Understanding intensity in endurance training.

Training to improve our physical performance is a game of breaking down and recovering. We are applying a stressor (training), to stimulate a response (recovery), that leads to an adaptation (gains).

In this game, we have several variables that we can manipulate to shape the type of stressor we are applying and ultimately affect the nature of the adaptation we are aiming for.

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In this current article, we will focus on intensity as our training variable of choice. This post will look specifically at cardiovascular training, with a future post covering resistance training.

Within those two training types, we will look at what intensity means, how intensity can be changed, and what the outcome is of changing intensity.

What does intensity mean in cardiovascular training?

Now, this can be dependent on the type of cardio training you are doing. For example, the metrics and understanding of intensity will differ between cycling and running. Whilst there are typically some shared measures that can be common to all modalities (heart rate, or RPE for example) there are also differences that aren’t commonly used between modalities (Watts on a bike versus speed in running).

Generally, though we look at intensity in a few common ways with cardio.

• Pace: Defined by a time per distance output. eg: Mins/km in running, or time/500m on the rowing erg.

• Speed: Just a simple measure of how fast you are going eg: 30 km/h

  
  

• Heart rate zones: Various ways to calculate these but these are usually calculated as %’s of maximum heart rate.

  
  

• RPE (rate of perceived exertion): A subjective measure of how hard you feel you are working. A 1-10 scale is common, but others are used.

Overall, intensity in cardio training is based on either absolute values like speed, or it is based on values relative to something like heart rate max or VO2 max.

Regardless of how you measure intensity, it’s still a measure of how hard you are working from a physiological standpoint.

We use different distinctions and classifications here such as aerobic or anaerobic, power or endurance, and sustainable or unsustainable. These are largely arbitrary, but they do help us to: a) define how hard we are working, and b) understand, to a degree, what our body is doing to do that work.

How can intensity be changed in cardio training?

This might now feel a little too obvious to be worth discussing much, but I think there is still some value in knowing how we change the intensity of our cardio training. Largely because how we change it can impact the adaptations we target.

So in a reductionist, simplistic view, we can change the intensity by either working harder or working easier. In the real-world metrics above, this means either increasing or decreasing the measures. For example, if we run faster our intensity increases. Or if we lower our heart rate, it is generally a case that we have lowered our relative intensity.

Where I think this matters on a deeper level is in how we change intensity in its subsequent impact on adaptations.

Let’s use a worked example to show what I mean. Say you are on a static bike erg, pedalling for intervals of 30 secs at a set power of 300 watts with 60 secs rest between.

That represents whatever individual level of intensity that exists for the person on the bike. However, that intensity can be dependent on how we look at the work they are doing.

If you take the intensity as just power output, then they are working at 200 watts. You could compare that to a maximum watt output or you could look at what speed that 200 watts is associated with. These are all objective measures of intensity that are largely static regardless of internal measures of intensity.

Now we can ask that person how hard they feel they are working, representing a subjective measure of internal intensity. Or you could measure their heart rate you can see an objective measure of internal intensity. These are independent of the power output, and although still relevant and related, they do not always directly align.

Moving forward, say that we keep that same 200 watts but adjust the time from 30 secs to 60 secs. Or we reduce the rest from 60 secs to 30 secs. Now from an objective standpoint, nothing much has changed intensity-wise. We are still working at the same power output and the same speed.

But internally, we have likely increased the intensity. RPE will begin to increase, and heart rate will be elevated.

So what has happened to the intensity? In one case, nothing is different, on the other we have increased.

We can see through this example then that how we choose to measure or change intensity can affect the adaptations we might be causing. To that end, it’s important to understand not only what intensity is and how it can be changed, but also what physiologically happens when we change intensity.

What happens when we change the intensity with cardio training?

Broadly, we are looking at what happens to our physiology from lower to higher intensities and how that then leads to different adaptations.

Traditionally, we would separate aerobic and anaerobic training where it is generally seen that aerobic training is a lower intensity and anaerobic is a higher intensity. Whilst these distinctions can help to show overall differences in training intensity, more modern understandings tell us that there isn’t a clear-cut line between aerobic and anaerobic.

