Maximal Neuromuscular Power Depends On Specific Characteristics Of The Central Nervous System : Part 1

Rhodes
Rohde

The understanding of [sport] movements requires the knowledge of neuroscience notions. Many people over the years have focused on mechanics of movement – that is biomechanics- and have offered very valuable insights into weightlifting technique. If one is trying to understand how movement efficiency is achieved only through the analysis of the angles of body segments , one can’t fully appreciate the reality of said movement without considering neural factors. After all, the central nervous system is the reason why those segments actually move in the first place. How to develop maximal neuromuscular power is one – if not the most- important concern for the weightlifter. Understanding how neuromuscular power is achieved, and how different neural factors can influence it, allows you to be as specific as you can so that real progress can be made. This educational article seeks to explain in layman’s term important notions of neurosciences that will positively change your training. 

1. Introduction

Boevski, incredible lifter with everything but big muscles.
Boevski, incredible lifter with everything but big muscles.

Most nations involved in weightlifting took a look, at some point, at how neural factors can influence the training of weightlifters (and other sportsmen). For example, the soviet union took a great look at this and of course most of the results were published only in Russian. The importance of understanding the neural factors involved in neuromuscular power reside in the fact that the CNS (central nervous system) produce planned movements and the characteristics of the CNS explains the performance (or not) of movements. This article should make it clear for the reader  that training recovery, strength/power gains, technical refinement, muscle recruitment, coordination, and more, all depends, at some levels, on characteristics of the CNS. It seems obvious that your program should take into account this reality and promote the right neuro muscular adaptations. 

As a side note, I would like to point out that mainstream media and many people promote the belief that the size of a muscle explains its potential for strength (ie : bigger muscle means stronger muscle). This is partially true. The truth is that it really depends on neural specific adaptations. A bigger muscle has better  contractibility potential but that potential may never be reached if the neural adaptations to allow this are not made (your body is unable to recruit a large percentage of total muscle fibers at the right time and speed). This is one of the reasons why you have light weight weightlifters that are more powerful than 150kg bodybuilders.  In the words of Cormie (2011) : ”The ability to generate maximal power during a movement is not only governed by the muscles morphology, but also by the ability of the nervous system to appropriately activate the muscles involved.”

2. The better the ability to recruit motor units is, the more force can be generated by muscle (s)

classical example of a motor unit.
classical example of a motor unit.

In order for the muscle fibers to be able to do their action – which is to contract- they are structurally linked to the central nervous system (as in they receive input from cells in located in the CNS). Motor neurons are cells that are located in the spine and innervate skeletal muscles. Skeletal muscles are made of a large number of muscle fibers. Various experiments have shown that a single motor neuron innervate a great number of muscle fibers at the same time. We also know that there are a lot more muscle fibers than there are motor neurons. This leads us to motor units. Therefore, a motor unit is characterized as a motor neuron and every single muscle fiber that is innervated by that motor neuron.

There are a few functional reasons why this is the way it is. Having a single motor neuron that  innervates many muscle fibers allows for the contractible force to be spread evenly (all fibers gets innervated equally). Also, if for some reason (ie: injury) a motor neuron is unable to innervate its target, then the whole muscle won’t be shut down as other motor neurons who innervate the rest of the muscle fibers still can do their job (the shut down will be partial).

The motor neuron act as a middle man between the central nervous system and the muscles. In layman’s term, once the person decide to produce a movement, a signal from the brain will be sent down the spine to activate the right motor neurons that will in turn innervate the muscle fibers needed to do the movement. There are other cases where it does not happen like this, but I will not cover this right now as it is not as relevant to the weightlifter. The brain will recruit the right motor neurons for the job according to a certain activation pattern that is movement specific.

2.1 Types of motor units and their behaviors (difference in force production) 

Motor neurons, as well as motor units, vary in size. As a rule of thumb, smaller motor neurons innervate fewer muscle fibers than the bigger ones. Smaller motor units therefore generate smaller forces since less muscle fibers are being innervated so the contractile force is lesser.In contrast, bigger motor neurons innervate more muscle fibers and produce more force.

Dimas jumping high is a good example of recruitment of fast fatigable muscle fibers .
Dimas jumping high is a good example of recruitment of fast fatigable muscle fibers .

There is also a difference in the type of muscle fiber innervated by motor units. Concretely, smaller motor units innervate muscle fibers that are said to contract slowly, are resistant to fatigue and that they have the ability to generate smaller force. These are the type 1 muscle fibers and we call those motor units Slow motor units. These motor units allow us to do low intensity activities like standing up and walking. They are implicated in all movements as we will see in a bit.

Larger motor neurons will innervate fibers that have the ability to generate more force. We consider these fibers fast because they allow for much more power to be developed. We distinguish between two categories. The first type of motor unit is the fast fatigue-resistant motor unit. The muscle fibers innervated are called type 2a fibers. We often consider them to be hybrid fibers because they produce more force than slow motor units but they are also resistant to fatigue. The second type of motor unit is The Fast fatigable motor unit. The motor units recruit the type 2b muscle fibers which generate the most force but they are highly fatigable. In other words, they generate very high force but only for a short period of time. Does it sound like a weightlifter need to be able to recruit that type of motor units?

