Basic Skeletal Muscle Physiology - The Motor Unit

The Motor Unit

Each fiber of a muscle can contribute to force production only if it is recruited by the brain. One motor nerve can branch into tens, hundreds, or even a thousand branches, each one terminating on a different muscle fiber. One motor nerve plus all of the fibers that it innervates is called a motor unit. A single muscle can consist of hundreds of motor units. For example, your rectus femoris (one of the 4 quadriceps muscles) might contain 1 million muscle fibers, controlled by 1000 motor nerves. So on average, each motor unit contains 1000 fibers. The fiber type composition of a single motor unit will always be homogeneous. So a single motor unit will consist entirely or either type I (slow twitch) or Type II fibers (fast twitch). The composition of the entire muscle will be heterogeneous. Every muscle will contain some combination of slow and fast motor units.

Regulation of Muscular Force

The brain combines two control mechanisms to regulate the force a single muscle produces. The first is RECRUITMENT. The motor units that make up a muscle are not recruited in a random fashion. Motor units are recruited according to the Size Principle. Smaller motor units (fewer muscle fibers) have a small motor neuron and a low threshold for activation. These units are recruited first. As more force is demanded by an activity, progressively larger motor units are recruited. This has great functional significance. When requirements for force are low, but control demands are high (writing, playing the piano) the ability to recruit only a few muscle fibers gives the possibility of fine control. As more force is needed the impact of each new motor unit on total force production becomes greater. It is also important to know that the smaller motor units are generally slow units, while the larger motor units are composed of fast twitch fibers.
The second method of force regulation is called RATE CODING. Within a given motor unit there is a range of firing frequencies. Slow units operate at a lower frequency range than faster units. Within that range, the force generated by a motor unit increases with increasing firing frequency. If an action potential reaches a muscle fiber before it has completely relaxed from a previous impulse, then force summation will occur. By this method, firing frequency affects muscular force generated by each motor unit.

Firing Pattern

If we try and relate firing pattern to exercise intensity, we see this pattern. At low exercise intensities, like walking or slow running, slow twitch fibers are selectively utilized because they have the lowest threshold for recruitment. If we suddenly increase the pace to a sprint, the larger fast units will be recruited. In general, as the intensity of exercise increases in any muscle, the contribution of the fast fibers will increase.
For the muscle, intensity translates to force per contraction and contraction frequency/minute. Motor unit recruitment is regulated by required force. In the unfatigued muscle, a sufficient number of motor units will be recruited to supply the desired force. Initially desired force may be accomplished with little or no involvement of fast motor units. However, as slow units become fatigued and fail to produce force, fast units will be recruited as the brain attempts to maintain desired force production by recruiting more motor units. Consequently, the same force production in fatigued muscle will require a greater number of motor units. This additional recruitment brings in fast, fatiguable motor units. Consequently, fatigue will be accelerated toward the end of long or severe bouts due to the increased lactate produced by the late recruitment of fast units.

Specific athletic groups may differ in the control of the motor units. Top athletes in the explosive sports like Olympic weightlifting or the high jump appear to have the ability to recruit nearly all of their motor units in a simultaneous or synchronous fashion. In contrast, the firing pattern of endurance athletes becomes more asynchronous. During continuous contractions, some units are firing while others recover, providing a built in recovery period. Inital gains in strength associated with a weight training program are due to improved recruitment, not muscle hypertrophy.

I’m just failing to understand what is being stated in the bold red writing above.

Is it initiating the way to go, is too perform light/moderate weights x 100 reps per set? as opposed to heavy/low reps for recruitment. It states: “Fast units will be recruited as the brain attempts to maintain desired force production by recruiting more motor units as slow units become fatigued and fail to produce force”. Does this suggest high work/volume (high reps/moderate weight) is better?.

Another really interesting article…

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.

Any thoughts???..

generally speaking:
12+ rep range= musclular endurance
8-12= hypertrophy (cross section)
1-5=strength ( Recruitment)

Methods of compensatory accleration= discharge rate

Please note this has much variance among individuals.

With regards to your question of whats best? well depends on the goal…but surel Max strength (low reps) is the ebst way to go for recruitment.

Right. But what is the difference between: 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.

