An Introduction to Motor Unit Recruitment and Training Methods to Maximize Recruitment
posted on August 21, 2024
First and foremost, before diving into motor unit recruitment and its importance in training, it is critical to define what a motor unit is. A motor unit is made up of two parts, one is the motor neuron and the other is the muscle fiber that it innervates. Often, one motor neuron will innervate a large number of muscle fibers, and each muscle will consist of many muscle fibers. Think of this as the link between the central nervous system and the muscular system. For a muscle to contract it needs to be stimulated by the motor neuron. These stimulations happen all throughout the day by the movements that we voluntarily, and sometimes involuntarily make. Any time a motor unit stimulates all the muscle fibers needed, it will contract in an all or nothing principle. Now it is time to talk about training.
When we exercise, we are forcing our central nervous system to stimulate a given amount of motor units to create muscle contractions allowing us to accomplish a task. Depending on the intensity of the exercise, this will dictate which kind of muscle fibers and how many motor units are recruited. Everyone has heard of type I and type II muscle fibers, but if not, we will keep it simple and say type I get recruited easily at low intensities, while type II requires much higher intensity levels before they come into play. Even though we are only talking about two types of fibers there is an entire spectrum of fibers to fill in the gaps between type I and type II that share some characteristics of both. So, the amount motor units recruited to do a body weight squat compared to a 500 lb. squat for an equally trained person will be drastically different. It is safe to say one of these movements will be more intense than the other. It is through this reasoning that I think it is safe to say that for the purposes of this article we can define intensity as the amount of motor units recruited.
So high intensity training, HIIT workouts high-intensity interval training… does this mean you are recruiting the maximum amount of motor units? Well, sometimes but that isn’t always the case for HIIT workouts. Those can be just from the buildup and/or poor clearance of lactic acid in the body which makes things hard but that can maybe be the topic of another blog post in the future. Let’s take a look at some basic principles of motor unit recruitment that the very smart Dr. Vladimir Zatsiorsky has come up with and then talk about intensity:
Now with this understood, you may think, “I need to recruit all the motor units of this muscle to get a training effect. I better lift as much weight as I physically can. I need to make sure the motor units fatigue”. Well, you aren’t totally wrong but is this only way? Is this this the best way for people just starting to workout or aren’t trying to set world records? The answer to this is no. There are other ways into maximal motor unit recruitment and let’s talk about those now.
If we aren’t lifting maximally then obviously we are into submaximal training. For the sake of this post, we are going to omit training submaximal loads at high speeds for motor unit recruitment, which can again be for another post. So, let’s talk about the repeated effort method. This is what most people strive for in the gym 3 sets of 8 to 12 reps 5 set of 5 reps. The goal here is to reach maximal recruitment after the allotted number of sets and reps. Sure this works but I am going to bring up another rep scheme to accomplish maximal motor unit recruitment that may be a bit unfamiliar but equally and if not more effective than those just mentioned and that is 1 set of 20.
This idea of 1 set of 20 comes from the mind of Dr. Yessis, if you don’t know who he is it’s not important for this, just know he has been in the world of sport science and training for a very long time and is highly respected. Anyways, this was something that at first I did not quite get, I was too caught up on things being heavy, force production, bar speed, etc. and really did not understand the essence of this method. It wasn’t till I got smarter and read Dr. Doug McGuff that I had this revelation of intensity and motor unit recruitment; then things became clearer. Don’t worry, we will get to McGuff shortly. Remember, we are talking about motor unit recruitment here not force production. The goal of this training is to reach total failure of a movement by the 20th rep within a single set. This, however, differs a little bit from other rep schemes to failure, as this is better suited for a wider range of the population. Hitting failure on sets of 5 and even 8 can often be quite difficult for novice lifters who may give up and not know how to push through those hard reps to truly recruit those higher threshold motor units. With sets of 20, starting at a submaximal load will initiate motor unit recruitment for the low threshold motor units. As the reps increase, those lower threshold motor units will fatigue, forcing high threshold units to be recruited to complete the lift. When a maximal amount of motor units is reached and fatigued, around the 20th rep, the movement will no longer be able to be executed and only one set is needed for that movement.
As I mentioned earlier, I didn’t totally understand the 1×20 method and actually thought it was a bit silly, but then I came across Dr. McGuff and his method of training. Then things began to make a bit more sense. The approach that Dr. McGuff takes to maximize motor unit recruitment is not based around a high number of repetitions to failure but by slow tempo repetitions, and I do mean slow. These sets can take anywhere from 90 seconds up to 2 and a half minutes to complete while only doing a few total reps. For example, a rep scheme using this method could be a 15 second concentric motion and 15 second eccentric motion therefore making each rep 30 seconds to complete. With 4 reps per set, you have truly put in quite a lot of work. Much like the 1×20 method the objective here is to hit total failure of the movement but the designated time of each set using submaximal weights. Once failure is reached it is time to move onto the next movement. One of the interesting things Dr. McGuff notes about these slow tempo reps is the depletion of muscle glycogen by the end of the set. This makes me wonder if the 1×20 is equally as effective as the depletion of glycogen stores as the slow tempos.
Regardless of how you want to train, what your goals are and what you find enjoyable, one of the main things you should be working towards each time you train is to maximize the number of motor units recruited. I will not deny the benefits and results of training heavy weights with low reps or classic rep schemes like 5×5 or 3×8. What I do hope I have done is shed some light on some other ways to go about training that can accomplish similar levels of motor unit recruitment without the high loads of traditional methods.
Ethan
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