In his recent article, “Everything is About to Change”, Chad Waterbury posted the following question:
If the last few reps of a high intensity set really do recruit extra muscle fibers, then why aren’t the last few reps the easiest?
I think that is a very important question to address. I applaud Chad’s efforts, and that of any other lifter or scientist, to further our current level of training knowledge. I am looking forward to Chad fleshing out his theory, and I am envious of his in-depth knowledge of the nervous system. I will do my best to try to answer Chad’s key question and give my rationale for those answers.
Really Chad is asking two points. The first is, do the last reps of a tough set recruit extra motor units (muscle fibers)? The answer is absolutely yes, the last few reps do recruit additional motor units.
I think the easiest way to look at that question is to think about it like this. What percentage of the muscle is left untrained after completing the set? It should be clear that you work a certain percentage of the muscle on the first rep, a larger percentage on the second, and third, etc, so after a tough set you have worked a very large percentage of the overall muscle.
Chad already explained the Size Principle in his article, but in a brief nutshell it says that you will only recruit the motor units necessary to perform the exercise, starting off with the smaller or weaker motor units. As those motor units get tired you then recruit the bigger and stronger motor units.
When you are trying as hard as you can (at the end of the set) you are attempting to recruit the biggest and strongest motor units that you have. There is a very strong correlation between difficulty and the number of motor units recruited. The more difficult something is (or gets) the more motor units over time that have been recruited. You have trained a greater percentage of the muscle with 10 reps than with one rep. You have recruited more motor units to finish a 100 M race than to start one. You have used more motor units in the fourth quarter of a tough game than in the first quarter. This should make common sense.
It is the recruitment of said motor units that contributes to the feeling of exhaustion.
You might ask if we recruit a larger percentage of the muscle as we do more reps and train harder, why don’t we take every set to failure and do 20 sets per exercise? The answer is simply that you would do so much damage that it would be very difficult for you to recover in a reasonable timeframe, and the damage done might not just be micro level damage but might instead be macro level damage, or something serious like a pulled muscle or injured tendon. The goal, which is nothing new, is to do enough to avoid undertraining (too few reps, too few sets) but to prevent overtraining. Of course, walking that fine line is easier said than done.
So if the first part of the question is true, which it definitely is, why then aren’t the last few reps the easiest? This is because some of the motor units that started out working in the set have become fatigued and they are no longer able to contribute effectively to the exercise. So even though a higher percentage of the total motor units are being recruited as the set continues, on a given rep particularly near the end of the set, fewer motor units are able to contribute to that specific rep because the others are fatigued.
Chad used the example of people pushing a car, which I think is a good one. I think about it like this. Let’s say you have 10 people that are available to push a car, so you start off but then it is harder than you thought it would be and some of the people are beginning to get tired. So you get 5 more people to help you push the car and they join in over time. Upon completion of the car push, 15 total people pushed the car (greater than 10) but pushing the car at the end was still hard because most of the people were tired and probably only 2 or 3 strong people were left to push the car.
I have included a chart that I hope will better explain how this phenomenon works. The chart tries to explain motor unit recruitment during a set. The motor units currently in use are in gray, the motor units fatigued are in orange, and the line at the top of the chart represents a person’s theoretical ability to recruit their motor units.
It is curved because I believe there is motor learning taking place during the set, so you can recruit a great number of motor units on rep 2, and 3, and 4 etc until fatigue sets in which limits your ability to recruit the motor units. Hopefully you can see from the chart that as a set progresses, a greater number of motor units are used (a larger percentage of the overall muscle is trained) but each rep gets harder because there are a larger number of fatigued motor units that are not contributing much to the exercise.
Another way to look at it is that each additional rep damages a new part of the muscle that had previously been unused prior to that rep.
Remember that even elite athletes can not recruit 100% of their motor units in most situations. Simply thinking that you want to recruit your motor units does not mean that you will, sometimes you must be forced to do so. Beginners have very poor motor unit recruitment and they must be taught or forced to learn how to recruit their motor units. Normally the toughest motor units to recruit, especially at the beginner or immediate level, are the type II fast twitch motor units, which are the ones that respond best to the training by getting bigger and stronger.
Only the motor units recruited during an exercise are the ones that respond to the exercise, so if you don’t hit that part of the muscle it will not make any significant changes.
I hope that makes some sense. These are important questions and I feel it is important to try to answer them. Given the number of responses to Chad’s article it is obvious that many people are interested in this topic. I am happy to go over anything in this post if desired. Thanks to Chad for writing the article and prompting the discussion, it seems he achieved his purpose of getting people to ask questions.