2.2 Motor units are recruited according to a size principle

Classical example of muscle unit recruitment according to a size principle. Smaller/less powerful motor units get recruited first and then bigger/more powerful motor units get recruited.
Classical example of muscle unit recruitment according to a size principle. Smaller/less powerful motor units get recruited first and then bigger/more powerful motor units get recruited.

Experiments dating from the mid 1900’s have shown that motor units are recruited according to a size principle. In other words, the central nervous system recruits smaller motor neurons first and then recruits bigger motor neurons if need be. Yet, another way to put it is that slow motor units are recruited first, then the fast fatigue resistant motor unit are recruited and finally the fast fatigable motor units are recruited.  This allows for the CNS to never waste energy and time : If it is not needed, it is not recruited.

2.3 No convincing evidences of preferential recruitment of high threshold  motor neurons

hi-res-2754136-evgeny-chigishev-of-russia-smiles-to-the-crowd-as-he_crop_650x440Results show that higher threshold motor neurons – that is motor neurons that innervate the fast and fatigable muscle fibers- produce the most force. Logic has it that a person would be more powerful if he could preferentially activate those motor neurons instead of the motor neurons responsible for lower force development. Concretely, to generate maximal neuro-muscular power, it has been theorized that preferential recruitment of motor neurons (ie : those who innervate the fast fatigable muscles fibers) happens. Unfortunately, there seem to be a lack of evidence in this regard. It seems like you can’t escape the size principle. 

However, a different theory has been proposed and some results seem to support it. This theory stipulate that with training, the threshold of the high threshold motor neurons lowers. In other words, if it takes a certain amount of excitation/input to activate the bigger motor neurons responsible for the highest force (you have to recruit the smaller motor neurons first), then that ”certain amount of excitation/input” lowers with adequate training and time (as in, there is neural adaptation going on to make you better suited for the task at hand, over time). Another way to put it, through training and neural adaptation,  it becomes easier to recruit the motor neurons responsible for the production of the highest forces.

2.4 Firing frequency of the motor neuron impacts its ability to generate force

If the motor neuron is more active – as in it fires faster thus send signals to the muscles at a higher frequency – the magnitude of force generated will be higher. In other words, because the firing frequency is high, the muscle gets a lot of input which means more force is developed (especially in the case of high threshold motor neurons). Cormie says : ”It has been estimated that the force of contraction may increase by 300–1500% when the firing frequency of a motor unit is increased from its minimum to maximum rate”. It is therefore one of the mechanism that could explain improvement in neuromuscular performance. Also, the higher the frequency, the better the rate of force development, which leads me to my next point.

2.5 Rate of force development : A concept that is of extreme importance for the weightlifter

Taner Sagir, one of the skinniest 77kg lifter who snatched above 170kg. He also had a long levers. He succeeded because he had the right neural adaptations for the sport.
Taner Sagir, one of the skinniest 77kg lifter who snatched above 170kg. He also had a long levers. He succeeded because he had the right neural adaptations for the sport.

Rate of force development (RFD) is a measurement of how quick an individual reaches maximal level of force output. The quicker you can reach maximal force output, the better your RFD is and vice versa. This concept is one of the most important concept in power sports and especially in weightlifting where you are trying to move a heavy load very fast. Your success in weightlifting is dependent on your RFD.

The RFD depends on how quickly you can activate the high threshold motor neurons – or in other words, on how quickly you can reach the threshold for the activation of high power output motor units. We consider the influence of RFD as follow :Over time, as the RFD increases, we see the threshold of the bigger and more powerful motor units decreases (see 2.3) and the result of this is more force is being produced at the beginning of the movement (The threshold is met faster). This translate in better performance of weightlifting (more weight being moved about).

Another picture of Sagir, showing his overall body composition. Credit Macklem
Another picture of Sagir, showing his overall body composition. Credit Macklem

RFD is a concept that explains why people who pulls slower than others still can get under the bars (they develop force very quickly in key positions). It also explains why lighter weights must be used in weightlifting. Finally, it explains why muscle size is not all that matters in weightlifting performance. In other words, RFD is a concept that explains why you can have someone as skinny as Taner Sagir snatch above 170kg as a 77kg weightlifter.

2.6 Conclusion

Finish lifter Milko Tokola is a very good example of high rate of force development.
Finish lifter Milko Tokola is a very good example of high rate of force development.

I highlighted a few concepts that are important. As Cormie said in his review on the very same topic, ”Development of effective training programmes that enhance maximal muscle power must involve consideration of these factors and the manner in which they respond to training”. In other words, if you want to be a good weightlifter your training has to take into account these factors.

It gives great insight into how to specifically train for weightlifting and it is another reason why training in slow way does not make you faster. Training in a slow way, according to these factors, will not promote the right neural adaptations which will limit your progress. This is why going from powerlifting to weightlifting is hard (or going from  marathons to sprints). Other neural factors are important too such as inter muscle coordination (ability to recruit agonist muscles and inhibit antagonists muscles).

In part 2, we will see how we can manipulate training variables to create the right neural adaptations to make you a faster and better athlete. 

 

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