“If you can’t explain it simply, you don’t understand it well enough” - Albert Einstein

As coaches, being able to share ideas and discuss strategies in training is a valuable endeavor. However, if you begin talking at any length with a coach that is not completely familiar with your training system you may find yourself having to continually define the terms you are using and be asked to clarify what you mean by a certain term. Some common terms or phases you may hear, relative to endurance based training would be “threshold”, “aerobic power”, “anaerobic”, “lactic endurance”, “lactic capacity”, and so on. Wouldn't it be better if we agreed to use a better set of terms/phrases to describe what we mean when referring to this area of training? Wouldn't it be better if we were able to speak with fellow coaches from around the world and immediately understand what they were talking about when they referenced certain terms or phrases? A gram of protein is clear. I know what it means, you know what it means and your clients/athletes probably know what it means. “Lactic Endurance” is not clear what is meant.

The goal of this months reading list is to challenge you, the Coach, to improve the language you use (myself included) and therefore hopefully improve the language we all use. I believe we should be striving to use terms and phrases that clearly explain what we mean, that allow Coaches to easily converse with one another and most importantly allow our clients/athletes to understand what we are saying without talking over them.

I am not suggesting the terms and phrases I will be introducing and inviting you to use here are going to solve everything and are all you'll ever need to use. I am not naïve. However, I am hopeful that this may be a small step in the right direction for us as Coaches and the fitness community as a whole. Lastly, I encourage you to look to other areas of training and devise a better set of terms and phrases. Ones that are clear and correct.

Exercise Intensity Domains
The following discussion pertains mainly to constant load exercise on the SkiErg, Rower, AirBike, EchoBike, BikeErg, Swimming and Running. I believe the following discussion pertains to more than that, but I'm being conservative in my estimation of the value of these terms and starting off with areas of fitness I know can be completely described by the existing literature. The beauty of these exercise intensity domains is that they mostly encapsulate all of the nitty gritty science that many people like to discuss. However, using these terms will allow you to convey the same message without having to use terms/phases that imprecise and not clear.

“If exercise intensities are to be defined based on their distinct metabolic profiles, there should be four domains. In ascending order: moderate intensity exercise is associated with a rapid mono-exponential increase in the VO2 to a steady state; heavy exercise engenders a slow component to the VO2 response and is associated with a delayed sub maximal steady state; (only) severe exercise is characterized by the attainment of VO2 Max; and extreme exercise is associated with the development of fatigue before VO2 Max can be achieved” (Hill, D.W., et al.).

Moderate Intensity or Moderate Exercise 
Generalizations:
Sustainable 1-4 RPE (very lite to moderate)
60-80% max HR relative to modality
Likely no exercise induced pain

Commonly referred to as “Zone 1" in the three zone training model, proposed by Stephen Seiler. Research consistently suggests the majority of training done by most elite endurance athletes (read non-CrossFitters) is performed at this intensity (>90% of training time for these athletes).

Although the moderate intensity domain is relatively easy to exercise within (i.e. it feels easy, low RPE), it is actually quite challenging to define. As a fitness coach (read, lacking laboratory equipment and research skills), the upper limit of the moderate domain is quite challenging to accurately define. What does define the upper limit of the moderate intensity domain is the "lactate threshold” (the point/intensity at which lactate concentration rises from resting levels). As you will see from the video example attached below it is not exactly a straightforward process. Regardless, Stephen Seiler has done a lot of valuable work to explore in this area by using RPE scale's and heart rate to try to establish training intensities. Each of these variables is impacted by the individuals experience and knowledge with training. Inexperienced and low trained individuals will likely not have a great grasp on using and RPE scale. It takes time and exposure. As well, the percent heart rate that may signify the upper limit of the moderate domain will also generally be lower than highly trained individuals. Here are some metrics you would want to start using to try to establish the upper limit of this intensity domain for your typical fitness client.

1 - Heart Rate - 70-75% max HR relative to modality
2 - Heart Rate is stable for extended periods (with 5 bpm as an example from minute 30 to minute 60, excluding the impact of environmental stress/heat/terrain)
3 - RPE is below 4/10 on the Modified Borg Scale
4 - RPE does not increase during the session or RPE increases very slowly (i.e. over the span of 30-60 minutes)
5 - Breathing Rate/Difficulty - low/stable throughout

The mechanisms of fatigue in the moderate intensity domain are primarily “central” in nature, meaning the brain (central drive/fatigue/motivation) and/or muscle damage due to excessive volume as exercise at this intensity can be sustained for hours on end. The mechanisms of fatigue are not generally due to peripheral or metabolite accumulation. One of the fundamental aspects of modern intensity exercise is the rapid rise to a metabolic steady state (within a few minutes) and the lack of a VO2 slow component in the oxygen kinetics. The importance of the slow component and its affect on metabolism can be found in the reading provided. Suffice to say, it leads to an efficient metabolism when considering very long duration events.

Some examples of endurance events that incorporate moderate intensity domain are ultra endurance running events or major portions of bike stage races (Tour de France) or Ironman. Moderate intensity in fitness training would most commonly be found during “warm-up” or recovery/rest intervals between more intense bouts.

