (2017) “Variable accuracy of commercially available wearable heart rate monitors,” Journal of the American College of Cardiology, 69(11), p. 336. Available at: https://doi.org/10.1016/s0735-1097(17)33725-7.
Jornet, K., House, S. and Johnston, S. (2019) “The Physiology of Endurance,” in Training for the uphill athlete: A Manual for mountain runners and ski mountaineers. Ventura, CA: Patagonia Books.
Koop, J., Rutberg, J. and Malcolm, C. (2021) “The physiology of a better engine,” in Training essentials for ultrarunning. Colorado Springs, CO: Koop Endurance Services.
