Lesson 7: Aerobic Training Terminology and Concepts

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As discussed in Lessons 1 and 2, aerobic training and resistance training utilize different systems in the body to different degrees. In a sense aerobic training is actually resistance training with very high rep ranges (ie, hundreds or thousands of reps if each step taken or each bicycle pedal rotation is considered 1 rep). However, there is such a distinction between the two that different terminology and concepts are needed to accurately describe aerobic training. Thus, I will go over these aspects in this lesson.

Note: The term “aerobic” training implies aerobic metabolism, however I will also discuss associated aspects such as high-intensity interval training (“HIIT”) and lump this in together with aerobic training even though HIIT significantly uses anaerobic metabolism as well.

Physiologic determinants of endurance performance

Three key factors seem to underlie endurance performance: VO2 max, running economy, and lactate threshold.(Alvero-Cruz, 2020)

VO2 max

When you start doing a cardiovascular session (“cardio”) initially you use your anaerobic energy production pathways as described in Lesson 2. However, if you perform cardio for several minutes to >1 hour, aerobic energy production plays a much larger role. This requires oxygen to generate energy, and your VO2 max is the maximum amount of oxygen you can utilize in any given period of time. This is frequently divided by one’s body weight to standardize values. VO2 max was discussed in Lesson 2 in more detail, but in summary many people consider it the best marker of cardiorespiratory fitness. Many aspects of physiology contribute to VO2 max (ie, cardiac output and red blood cell number), and people with a higher VO2 max generally (though not always) perform better in endurance events.(Valenzuela, 2020)

Running economy

Individuals with similar VO2 maxes may perform differently in endurance events, and endurance performance can improve without an increase in VO2 max.(Kramer, 2020) While multiple factors contribute to this, one of the major ones that is generally considered is running economy.(O Sullivan, 2019) Running economy describes the oxygen uptake required at a given submaximal velocity. When your running economy improves you will need less oxygen and thus use less energy to perform at a specific submaximal pace regardless of your individual VO2 max.

Thus, if person A & B have a similar VO2 max but person A has a better running economy you can expect person A to perform better than person B. The concept of running economy applies to non-running endurance activities as well. Various factors contribute to running economy, such as movement efficiency (ie, running in a straight line without incorporating horizontal motion). Another example would be if person A has increased connective tissue stiffness in the foot/ankle to help prevent excessive energy loss into the ground with each foot strike.

Lactate threshold

While performing endurance activity at a level greater than your VO2 max is possible, this requires an anaerobic energy production component. However, even when performing at a level less than your VO2 max there is still an anaerobic contribution to energy production. As described in Lesson 2, this generates lactic acid. Up to a certain intensity of exercise the lactic acid is broken down as quickly as it is created; however, beyond the first lactate threshold (“LT1”, also referred to as the “aerobic threshold”) the blood lactate level begins to rise as lactate accumulates. As exercise intensity further increases there is eventually an acceleration of blood lactate accumulation; this point of acceleration is referred to as the second lactate threshold (“LT2”, also referred to as the “anaerobic threshold”).

Lactate accumulation negatively impacts performance. As lactic acid accumulates, acidity increases (measured as a decrease in pH) at the site of accumulation, and this worsens the function of various enzymes in the body that rely on a specific pH to work optimally. Thus, when lactic acid accumulates the affected skeletal muscles are not able to contract as well and performance suffers. A range of cardio modalities from shorter interval training to longer distances can increase the lactate thresholds.(Girard, 2018; de Waal, 2021) Additionally, an increase in skeletal muscle mitochondria production, capillary density, and fatty acid utilization may all increase your lactate threshold.

Overall, an increase in VO2 max, running economy, and lactate threshold(s) will increase endurance performance.

How to gauge aerobic exercise intensity

Special equipment is needed to directly measure VO2 max, running economy, and lactate threshold, and thus these are not practically useful metrics for most individuals. A much more commonly used metric that is easily accessible is heart rate, generally measured in heart beats per minute.

Heart rate max and reserve

As you increase your exercise intensity and need to transport more oxygen to your muscles, your heart rate will increase to do this since oxygen is transported by blood. Your heart rate maximum (“HRmax”) is the maximum amount that your heart rate can elevate. You can determine this experimentally while wearing an accurate heart rate monitor; however, not everyone has access to this and many people cannot safely perform maximal intensity cardiovascular activity. Thus, you can estimate our HRmax with one of several different equations. The simplest is HRmax = 220 – age, as described by the CDC. If you take your heart rate max and subtract your resting heart rate then you get your heart rate reserve (“HRR”). Both HRmax and HRR are metrics that can help define endurance exercise intensity, though as discussed below neither is very accurate from a physiologic perspective.

