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Neural Plasticity and Strength Training

When it comes to strength training it’s really common to hear people refer to ”Neurological Strength” or ”Neural Drive” when discussing the importance of the the neurological system in strength. This statement has a lot of basis but unfortunately the majority of people that mention it can’t really go much deeper than saying ”Yeah, it’s really important.” What is actually happening when we talk about neurological adaptations to strength training is a couple of different forms of neural plasticity; primarily the reorganization of the motor cortex, via cortical synaptogenesis for skill development, and spinal synaptogenesis (synaptogenesis = creation of new synapses between neurons) in direct response to strength training (Adkins, 1985, Swain et al, 2012). The specifics of these terms is not important for the purpose of this article, just know that the motor cortex acts like a switchboard operator when it comes to determining which motor neurons need to fire.

You may be wondering what does the nervous system have to do with strength anyways? Isn’t it all about muscle size? Well no, not at all. You see muscles have one really simple action they perform; contraction. Muscles contract in response to the electrical stimulus they receive from their associated motor neurons. So while it is true that bigger muscles can produce more contractile force, they can’t produce that force at all without the “Go” signal from the nervous system. So now we have a very simple framework to understand how the nervous system plays a role in strength training.

 

Next, consider the first time you ever tried to bench press. Chances are you looked, and were, pretty awkward. This is because even though you may have known which muscles to use, you didn’t really know the how or when that comes with experience. Basically you grabbed that weight, lowered it to your chest, and then fired every motor neuron connected to your chest, triceps, and shoulders to try to launch it. Over time you, hopefully, learned how to bench with a little more finesse, and now you press the weight off of your chest by first contracting with your chest and finish with a strong tricep contraction. The experience that fed into learning this better sequence also worked to reorganize the motor cortex to encode for an optimal set of sequences to fire the motor neurons used to bench the weight.

Now that we have a general framework and a simple example of how neural plasticity and strength training work together let’s get into the details.

Behavioral Drivers of Neural Plasticity

According to Kleim (2012)* the behavioral drivers of neural plasticity are:

  • Repetition
  • Intensity
  • Timing (Coordination)
  • Difficulty
  • Specificity
  • Salience

In the following video I talk about each of these behavioral drivers and how they relate to strength gains:

 

While the wording may be somewhat different, these factors are something that strength coaches everywhere have known for a long time. Essentially, to stimulate an adaptation to something (like lifting heavy weights) the stimulus needs to reoccur frequently (repetition), has to take significant concentration (intensity), be at a sufficient difficulty to require adaptation, and must be important to the individual (salience). Timing, or coordination, refers to what is best described by the adage of “Neurons that fire together, wire together,” often called “Hebbian Learning.” Lastly, specificity refers to the fact that the action that we are adapting to must be specific.

Let’s now revisit our previous example of training the bench press and compare it to each of these factors:

  • Repetition – Bench pressing 1-2x per week
  • Intensity – Regularly benching 80-95% of your 1RM
  • Timing – Ensuring your form is on point and muscles move in a coordinated fashion
  • Difficulty – Continually trying to incrementally increase performance
  • Specificity – Benching more will make it so you can bench more, but not so that you can run more efficiently
  • Salience – “Benching is important to me because it will get me the strength I want!”

Using these factors to structure your training

As you can see these behavioral factors are very similar to the metrics we commonly manipulate in the gym to improve performance. Most commonly, the metrics we manipulate are Repetition, Intensity, and Difficulty. These are the most important factors to manipulate for experienced lifters. In fact, it is advisable to try to optimize your gains by rotating which of these factors you emphasize. For example competitive powerlifters will often employ a high volume training strategy (High repetition factor) for a length of time and then go into a ’taper’ period in which they systematically decrease the volume and increase the weight (High intensity factor), resulting in an overall better performance at their competition. This kind of plasticity is a direct result of spinal synaptogensis; the creation of new synapses in the nerves extending from the spinal cord into the muscles. Essentially, this creates a more powerful driver for muscle contraction.