What is the importance?. Will fatiguing a lot of MUs have a better effect on sprinting than recruiting a lot of MUs via heavy loads?.

What are the benefits of fatiguing?.

“But what is the difference between: 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”

the difference is waht you just stated. heavy weights recruit, mod weights fatigue (with enough volume.)

Your next question is a complicated one. Because it is a question relative to the individual.

generally speaking however, it is best to work on cross section through moderate weights as a developing athlete. Once cross section requirements are met shift to a focus more on Max strength. Of course in either case there will be work in both zones at certain time periods.

Additionally, when you think about weights with respect to sprinting, think more about stimulation than fatiguing.

The question I should have asked was: Does fatiguing gain an individual any additional benefits to training (Elastic strength, Explosiveness, CNS Stimulation etc etc) instead of recruiting a lot of MUs via lifting heavy loads.

I think what I’m suggesting is what Charlie suggests for core work. But I’m referring to extremities.

Also most texts state that the gluteus maximus are a 60-68% slow-twitch muscle. This lends support to going moderate for high reps???. However, as slow units become fatigued and fail to produce force, fast units will be recruited as the brain attempts to maintain desired force production by recruiting more motor units. Consequently, the same force production in fatigued muscle will require a greater number of motor units.

Thanks for the responses.

Depends on the definition of fatigue.

but anyway to answer the question: fatiguing work results in a loss of elastic strength, explosiveness, CNS stimulation.

Also not sure what text you have that says the glutes are 60-80 percent slow twitch…even if it were it is too variable within each person to go by…even if you take a biopsy of the muscle it cud come up all slow twitch but if you moved over a centimeter it could come out all fast twitch.

More recent research shows that elite athletes can preferentially recruit fast twitch anyway… so balls to this very basic introduction.

I’ve just realized, guys like Bruce Lee, Mas Oyama, William Cheung etc put a lot of emphasis on fatigue training as opposed to going Heavy/Low Reps to develop there insane speed. In fact Bruce Lee very rarely went heavy.

Grandmaster William Cheung was said to throw 3,500 punch repetitions with resistance in each hand per workout. Hes holds the speed record (8.3 punches per second)/(10 punches per 1.1 second).

Wonder how 10 punches per second could relate to stride rate through this method of training?.

These guys are obviously some of the powerful guys of all time, so I think fatiguing could have a place in a program.

of course there is a place for fatiguing work. BUT fatiguing work will decrease power output in all but beginners, just becasue the MUs are being recruited, doesnt mean they are being recruited in the right way. You have to think how they are being recruited and what adaptations it will cause.

“Wonder how 10 punches per second could relate to stride rate through this method of training?.”

talk to youngy

Topcat any chance you could post a reference or 2 regarding the preferential recruitment of FT muscle in elite athletes, sounds interesting.

Ive seen a few…look though pubmed central or highwire

isnt this a process thats taken advantage of when doing depletion sets such as press ups?

Speedster, Looking through a lot of the training journals people keep updating, the trend seems to be no fatiguing work at all. But…

If you happened to include fatiguing work in your program (which you probably don’t), would you schedule it before? or after? heavy work. Or would it done on separate days?.

Take Upright rows just as an example for delts. Doesn’t matter if its a good or bad exercise. Would it be better to do fatiguing sets first then progress to heavier sets?, or vice-versa?

Would you happen to know…?

It depends on your goal(s)/what your are training for.

If you follow the size principle and want to recruit the highest amount and largest MUs, than fatiguing sets should precede heavy sets, in order to fatigue the medium sized MUs and thus be able to recruit the largest MUs on the final sets with a quasi-maximal weight, of course all in the same exercise (ex. Sets of 5 with Sub-Max weight leading to a ~90% attempt will stimulate the highest threshold/largest MUs).

Zatisorsky is against pyramid training for this reason if your goal is to increase intramuscular coordination. If your purely training for hypertrophy, than it really doesn’t matter.

But then again elite athletes are known to recruit out of order and skip right to the high level MUs.

When I use the word ‘fatiguing’ that doesn’t mean ‘failure’ but rather, as Verkhoshanksy puts it, ‘volitional fatigue’.

Get your hands on Science and Practice of Strength Training, all of your questions will be answered.