The Secret to Motor Unit Recruitment
by Chad Waterbury
It's time to cut the crap about motor unit recruitment.
A fundamental misunderstanding about this subject is running rampant in the strength and conditioning community. I don't know whether the truth was simply forgotten, or never learned in the first place, but understanding motor unit recruitment is absolutely essential to your training success. And a huge part of understanding motor unit recruitment is understanding its relationship to lifting speed.
Before I get to the goods, I'll define what a motor unit is, so you know what I'm talking about. A motor unit is your motor neuron and all the muscle fibers it innervates. Here's an illustration of a motor unit to give you a better idea.
The key is to recruit as many of these suckers as possible with each rep. Do that and you'll gain size and strength faster than ever before.
15 Seconds is Enough
Read carefully, because I'm about to give you one of the most valuable pieces of information you'll ever learn: you can't sustain maximum motor unit recruitment for more than 15 seconds. The neuroscience-based size principle states that motor unit recruitment is fixed and orderly. Your smallest motor units will always be recruited first; your largest will always be recruited last.
This is the first place that most people get into trouble. Since the largest motor units are recruited last, people equate that with meaning "the end of a set." This is wrong, wrong, and more wrong. You can recruit all of your recruitable motor units within the first or second rep, if the load is heavy enough and the speed is fast enough. As a gross generalization, you can assume that the load must be 60-100% of your one repetition maximum (1RM) to recruit all your motor units.
If you always lifted with 100% of your 1RM, you wouldn't hear me on my soap box about lifting speed. That's because you're always forced to recruit all your motor units when the load is your 1RM. However, any load less than 100% of 1RM will not recruit all your motor units unless you attempt to lift that load as quickly as possible.
The difference between maximum lifting speed and any speed slower than maximum is directly due to motor unit recruitment.
I'm going to bastardize some force numbers in order to break things down as simply as possible. Imagine you're holding a dumbbell in your hand and you're ready to perform an arm curl. You decide to use 70% of your 1RM since you want to get around 12 reps. Now, let's say that you've fallen victim to the slow contractions dogma and decided to lift the load for a count of four (a four seconds concentric). The load would move slow and it would be easy.
Now, let's say for the second rep you try to lift as fast as possible. Bam! The dumbbell races to the top position. The difference between your first and second repetition is this: with rep one you recruited 70% of your motor units, with rep two you recruited 100% of your motor units.
Lift as fast as you can for maximum recruitment.
But the picture gets even more interesting and telling when you look at it closer. That 30% you neglected with your first rep? That 30% represents your biggest and strongest motor units that have the most potential for size and strength gains! Remember, the largest motor units are recruited last, as explained by the size principle. Lifting slowly with any load less than 100% of your 1RM will not recruit all of your motor units. You must make up the difference by attempting to lift all loads as fast as possible.
Of course, some loads will still move slowly, even though you're exerting maximum force against them. You can't make heavy loads move quickly, no matter how hard you try. So this isn't an issue of "Waterbury says that all loads should move quickly." Anyone who thinks that has a fundamental misunderstanding of my methods, motor unit recruitment, and muscle physiology, not to mention the laws of physics.
You can't make heavy loads move quickly, no matter how hard you try.
Speaking of muscle physiology, let's move to the next critical point that I alluded to earlier. You can't maintain maximum motor unit recruitment for more than 10-15 seconds, based on the physiological limitations of your largest muscle fibers. Your largest muscle fibers rely on the ATP-PC energy system that supports short, powerful bursts of activity (think of a 100M race).
All you need to understand about this system is that it has a limited energy supply and it runs out within 15 seconds. This is why you can't lift 90% of your 1RM for 20 reps.
With that in mind, get ready for a shocker. I'm always harping on failure training by saying it's an ineffective way to train since it works against the size principle and muscle physiology. However, if your sets lasts less than 15 seconds, and if you always attempt to lift the load as fast as possible, failure training is a very effective way to train! In fact, that's what many power athletes do, because it works.
Since a typical HIT set involves slow contractions and sets that last much longer than 15 seconds, I'm not a fan of the system. In other words, it's not so much the "failure" part that disturbs me, it's how and when the failure comes about.
Motor unit recruitment research and muscle physiology put a big nail in the notion that 40-60 seconds of continuous muscle tension is optimal for hypertrophy.
8x3 and Muscle pH
When I started writing articles, the dogma of hypertrophy training was to perform 3 sets of 8-10 reps. I recommended that lifters flip those numbers in favor of 8x3. There are two important benefits of lower reps for hypertrophy. First, you can use a larger load, and therefore recruit more motor units. Second, and most important, is that lower rep sets are shorter.