However, when you are first learning a new lift/movement it’s smart to focus on the repetition and timing factor above all else. Keep intensity and difficulty low so that you can do the movement frequently and do your best to keep your form on point. These early days are when you are actively reorganizing the motor cortex to learn how to perform the movement. In these early times of learning a movement you are engraining the foundational neural pathway for that movement, so if you build good form now you don’t have to go back to fix it later. If you build your foundation with shitty form it’s extremely difficult to back and fix it because you’re not just learning good form, you have to forget bad form (which is another entire plasticity article in itself).

Summary

When it comes to Neural Strength in strength training we are talking about neural plasticity applied to strength. When we first learn a new movement the predominant form of plasticity is motor cortex reorganization and it is literally our brain learning the optimal set of sequences for firing the various nerves controlling the muscles to control the movement. Later on, the motor cortex does not see as dramatic of a reorganization, instead the predominant form of plasticity becomes spinal synaptogenesis; an increase in the number of synapses in the motor neurons controlling the muscles. This essentially creates a more powerful driver to the connected muscles stimulating more powerful muscular contraction.


*This reference is a textbook.

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Analyzing the neuromuscular controls of grip and application to lifting weights

 

An example of a precision grip. Credit to Stocksnap.io

An example of a precision grip. Credit to Stocksnap.io

Fluid movement and strength development depends heavily upon the brain. Pretty much everything in life does. In this article we are going to explore a few aspects of grip force and load force with respect to their neurological bases. Specifically we are going to look at and summarize a series of research articles related to the neuromuscular control bases for grip and lifting force. The information presented in this article should be taken with a grain of salt. Many of the studies I reference here were looking at lifting a relatively light object, between 100 g and 1 kg, with a precision grip, using only the index and the thumb. This is the most comprehensive information I have found relating lift, grip, and neuromuscular controls. I hope to see more direct research performed on neuromuscular controls and weight lifting in the future.

The direct correlation between grip force and load force

In a 1984 study by Westling and R.S. Johansson researchers measured the grip force applied by subjects lifting an object with varying weights and 3 different grip surfaces. As you’d imagine the grip force increased parallel to the load force and the grip force was significantly higher on slippery surfaces than it was on rough surfaces. The seemingly direct relation between grip force and load force is a reaction to the increased force necessary to keep the object from slipping, the “slip force.” There were 2 interesting this to note from this:

  1. The higher the slip force the higher the difference between the slip force and grip force.
  2. Slippery surfaces, suede & silk, required significantly more grip force than the rough surface, sandpaper. As we’ll see later on this requires much more effort from the nerve pathway that controls grip.

Essentially, when we’re applying a grip to pick something up we adjust our grip based on the objects weight and the texture of the object (load and friction).

Our previous lifts determine the efforts of our current lift

In a follow up 1988 study researchers found that the experience of a previous lift created a type of “sensorimotor memory.” Subjects lifted an object whose weight was altered by experimenters in a pseudorandom manner or left constant. Researchers were looking to compare the effects of lifting a heavy object after a light one, vice-versa, and lifting the same weight repeatedly.

When the weight was kept constant (control) the applied grip force was close between trials (no significant difference). Essentially subjects knew the correct force to apply and successfully did so with minimal variation.  Additionally E.M.G. imaging showed an drastic increase in activity during the time in which grip was being applied with a small reduction once the grip force stabilized (Fig 1A, third row).  This suggests that grip force, and the rate at which it is applied, is correlated with the output of electrical activity in the muscle.

The interesting findings of the study came when the weight was switched. When subjects previously lifted a heavier weight the grip force was greatly overshot in proportion to the difference between the two weights. This was exactly what you think it is: recall a time you prepared to lift a heavy box only to find it was empty. On the other hand when subjects previously lifted a lighter weight they gradually increased their applied grip force and load force until the object moved.