Note: There are several different HRmax formulas; however, to my knowledge none of them are exceptionally more accurate than the others and their accuracy varies by characteristics such as age. A recent review noted that using 220 – age has a standard deviation of 10-12 and the inaccuracy is even greater in individuals with coronary heart disease.(Franklin, 2022) As discussed below it is questionable if you should rely solely on HR to determine training intensity, and thus it is not imperative to accurately determine your own HRmax. For people who want to use % HRmax to determine a training pace you can use any formula to generate a rough estimate and then make changes depending on how training sessions are going (as indicated below).

Defining intensity range

Aerobic exercise intensity can be classified in several ways.(Jamnick, 2020) For example, one classification system involves moderate, heavy, and severe domains. The idea of domains is to demarcate different physiologic affects within the body.

  • In the moderate domain there is a plateau of VO2 with blood lactate concentration near baseline levels. This implies most ATP production is coming from oxidative phosphorylation with mostly type I muscle fiber recruitment and a low rate of muscle glycogen depletion.
  • In the heavy domain there is a rise in blood lactate above baseline that then plateaus while VO2 has a delayed increase (a “slow component”) prior to also plateauing. This slow component of VO2 represents a greater amount of aerobic metabolism than expected for the given activity and occurs in part due to the decreased efficiency of energy production as energy requirements increase (due in part to increased lactic acid accumulation and alterations in mitochondrial efficiency). Additionally, in this heavy domain there is greater recruitment of type II muscle fibers, a moderate rate of glycogen depletion, and a decrease in muscle pH with a subsequent plateau.
  • In the severe domain there is the same slow component of VO2 increase but this time it does not plateau. Additionally, there is a continual increase in blood lactate without plateau. There is a greater contribution of phosphocreatine stores to ATP turnover, greater recruitment of type II muscle fibers, rapid rates of muscle glycogen depletion, and a continual decrease in muscle pH.

The figure below shows the moderate, heavy, and severe domains demarcated by grey lines, VO2 max at the top with the black line, and indicates how VO2 and blood lactate differ between the domains. Each domain is thus physiologically distinct.

an image showing how VO2 and blood lactate differ by the domain classification of exercise intensity

Another classification system uses training zones, which use some of the metrics described above even if they do not correlate perfectly to physiologic impact.

  • Zone 1 implies low intensity (<80% HRmax, 65-75% VO2 max, below LT1)
  • Zone 2 implies moderate intensity (80-90% HRmax, 75-85% VO2 max, above LT1 but below LT2)
  • Zone 3 implies high intensity (>90% HRmax, >85% VO2 max, above LT2)

Note: Some references will divide training into more than 3 zones (potentially up to 7)(Kenneally, 2018). The main point is that zones are frequently derived by measurable metrics such as % of HRmax even though these metrics may not represent accurate thresholds for physiologically distinct states.

Clearly there are several different metrics you can utilize when trying to define exercise intensity. However, a recent review highlights how none of these are ideal as different individuals may have different physiologic responses at different thresholds.(Jamnick, 2020) For example, the % VO2 max that corresponds to LT1 or LT2 varies between individuals, as does the % HRmax. HRR also does not do a great job of delineating domain-specific physiology. For this reason, using any of the above descriptors to define physiological distinct training zones is imprecise. Using domains makes more sense as each domain corresponds to different physiology, but there is no way to accurately assess what domain you are in outside of a laboratory setting where pulmonary gas exchange, blood lactate, and other markers are readily measurable.

Thus, using a subjective method to estimate your intensity domain is reasonable, as discussed in the next session.

Tip: For the above reasons, any aerobic training plan that prescribes training based on specific zones or specific measures of exercise intensity such as % HRmax should be seen as providing a generic starting template that can be modified for specific individuals. This is important as many people will benefit from making modifications based on how their training is progressing.

Example: Let’s say a person age 30 calculates their HR max as 220-30 = 190 beats per minute (“bpm”). As discussed above there is some variability here so perhaps the HRmax is actually in the range 182-198 (190 ± 8). Let’s say their resting heart rate is 60 bpm. Thus, their HRR = 190 ± 8 – 60 = 130 ± 8 (122-138). Let’s say their running plan calls for them to perform a run at 70-80% of their HRR. We thus need to calculate this HRR range and add it to the baseline HR to determine the desired HR interval when training. Calculating the lower range:

  • 130 * 0.7 = 91;     60 + 91 = 151
  • 122 * 0.7 = 85;     60 + 85 = 145
  • 138 * 0.7 = 97;     60 + 97 = 157

Thus the lower range is anywhere from 145-157. Calculating the upper range:

  • 130 * 0.8 = 104;     60 + 104 = 164
  • 122 * 0.8 = 98;       60 + 98 = 158
  • 138 * 0.8 = 110;     60 + 110 = 170

Thus the upper range may be anywhere from 158-170.

