Your make-up of muscle fibers will determine a large amount of your running success. Two otherwise identical runners with different percentages of slow and fast twitch muscle fibers will have wildly different strengths. The good news, however, is that whatever your current state physiologically, it’s possible to change it!
Slow Twitch vs Fast Twitch
First, we have to lay the groundwork. All of us have a spectrum of muscle fibers in our muscles. At one end, we have the purely slow twitch fibers. Slow twitch fibers get activated with even the smallest amount of force (ie: a slow jog, the beginning of a run, easy running, etc.). They are very fatigue resistant, which means they are built for endurance activity and will last longer.
Purely fast twitch fibers are at the opposite end of the spectrum. They are only recruited when a high amount of force or speed is produced (ie: a sprint, hill repetitions, fast surges, etc.). They are also recruited once the slow twitch fibers have been exhausted (ie: end of a race or long run). Fast twitch fibers are powerful, but they are not so fatigue resistant. They tend to tire out quickly, which is why even the fastest runners can only maintain their top speed for a short amount of time.
The Important Part | The Middle!
In the middle of the muscle fiber spectrum, we find the those fibers with a mixture of both slow and fast twitch qualities. For better or for worse, the make-up of these is largely determined by genetics. But here’s the crucial part – we can actually change the makeup of these fibers to be more slow twitch or fast twitch depending on our training!
What does this mean, practically speaking? Well, at a high level, if you want more speed, you should seek to recruit more fast twitch fibers in their training. Doing so will cause those blended fibers to shift towards the fast twitch end of the spectrum.
Likewise, if you need endurance for longer events (5k or longer), you should seek to recruit more slow twitch fibers. More long runs, general endurance runs, easy paces, etc. Keeping the fast twitch recruitment to a minimum.
Or better yet, you’ll often hear of an endurance athlete doing hill repetitions or fast surges in the beginning of a season. This recruits more fast twitch fibers early in the training cycle that will be able to be converted to slow twitch throughout the rest of the cycle as more specific endurance work is performed.
What Kind of Runner Are You?
Most of us have a mix of both types of fibers. As such, no significant training changes need to be made. Others may find that they are strongly slow or fast twitch oriented! Here’s the best two ways I know to identify it.
PR Comparison. Look at a Jack Daniels VDOT chart and see how your PRs across different race distances compare. Are you able to hit a particular VDOT score for short distances but nowhere close for the half marathon and marathon? This indicates a fast twitch runner and vice versa for a slow twitch runner. If your PRs are somewhere in the middle, then you are a well-balanced runner!
Beware though, this method of comparison assumes that you’ve trained adequately for each distance. If, for example, you’ve only run one marathon and you were poorly trained for it, then it’s not fair to compare that to shorter distances that you’ve raced much more frequently.
How Do You Handle Different Workouts. The other method looks at what workouts are your strengths. Do you consistently hit your interval workouts and even look forward to them? Fast twitch! Do you love to run long and seem to never run out of gas? Slow twitch! If you’re on the ends of the spectrum, usually a strong pattern emerges.
I’m Slow Twitch | Now What?!
Congratulations! Your abundance of slow twitch fibers has several implications. For one, it’s going to take a lot to recruit those fast twitch muscle fibers. The average runner will get good fast twitch fiber recruitment at the end of a long run once slow twitch fibers are exhausted. But not you!
To fight against this, consider adding something like strides at the end of your easy runs 2-3 times per week. This will activate those fast twitch fibers and put them on standby for a finishing kick, should you ever need it!
As a slow twitch runner, if you want to run a little faster on an easy day, don’t sweat it! If you’re able to keep the pace conversational and comfortable, then this is fine. You’re likely still far from recruiting any fast twitch fibers, which is unwanted on an easy day.
The benefits of slow twitch runners are vast though. These runners tend to perform more consistently in races and can maintain peak form for longer. Furthermore, they can burn fat more efficiently, which is very helpful for those running marathons.
I’m Fast Twitch | Now What?!
You have the gift of speed! This can be a great asset but it can also come with a price. Namely, it doesn’t take much to start recruiting those fast twitch fibers so if you want to train for a longer race, you need to make some adjustments.
I would start by slowing down your easy runs. This will help you recruit and build more slow twitch muscle fibers which are more fatigue resistance and built for endurance. Don’t pay attention to your friends who have the same race times as you. You may be well slower than them on easy days!
Furthermore, those intervals that you run so well? Slow down the recoveries. This will keep the workout from becoming purely a fast twitch exercise.
All the opposites are true when compared to a slow twitch runner. Your performances may be less consistent, peak season may be shorter, and you’ll burn fat less efficiently. All things that will improve if you make some of the changes noted above, which will recruit more slow twitch fibers.
Don’t Overthink It
Training is more about art than science. A good solid training plan will prepare you for the specific demands of the race. A marathon plan, for example, is going to include workouts that draw out and increase slow twitch fibers. It’s going to get you most of the way there in preparing for your goal. But if your muscle fiber type is predominantly fast or slow twitch, the tweaks and changes noted above will help get you over the hump!
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