The Science of Motor Unit Recruitment Part 2

In my first installment of this series, I explained how the incredibly important size principle works. It’s crucial to understand that information, so if you haven’t checked it out you can do so by clicking here.

To recap, all movements start in your brain, and then travel down your spinal cord where the signal stimulates the motor neuron to contract your muscles.

Now, I’m going to take things a step further by explaining the essential component of any training program that’s designed to build muscle, boost strength, or burn body fat.

What is the essential principle that I live by? You must recruit as many motor units as possible with every rep of every exercise. The only way to do this is by following training parameters that tap into your largest motor units.

Here’s why that’s important. You see, according to Henneman’s size principle, if you’re tapping into the largest motor units you’re also recruiting all of the other motor units (from smallest on up).

Check out the graph on the right that shows the relationship between force and motor unit recruitment. The highest levels of force coincide with the highest level of motor unit recruitment. There’s no reason to ever do a rep, set, or workout that doesn’t target as many motor units (muscle fibers) as possible, therefore, keep this graph in the back of your mind.

Ok, at this point a logical question surely popped into your mind: “How do I recruit all my motor units?”

There are two ways: lift as heavy as possible, and lift as fast as possible. Now, keep in mind that heavy weights won’t move quickly, no matter how hard you try. But they don’t need to. When the weight is heavy enough to only allow three or four reps, you’re recruiting all your motor units because it takes every ounce of effort to get the weight moving.

Where many people screw up, however, is with submaximal weights. I’m talking about lighter weights that you could move faster, but don’t. If you start with a weight you could lift 12 times, and if you lift with a slow tempo, you’re probably recruiting about 60% of your motor units. But if you accelerate the lift from the first rep as fast as possible – wham-o! – you’ve tapped into all the motor units that were sitting on the bench.

So there are two ways to recruit all your motor units: lift heavy weights some of the time, and lift lighter weights as fast as possible other times.

There’s another essential aspect that I want to mention with regard to lighter weights. It’s not the load of the weight that’s important – it’s the effort and intensity that will make or break your results. With enough focus and drive you could theoretically recruit more motor units with 50% of your one-repetition maximum compared to, say, 75% of your one-repetition maximum.

Now, here’s a critical component of the equation that many people don’t think about: “How long can I sustain maximum motor unit recruitment?” This, my friends, is the key to getting this powerful training concept right.

Remember I said that the goal of any rep, set, or workout should be to tap into your largest motor units (aka, largest muscle fibers)? Well, your biggest, strongest muscle fibers can’t maintain their activity for long since they rely on the short-acting ATP-PC energy system. This is a readily available supply of energy in your muscles that allows you to immediately tap into the “fight or flight” response. However, since it’s a small pool of energy, it can only maintain your maximum efforts for 10 seconds.

In other words, you only have 10 seconds of available time to recruit all your motor units. This is why so many lifters immediately got results when I told them to switch from 3 sets of 10 reps to 10 sets of 3 reps. With 3 reps, you’re finishing each set in less than 10 seconds. In either case you’re performing 30 total reps, but with 3 reps instead of 10 reps per set, you’re able to take advantage of maximum motor unit recruitment with every set.

Now, keep in mind that the “10 seconds maximum” I mentioned can differ from person to person. Some lifters will only be able to sustain maximum motor unit recruitment for 6 seconds, whereas others can pull it off for 12 seconds. That’s why it’s important to know which cues tell you that motor units are dropping out of the task. It’s important to keep in mind that the largest motor units are recruited last, but drop out first. So you must first tap into them as quickly as possible by lifting heavy or lifting lighter weights fast. Then, you must know when those largest motor units are dropping out.

How do you know? There are three cues. First, your speed will slow down dramatically. Second, your range of motion will shorten. Third, your technique will break down. These don’t all happen in the order I mentioned. For some exercises, such as the pull-up, your range of motion will typically shorten before your speed slows down. For technical lifts such as a hang snatch, your technique will falter before your speed decreases. However, for most common strength exercises your speed will dramatically slow down when motor units start dropping out.

