Motor Units and Mad Libs

Waterbury's Mad Libs

While waiting at the DMV for my moped license renewal form, I decided to kill some time with one of my favorite childhood pastimes. No, not chewing tobacco and throwing rocks at whores. I'm talking about that crazy little word game known as Mad Libs.

Here's how my sentence looked:

Today, (name of person) and I rode a (noun) across town to see an (adjective) (noun) swallow a (noun) with her (noun).

Here's how it turned out:

Today, Chris Shugart and I rode a unicycle across town to see an illiterate stripper swallow a Siamese cat with her plunger.

Okay, so it really didn't turn out right. But you know what? Every time I read the cover of a newsstand muscle magazine, I see statements that make about as much sense as my Mad Libsdo. I swear the editors must write up a prefabricated sentence and, each month, do nothing more than drop a training method in the blank.

The key to building bigger muscles is with ___________!

Let's see, in May the blank was filled with slow eccentrics. In June, it was triple drop sets. In July, it was isometrics.

You see, there's a constant cycle that occurs throughout the year in the fitness industry. Strength coaches, fitness coaches, muscle magazines – they all follow this cycle. The cycle is this: take a half-dozen training methods and write an article or two for each method. The next year? Start the process over.

Now, there's nothing inherently wrong with any of those methods, but it's important to understand that every muscle-building method is intended to do the same thing: recruit more motor units. So I'm going to give you some background in motor unit science, along with the research that either supports or debunks the latest newsstand training method.

Here we go!

200 Strangers

You've probably heard of motor units, and maybe you've even sat through a lecture or two about them. But have you ever really given motor units serious thought? In other words, what if I told you to stand up in front of 200 strangers and explain what a motor unit is and how it works. Could you do it?

While getting my first graduate degree, my mentor at the University of Arizona would make me do just that. I'd stand up in front of a group of people and explain in detail whatever process we were studying that week. You'd be surprised how difficult it is, even when you think you know what you're talking about.

Even if you don't have an aversion to strangers and public speaking, it's easy to merge into an explanation that's anything but lucid. So instead of rehashing a textbook definition, I'm going to put motor unit science into real-world language.

Motor Units 101

You have nerves that exit your spinal cord and innervate your muscles. These nerves are known as motor neurons and they're controlled by your brain and by reflexes throughout your spinal cord.

A motor neuron's job is to make your muscles contract in a way that the brain sees fit for whatever task you're trying to do. In essence, a motor neuron is nothing more than a translator between your brain and the actions of your muscles.

So, if your brain is being told by your eyeballs that an angry, SUV-wielding ex-girlfriend is chasing you through an Applebee's parking lot, your posterior chain muscles need to find out this valuable information so you can hightail the hell out of there. Without your motor neurons acting as a link between your consciousness and skeletal muscles, you'd be flattened right in the middle of the parking lot during happy hour.

I'm vilifying your ex not for the sake of entertainment, but for the sake of science. You see, most people don't take time to contemplate how a motor unit, and all its parts, works. You might see an illustration like the one below and think, "Yeah, I get it," but do you really? I ask this because most strength coaches can't explain how a motor unit really works.

It's imperative, however, that everyone from a fitness professional to a weekend warrior understand motor unit science. Why? Because recruiting more motor units is what the hypertrophy game is all about! Learn to recruit more motor units, and you'll build bigger, stronger muscles.

Motor Unit Recruitment and Rate Coding

We all lift weights to get bigger and stronger muscles, and we do this by lifting ever- heavier loads. Our nervous system is what's driving our muscles' ability to lift these ever-heavier loads, but what mechanisms are your nervous system using to develop greater strength and muscle mass?

There are two primary processes the nervous system uses. The first process is motor unit recruitment. Specifically, I'm referring to your nervous system's ability to recruit more motor units. The more motor units (muscle fibers) you recruit, the more force you'll produce.

Small force tasks recruit few motor units; large force and/or explosive tasks recruit many motor units. (1) So when you lift heavy or fast, you recruit the most motor units and your body responds to this stress by increasing the percentage of motor units that you can recruit.

It's safe to assume that no one can voluntarily recruit all of his motor units. Let's say you perform a lying leg curl, and let's say that you have 1000 motor units in your hamstring muscles. An untrained person might recruit, say, 500 motor units; a highly-trained person might recruit, say, 800 motor units.

So with training, you increase your ability to voluntarily recruit more motor units. And that's why many researchers don't report muscle mass gains during the first few months of a strength training study (the people are simply training their nervous systems to recruit more motor units, among other things).