I think additionally, it’s important to understand that within each of these intensities, you could further differentiate between higher and lower intensities relatively. So you could do more challenging, low-intensity work. Or you could do moderate high-intensity work. We can look at this across a spectrum where we go from the easiest, lowest-intensity work to the hardest, highest-intensity work.

This relates in part back to our objective and subjective measures; we need to consider both the external values, like speed or pace, and the internal values, like heart rate or RPE.

We can have a higher power output, done for a short time with lots of rest. So it might feel relatively lower intensity than moderate power output but done for a longer time.

Equally, we could have a low-intensity output but done for a very long duration and this would then feel relatively hard.

These inherently target different types of adaptations and are not typically good substitutes for each other.

Broadly, let’s think about what the different types of intensity will achieve from an adaptation standpoint.

Low-intensity training, done for longer durations is working largely on our ability to produce aerobically fuelled efforts. This is where we might traditionally think of aerobic capacity being the aim. What we are doing here is making our heart and lungs more efficient at taking oxygen in, getting it to our working muscles, and using it to produce lots of energy, but slowly, and at lower intensities. Our body here is working predominantly in an aerobic state, whilst still utilising smaller percentages of anaerobic energy production. We can sustain these outputs for long durations and so long as we keep fuelling our body, can keep going without much drop-off.

At moderate intensities, we are working on our bodies’ ability to maintain a sustainable output whilst getting closer to the tipping point or threshold whereby we are working unsustainably and will then be forced to slow down or decrease our level of output to continue further. Our body is producing a higher amount of lactate which is being used to buffer the increasing levels of acid buildup in our muscles and reform glucose as additional fuel. Here we are still using oxygen and working aerobically, but the relative proportion of anaerobic processes is increasing and our bodies are working hard to maintain output without it dropping off. We are burning lots of glucose here and this has a cost associated with it that as we start to climb towards higher intensities begins to become unsustainable as we fatigue faster than we can clear waste buildup and use oxygen.

At high intensities, we are pushing our bodies’ physiology closer to its maximum levels. We are approaching our VO2 max which is a measure of the maximum amount of oxygen our body is capable of taking in and using before this reaches a peak and no further increases in oxygen usage are possible. Between the moderate intensities and VO2 max, we have surpassed the ability of the body to maintain a sustainable output and we are fighting a losing battle on maintaining this level of work. Past this threshold of maximal steady state ability, we are beginning to accumulate more waste than we can clear, we cannot supply and use enough oxygen to keep aerobic processes going at a fast enough rate and anaerobic processes are beginning to get close to their max outputs. Once we reach VO2 max, we hit the ceiling of our ability to effectively use any greater amount of oxygen to fuel aerobic energy production processes. Importantly, we are still using a high amount of aerobic energy production here. We are still working aerobically, but anaerobic processes are contributing a relatively much higher amount of the total energy production. We nearly never don’t use oxygen at any intensity, despite traditional viewpoints and teachings.

Beyond VO2 max, we can push slightly further to a true max effort. But this capacity is severely limited and has a very short time frame; a matter of a couple minutes at most of the majority of individuals. At this stage, we are near max heat rates, and our body has no more ability to produce further energy to supply the demand through aerobic or anaerobic means. These higher levels of intensity from moderate to max, are working on the efficiency of our body to produce anaerobic energy production, we are also working on elevating our VO2 max and pushing up the ceiling of our maximum oxygen usage through increased efficiencies in the processes of metabolic fuel usage.

Hopefully, this shows how there is overlap between these different intensities, but they are not good direct substitutes for each other. We can’t just use high-intensity training to negate having to spend more time at lower intensities. Equally, we can’t just do low-intensity work and hope that it will improve our ability to go fast and hard when needed.

A good training program will use a mix of intensities to achieve the desired outcomes, specific to the individual, their training modality/discipline, and the event/race they’re training for.

Hopefully, this article has helped to give you a deeper and broader understanding of intensity within cardiovascular and endurance training. You should now know more about what intensity is, how it can be measured, and the effects it has on your body.

If you have any questions please comment below, DM me on Instagram @apexdeltacoaching or email me at kieran.apexdelta@gmail.com