Issue #5: Muscle Fiber Type and Recovery Demands

✍️ Author’s Note

Welcome to this edition of the Threshold Lab newsletter! I’m Stephen Pelkofer, an experienced HYROX athlete with a men’s pro personal best of 59:41 and men’s pro doubles best of 51:39. If you want to learn about my story and how I got here, check out this Instagram post here.

The goal of this newsletter each week is to pick a training topic related to running or HYROX, do deep research into it, and provide actionable protocols that the reader can take away and apply to their training immediately. Let me read the research, listen to the experts on podcasts and youtube, and then give you the tools to make it work for you.

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👨‍💻 Introduction

Topic: Muscle Fiber Profile and Recovery Impacts

Have you ever noticed how some people bounce back from a brutal workout much faster than others? One athlete might crush a morning interval session and feel ready to go again by evening, while another is still feeling the effects the next day. Understanding these recovery differences is crucial. Today we’re going to dive into fast-twitch vs slow-twitch athletes, and how an athlete’s muscle fiber profile can have a huge influence on recovery needs. The majority of the following is based on this specific paper: Muscle fiber typology substantially influences time to recover from high-intensity exercise; Lievens, Klass, Bex, Derave [1].

⚡️ Fast Finds for the busy reader

• Fast-twitch dominant athletes take longer to recover from high-intensity workouts than slow-twitch athletes.

• Fast-twitch = power + fatigue. These athletes shine in explosive efforts but need more recovery and careful programming.

• Slow-twitch = steady + resilient. These athletes can handle more volume and recover quicker, even from back-to-back hard days.

🔬 Deep Dive

The Fast vs. Slow Twitch Basics: Human muscles are composed of a mix of Type I (slow-twitch) and Type II (fast-twitch) fibers. Fast-twitch fibers contract quickly and produce high power (think of a sprinter or leaper’s explosive ability), but they also fatigue rapidly. Slow-twitch fibers are the opposite – they generate less peak force but are far more resistant to fatigue, making them ideal for endurance efforts. These basic differences have been known for awhile, but applying the knowledge to specific athletes can be difficult because measuring an athlete’s fiber makeup requires an invasive muscle biopsy.

In this particular study that I reviewed, scientists used a noninvasive MRI-based test (proton magnetic resonance spectroscopy) that can estimate your muscle fiber typology by measuring carnosine levels in the muscles. Carnosine is naturally higher in fast-twitch fibers, so it serves as a marker for fiber composition. With this tool in hand, researchers set out to see just how much fiber type influences fatigue and recovery in real athletes.

The Experiment - Sprint, Suffer, Recover: The study divided a group of male athletes into two camps based on their muscle scans: one group with predominantly fast-twitch muscle profiles, and another with predominantly slow-twitch profiles. Each athlete then underwent a challenge: three back-to-back 30-second all-out sprints on a cycling ergometer (the Wingate test). These repeated sprints are extremely taxing and create heavy fatigue. The researchers measured how each athlete recovered by testing their maximal knee extension force at various intervals for up to five hours after the sprints.

What They Found: Both groups performed similarly during the sprints in terms of average power output, but the pattern of fatigue was very different. The fast-twitch dominant group saw their power drop by about 61% from the first sprint to the third, whereas the slow-twitch group’s power declined only about 41% over the same series. The fast-twitch athletes faded much more within the session. Even more striking was the difference in recovery afterward: the slow-twitch group’s muscles had fully recovered their strength within 20 minutes, while the fast-twitch group’s muscles were still not fully recovered even 5 hours later. So five hours post-exercise, the fast-twitch athletes hadn’t regained all their muscle force, whereas the slow-twitch athletes were basically back to normal after a quick breather. This suggests that an athlete’s muscle fiber profile might have a substantial impact on the time it takes to recover after high-intensity work.

Athletes with an endurance based muscle profile (more slow-twitch fibers) bounce back faster from a hard interval session than those with a power or explosive based profile. One workout might barely dent a “diesel engine” slow-twitch athlete, but completely floor a more explosive, fast-twitch athlete. This is why one-size fits all training plans can have issues, specifically for fast-twitch athletes.

It’s not just about single workouts, either. Additional research from the same group of scientists examined how athletes responded to a high-volume training block. After three weeks of progressively intense training, the athletes with more fast-twitch fibers were far more likely to end up overtrained compared to the slow-twitch athletes [2]. In that study, the slow-twitch-dominant runners handled the overload just fine and actually came out faster after a taper, whereas the fast-twitch runners struggled and saw their performance dip. This reinforces the point that your fiber makeup influences not only how you feel later that day after a hard workout, but also how you adapt over weeks of training.

Time for the lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

• Know Your Bias: Power and sprint athletes are likely fast-twitch dominant. Endurance athletes lean slow-twitch. If you played and excelled at sports that required explosive ability growing up, you might lean more fast twitch. If you have been a distance runner since you were in middle school, you probably lean slow twitch.

• Match Intensity to Recovery: Fast-twitch = longer recovery between hard sessions. Slow-twitch = more frequent quality work is fine. If you are a fast-twitch athlete, two hard quality sessions per week might be the most you should program for now.

• Scale Volume Wisely: Fast-twitch athletes should avoid big volume jumps. Slow-twitch types can handle high training load weeks better.

• Train the Weak Link: While you should respect your fiber giftedness, you can also improve your weaknesses with smart training. If you’re fast-twitch dominant, you might have great anaerobic capacity but a relative weakness in aerobic endurance. Focus on building an aerobic base with a lot of easy work surrounding your quality sessions.

Remember, muscle fiber makeup is not destiny. You can train to become a more well-rounded athlete, but it does influence how you respond to training. By listening to your body and applying these strategies, you’ll train smarter, unlock better performance, and reduce the risk of injury in your athletic pursuits.

That’s it for this edition of the Threshold Lab. If you enjoyed this and found it useful, please share it with a friend!

What I Read and Researched for this Issue

• [1] Muscle fiber typology substantially influences time to recover from high-intensity exercise

• [2] Muscle fibre typology and the individual responses to training