Low Intensity Training and the Maximal Aerobic Function Test: Measuring Aerobic Fitness and Nervous System Function

Testing strength standards is fairly straightforward: can you lift a certain weight for a certain number of reps, or not?

However, measuring cardiovascular and aerobic function can be more complex. Of course, there's a straightforward race or time trial -- how fast can you go over a certain distance -- but this does not consider different cardiovascular energy systems, or the role of mental fortitude, for example.

Even more complex is measuring nervous system function. Do you live and work primarily in a sympathetic state (more hype, more stress, faster-paced sprint efforts), or a parasympathetic state (resting and digesting, slower aerobic efforts)? This is important to consider as it can be a window in both the effects of our training, and how we are coping with all of the other stresses present in our lives. 

Our culture tends to push most people towards a sympathetic state. We're talking about chronic stress: The stress of constant emails, notifications, finances, interpersonal conflict, traffic, commuting, environmental exposure, processed foods, and more -- these are all stressors that our body must attempt to adapt to. When the body senses a potential or actual disruption to its homeostasis, a sympathetic stress response occurs to help the body deal with that stress. This kicks off a complex chemical cascade that affects everything: our hormone expression, how we feel, our blood pressure, our glucose response, our hunger. High intensity training is often more sympathetic in nature.

Normally, our body evolved to handle the stress, and then come down from the sympathetic state with a parasympathetic response. This is where you chill out. Crisis over, you can relax. Your system resets and repairs, digestion restarts, blood pressure lowers, and things go back to normal. However, our bodies did not evolve to live in the 21st century. We are surrounded by things that are potential stressors, and many people live in a sympathetic state, never getting that parasympathetic response. This can send many of our body's systems awry, and is something that often underlies chronic gastrointestinal issues, overuse injuries, or other pain conditions. Lower intensity aerobic training is often more parasympathetic in nature.

Training at a low intensity:

Why does all of this matter? Exercise is exercise, and the more of it, the harder it is, the better, right? Actually, not really. Being in a sympathetic state all the time can cause problems in the body. Yes, we need to go hard and go heavy, but the body must also function efficiently at a lower intensity. Even if you're super strong, if you don't have a robust aerobic system, you'll be missing out on performance and health benefits.

There an expansive (and growing) body of research demonstrating a few things:

  • The most successful athletes, even in high intensity sports, spend about 80% of their time training at low intensities, and only 20% training at a high intensity (reference). The same reference demonstrates that spending more time at higher intensity levels does not improve performance.
  • Spending too much time training at high intensity increases risk of injury and overtraining syndrome as well (reference). Overtraining syndrome and overuse injuries are caused by excessive loading. Loading = Frequency x Duration x Intensity, and there's some research indicating that higher intensity workouts are significantly harder to recover from than low intensity workouts of an equal or even longer duration. (See here and here). So, while we need to spend some time at a high intensity, we must be aware that those workouts will take more time to recover from.
  • At 7,000 feet elevation in Flagstaff, there is less oxygen, and this makes the heart pump faster at a given workload to deliver adequate oxygen to the system. Even if you don't feel like you are working hard, stress is still stress, and a high heart rate means high intensity. So, we must be even more aware of our training intensities at elevation than we might be at sea level, as it is very easy to train too hard too often.
  • What's training too hard? Check out the test below. 80% of your training should be at or below the maximal aerobic threshold determined by the 180-formula. 20% should be high intensity, go-for-it, but with planned subsequent recovery or lower intensity work.

Measuring low intensity performance

One problem that athletes run into when doing low intensity training is that it doesn't feel hard, or that you are making progress. It can feel hard to measure the effectiveness of your low intensity training. Enter the MAF test. It is a simple at-home test is called the Maximal Aerobic Function Test that measures lower-intensity aerobic performance, and by extension, nervous system function.

