A Scientific Overview of Exercise Intensity Domains, Power Zones, and Physiological Boundaries.
Exercise Intensity Domains refer to distinct intensity categories in cycling and endurance sports, each defined by measurable physiological thresholds. These domains—Moderate, Heavy, Severe, and Extreme—provide a structured way to understand exercise intensity based on specific physiological events, offering an alternative to the traditional “training zones” used by coaches and athletes.
Unlike training zones, which are often defined differently by various researchers, Exercise Intensity Domains are grounded in measurable physiological markers, reducing confusion and providing a more consistent framework. For instance, Zone Two is often debated, as different definitions of this zone exist, based on power, heart rate, lactate levels, or expired gas. In contrast, Exercise Intensity Domains provide clarity by focusing on the physiological turn points where energy systems shift. This article explores the four main Exercise Intensity Domains, when they occur, their applications in training, and how they can be measured effectively.
Exercise Intensity Domains are distinct domains separated by quantifiable physiological boundaries and turn points. They are split into the Moderate, Heavy, Severe and Extreme Exercise intensity Domains. They offer an alternative to more traditional training ‘Zones’ of which there are multiple different methodologies of determining, and therefore confusion over what a specific ‘Zone’ is.
Zone Two, for example, is a difficult one to state exactly since various researchers/coaches have different definitions of Zone Two, and set the boundaries of it at different power, heart rate, expired gas, or lactate levels. You’re almost better off finding a BB standard than a Zone Two standard. This is where Exercise Intensity Domains come in, as they base the change of ‘Zones’ on specific physiological occurrences within the body. But what are the Exercise Intensity Domains, when do they occur, what use are they to individuals, and how do we measure them?
Moderate Exercise Intensity Domain
The Moderate Intensity Domain corresponds to exercise intensities up to the first lactate threshold (LT1). This domain is typically represented by traditional training zones ranging from Zone One to Zone Two, and in some models, it may extend into the lower part of Zone Three, such as in Andy Coggan’s training model. In this domain, the body predominantly uses fat as its primary fuel source, generating energy through aerobic pathways.
The key transition point that marks the end of the Moderate Intensity Domain occurs when the body shifts from primarily fat oxidation to greater carbohydrate use, signalled by an increase in lactate production above baseline levels—this is known as LT1. Testing for LT1 can be performed using a lactate test, where lactate concentrations are measured during long intervals at low intensities, or via the gas exchange method (GEX), where the intersection of VO2 and VCO2 lines marks this threshold. An alternative, less accurate method is to estimate LT1 as approximately 70-80% of Critical Power (CP), although training conservatively below this threshold is advised.
The Moderate Intensity Domain is primarily utilised in long endurance rides aimed at improving mitochondrial density, fat oxidation, and muscle capillarisation. Fatigue in this domain tends to be more psychological, arising from mental fatigue or saddle discomfort during prolonged steady-state efforts. Additionally, environmental factors such as heat, altitude, fatigue, and substrate availability can impact performance, leading to increased reliance on carbohydrates even at lower intensities.
Heavy Exercise Intensity Domain
The Heavy Intensity Domain spans from LT1 to Critical Power (CP) and includes intensities associated with tempo, sweet spot, and threshold training—typically Zones Three and Four. CP can be measured through various testing protocols, including the 3-minute and 12-minute methods or the all-out 3-minute test. Within this domain, exercise is theoretically sustainable for an extended period, provided that fuelling with carbohydrates is maintained. However, in practice, athletes can typically sustain efforts at or near CP for 35-60 minutes, depending on the individual’s fitness level. Lactate accumulates steadily within this domain but generally stabilises at a steady state, unless the intensity reaches higher levels, where the accumulation of fatigue metabolites begins to reduce performance capacity.
Training in the Heavy Intensity Domain is essential for improving lactate clearance, increasing the capacity for sustained high-intensity aerobic efforts, and enhancing the athlete’s ability to maintain performance at a reduced perception of effort. Fatigue in this domain is primarily driven by substrate depletion (carbohydrates), mental fatigue, and heat generation, particularly during longer sustained efforts. The duration of efforts in this domain can range from 35 minutes to 3 hours, depending on whether the intensity is closer to LT1 or approaching CP.
Severe Exercise Intensity Domain
The Severe Intensity Domain begins above Critical Power and is characterised by maximal aerobic capacity efforts and longer glycolytic activity, corresponding to Zones Five and Six. These are high-intensity efforts that push the athlete to their physical limits, such as VO2max intervals, Glycolytic capacity, or efforts such as 10mile time trials. Efforts in this domain are typically maximal, requiring a substantial energy expenditure that relies on both aerobic anaerobic energy systems. These efforts are crucial for improving the athlete’s ability to perform high-intensity work, including breakaways, climbs, and final surges in races.
Fatigue in the Severe Intensity Domain arises from the depletion of W’ (Work Prime), a finite resource that limits the ability to maintain efforts at intensities above CP. As fatigue builds, the capacity for high-intensity work decreases, and the athlete must reduce intensity to recover. The W’bal model is useful for tracking this decline in available W’, but it is important to note that W’ depletes more rapidly as fatigue accumulates.
There are also many ongoing studies investigating W’ depletion and replenishment currently (2025). Training in the Severe Intensity Domain stimulates mitochondrial biogenesis via the AMPK signalling pathway, which plays a key role in enhancing aerobic performance. This domain is also commonly utilised in polarised training models, where athletes perform a large proportion of their training at low intensities (Moderate Intensity Domain), with occasional high-intensity efforts (Severe Intensity Domain).
Extreme Exercise Intensity Domain
The Extreme Intensity Domain is an additional proposed domain to the traditional Three Domain Model, characterised by energy derived from anaerobic pathways, such as phosphocreatine and anaerobic glycolysis. This domain fuels high-intensity sprint efforts, typically at the end of a race or a maximal attack. A key distinction is that energy production leads to the release of inorganic phosphate, a fatigue metabolite that impairs muscle contractile function and isn’t fully recoverable by simply dropping below Critical Power (CP). Repeated efforts in this domain can lower your functional Critical Power during a session and reduce performance across other intensity domains.
For example, during under/overs (riding at 90% and 110% of CP), recovery is possible even with an elevated heart rate, as the body can dip into W’ to recover. However, maximal sprints in the Extreme Intensity Domain, such as 10-second sprints, may require lower average power, result in a higher heart rate, and require longer recovery times.
The muscles will experience greater lactate and associated hydrogen ion accumulation, causing muscle burn, typical of efforts lasting 20-60 seconds. These efforts are best saved for key moments, such as a final sprint in a race, or for training to improve peak power and torque generation and psychological resilience to high-intensity efforts. The main fatigue element is inorganic phosphate, which limits both the immediate effort and the ability to repeat maximal efforts.
Limitations of Exercise Intensity Domains
A major limitation of the Exercise Intensity Domains is that the physiological turn points defining these domains are not fixed and can be influenced by a range of factors, including fatigue, environmental conditions, psychological state, diet, and supplementation. As a result, the boundaries between domains can be blurry and vary even within a single workout.
For example, an athlete’s 20-minute maximum power may decrease significantly after 2000-2500kj of work or following a poor night’s sleep. Similarly, pre-workout nutrition, such as consuming fructose, may lead to a higher reliance on carbohydrates and less fat oxidation. While these physiological boundaries are not absolute, they provide a more reliable framework for assessing and monitoring training intensity compared to traditional zone-based methods, which are often subject to greater variability.
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