The second step, once you've tapped out your recruitable motor unit pool, is something called rate coding. Rate coding is another way for your nervous system to make your muscles develop more force by enhancing the rate at which your motor neuron sends an electrical signal to your muscles.

Your motor neuron sends an electrical signal to your muscles that, through a series of reactions, causes them to contract. Importantly, this electrical signal is known as an action potential. The more action potentials that your motor neuron discharges, the more muscle fibers you'll potentially recruit. To iterate: rate coding sends a more powerful signal to your muscles, thus more powerful muscle actions usually follow.

So in the above example with your ex-girlfriend and her SUV, your motor neurons are discharging a huge amount of action potentials to your sprinting muscles. Why? Because you need to contract a lot of muscles, very quickly. And in order to avoid getting steamrolled by 6000 pounds of Cadillac Escalade, you need to quickly contract a lot of glute, hamstring, and calf muscle fibers. Again, this increase in rate of discharge of the motor neuron is known as rate coding.

Research on rate coding has demonstrated minimum and maximum values during contractions. On the low end of the scale, it appears that the minimal rate at which we can discharge action potentials to our muscles is 5 to 8 pulses per second (pps) during very slow voluntary contractions. (2, 3) With isometric contractions, average rates of 30-50 pps have been recorded. (4) With regard to rapid contractions (my favorite, of course), values have been recorded in the 100-200 pps range! (5,6,7)

Importantly, both motor unit recruitment and rate coding are taking place as you produce high levels of force with your muscles. Research has demonstrated that the first 85% of your maximal force development occurs by recruiting more motor units. The last 15% is accomplished entirely by rate coding.

For example, if your 1RM for the leg extension is 200 pounds, the force you produce for the first 170 pounds is accomplished by recruiting more motor units. To lift the last 30 pounds, your muscles are relying on rate coding to jack up the force production. And this is as good of a reason as you'll ever find to work with loads greater than 85% of your 1RM (you'll theoretically enhance rate coding).

So if you're trying to recruit more motor units, it makes scientific sense to focus on rapid contractions and isometric contractions since both have been shown to result in the many pulses per second. And rate coding becomes a factor when you train with heavy loads, or any load that allows you to develop maximal force.

Key Points: Focus on fast contractions and isometrics for maximum motor unit recruitment. Focus on heavy loads and fast contractions to enhance rate coding.

Motor Unit Synchronization

If you open up any decent text on neural adaptations that occur with strength training, you'll undoubtedly see the term motor unit synchronization. What does that mean? Well, if two actions are happening at the same time, they're said to be "in synchrony." So when two or more motor units fire together, they're firing in synchrony; hence, the term motor unit synchronization.

Still confused? Let me explain.

Let's take, for example, your hamstring muscles. Since they represent one of the largest muscle groups in your body, it's not surprising that your hamstrings are made up of many motor units. If you only need to produce a small amount of force in your hamstrings (like when you're walking) you only need to recruit a few motor units.

If, however, you're performing a leg curl with 90% of your 1RM, you need to recruit most of your motor units. And when you're recruiting most of your motor units, intuitively it seems that you'd produce more force if those motor units fired together.

So it's assumed that the increased strength from resistance training is accompanied by more motor units firing together, a.k.a. motor unit synchronization. Specifically, it's assumed that strength training synchronizes your motor units' discharge rates. Again, let me explain what that means.

Let's say you're driving home from a football game and you've got five college linemen in your SUV. While hanging your head out the window and yelling naughty stuff at a busty babe on campus, you end up driving into the ditch. Luckily, you've got five strong football players who are willing to pull your massive gas-guzzler out. You pull out a rope, tie it to your SUV, and tell the big dudes, "Pull!"

Unfortunately, three of the five guys are foreign exchange students so they don't know what in the hell you just said. In essence, only two guys are pulling on the rope. The two "pullers" eventually peter out when, low and behold, the three foreign exchange football players pull their heads out of their asses and start pulling on the rope.

What happens to your SUV? Nothing, because you need to get all five dudes to pull at once.

Once you finally got all five guys to pull on the rope at once, they were able to pull you out of the ditch. The key point to understand is that it took the combined effort of all five football players to generate enough force to pull your SUV out.

It's been assumed that this is how strength training and motor unit synchronization work. When you first start training, it was assumed that the motor units weren't firing in synchrony as you approached maximal levels of force. So, in essence, you couldn't produce maximal force. Over time, though, your nervous system was thought to learn to fire your motor units in synchrony, thus producing more force.

