How Hard: Training Intensity Explained

Rosemary after a severe intensity effort up Arthur's Seat
Rosemary after a severe intensity effort up Arthur’s Seat


I often get asked questions on what intensity level an athlete should be training at or what is the best session to be doing to optimise performance. Such questions are never easy to answer because everyone is individual and responds to training differently. However, understanding a bit about the theory behind intensity and what that intensity should feel like for an athlete is essential to get the most from training sessions. Overcook it and the onset of fatigue will be rapid. Under-do it and you may be wasting valuable time.

Despite everything written in magazines, books and academic publications, there is limited evidence on whether one type of training works better than another. Trends come and go, especially in triathlon, with lots of athletes wanting to get that ‘marginal gain’ from by following the latest fad.

However, collecting good evidence on specific types of training is quite a challenge. That’s because ‘controlling’ athletes and monitoring their training adaptations over time, as part of a scientific study, is notoriously difficult to do. Rather, as a coach, I use my understanding of the underpinning theories of exercise intensity and apply that knowledge to solve training ‘problems’ on an individual basis. I.e. asking myself:

  • does a type of training make theoretical sense?
  • what are the mechanisms that explain the likely training adaptation?
  • does it fit in with my athlete’s lifestyle or likes/dislikes?

Even if ‘evidence based’ answers aren’t available to the first two questions, simply exploring them is likely to give you a pretty good idea of whether the training will work or not. In this Blog, I’ll cover the theory behind exercise intensity, how it relates to training level/zones and how it can be applied in your day-to-day training or coaching.

NY Day Tri70

The Theory of Exercise Intensity

How hard? That’s what exercise intensity comes down to. The corresponding physiological (and psychological) responses determine what we feel, how quickly we fatigue and the training adaptations that are likely to occur. Much as theory doesn’t float everyone’s boat, a basic understanding of human energy production is essential to get to grips with the subject.

Human movement and muscular contraction involves complex interactions between the brain, vital organs, muscles and bones.  All movement requires energy to be transferred and we get that energy from food. We break it down through our digestive system into an  energy rich compound called adenosine triphosphate (ATP). This compound is used within the cells to fuel muscular contraction.

Figure 1. ATP
Figure 1. ATP

However, the muscles can only store enough ATP to fuel the muscles for 2-3 seconds. Therefore, ATP must be re-synthesised for the muscles to continually contract. There are three different energy pathways, known as energy systems, used to re-sythesise ATP. These are:

Aerobic oxidative system, which requires oxygen.

Anaerobic glycolytic system, also known as the lactic acid or anaerobic system. Anaerobic refers to a process that does not require oxygen.

ATP-Phosphocreatine (ATP/PCr), also known as the phosphagen or alactic energy system.

The contribution from each of these systems depends on duration of exercise and the intensity at which it is performed. I.e. as exercise increases through from rest, up to maximum there are marked changing relative contributions from each of these systems.

Exercise Intensity Domains Defined

I developed my understanding of exercise intensity through the study of oxygen uptake kinetics (VO2 kinetics), a very holistic area of exercise physiology. Exercise intensity domains are identified using terminology that is easy for everyone to understand: moderate, heavy, severe and extreme. (Gaesser and Poole,1996; Hill et al. 2002). It’s a shame that there’s not one called easy too, but science isn’t always convenient.

Oxygen Uptake Kinetics
VO2 kinetics reflect changing metabolic processes during exercise.  The related theory takes a holistic approach insofar as parameters such as VO2max, Critical Power (think FTP or CSS), exercise economy/efficiency, the mechanisms of aerobic energy transfer and their performance correlates are all reflected in the VO2 kinetic response. In turn, this response affects the rate of carbohydrate oxidation, the rate of heat storage, the rate of anaerobic work capacity utilisation, metabolite accumulation and, ultimately, performance (Gaesser and Poole, 1996; Xu and Rhodes, 1999; Poole and Jones, 2005; Burnley and Jones, 2007). Thus, understanding VO2 kinetics is useful in optimising training.Prof. Andy Jones has written an excellent article for Peak Performance which is well worth a read if you would like a more in-depth overview of  the topic.
Dave doing a Fatmax test at GSSI
Dave doing a Fatmax test at GSSI

Intensity domains, and thus metabolic changes, are demarcated by threshold* points. These points can be used to identify training zones. Table 1 summarises these threshold points and how they relate to the energy systems used. Note the number of thresholds terms used. I find lactate threshold particularly confusing because some people use this term to describe two different physiological responses. My Top Tip is to avoid using any physiological term unless you can describe exactly in what context you are using it, otherwise the confusion may continue, and I’ll get shirty if you’re my coach.

Table 1: Intensity Domains

  Intensity Domain





Upper Boundary of Domain/Threshold

  • Lactate threshold
  • Anaerobic threshold
  • 1st ventilatory threshold
  • Lactate threshold
  • Critical Power
  • Critical Swim speed
  • Functional Threshold Power
  • Maximal lactate steady state
  • Lactate breakpoint
  • 2nd ventilatory threshold
  •  VO2max
Fatigue occurs before VO2max is attained

Predominant Energy System

Aerobic oxidative Aerobic oxidative and anaerobic glycolytic Aerobic and anaerobic glycolytic Anaerobic glycolytic and ATP-PCr

Estimated Range of Endurance Time within Domain

3–4 hours or more Upper- Up to around three hoursLower- 30 minutes Upper- 30–45 minutesLower- 3- 5 minutes Upper- 3 minutesLower- several seconds

‘Cause’ of Fatigue

  • Too hot
  • Tired muscles
  • Lack of motivation
  • Depletion of carbohydrate stores
  • Too hot
  • Depletion of carbohydrate stores
  • Accumulation of the by-products of carbohydrate breakdown (metabolites/ H+) 
  • Metabolite accumulation
  • Inability of nervous system to cause muscles to contract

The Physiological Demands of Events

One of the principles of training is called specificity. That is, the training we undertake should be specific to the demands of the event(s) we are training towards.