Here's how it goes:

  1. Calculate your maximal aerobic heart rate using the 180-Formula. This is the rate above which your body stops relying primarily on aerobic energy systems. For our purposes, we are going to assume that it's also the level above which your sympathetic system is going to kick in. There are a lot of heart rate formulas out there, but Phil Maffetone's 180-Formula is fairly simple and gives a range that's approximate to other heart rate zone calculations. Here's the calculation, taken from Maffetone's literature:
    • Subtract your age from 180. Then, modify this number by selecting among the following categories the one that best matches your fitness and health profile:

      • If you have or are recovering from a major illness (heart disease, any operation or hospital stay, etc.) or are on any regular medication, subtract an additional 10.

      • If you are injured, have regressed in training or competition, get more than two colds or bouts of flu per year, have allergies or asthma, or if you have been inconsistent or are just getting back into training, subtract an additional 5.

      • If you have been training consistently (at least four times weekly) for up to two years without any of the problems in (a) and (b), keep the number (180–age) the same.

      • If you have been training for more than two years without any of the problems in (a) and (b), and have made progress in competition without injury, add 5

    • For example, let's assume I am 35 years old, do not have any regular medications or recent illnesses, but am recovering from an injury. So, my maximal aerobic heart rate would be 18- - 35 - 5 = 140.

  2. Choose a repeatable, consistent aerobic effort that's approximately 30 - 60 minutes long. You can change the duration based on your goals: if you are an endurance athlete, choose a longer test. If you don't often do endurance events or this is not your training goal, something around 30 minutes is OK. We do want at least 30 minutes so we can see how you respond as you get into an endurance time-frame.

  3. The test should take place over should be something that is fairly consistent in grade/elevation (not many big climbs or descents), surface (paved or smooth trail) so that your heart rate does not have any unpredictable spikes during the test. Additionally, it should be something that you can repeat a few times a year in a similar fashion.

  4. Warm up for about 5 minutes. Then, using a heart rate monitor, complete your course, keeping your heart rate at or below your maximal aerobic heart rate. In the example above, I would have to keep it below 140.

  5. Track how long it takes you to complete your course, and if possible, standardized intervals during the test. For example, I chose to run 3 miles. My first mile was 9:30, then 10:15, then 11:00 for 30:45 total.

  6. These times are the result of the test. Over time and with additional training, we would expect you to be able to complete the test at a faster pace without exceeding your maximal aerobic heart rate.

A few notes on interpretation:

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  • You might have to go at an agonizingly slow pace to keep your heart rate below your maximal aerobic rate. Almost everyone who's done any training can usually "push" to go faster, and may feel like it's almost a waste of time to go slow enough to keep their heart rate in the correct range. Resist the urge to go faster. We want to see how fast you can go in your aerobic range. We're also using this as a test of nervous system function. 
  • If you do indeed find that you have to go slower than expected, that's a sign that you may have more sympathetic dominance in your system, and that you are potentially lacking a true aerobic base. You will likely benefit from additional low intensity training at or below your maximal aerobic heart rate to develop aerobic fitness and parasympathetic balance.
  • With appropriate training over time, we want to find that you can complete your course faster while maintaining your heart rate. This demonstrates aerobic fitness improvements and more nervous system balance.
  • While a ventilatory or V02 test with gas and blood marker measurement might be the gold standard for determining your low, moderate, and high intensity levels, these tests are expensive, time consuming, and for most not nessesary. The 180-Formula or other heart rate zones tend to be close enough.
  • If you do not have a heart rate monitor, we do recommend purchasing one, as they are a valuable training tool, especially if you live here in Flagstaff at 7,000 feet. At altitude, it is easy to spike the heart rate high, potentially leading more easily to overtraining. A heart rate monitor can help to track your training intensity.
  • If you cannot purchase a heart rate monitor, then you can perform the MAF test breathing in and out of your nose the entire time. This will automatically limit your pace as if you go too fast, you will have to begin breathing in and out of your mouth. It's not perfect, but nasal breathing is more parasympathetically driven, so it will give us some insight. 

Entire libraries can be filled with books on training volume, frequency, and intensity, and we are just scratching the surface here. The key takeaway: Use the MAF test to measure your aerobic system. You might be training harder than you think.

Here are some additional articles about this concept in addition to the peer-reviewed references above:

We currently recommend formally doing your MAF test every 8-10 weeks when we test our other Performance Training benchmarks. This allows for time for improvements, but testing frequently enough to catch changes and tweak the training program if needed.