“The last few reps of a set is where the results happen,” has long been the dogma in resistance training circles. The theoretical reason why some coaches said this was true is because they figured that additional motor units were recruited at the end of a long, agonizing set to failure. However, if you look at the neuroscience research it’s clear that this postulate holds no water.

If you recruited more motor units in the last few reps to failure, the set would get easier and the speed would increase. Since this doesn’t happen it’s time to look at a more progressive way of training. Lift heavy, lift fast, keep the sets shorts, and avoid failure. Those are the keys to maximum motor unit recruitment.

In my next installment I’ll discuss some exciting new research that uncovers new ways to tap into your largest motor units.

Stay focused,
CW

18 thoughts on “The Science of Motor Unit Recruitment Part 2

  1. Chad, does this mean that 3×8 are still useful provided one lifts the weight as fast as possible?

    Is 3×12 getting to light re: the weight lifted?

    DS

    CW: Yep, as long as you lift as fast as possible, and stop the set once the speed slows down significantly, you can get results on many different set/rep schemes.

  2. Very nice Chad! I’ve been following your scientific methods for a few years now and have found that my workouts produce much greater results in much less time, so thank you for that 🙂 The steady stream of articles is really nice to see – I’m learning something new and remembering something previously learned quite frequently now. Thanks!

  3. Hi Chad – great stuff!
    Quick question – when lifting sub-maximal weights fast how do you avoid the issue of momentum doing a large portion of the work?

    CW: You avoid momentum by maintaining perfect technique. Lifting fast is no substitute for poor/slopping technique.

  4. Chad, I’m training to enter the military. Does this principle hold when training for more endurance-style results? Is there some other way I should be working?

    CW: Training for endurance vs. maximum strength or muscle growth are not the same things. It doesn’t make endurance training wrong, it just requires a different approach. For endurance you should measure the time you’re looking to maximize (say, 45 seconds) and do sets that last that long but are still based on lifting fast and adding weight over time.

  5. Time Under Tension

    What is your belief on the time under tension theory? Is the theory of time under tension bogus?

    CW: For endurance, time under tension is valid. For strength and muscle growth, forget it. The notion that you need 45-60 seconds of time under tension for optimal muscle growth is bogus, and it defies what the neuroscience research shows is optimal for maximum motor unit recruitment.

  6. Why do profs teach that going to failure recruits more MUs at the end? As the fast MUs fatigue and drop out speed decreases, that we’ve all witnessed ourselves, so what do the profs base their lesson on? Was it bad research? I can see the possibility that speed could decrease with the fast MUs dropping out while more, even bigger MUs get recruited, it would only need to be that the new MUs are not enough to cover the amount of MUs that are dropping out. I.e. if five MUs drop out that produced a total of 10 units of force, while two MUs start that only produce 8 units of force, you end up slower even while recruiting more, larger MUs.

    CW: Why do professors teach it? Because they’re relying on old research, and because they don’t train anyone. My professors never taught me that – part of the reason is because they understood the mechanisms better than anyone else in the world. Based on Henneman’s size principle, motor units can’t drop out while other “bigger ones” take over – that goes against the biochemical and physiological properties of the motor neurons and muscle fibers.

  7. What is the target rest between sets when performing the 10 X 3 that you refer to? Thanks—— Bill

    CW: I like to alternate between antagonist exercises and rest about 30-45 seconds between each movement. For example: chin-up for 3 reps, rest 30 seconds, dip for 3 reps, rest 30 seconds, chin-up for 3 reps, rest 30 seconds, dip for 3 reps, etc.

  8. Hey Chad, this is some groundbreaking stuff! Awesome.

    I just have one question, I know lifting heavy weights all the time isn’t good on your CNS so how would you do a medium or even a light set in under 10 seconds?

    CW: Simple – lift that load as fast as possible. If it’s very light you might get 15 reps in that 10 seconds. If it’s a medium load you might get 8 reps. If it’s heavy you might only get 3 reps.

