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The Science Of Speed – Part 1 Fast Twitch Muscle Fiber Recruitment


The goal of this series is to provide a summary of the adaptations that occur from maximal effort training at high speeds. I hope to explain how these adaptations can lead to improved performance in athletic tasks requiring maximum forces at fast movement speeds. For you and I, the particular focus will be on club head speed. These adaptations are important for all high force high speed athletic activities however.

Having an understanding of these adaptations and making sure to stay on top of them as part of your training throughout life (all ages) will also have great transfer to your movement capacity in everyday life.
I like to call this “aging athletically”.

Most of these adaptations are distinctly different from those that occur as a result of heavy strength training with low reps, and moderate or light load strength training with higher reps. Unless someone is specifically training for speed, they will be leaving these adaptations on the table, which is a mistake.

This is not to say there aren’t benefits to heavier strength training when speed is the goal. There most certainly is and I will get to them at a later time. (I also post about them on Twitter almost daily!)

High speed movements require light loads, relative to the maximum amount of load we can move in that movement pattern.

The most common light load exercises you will come across when training for increased club head speed are:

  • Maximum speed golf swings, whether this be with driver or speed tools that enable over and underload training.
  • Jumps, med ball throws and slams.
  • Band exercises at very high speeds.
  • Weight lifting exercises performed in a ballistic fashion.

Maximum speed swings with speed feedback from a reliable device like the PRGR are by far the most important of those listed above.

Chris Beardsley, a strength and conditioning researcher from the UK, has put together extremely good information about the adaptations I am going to write about. Much of what I write will be taken from work he has done and I will try my best to credit him whenever possible. His material is very in depth and one of my goals with this series is to try and simplify it to help my understanding, and hopefully yours too.

With good understanding we can start to apply it to training programs for hopefully better results.

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What happens when we train with maximum effort at high speeds that makes us better at producing force at high speeds?

The short answer to this is a combination of neurological and physiological adaptations. A common analogy used to explain the difference between these two types of adaptations is to think of neurological adaptations as improvements in the software, and physiological adaptations as improvements in the hardware.

Neurological adaptations mainly improve our ability to send better signals from our brain to the muscles.

Physiological adaptations are improvements to the muscles and connective tissue structure and function.

I am going to start with neurological adaptations.

Neurological Adaptations

The nervous system is our command center. It is primarily made up of the brain and spinal cord. When we want to perform a movement, like swinging a golf club fast, we send a signal from our brain to our spinal cord. This message is then transferred from our spinal cord to our muscles via a motor neuron. When the signal from the motor neuron reaches the muscle, the muscle contracts.

As we practice / train specific tasks (high speed activities at maximal effort in this case) there are a number of enhancements to our nervous system that can occur to increase the force that is produced when this muscle contracts.

These adaptations are very important for increases in performance and are what we try to stimulate in training. We probably don’t need to worry about trying to have a certain training method to hit each adaptation. They are all going to happen from the high speed training stimulus (I think), but it is interesting to learn about what is happening. It’s also unrealistic we will actually be able to track our improvements in these specific nervous system functions, as we won’t be getting assessed in a physiology lab.

That’s not overly important though, as we are more interested in performance improvements. The most practical and important feedback we can get is club head speed (and ball speed).

(1) Messages get sent from the brain to the spinal cord. These messages are then sent from the spinal cord to the muscles via a motor nerve. The different colour motor units represent different motor unit types (explained below). The motor units are not to scale. The low threshold would be smaller and the high threshold, larger.

Adaptation 1.
Increased recruitment of high threshold motor units / fast twitch muscle fibers.

This is one of the few adaptations I will cover that occurs from both heavy strength training and high speed training. It is also extremely transferable across different speed activities. Getting better at this increases your maximum strength and maximum speed. This is because both of those activities rely on the ability to use our “fastest twitch” muscle fibers, and both fast and heavy training with maximum effort enhances that ability.

We all have three types of muscle fibers, summarized in the table below.


Muscle Fibre Types
Fiber Type Classification Resistance To Fatigue Ability To Produce Force Good For
Type 1 Slow Twitch Excellent Low Sustained endurance activities
Type 2a Fast Twitch Low High Strength & explosive activities
Type 2x Very Fast Twitch Very Low Very High Very high speed & explosive activities


There will be differences in fiber type proportion between people.
This is largely down to genetics, but also training & activity. There will also be differences in fiber type proportions between muscles. This will be dependent on the muscles primary function.