Therefore, this person can aim for a range of 151-164 assuming their HRmax was accurately calculated, but the real range may be anywhere within 145-170. Additionally, as indicated above, the limits of these ranges may present different physiologic states. Thus:

  • initially they could aim for somewhere in the 145-170 range
  • after the run they can subjectively determine how difficult the run felt
  • next they can compare this to how they think it should feel given the prescribed training intensity
  • finally, in a future run they can aim for the lower or higher end of this range based on the information obtained from this run

If the run seemed too easy they could aim for the higher end of the range and if the run seemed too difficult they could aim for the lower end of the range.

Subjective methods to define exercise intensity

As mentioned above, the absolute methods to define exercise intensity are either not very accurate, not very practical, or both. Thus, it may be more prudent to use subjective methods. This was recently reviewed, with the primary focus pertaining to the “talk test”, “feeling scale”, and rating of perceived exertion (“RPE”)(Bok, 2022):

  • Talk test: gauge intensity by your ability to talk
    • The idea is that talking increases carbon dioxide retention and increases your ventilatory drive; as your exercise intensity increases talking becomes uncomfortable and then impossible.
    • The highest intensity where you can talk normally is at the border between the moderate and heavy domains. The lowest intensity where you can no longer speak is at the border between the heavy and severe domains.
  • Feeling scale: gauge intensity based on how you feel
    • People who are more fit will likely still feel “good” at a higher exercise intensity than people who are less fit.
    • This has an 11-point scale form -5 to 5, with -5 = “very bad”, 0 = “neutral”, and 5 = “very good”.
    • Generally a score of 3-5 is in the moderate domain, -2 to 2 in the heavy domain, and -3 to -5 is in the severe domain.
  • RPE: rate the exercise based on the required exertional effort
    • This can be scaled from 1-10 or from 6-20 (this was created at 6-20 to reflect heart rates ranging from 60-200 with exercise)
    • A score of 6-10 corresponds to the moderate domain, 11-14 corresponds to the heavy domain, and 15-17 corresponds to the severe domain.
The above domain demarcations for the talk test, feeling scale, and RPE are not perfect, but they can serve as general guidelines that will be “mostly” accurate for “most” people.

The talk test should be fairly reproducible and you can use this to gauge your intensity between sessions. The feeling scale can vary between individuals based on the interplay between their cognitive factors and physical limitations, but for anyone who is sedentary and just starting to exercise purposefully staying in the “good” range (minimum score 2 or 3) will aid compliance and habit-building. RPE can vary widely between individuals as well as based upon changes in your physiologic state any given day and the length of the session, but with practice you should be able to use it fairly reproducibly if desired.

Tip: With subjective methods, even if you keep the intensity the same from session-to-session, as you become more fit your performance will increase over time. For example, with the talk test you may be able to run a 12-minute mile pace while still being able to talk without difficulty; with continued training you will be able to run a mile more quickly while still not having any trouble talking. You can perform intervals at different scores of the feeling scale or RPE metric. There are many ways you can structure your conditioning program.

Periodically you may want to perform some type of test to help track absolute changes in performance over time (ie, every 2 weeks row 1,000 meters as quickly as possible and see how much you improve). This can help you ensure that your training is productive for increasing your fitness, assuming this is one of your goals.

Types of training sessions

When doing cardio with any modality (ie, running, elliptical, rowing, bicycling, etc), you can manipulate multiple parameters based on your goals for the training session. These can include the duration of the session, the resistance (ie, running up a hill versus on flat ground, setting the actual resistance level higher with an elliptical machine, etc), the velocity or speed of movement, and you can even vary these parameters throughout a session. Regardless of the training modality and the above parameters, you can choose to perform cardio continuously or in intervals.

Continuous training

Continuous cardiovascular training is typically referred to as low- or moderate-intensity. Various forms of continuous training include:

  • “easy runs” – typically low intensity longer distance runs done at a relatively easy pace. This generally comprises the majority of training time when training to perform a longer-distance event. The pace may correspond to 70-79% of one’s best 5k time(Hamilton, 2018) or for elite long-distance athletes between 62-82% of their HRmax.(Casado, 2021)
  • “fartlek runs” – involve going at an easy pace, then speeding up for some period of time, and then dropping back to the easy pace; this can be repeated.
  • “tempo runs” – can also be referred to as “threshold runs” and involves running at the desired race pace. These are generally moderate-intensity.

