Motor Unit Recruitment and Training the “Fast Twitch” Muscle Fibers
This article explains the science of motor unit recruitment, and what this means for improving your ability to use your fast twitch muscle fibers. This ability is highly relevant for club head speed.
What is a motor unit?
A motor unit is a motor neuron and the muscle fibers it innervates. We have three primary types of muscle fibers, and all of the muscle fibers in a single motor unit are of the same type. When we carry out a movement, a signal is sent from our brain, through our spinal cord, to the relevant motor units. This signal is what excites a motor unit, causing the muscles to contract.
The table below outlines the three primary types of muscle fibers.
Muscle Fibre Types
| Fiber Type | Classification | Resistance To Fatigue | Ability To Produce Force | Good For |
|---|---|---|---|---|
| Type I | Slow Twitch | Excellent | Low | Sustained endurance activities |
| Type IIa | Fast Twitch | Low | High | Strength & explosive activities |
| Type IIx | Very Fast Twitch | Very Low | Very High | Very high speed & explosive activities |
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 color motor units represent different motor unit types.
*The motor units are not to scale. Low threshold motor units would be smaller and high threshold ones larger.
Motor Unit Classifications
Motor units are groups of the same muscle fiber type. In the previous section we learned how there are three primary types of muscle fiber:
type I, type IIa, type IIx. Recruitment of motor units is dependent on the force requirement for the task.
For this reason, motor units are classified by force or intent threshold. For low force or low effort tasks, we only recruit type I / slow twitch fibers, and these are labeled as low threshold motor units. They are sufficient for carrying out the task, and the faster twitch fibers of the higher threshold motor units do not need to contribute.
As the force requirement of a task, or our intent to produce force rises, we begin to recruit more of our fast twitch fiber / high threshold motor units, on top of the lower threshold ones already recruited. Only in cases where our intended force production is maximal do we recruit our fastest twitch fibers and highest threshold motor units.
Table showing 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 |
In high force or high effort activities slow twitch fibers are contributing, but how much they can help is very limited. They typically have a smaller cross sectional area than type II fibers which limits their maximum force potential, and due to their make up, their power output is a fraction of what a fast twitch fiber can produce.
Additionally, in very high speed activities the speed of movement can exceed the maximum shortening speed of the slow twitch fibers. When that happens they can’t produce any force at all. They’re still recruited and still firing, they just can’t shorten fast enough to help. (1)
Motor Units and Aging
It’s well established that as we age we lose muscle size, strength, and speed. Grosicki et al (2) reviewed 27 studies in their paper “Single muscle fibre contractile function with ageing” and found this is predominantly a fast twitch fiber problem.
Slow twitch fibers were well preserved into later years, while fast twitch fibers showed profound reductions in size and function.
The decline was primarily down to the fast twitch fibers getting smaller rather than any loss of quality. Individual fast twitch fibers from older adults shorten just as fast and produce just as much force for their size as those from young adults.
Motor Unit Recruitment Increases With Training
Our highest threshold, and most powerful motor units are reserved for maximum efforts. An all out sprint, jump, lift, punch, kick, or in our case, golf swing. As these levels of effort are not required in regular daily living, we seldom or never recruit these motor units, and it takes training to develop the ability to recruit the highest threshold and most powerful ones.
Increasing how many of their high threshold motor units one can recruit is one of the biggest adaptations that leads to people getting stronger, faster, and more powerful as they begin strength or speed training.
When someone begins strength or speed training they begin to train movements during which they produce as much force as they can. This intent produces a stimulus that leads to a change in the nervous system. It starts to tap into more and more high threshold motor units.
Beginner and advanced trainees have very different levels of motor unit recruitment. When we are new to training and attempt a maximal effort, we will recruit a certain percentage of our high threshold motor units, but a large amount will remain untapped. As we progress in our training and become more practiced at truly giving maximum effort, we begin to be able to recruit more and more high threshold motor units.
This adaptation is transferable across tasks, as it is a central adaptation between the nervous system and muscles. This is an example of how training that does not mimic the golf swing can have a direct impact on our force production capabilities in the golf swing. (3,4)
For example, lower body power training will help you develop the ability to recruit more high threshold units in your legs. When you perform another high effort task involving the lower body, like swinging a golf club, you can tap into these motor units you could not use before.
If we stop training we will start to lose our ability to recruit high threshold motor units / fast twitch fibers. This starts to occur within a few weeks of training being stopped. (5)
Summary
The ability to recruit our highest threshold motor units and fastest twitch muscle fibers is essential for performance in explosive 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 come close to exhausting the gains from this adaptation. You are already recruiting close to all of your fast twitch fibers. This means further improvements from this adaptation will be small or zero.
That doesn’t mean you can’t improve your performance. Once you can access close to all of your motor units, further force has to come from how fast those units are firing and the size of the muscle fibers in those motor units. Both are covered in the chapters that follow.
References:
Nervous system diagram screenshotted from this great video: https://www.youtube.com/watch?v=UnNGGD4-IHU
1) Bottinelli, R., Pellegrino, M.A., Canepari, M., Rossi, R. and Reggiani, C. (1999) “Specific contributions of various muscle fibre types to human muscle performance: an in vitro study,” Journal of Electromyography and Kinesiology, 9(2), pp. 87–95. Available at: https://doi.org/10.1016/S1050-6411(98)00040-6
2) 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: https://doi.org/10.1113/JP282298
3) Behrens, M., Mau-Moeller, A., Mueller, K., Heise, S., Gube, M., Beuster, N., Herlyn, P.K.E., Fischer, D.-C. and Bruhn, S. (2016) “Plyometric training improves voluntary activation and strength during isometric, concentric and eccentric contractions,” Journal of Science and Medicine in Sport, 19(2), pp. 170–176. Available at: https://pubmed.ncbi.nlm.nih.gov/25766509/
4) Ema, R., Saito, I. and Akagi, R. (2018) “Neuromuscular adaptations induced by adjacent joint training,” Scandinavian Journal of Medicine and Science in Sports, 28, pp. 947–960. Available at: https://doi.org/10.1111/sms.13008
5) Mujika, I. and Padilla, S. (2001) “Muscular characteristics of detraining in humans,” Medicine and Science in Sports and Exercise, 33(8), pp. 1297–1303.