Heavy Intensity or Heavy Exercise
Generalizations:
Sustainable 4-6 RPE (moderate to hard)
80-90% max HR relative to modality
Low to moderate exercise induced pain

The heavy intensity domain begins where the moderate domain ends, just above the lactate threshold. If assessing gas exchange/oxygen consumption, the presence of a VO2 slow component will appear at these intensities. Meaning, after the start of exercise in the heavy intensity domain a metabolic steady state will. It be achieved for 10-20 minutes (much longer than moderate intensity). The upper limit of the heavy intensity domain is the critical power (or critical speed). The critical power delineates the highest power output at which a metabolic steady state can be achieved and therefore is the highest sustainable power output. As a coach, the best way to figure out the heavy intensity domain is the first figure out the upper limit of the moderate intensity domain (take a few clients and really try to figure it out) and then try to figure out the critical power (there are a few ways to do this as explained in the reading below, but you will likely be using your best guess regardless).

“Too much pain, for too little gain”. When discussing polarized training, Stephen Seiler commonly states that most amateurs spend too much time in this domain. However, much of “Fitness” training is spent at this intensity. Many individuals “prefer” to exercise at this intensity, for one reason or another. Many times when you look at EMOMS or work intervals utilizing the aforementioned modalities (ergs and running) they will likely straddle the Heavy and Severe (see below) Intensities. As well, due to the unsustainable nature of work above the critical power, when the ergs and/or running are large sections of certain tests or events, athletes naturally move to these intensities so as to not “blow up” before the subsequent more important portion of the event.

The mechanism of fatigue in the heavy intensity domain is primarily glycogen depletion. Exercise at these intensities can persist for a few hours and is typically the intensity domain that the best Marathoners/Half Marathoners would be running in. If you were to perform a half marathon or marathon row events from the 2013 or 2018 CrossFit Games this is also likely the exercise intensity domain you would be in throughout the duration of the event.

Severe Intensity or Severe Exercise
Generalizations:
Unsustainable
6–>10/10 RPE (very hard to maximal)
90+% max HR relative to modality
High to very high exercise induced pain

Work rates above the critical power enter into the severe intensity domain where a metabolic steady state can no longer be achieved and task failure occurs at a predictable time and rate. At these intensities, if continued, a VO2 Max will be achieved followed closely by task failure (think a 1k, 2k or 3k Row Time Trial). When competing in events at these intensities (lasting anywhere from ~2 minutes to ~40 minutes) getting your pacing right is imperative.

The mechanisms of fatigue in the severe intensity domain occur with depletion of the W’ (Work Prime - the fixed amount of work that can be performed in one continuous bout above the Critical Power/Speed). It is at these intensities when metabolically mediated fatigue occurs (Increase H+, decreased PCr, increased Pi, decreased PH, decreased SmO2, etc.) along side high levels of exercise induced pain.

This is the intensity domain many people would refer to as anaerobic or lactate endurance or the like. So, instead of using those words/phrases you can simply refer to as it as “severe intensity” or “severe exercise” and you will both clear with your language and correct in your assessment.

This intensity domain is also commonly prescribed and can be found in EMOMS and other types of work/rest sets. One of the confounding aspects of work and rest sets is a can give the appearance of sustainability because the sets are repeatable. For example, 500m Row at your 2k PR x 5-10 sets, 1-2 min rest. Remember, ”sustainable” refers specifically to the metabolic rate one is engaged in, if that metabolic rate is above the critical power and therefore in the severe domain it is not sustainable, regardless if the sets are repeatable.

Extreme Intensity or Extreme Exercise
Generalizations:
Unsustainable
9–>10/10 RPE (very hard to maximal)
90+% max HR relative to modality
High to very high exercise induced pain

Extreme intensity is defined as exercise intensities that reach task failure PRIOR to the attainment of a VO2 Max. This would be reserved for events and intensities that reach task failure in 90-120 seconds or less (100m—>400m Sprint, 250m Row, 500m Row, 500m BikeErg, 1000m BikeErg, 1 min AirBike max cals, etc.).

The same mechanisms of fatigue in the severe intensity domain would apply to the extreme intensity domain alongside complicated processes (which you do not need to know about associated with excitation contraction coupling failure.
Commonly referred to as sprint intervals, alactic power, alactic endurance, lactic power, etc. You can replace those terms with extreme intensity or extreme exercise.

Reading:
1 - Exercise Intensity Domains and Phase Transitions: The Power-Duration Relationship by Mark Burnley (2020)
2 - Jones, A.M, et al. Muscle Metabolic Responses to Exercise Above and Below the Critical Power Assessed Using 31P-MRS (2008)

3 - Burnley and Jones. Oxygen Uptake Kinetics as Determinant of Sports Performance (2007)

4 - Hill, D.W., et al. The Relationship Between Power and Time to Achieve VO2 Max (2001)

Video:

Thresholds and Intensity Domains by Mark Burnley

Lactate Threshold by Mark Burnley

Thanks!

Michael