From my understanding most of us do not have many type 2x fibers to begin with, and most types of training convert them to type 2a. This is something that I need to learn more about, and then try to write about. I have asked Chris Beardsley about this and he suggests it’s not a particularly big problem. The golf swing occurs over a long enough time period that the type 2a muscle fibers can hugely contribute to lots of speed production. This would be in contrast to something like maximal speed sprinting, where the foot is only in contact with the ground for about 0.1 of a second.

Muscle fibers are part of something called a motor unit.
A motor unit is a motor neuron, and all of the muscle fibers it innervates. All muscle fibers in a motor unit are of the same type.

Motor units are categorized by the threshold of force required to recruit them.


Motor Unit Classifications
Force requirement / effort level Motor unit name Contains which fibers?
Low Low Threshold Type 1
Intermediate to high High Threshold Type 2a
Very high to maximal High threshold Type 2x


The amount of muscle fibers controlled by different types of motor units varies greatly. Smaller muscles that control very precise movement, like the eye for example, might have as few as 5 muscle fibers in a motor unit.
The soleus which is one of the calf muscles and very important for posture has an average of 180 fibers per motor unit. The other calf muscle, the gastrocnemius which is used for more powerful movements like running and jumping has 1000-2000 fibers per motor unit. (2)

For low effort activities we will only recruit slow twitch muscle fibers.
As we gradually try to increase force we will still recruit our slow twitch muscle fibers but start to move down the continuum towards our faster twitch muscle fibers and recruit them too. When effort is maximal, we will recruit all of our muscle muscle fibers. Type 1, Type 2a, and Type 2x will all be pitching in to help!

In high force / high effort activities the slow twitch fibers won’t be able to help much, as their force production capacity is low, especially at high speeds. In high speed activities the speed of movement may be faster than the maximal contraction / shortening speed of slow twitch fibers, meaning they cannot tribute (3). This is why the specific training of our type 2a and type 2x fibers is so important in enhancing power output in explosive activities, like swinging a golf club faster.

The force production capacities of type 2a and 2x fibers do not get conditioned with everyday activities or cardiovascular / aerobic emphasized activities. Additionally, the loss of muscle fiber size and function with age is predominantly a type 2 – fast twitch problem (4). To maintain their size and functional capacity we must do some challenging resistance training, and some light explosive training.


Untrained VS Trained People

People who have not trained for strength or speed do not have the capacity to recruit their highest threshold motor units / fastest twitch muscle fibers.
To become proficient at this we need practice at trying to produce maximum force.

There is a huge difference between an untrained person and an advanced trainee both trying to produce maximal force in a movement. The advanced trainee will of course have bigger and stronger muscle fibers, but they are also much better at recruiting and using all of their type 2 muscle fibers. When they exert a maximum force all of their type 2 fibers are contributing. This is not the case for the people who have never trained for strength, speed or explosiveness. When they are trying their hardest, they are unable to access all of their muscle fibers, inhibiting their performance.

The good news is that we will quickly get better at this as we train, provided we are trying very hard with appropriate training.

If we stop training for a few weeks, or longer, we will start to lose our ability to recruit high threshold motor units / fast twitch fibers. We won’t return all the way to baseline, and we will improve the ability quickly after we resume training.


The ability to recruit our highest threshold motor units / fastest twitch muscle fibers is essential for performance in high force activities (like swinging a golf club faster).

If you are new to training for strength, speed, or explosiveness, you have lots of room for improvement in this capability.

If you have been training with high effort for a long time, you have probably exhausted or close to exhausted gains from this adaptation. You already know how to recruit all of or close to all of your fast twitch fibers. This means more improvements from this adaptation will be small or zero.
This doesn’t mean you cannot improve your performance, it just means that performance improvement will likely need to come from other adaptations.

Keep an eye out for future articles which will dig into the details on those.

I hope you enjoyed this article and please let me know if you have any feedback or questions.

Interested in a PRGR Radar for speed training? You can get 10% off with the code fitforgolf22 at this link.


Cover image: Golf Anatomy book by Craig Davies & Vince Dasaia

1) Nervous system diagram screenshotted from this great video:

2) The Motor Unit – Neuroscience 2nd edition:

3) Chris Beardsley Patreon:

4) Grosicki, G.J., Zepeda, C.S. and Sundberg, C.W. (2022) “Single muscle fibre contractile function with ageing,” The Journal of Physiology, 600(23), pp. 5005–5026. Available at: