Table of Contents
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 them that different terminology and concepts are needed to accurately describe aerobic training. Thus, we 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)
When we start doing a cardiovascular session (“cardio”) initially we use our anaerobic energy production pathways as described in Lesson 2. However, if we perform cardio for several minutes to >1 hour, aerobic energy production plays a much larger role. This requires oxygen to generate energy, and our VO2 max is the maximum amount of oxygen we 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)
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 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 we 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 increased connective tissue stiffness in the foot/ankle to help prevent excessive energy loss into the ground with each foot strike).
While performing endurance activity at a level greater than one’s VO2 max is possible, this requires an anaerobic energy production component. However, even when performing at a level less than one’s 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 in muscles they 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 one’s lactate thresholds.
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 we increase our exercise intensity and need to transport more oxygen to our muscles, our heart rate will increase to do this since oxygen is transported by blood. Our heart rate maximum (HRmax) is the maximum that our heart rate can elevate. One 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, we can estimate our HRmax with one of several different equations. The simplest is HRmax = 220 – age, as described by the CDC. If we take our heart rate max and subtract our resting heart rate we get our 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. As discussed below it is questionable if one 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.
Defining intensity rangeWe can classify aerobic exercise intensity 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.
- 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 we 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.
Tip: For these 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 going.
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 we calculate 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% 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, one 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 one 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 one could aim for the higher end of the range and if the run seemed too difficult one could aim for the lower end of the range.
Types of training sessions
When doing cardio with any modality (ie, running, elliptical, rowing, bicycling, etc), we can manipulate multiple parameters based on one’s 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, as well as to what degree one varies these parameters during a session. Regardless of training modality and the above parameters, one can choose to perform cardio continuously or in intervals.
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 bulk of one’s training when training to perform a longer distance event. This 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, 2019)
- “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.
While we can theoretically perform intervals at any intensity, high-intensity interval training (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 VO2 max while SIT and RST are performed at greater than VO2 max. The amount of rest between each interval can vary.(Rosenblat, 2020)
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 depends on one’s goals. If the goal is to perform in a competition (ie, a half-marathon) then one 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 one is 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)
We can divide 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, we can classify training zones by(Barrie, 2020):
- <95% of goal race pace
- 95-105% of goal race pace
- >105% of 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 we develop both resistance and aerobic training attributes simultaneously. When doing this one 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.
In this lesson we have 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 we 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 engaging in moderate and vigorous physical activity, not activity at a specific % of one’s HRmax or HRR.
Thus, for practical purposes when performing endurance activity for general health, the more relevant questions become how to divide up time between continuous training and interval training while minimizing any interference effect with resistance training. We will discuss these topics, as well as the safety of more intense exercise sessions, in the next lesson.
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