Unfortunately, research is demonstrating that motor unit synchronization isn't a key to developing more strength. (8,9) To make matters even more complicated, it's been demonstrated that asynchrony of motor unit firing can result in higher levels of force. (9)

Key Point: Motor unit synchronization doesn't appear to influence the maximal force that a muscle can develop.

Increase to Maintain

Weight training for maintenance can be as important as weight training for increased performance. At some point, most of us are on some type of maintenance plan for our genetically-gifted muscle groups. That's because most of us don't need to increase the size and strength of every muscle group on our body. We all have certain muscles that respond well to very little training, thanks to Ma and Pa.

Let's say your Ma and Pa are Tom Platz and Kim Chizevsky. It's obvious, then, that you don't need to spend much time training your quads. So let's say you go on a hamstring specialization program to balance out your thigh development.

So you lay out a plan: focus on the hamstrings and maintain the quadriceps. Fine, that's a decent plan. But how are you going to maintain your quad size and strength? If you're like most lifters, you'll train your quadriceps with a constant load over, say, eight weeks while you're bringing up your hamstrings.

If your 5RM for the front squat is 315 pounds, you'd probably decide that you'll do something like 3 sets of 5 reps with 315 pounds, twice each week to maintain the size and strength of your quadriceps.

Research, however, demonstrates that such a plan is a bad idea.

It's been demonstrated that you'll reduce your motor unit activity, over time, if you train with a constant load. (10) So the number of motor units in your quadriceps that it takes to lift 315 pounds for the front squat will decrease over time. Basically, you'll be recruiting fewer motor units, or less muscle mass, to do the same task. That's not good.

Key Point: While maintaining a muscle group, you should vary the loading and attempt to increase your intensity, volume, or speed of contraction, even if it's minimal compared to your other muscle groups.

Eccentrics and a Little Known Strength Booster

There are three types of muscle contractions: concentric (shortening), eccentric (lengthening), and isometric (unchanging). In terms of research, concentric and isometrics are pretty damn similar in terms of the nervous system's control over them.

Eccentrics, however, appear to be a completely different monster. It appears that the nervous system modulates eccentric contractions (negatives) differently than concentric and isometric contractions. According to the research, eccentrics aren't simply a mirror-image of concentrics. Compared to concentric contractions, eccentric contractions activate less muscle, alter the recruitment order of motor units during submaximal contractions, and have a greater resistance to fatigue, just to name a few. (11)

But I don't want to turn this article into a review of research pertaining to eccentric contractions. We all do them; it's not like we're trying to avoid them. They certainly have some benefits for gaining strength and size, so I won't beat this dead horse.

However, I do want to mention one study that you probably aren't familiar with. And by God, it just might help you gain some strength and mass in the gym! If you're someone who has a strength discrepancy between limbs, listen up.

It's been demonstrated that you can increase the strength of one limb approximately 11% if you first train the contralateral limb with eccentric contractions. (12) I've experimented with this technique quite a bit and I've found that 4-5 eccentric reps with 80-85% of your 1RM works well.

Let's say your left triceps is stronger than your right. You can take advantage of this method by performing 4-5 eccentric contractions for the lying dumbbell triceps extension on your left side. Then immediately pick up a dumbbell and do the same movement with your right side. You will be stronger.

This totally goes against the notion that you should train your weakest limb first, but it works. Give it a try and see for yourself!

Conclusion

Your motor unit IQ should be substantially higher than it was at the beginning of this article. Now I've got to get to work on a book that's sure to make me rich: Mad Libs for Neuroscientists!

References

1. Walmsley B, Hodgson JA, and Burke RE. J Neurophysiol 41(5):1203-1216, 1978.

2. Sogaard K, et al. Electroencephalogy Cin Neurophysiol 101:453-460, 1996.

3. Van Cutsem M, et al. Can J Apply Physiol 22:585-597, 1997.

4. Enoka R and Fuglevand A. Muscle Nerve 24:4-17, 2001

5. Desmedt JE and Godaux E. J Physiol 264:673-693, 1977.

6. Van Cutsem M and Duchateau J. J Physiol 562:635-644, 2005.

7. Van Cutsem M, et al. J Physiol 513:295-305, 1998.

8. Semmler JG, Sale MV, Kidgell DJ. Proc Int Australas Winter Conf Brain Res In press, 2006.

9. Yao W, Fuglevand A, Enoka R. J Neurophysiol 83:441-452, 2000.

10. Ploutz LL, et al. J Appl Physiol 7:1675-1681, 1994.

11. Enoka R. J Appl Physiol 81(6):2339-2346, 1996.

12. Owings T and Grabiner M. Med Sci Sports Exercise 28:S141, 1996.