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Writer's pictureJordan Metcalf

Running Fast and Slow: Understanding Energy Systems to Help You Run Faster

Updated: Oct 7

If you’ve ever used an online run training program like Garmin or Hal Higdon, you know that when you set your race goal pace, the easy runs are considerably slower. Have you ever wondered why? If yes, you’re in the right place, because I will seek to answer the question of why you should make your easy runs considerably easy, and why you should make your tempo or faster runs challenging.


The simple answer comes down to how the body processes energy.


When you run slowly or even walk, you use your aerobic energy system. You’ve likely heard of this before, it’s the system of energy consumption that uses oxygen. This system is very efficient and can go for a long time, but it doesn’t produce a lot of power. Think of rocket boosters vs. a Prius. Rocket boosters can produce an immense amount of power, but they can’t run for particularly long before they run out of fuel, while a Prius can run for who knows how long before it needs to fill up on fuel.


When you run fast or sprint, you use your anaerobic energy system. This is where the body uses its rocket boosters. This energy system burns fast and bright, and does not need oxygen to function. This system is slightly less efficient in that it doesn’t fully break down glucose molecules into energy, and it takes longer to recharge.


When you are training for a race, you can bias or train one or both of these systems depending on what parameters you use during your runs. The primary characteristics that you will see of many high quality run training programs is that the slow runs seem very slow to train the aerobic system, and the interval or tempo runs are quite challenging to train the anaerobic system, but there are not many long training runs at moderate intensity.


Why is that?


Because training between the energy systems by running your long/easy runs too fast is more likely to leave you depleted without maximizing the benefits of improved aerobic capacity or improved power and speed. Most likely, the primary time that you will run between the two energy systems is on race day.


So how do you gauge the intensity of your easy runs to make sure that you are not going too fast?


  1. Heart rate: if you use a heart rate monitor when you run (recommended) you should shoot for zones 2-3 which is ~ 65-80% of your heart rate max (HRmax). The equation to calculate your HRmax is 220-age. For example, let’s say you are a 30 year old runner:


220-30 = 190 x 0.65 = 123.5 bpm


220-30 = 190 x 0.8 = 152 bmp


So your average heart rate should be between 124-152 bpm for your easy runs.





  1. Pace: There are pace calculators out there that will help you determine what pace you should strive for depending on the goal of the run, such as this one from Runner’s World. These work well, however they don’t take into account environmental factors like hills, heat, and humidity.

  2. Rate of perceived exertion (RPE): This is a 0-10 scale used to assess one’s own exertion level with a particular activity. Pros: it doesn’t require using a heart rate monitor, it’s not sensitive to environmental factors like heat, humidity, and hills, and it is also not sensitive to the ebbs and flows of how you feel. If you didn’t get great sleep the night before a run, something that might usually be a 3 on the RPE scale might be a 5 that day. Cons: it may be a little less scientific, and it requires you to be intuitive with your exertion levels, which some people struggle with. Read more about RPE here.


For distance running, most training programs recommend at least 80% of your weekly miles to be easy miles.


At Alterra, we specialize in helping runners resolve any and all of their injuries that limit their running ability or performance. If you're local to Boston's Back Bay area and are looking to dial in your training, we'd love to help! Book a free discovery call to chat with a therapist today.




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Disclaimer: The information in our blog posts is not a substitute for professional medical advice. Consult a healthcare provider before making decisions related to your health. Use our content at your own risk.






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