A three-rep set will never take more than 15 seconds, unless you purposely slow down the movement. The shorter the set, the greater your potential to recruit all your motor units with each rep.
Also, when you keep the reps low your muscle pH doesn't drop nearly as much. With higher rep sets your muscle's pH can drop substantially. During muscle contractions, glycogen is broken down for energy, and it releases one unit of hydrogen (H+). As you perform more reps your body must switch over to glucose, and it releases two units of H+.
At this point your muscle pH is dropping and your muscles are burning. The biggest problem with acidifying your muscles is that it limits your force-generating potential by interfering with cross-bridge formation. Lower rep sets offset this acidification.
The Truth About Lifting Speed
I've been hemming, hawing, and hand waving about lifting speed. But it's clear to me that I haven't been very clear. Here's hoping the following paragraphs take up the slack.
Fatigue is an inevitable and necessary aspect of training. To build bigger muscles you need a certain amount of it. The key is to understand whether you're fatiguing a portion of your motor units, or all of them. No one would argue that the latter will result in more size and strength than the former.
And this, my friends, is the genesis of the confusion: terminate each set once the speed slows down noticeably.
Fatigue is an inevitable and necessary part of training.
Let's say you're performing a set of squats with 80% of your 1RM. If your goal is size and strength I'll recommend a target number of total reps for each lift. In many cases that number is 25.
So you perform your first set. If you follow my principles, you'll terminate the set as soon as you notice the speed is slowing down, which means that the last rep will always be the slowest in the set. If you stop the set with the speed you start with, then you've stopped too soon. Don't stop until you actually notice the bar slowing down.
And you should never stop in the middle of a rep, even if you do notice that you're slowing down. Finish the rep, then stop. Got it?
Terminate the set as soon as the bar starts to slow, after you've finished the rep.
My position is that if you keep pushing beyond that initial slowing, you aren't recruiting more motor units. You slow down because your ability to generate high levels of force has diminished. And force is positively correlated with motor unit recruitment.
When your speed slows down you're producing less force as a direct result of recruiting fewer motor units. You know all those gut-wrenching, high intensity sets with slow contractions that result in nausea? Essentially, at the end of those sets you've simply forced your smaller motor units to do the brunt of the work. If the set lasts any longer than 15 seconds, the biggest motor units have taken a break.
So doesn't it make sense to keep your lifting speed as fast as possible?
Back to the squat example with 80% of your 1RM. With your first set you might notice your speed slowing down at rep five, so you stop. You rest for a specified amount of time and start your next set. That set might result in four reps. You continue until you reach 25 reps.
It's important to understand that each subsequent set will have a lower top speed. In other words, you won't be able to lift as fast on set four as you do on set one. That's fine, and that's how it should be because some fatigue is necessary for size and strength gains. What I'm trying to do is show you how to get the most out of every rep of every set.
My maximum recruitment training guidelines are basically catered to loads that allow 5-20 reps per set. Any talk of loads heavier than a 5RM is a moot point. With, say, 90% of your 1RM very little of this information makes any difference. Why? Because the bar will always move slowly, and you can't get enough reps to compare the speed to other reps.
However, you should always attempt to lift all loads as fast as possible, even when the load is very heavy. If you do, you'll augment the descending neural drive from your brain to your muscles. This results in greater motor unit recruitment (force-generating potential).
There are many other benefits to lifting as fast as possible that I've already addressed in other articles, but I want to mention a few more.
First is your motor unit recruitment threshold. For the last 30 years, research has clearly demonstrated that fast lifting tempos reduce your motor unit recruitment threshold. (1) This means you can train your nervous system to recruit your motor units sooner. That's great for strength and subsequent size gains.
Second, fast tempos will train your nervous system to recruit more motor units. There's always a reserve of motor units that, usually, can only be recruited in life-or-death situations. But with fast contractions, you can tap into this reserve.
Take-Home Points (yes, there will be a quiz)
• Train with loads between 60-100% of 1RM.
• Each set should last less than 15 seconds.
• Perform the concentric (lifting) phase as fast as possible.
• Terminate each set once you notice the speed is slowing down.
• Use compound movements and focus on 25 total reps for size and strength.
• Use compound movements and focus on 15 total reps for maximal strength.
Chad Waterbury is a strength and conditioning coach with Bachelor of Science degrees in Human Biology and Physical Science. His clientele ranges from members of military special forces units to non-athletes seeking exceptional physical performance and development. You can contact him through his website.
Chad's new e-book for losing fat and building muscle, The 10/10 Transformation, is now available here for only $19.95!
His book, Muscle Revolution, is available at the T-Nation online store.
1. Desmedt JE and Godaux E. J Physiol 264: 673-693, 1977.
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