The Role of Muscle Fiber Recruitment in Strength

If you’re interested in increasing your physical strength or your general athletic performance, then there’s a good chance that you’ve heard about muscle fiber types. You know the score by now: there’s the slow and steady type 1 fiber for endurance and then the fast twitch 2a and superfast twitch 2b for explosive movements and sprinting.

But there might also be a few things you don’t know about your muscle fiber. For instance, did you know that the speed of motor unit recruitment is more of a limiting factor than the number of fibers you have? Did you know that you can create muscle fiber?

How Muscle Fiber Works

When you eat cereal with a spoon, you will move your arm using only a few slow twitch motor units. Motor units are clusters of muscle fibers that your central nervous system activates as needed.

Muscle fibers are binary meaning that they either contract/fire or they don’t – there’s no such thing as a partial contraction, or partial firing. Thus, the most important factors in determining power output are the types of muscle fibers you fire and the number of motor units used.

Now if you load the resistance up and start curling weights, you’ll use more and more motor units and if the weight is heavy enough and you try to accelerate through the movement, some of those motor units will consist of more fast and superfast twitch muscle fiber.

But did you know that you only ever recruit around 50% of muscle fibers at once? And this is true for athletes. The rest of us likely are only capable of recruiting about 20-30% of our fibers! That means there’s a ton of untapped potential in our muscles when it comes to power output that we simply can’t access!

What’s more, the amount of time it takes for us to recruit all of our motor units is also a limiting factor. Specifically, it takes about .6 seconds for us to engage max strength, whereas throwing a punch or performing a vertical jump takes significantly less time (about .2 seconds). This means that for something like a punch or a jump, we’re now using even less than 50% of our power!

The reason that so much of our strength is ‘off limits’ is that we otherwise might be able to injure ourselves – ripping our tendons and potentially even breaking bones through our own strength alone. What’s more, it’s important to remember how quickly fast-twitch muscle fiber fatigues – if you could use it all in one go, then you would be completely vulnerable and defenceless in the immediate aftermath!

Of course these days we can be trusted to use our power wisely and being able to bench press twice or three times as much would make for an amazing party trick.

So how can you recruit more motor units?

Crisis Strength

One amazing example of humans using a greater proportion of their strength is during what’s known as ‘crisis strength’. While there isn’t strong evidence for crisis strength, it’s generally accepted that it does exist and there are many anecdotal stories about it.

Crisis strength is the term given to the brief moments of incredible power we gain when we are in life-or-death situations. The most common example is the oft-told story of the mother who lifted a car off of her child trapped underneath. Another example is the rock climber who reported managing to throw a boulder off of their body that would have otherwise crushed them.

It appears that under certain circumstances, hormones and neurochemistry related to the ‘fight or flight’ response can provide us with access to a greater percentage of our muscle fiber – can strengthen the ‘mind muscle’ connection to the point that we gain ‘super strength’. This is also known as ‘hysterical strength’. Possibly related, is the fact that amphetamines can increase physical strength via the sympathetic nervous system (1).

Increasing Motor Unit Recruitment

Now of course it’s unlikely that you will want to trap your firstborn under a car in order to gain super strength in the gym. Seems a bit extreme…

Instead, you might be more inclined to look for alternative ways to engage your muscle fibers more fully.

One way you can do this is by training using ‘overcoming isometric’ techniques. This means pushing or pulling against an immovable force – higher than your 1RM (1 rep max). For instance, trying to push down a wall would be an example of an overcoming isometric.

This is the type of training used by strongmen and steel benders. It is also the type of training used by Dennis Rogers – who is able to recruit over 50% of his muscle fiber with great speed and who some people consider to be the world’s strongest man pound for pound. Dennis’ feats include such things as preventing planes from taking off by holding onto a chain attach to them…

When you train in this matter and try and push against something that won’t move, your brain sees this as you trying to lift something very heavy and as such, it recruits as much muscle fiber as possible and you strengthen neural pathways allowing you to do this. Essentially, it’s basic SAID – specific adaptations to imposed demands. Put yourself in a situation where your body perceives your muscle fiber recruitment to be insufficient and you will learn to recruit more!

Meanwhile, lifting at close to your 1RM can help you to increase your mind-muscle connection in a similar way. Lifting explosively meanwhile (as is necessary for your 1RM, or for plyometric exercise) will help to increase your proportion of fast twitch to slow twitch muscle fiber (2).

Bruce Lee, another of the world’s strongest men pound-for-pound, interestingly used both overcoming isometrics and ‘speed training’ where he would lift weights as fast as possible to increase type 2b muscle fiber.

In conclusion then, there’s much more to muscle fiber than first meets the eye. And if you want to increase your strength as much as possible, there are some interesting techniques you may be able to use to accomplish that!

Comments 1
  1. Hi, very interesting!

    Could you please point me to the source where you got the information below from? I would be interested in reading more and having a better understanding of this concept. Are there studies you drew from?

    “But did you know that you only ever recruit around 50% of muscle fibers at once? And this is true for athletes. The rest of us likely are only capable of recruiting about 20-30% of our fibers! That means there’s a ton of untapped potential in our muscles when it comes to power output that we simply can’t access!”


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