Interval training

While you can theoretically perform intervals at any intensity, HIIT has become very popular in recent years. This is defined in various ways, with one classification scheme as follows(Wen, 2019):

  • long-interval HIIT (LI-HIIT): 2-4 minute intervals typically at 85-95% HRmax and <VO2 max
  • medium-interval HIIT (MI-HIIT): 0.5-2 minute intervals typically at 85-95% HRmax and <VO2 max
  • short-interval HIIT (SI-HIIT): <30 second intervals typically at 85-95% HRmax and <VO2 max
  • sprint-interval training (SIT): 10-30 second intervals typically at ≥95% HRmax and >VO2 max
  • repeated-sprint training (RST): ≤10 second intervals at ≥95% HRmax and >VO2 max

Additionally, the total duration you perform higher intensity activity (excluding recovery periods between the intervals) is described as low (≤5 minutes), medium (5-15 minutes), or high (≥15 minutes) volume.

Note: There are several other classification schemes for HIIT. Most will state that HIIT is performed at close to but not over your VO2 max while SIT and RST are performed at greater than your VO2 max. The amount of rest between each interval can vary.(Rosenblat, 2020)

Training distribution

When determining how to divide time allocated for aerobic training between continuous and interval training as well as training at different intensities, much of the decision-making process should depend on your goals. If your goal is to perform in a competition (ie, a half-marathon) then you should devote a significant amount of time to this; an organized strategy prioritizing aerobic training over resistance training is likely warranted. On the other hand, if you are simply training for general health and fitness this requires less time devoted to aerobic training. In this situation it is more important to consider how to balance aerobic and resistance training such that they do not hinder each other.

If training for a competition

For individuals who are training for a competition and intend to train over a 3 month time frame or so, usually the total training volume (ie, miles per week) will increase over time, as will the percentage of training done closer to the desired race pace. Then most athletes will taper for 1-2 weeks prior to the actual event by decreasing total volume while keeping training intensity similar.(Hamilton, 2018)

You can divide your overall training distribution within the 3 zones listed above in many different ways. The most commonly described methods include:

  • traditional pyramidal approach: ~80% in zone 1, 10-15% in zone 2, 5-10% in zone 3
  • polarized approach: ~80% in zone 1, very little in zone 2, ~20% in zone 3
  • threshold training: majority in zone 1, >20% in zone 2 (around race pace), a variable amount in zone 3

Historically most well-trained and elite long-distance athletes have followed the traditional pyramidal approach, though there has more recently been a shift to the polarized approach. Threshold training is generally considered suboptimal although some world-class marathoners utilize this method; these marathoners design their training based on their race pace as opposed to physiologic measurements.(Kenneally, 2018) In this situation marathon runners at the world class level have a race pace that falls within zone 2 and thus a large amount of their training is done in this zone. For individuals competing in shorter events their race pace may fall outside of zone 2 so less of their time will be spent in that zone.

Instead of using the specific zones listed above, which as indicated previously are not physiologically consistent between different people, a different strategy is to divide zones by goal race pace. For example, you can classify training zones by(Barrie, 2020):

  • <95% of your goal race pace
  • 95-105% of your goal race pace
  • >105% of your goal race pace

As different length races have race paces that can fall in different traditionally defined zones, and as physiologic characteristics within these zones can vary between people, simply changing the training design paradigm to zones based on goal race pace will help eliminate inconsistencies.

Note: As this course primarily focuses on general health, I will not spend additional time discussing how to specifically prepare for races or competitions of different lengths. The above information is for general knowledge and to provide a framework when attempting to design a training plan. For a general overview of considerations and sample training plan regarding training for a half-marathon, click here. For other types of endurance events, there are many internet forums and websites that provide useful information.

If training for general health and fitness

When training for general health and fitness both aerobic training and resistance training are typically incorporated. This is called “concurrent training” as you develop both resistance and aerobic training attributes simultaneously. When doing this you must be weary of the “interference effect”; this describes the situation where training both attributes simultaneously proves detrimental relative to training each attribute in isolation.(Sousa, 2019) This interference effect varies by aerobic training modality, exercise intensity, volume, and proximity to resistance training sessions.

Note: I discuss how to structure an aerobic training routine with a resistance training routine to help minimize the interference effect in the next lesson.


This lesson has covered some of the key terminology and concepts for describing and understanding aerobic training. The main factors influencing endurance performance are difficult to measure, and many of the physiologic characteristics at any specific exercise intensity differ between different people. Thus, it is challenging to prescribe a one-size-fits-all training program where you can expect similar results between different individuals. Using pace-based strategies may help mitigate this issue; more research is required on this strategy. However, while these considerations may influence training for a specific competition, for general health purposes they are less pertinent; after all, the general guidelines simply suggest that you engage in moderate and vigorous physical activity, not activity at a specific % of your HRmax or HRR.

Thus, for practical purposes when you perform endurance activity for general health, the more relevant question becomes how to divide up your time between continuous training and interval training while minimizing any interference effect with resistance training. I will discuss these topics, as well as the safety of more intense exercise sessions, in the next lesson.

Click here to proceed to Lesson 8


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