In the case of previously lifting a heavier weight the E.M.G. imaging showed a much longer period of increased activity before reducing (Fig 2A, third row). This is probably due to subjects preparing to grip the object and having to readjust the grip once they realize they have applied too much force.

Internal Models of Grip and Load Forces

The previously mentioned study suggests the existence of an internal model that acts to predict the magnitude and timing of forces necessary to control an object. As we have seen our previous experiences influence the rate we apply grip and load forces in preparation of lifting an object. This isn’t our only model for predicting the force necessary to lift an object. We can also use visual cues to scale our applied forces.

In a 2005 experiment by Chouinard et al researchers found that subjects could accommodate for unexpected weight changes if a visual cue, in this case an arbitrary color flash, indicated the magnitude of weight to be lifted. Suggesting our internal model for visual cues can take priority over the tendency to scale applied forces based on the previous lift.

Researchers were effectively able to interrupt this process by directing transcranial magnetic stimulation (TMS) over the dorsal premotor cortex. Similarly the tendency to begin a lift based on previous experience was also interrupted by applying TMS to the primary motor cortex. Suggesting the locations of these two internal models are very close but not the same.

Now that we have reason to believe that these models take place in distinct parts of the brain we have reason to believe that they must have a way to interact. In their 2010 research paper Loh et al sought to solve this problem. Researchers found that our previous experience acts as a preset approach to lifting an object and we can switch based on visual cues after 50-150 ms.

What the hell does this have to do with exercise and training?

Let’s review:

  • When we lift something heavy or slippery we use a stronger grip.
  • The force of our grip and the rate at which we apply it can be considered good indicators of neuromuscular exertion (at least with regards to the hands).
  • We base our approach to lifting an object on our most recent experience.
  • We are capable of adjusting our approach based on visual cues, however the model based on our previous experience appears to be more innate.

Many strength coaches are familiar with the “Law of Irradiation.” Essentially this law states as the tension of a muscle increases it will recruit additional tension from nearby muscles. This law makes a connection I’ve been hinting at throughout this article: grip exertion is a good indicator of neuromuscular exertion. Using this connection in combination with the information presented in this article we can derive a handful of training advantages.

Incremental increases in weight can effectively increase the total time under tension.

Incremental increases in weight can effectively increase the total time under tension.

Firstly, suppose you want to maximize the amount of volume within a given workout. In this case we wouldn’t want to be constantly performing at the edge of our nervous systems capabilities. We can do this by lowering the demands we make on grip with regards to weight and texture; using less than maximal weight (near 70%), gradually increasing the weight prior to the working set, and putting chalk on the barbell. This one is obvious but nonetheless critical. We can extend the length of our lifting sessions by reducing the strain on our grip.

Secondly, if we are trying to perform near-maximal sets we can actually set ourselves up to overshoot our grip and load force application, effectively making the heavy lift a little easier. This is done by performing a preceding set that is heavier than the working set, what is sometimes called a “proprioceptive set.” Suppose you want to perform a set of 6 deadlifts at 315 lbs, then you perform a single at 345 lbs beforehand. The caveat is that you’d have to do this before each set and this could eat up more time than is worthwhile.

Lastly, we can use large incremental changes in weight to increase the amount of total time under tension per lift. For instance, suppose you go from squatting 185 to 225 to 275. The lower preset approach to the lift will not be sufficient to lift the heavier weight and the rate of force will gradually increase to accommodate the new weight. Essentially you will apply the same total force to move the weight but the rate of force will increase over a greater period of time.


Please keep in mind that I am not a neurologist and none of the research cited here pertains directly to resistance training. I’m interested to see emerging research come out but in the mean time the research I have shown here, when applied to resistance training, agrees heavily with many of the lifting and coaching principles laid out by the likes of Pavel Tsatsouline, Charles Poliquin, Jim Wendler, etc…