Rep Speed, Fatigue, and Motor Units

It’s been awhile since I’ve written a good old-fashioned hater post, which is ironic since that’s one of the reasons I started this site: to serve as a place to rebuke claims of guru bullshit. I’ve pretty well gotten away from that lately, for better or worse, but today I saw something that got me interested enough to write up a response.

Namely I want to talk about this recent obsession with movement speed (tempo) and motor unit recruitment. This is being touted as “revolutionary” by certain parties; although it might be new to the current crop of would-be Internet lifters and bodybuilders, this is not a new concept. In fact, in my brief decade in the gym, I can remember running into that same idea in half a dozen places in the early days of the ‘net, back when places like Deepsquatter and MFW were the only real places to get decent info.

At any rate, I’m not hear to harp about that per se. What I want to talk about is some comments I read recently that were made by an individual that supposedly holds a graduate level degree in neuroscience, an individual who is touting his “revolutionary” new approach based on using a change in movement speed.

The basic claims being made seem reasonable enough at first glance. Motor units (the nerves that link your muscle fibers to the brain, basically) are recruited according to the size principle; smaller MUs, which have less force potential than larger MUs, are going to be recruited first in any motion. The higher the force requirement of a movement, the greater the number of MUs recruited. The caveat is that the bigger MUs also have lower fatigue times – once activated, they don’t last long before they wear out.

You may also be familiar with the old F=ma formula from physics class. Force acting against mass creates acceleration (Keep that in mind, as well. Forces create acceleration, not the other way around). OK, so the idea here is that high forces are created in your muscles when you 1) move something heavy, 2) move something quickly, or 3) some of both. Of course it makes perfect sense that you’d want to train with these concepts in mind when your goal is maximum MU recruitment.

And for most people, that should be the goal. Strength ability, and by extension your ability to stimulate growth, will directly correlate with how many MUs are both recruited and trained in a workout session. It makes sense that you’d want to use both heavy loads and optimize movement speed – in other words, you’d want to lift a given weight as quickly as you can. The rationale is still working just fine thus far – in fact, the idea of fast or explosive lifting is quite old. I can point you to sources from the 70s, 80s, and even older that emphasized explosive tempos in training, for this very reason. It was called “compensatory acceleration training” for those that care.

Now, if you go ask the average bodybuilder or even your recreational gym-rat how to lift, you’re probably going to get a different answer. You’re most likely going to be told to lift until the set is painful; more likely you’ll hear somebody telling you to push until you can’t lift the weight under your own power – the state of muscular failure. Bodybuilders seem to instinctively push towards that style of “grinding” reps, and there’s something to it for their goals (i.e., larger muscles).

The conventional wisdom in that regard tells us that MU recruitment will increase across a set, so by the end of a set where you have to start fighting each rep (and the reps slow down) you’ll be working with a different set of MUs. Here’s where I have the first issue with Mr. Neuroscience. In recent comments, he stated:

“You see, since acceleration is a component of force a drop in acceleration results in a drop in force. Why is this important? Because of the Size Principle. A drop in force tells us that fewer motor units are being recruited.”

Well, OK, let’s think about this. A drop in force does indeed tell us that there are fewer MUs active in the movement. Fewer recruited MUs means less force. That’s not in question. What it doesn’t tell us is why force decreased in the first place. The drop in force happened because the MUs that were carrying the movement have fatigued and dropped out – they’re no longer contributing force.

It’s suggested that, once this happens, the nervous system compensates by recruiting MUs that wouldn’t normally be active in the set. That’s when you reach those “grinding” reps at the end of a set, and it’s also why you can’t manage but maybe 2-4 reps once that happens. The high-threshold MUs fatigue too quickly to last any longer.

Look at it this way. A very heavy set, say your best set of 3-5, is going to recruit all available MUs right off the bat (at least, all the ones you can voluntarily recruit) because the weight is heavy. If you didn’t do that, the bar wouldn’t move. Those MUs have to be activated in order to create enough force to lift the weight. A lighter set of say 8-10 reps, that’s a different story. You’re only going to recruit a smaller sub-section of the total available MUs, because of the size principle – the smaller MUs are all that’s required to lift the weight. The thing is, during a longer set, more of those MUs are going to be fatigued, which is where the idea breaks down.

That’s the difference. A set with a very heavy weight is going to recruit a lot of MUs; a longer set with a moderate weight is going to fatigue a lot of MUs. It’s a subtle distinction, but very important – if you’re saying that a set should get easier if you recruit more MUs, you can’t just ignore the role of fatigue. A fatiguing set may not recruit more MUs simultaneously (as is the case with a very heavy weight) but it most certainly can fatigue more MUs over the course of a set.

Since fatigue seems to be conditionally important to both strength and size gains, in some contexts, it’s a bit short-sighted to categorically rule out the idea of “grinding” or fatiguing sets.

I’m also not saying the idea of compensatory acceleration itself isn’t sound; I’ve personally made fatigue-regulating explosive-type training a mainstay of my own lifting over the last 10+ years. So have lots of other lifters and coaches, people from Doug Hepburn to Bill Starr to Ed Coan to Louie Simmons. Mike Tuchscherer’s Reactive Training System is a more recent and very solid example. Compensatory acceleration is a sound practice and as far as strength training goes, you should be regulating your performance based on the difficulty or feel of a set.

It’s just not revolutionary. Or something that a person with a graduate degree should have problems understanding.

Come discuss this on the forum.

Written by Matt

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One thought on “Rep Speed, Fatigue, and Motor Units”

  1. There is some discussion by Charles Statley using his EDT training system which is disigned to manage fatigue during a session so the forced produced will remain high. have you looked into his thoughts on this at all.

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