Do Micro-Bursts on Endurance Rides Help?

Here’s why they’re key to improving your performance

Luke Hollomon, M.S., DPT
The Cycling Physio
4 min readJun 9, 2021

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Nearly every cycling training manual, guide, and book includes a version of “3hr Endurance with Micro-Bursts” somewhere within it. Based on the famous City Limit Sprints that mark just about every group ride, this type of workout has been around since structured training began. But what does it do? I don’t believe in doing any training blindly. Everything should have a reason and you should know why I’m asking you to break up your 3 hour cruise around town with random 15 second sprints every 30 minutes. Fortunately, this type of work now has great data to back it up. Let’s explore it.

What Micro Sprints Do To Your Body

The immediate effects of the micro-sprints are obvious. Shortness of breath, muscle burn, increased fatigue, etc. All of these are markers of a workout’s effectiveness, but none actually demonstrate what a workout is doing to your body. A pile of researchers from Norway recently dug into the details of this and discovered the long-term, ergogenic effects of this type of workout. They put twelve elite cyclists (VO2max 73.4±4) through two separate workouts, one of 4 hours of cycling at 50% VO2max and another with 4 hours of cycling at 50% VO2max with three sets of 3 x 30sec sprints, one per hour after the first. I’d argue that these are a bit long for sprints, but they do match the City Limit style of sprint that we’re familiar with. After the workouts, they took muscle biopsies and blood samples, then dug into the data to see what happened.

The goal was to see if the sprints created greater gains while keeping recovery time equal, and they did indeed. I’ll break them down by category.

What Didn’t Change

There’s a long list of things that didn’t change, but here are the highlights:

  • Time to recovery stayed the same.
  • Short sprints didn’t seem to change how quickly the legs came back.
  • Hormone levels
  • There were very few differences in hormone levels, indicating essentially no difference here.
  • Training effect
  • There was no difference in the markers of fatigue used by the study

There were essentially no differences in these areas, but that doesn’t mean the exercise was ineffective. The quite interesting changes are next up.

What Sprint Efforts Did Change

Genetics

The sprint efforts made many genetic and gene expression differences, indicating that the body was responding to the different effort and adapting to it. Genetics are often misunderstood in exercise science, so here’s a quick breakdown:

Gene sequences are innate and unchangeable. These are the differences between you and Taylor Phinney. His genetic sequences are always going to push him ahead with the same amount of work (or even less).

Gene expression, on the other hand, changes all the time. Gene expression changes in response to hunger, thirst, exercise, stress, and more. Gene expression is the amount of protein synthesis your body does at each gene. For example, when you exercise a lot, you’ll need more proteins associated with strength and cardiac endurance, so your body will express those genes more often. This is what changed in the study.

Specifically, the gene expression that changed in this study was related to angiogenesis (new blood vessels), ion transport, mitochondrial function, metabolism, and protein synthesis. The scientists saw this through mRNA differences in the blood samples they took.

Here’s a quick breakdown of the changes:

  • The sprint group had greater angiogenesis gene expression, indicating that their bodies were building and repairing more blood vessels, a huge part of increasing exercise endurance.
  • Ion transport expression increased in the sprint group as well, another essential element of exercise. Ion transport is needed to cause muscles to contract and relax and send nerve impulses throughout the body. Increasing ion transportation could lead to an increase in recovery capability and muscle contraction force.
  • Myostatin expression decreased in the sprint group as well. This is great news for muscle development. Myostatin is a protein that inhibits muscle growth, so lowering its expression increases the amount of muscle synthesis performed after a workout.
  • Lastly, fat metabolism gene expression increased in the sprint group, indicating that their bodies were switching to more fats for fuel, an essential part of endurance athletics ability.

Why You Should Sprint in Endurance Rides

As a quick summary, sprints in the midst of long endurance rides don’t increase the amount of recovery required, do increase angiogenesis, ion transport, muscle grown, and fat metabolism. This simple addition to your endurance rides can have huge effects that researchers are just now digging into. In the words of the authors:

Inclusion of sprints to prolonged LIT exercises might thus be an efficient training strategy for elite athletes, potentially leading to beneficial adaptations in skeletal muscle without affecting muscle performance on consecutive days.

This is great news for such a simple addition to a workout and I’ll be adding this to my rides often. The sprints don’t have to be perfectly programmed to get the same effects, so feel free to adjust as needed. If you have a few small hills, sprint up those instead of rolling them easily, that’s all you need to get these ergogenic boosts.

The information used in this article comes from:

Almquist, N. W., Ellefsen, S., Sandbakk, Ø., & Rønnestad, B. R. (n.d.). Effects of including sprints during prolonged cycling on hormonal and muscular responses and recovery in elite cyclists. Scandinavian Journal of Medicine & Science in Sports, n/a(n/a). https://doi.org/10.1111/sms.13865

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Luke Hollomon, M.S., DPT
The Cycling Physio

A science communicator and physical therapist with a master’s degree in physiology and a background in science education. I write about life science and health.