  9. Good article Chad.. Always thought going to failure was the way to go for getting to the next level. Thanks for enlightening me!!

    -Charlotte

  10. As always, Chad… great info. I recommend your book, Huge In A Hurry, to EVERYBODY. Thanks for everything. Keep up the good work.

    Doberman Dan

    CW: Thanks Dan! Much appreciated.

  11. As to Selective Recruitment, I have read various texts that have different findings. I know that DEs help teach the CNS to bypass the size principal and immediately recruit Type II fibers as opposed to recruting them only after the entire pool of Type I fibers have been recruited. My questions is this, are the Type I fibers actually bypassed or is the body simply recruiting Type Is and IIs simultaneously?

    CW: I covered this in part 3. The type 1 fibers are partially inhibited by the Renshaw cell, thus making the type 2 fibers dominant.

  12. waiting for the next installment chad. was reading ” Vladimir M. Zatsiorsky: Science And Practice of Strength Training” on the same topic and this blogs of yours has made it easy for me to understand motor unit factors. It all depends on the force. higher the force higher the number of motor unit recruited. so if i am doing bench press i come down slow on my eccentric and push explosive on concentric with maximum force to recruit max number of MU.

    CW: Yes, control the eccentric and accelerate the concentric. Sometimes it’s good to lower quickly, too, but only after you’ve spent a few weeks with a slower eccentric to build your base.

  13. Chad,
    Enjoyed the Article, good stuff
    So what is the average time that the “short-acting ATP-PC energy system” is replenished ? I was thinking of doing a routine based on the “Rest Pause” or clustered principal. Something like 3 fast reps @75-80% of RM, rest 15 seconds and then repeat for as many reps and cluster until speed and/or form breaks down.

    Thanks,
    Frank

    CW: You can replenish a good portion of it within 10 seconds, however, full recovery can take minutes depending on the exercise (a snatch requires more recovery than a curl, for example). That’s why I like to use circuits – it allows for minutes of recovery between the same exercise without making you sit around and wait. Rest pause training is a great way to keep the training load as high as possible.

  14. I have an issue I would like advice on: I cannot perform 1, 2 or 3 rep max sets. Either I cannot lift the weight at all, or I can do 10 or 12 reps before starting to tire. In other words, for example I can do a 10 rep set of bicep curls with a 60 lb dumbell in proper form but cannot even start lifting a 65 lb one (the next available size in our gym). With push or pull and leg exercices it’s even worse. Can you advise on why this is and how to work with it?

    CW: For your example of a 60-pound dumbbell, perform only 3 reps, even though you could do more. 2-3 days later do as many reps as you can with that weight. Alternate between these two methods. Make sure your form is perfect. The fact that you can do 10 reps with 60 pounds but not one rep with 65 pounds is extremely odd – stick to perfect form, accelerate the lifts, and your strength will go up.

  15. Great article Chad! Your graph corresponds very much with my own experiences: I measure the power (and force) of my lifts with my app IronChart. High power requires either big weights or fast lifts and I tripled my power when switching to series of 12 reps to series of 5. My total work (effort) would also be tripled without feeling more exhausted.

  16. Hi Cad
    this is a late post on the topic but i ll give it a try -)
    i am slowtwitch dominant
    bulking just makes me fat
    and doing bodybuilding splits of 8-12 slow reps till failure did not thing
    after reading about you on t-nation i ordered muscle revolution and started to lift heavy zeights fast
    most of the time 5X5 from reg park but also your 10×3
    i got alot stronger in the low rep ranges and my 1RM got up
    but muscle wise not much happend.
    for a slowtitch domiant person would you still advice 25 totza reps, or go for the 50 reps with a lower percentage of 1RM?

    greetings

    CW: Yes, start training with a higher volume, say, 40 reps per exercise. Then increase the volume up to 50 over the course of a month (add 2-3 reps each workout). That should do the trick.

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