The physiological systems engaged within an event are primarily dependent on the duration of the event, as shown in Figure 1.

Figure 2. Postulated relative contributions from different metabolic energy systems. Adapted from Noakes and Durandt (2000)

Therefore, the training we undertake should be closely related to the intensity of the events that are being trained towards. Most prolonged training for cycling and running will be in the moderate and heavy domains, with typical interval sessions going into the severe domain. Very little training for triathletes should venture into the extreme intensity domain.

Figure 3. Substrate use in triathlons

In common with others, I have observed that most triathletes swim regularly in the severe intensity domain. This is probably due to poor pace regulation and limited understanding of exercise intensity, rather than it being done on purpose. However, it is important to recognise that there  are good arguments for swimming close to VO2 max; the aerobic system will be pushed to the limit, resulting in greater aerobic adaptations than exercising in the heavy domain. The downside is more recovery time during and post-session is required and fatigue is more likely to impact on subsequent training.

The Importance of Identifying Thresholds

It’s good to be able to identify roughly where there are marked physiological changes occur as this helps with setting intensity for training sessions. I prefer measures that relate to performance rather than tests performed in a laboratory because:

  1. They are more specific to the demands of an event
  2. They can be repeated easily and more cheaply
  3. Athletes tend to be more motivated when performing them.

The most important threshold to identify is the boundary between the heavy and severe exercise domains. This is because this point is most commonly used to identify specific cycling  and runningtraining levels or swimming pace.

Physiological Threshold Caveat
I’m not keen on the term Threshold when it is referring to a biological/physiological response. That’s because of what I continually observed in the laboratory i.e. that thresholds whether using blood lactate or gas-exchange were normally a pig to identify.The lack of consensus in scientific literature on the best/most valid ways to identify threshold intensities is probably due to the ‘fact’ that physiological processes occur on a sliding continuum, without a clear threshold, but as an inflection point (Meyer et al. 2004).Therefore, being too stringent in adherence to using training zones is probably not overly useful. Rather, I would suggest using them as a guide during training.

My preferred methods to do so for each of the disciplines is to identify power, heart-rate or pace (depending on how you record intensity)using the following tests:

Cycling-  Functional Threshold Test. Perform a 1 hr time trial and use average heart-rate or power to identify the threshold point, termed by Allen and Coggan as Functional Threshold. If this sounds too daunting, 20 minute time trial performance less 5% provides an acceptable estimate.

Figure 4. FTP Test

Running- 10km race pace/ heart-rate unless you’re really fast and you may wish to go a bit further.

Swimming- T-30 test or CSS test which involve doing an even-paced but maximal 30 min swim or a 400m and a 200m swim and using the calculation to identify Critical Swim Speed.

There are many other alternative tests which you could use to identify threshold intensities, but most are based on similar theoretical physiological underpinnings. Therefore, I don’t think it’s of much consequence of which ones you choose. Rather, consistency of the protocol you use is most important as it will allow you to reliably monitor changes in performance (and make inferences on the physiological adaptations that may have occurred).

I want one of these pools!
I want one of these pools!
Watching for Thresholds
Sometimes I get grumpy in my swimming lane because I don’t think my training mates are doing what is asked of them. Most of the time, they appear to be swimming well above their CSS. But how do I know? Well, quite simply there are outward signs in their breathing provides a rough indication on exercise intensity:

  • In the moderate exercise domain, breathing will normally be relaxed with no outward signs that the athlete is working hard.
  • In the heavy exercise domain, breathing will be slightly deeper (reflecting an increase in carbohydrate oxidation) but it will still remain controlled and steady.
  • In the severe exercise domain, above FTP/CSS, breathing will be more forced. Lactate is produced more rapidly than it is cleared, resulting in hyperventilation.

Therefore, if a swimmer is struggling to speak immediately on completing a length because of hyperventilation, it is more than likely that they are above CSS. The same is true for FTP when cycling or running.

Identifying Training Zones/Levels

Traditionally, exercise intensity, training zones/levels was expressed in relation to VO2max or maximum heart rate. However, such methods do not take into account that thresholds occur at different percentages of maximum between individuals. For example, at 70% of VO2max, exercise tolerance may be limited for one individual because they are working very anaerobically at this intensity; whereas, for another individual, energy transfer may be almost wholly aerobic with minimal associated fatigue at the same relative intensity. If you ever consider using determining your training zones using the 220-age equation, don’t. It is nonsense. A good review article is found here on that topic.

Rather, I much prefer Allen and Coggan’s training levels using an FTP test, as shown in the Table 2 below.

Table 2: Allen and Coggan’s training levels


FTP (%)

FT  Heart rate (%)






Active recovery Increased blood flow




Endurance Increased fat metabolism




Tempo Increased oxygen use and efficiency




Threshold Increased carbohydrate metabolism




Aerobic power Increased VO2max




Anaerobic capacity Increased anaerobic pathways



Neuromuscular power Increased maximum power


So there it is. A whistle-stop tour of exercise intensity.  The key take-home messages are:

  • Develop a basic understanding of the physiology related to training intensity.
  • Threshold intensity is a good indication of relative aerobic and anaerobic contribution to energy turnover.
  • The relative contribution from aerobic and anaerobic systems is time-dependent.
  • Simple performance tests are best for identifying specific training zones.

In my next blog, I’ll introduce variable intensity exercise and pacing strategy, discussing how these topics should influence how we coach or train.


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