Analogies for Understanding: Origins of Strength
By
Dustin Kuhl
Motor units, rate coding, fast-twitch muscle fibers, recruitment capacity—do
you have this all down? Do you understand what is going on in your body when you
are lifting? Sometimes it feels like you need a degree just to figure out what’s
going on with your body. Most people have an understanding that a bigger muscle,
all things being equal, is a stronger muscle. This isn’t always the case due to
one major factor of strength potential—the neural system.
To help you understand this often overlooked aspect of strength, I’m going to
take a page (actually a few pages) from Zatsiorsky and Kraemer’s Science and
Practice of Strength Training to explain these neural factors. Because
throwing various terms and definitions might only serve to confuse, I’m
including some analogies to help with understanding.
Intramuscular coordination
Muscle strength through intramuscular coordination comes from three different
factors—recruitment, rate coding, and synchronization. All of these are
accomplished with the use of motor units. To put it the fancy way, a motor unit
is comprised of a motoneuron and the muscle fibers that it innervates. An easier
way to think of this is a light switch and lights. The motoneuron (light switch)
turns on or off the muscle fibers (lights). Just like a light switch can control
one light or multiple lights, a motoneuron controls a various amount of muscle
fibers.
There are many muscle fibers within each muscle, but the number of muscle
fibers isn’t necessarily related to how many motoneurons there are controlling
them. One might control 100 fibers while another controls 1,000. The determining
factor for this is the level of control needed. Controlling finger movements
uses more motoneurons than bending your leg due to the small, accurate movements
needed. To continue with the light switch analogy, think of a gym or office
level floor. There will only be a handful of light switches to turn on all the
lights (few motoneurons controlling many fibers). Contrast this with a
photographer’s studio where certain individual lights are turned on or off to
get the correct lighting (many motoneurons controlling few fibers).
Now that we know the basics of intramuscular coordination, let’s look at the
three main factors.
Motor unit recruitment
To understand recruitment, we need to build on our knowledge of motor units.
There are two types of motor units (for our sake of explanation)—slow twitch and
fast twitch. Slow motor units are characterized by having low force and high
endurance muscle fibers. I bet you can guess what the characteristics of the
fast motor unit are. You got it—high force and low endurance muscle fibers.
Examples of these are usually a sprinter and a marathon runner. The sprinter has
speed but can’t sustain that effort for long because fast twitch motor units are
used. The marathoner can sustain his effort for a very long time because the
speed is lower and he’s using the slow twitch motor units.
Time for another analogy. This time we’re going to use a platoon of soldiers,
where each soldier represents a muscle fiber. Some in the platoon have a M16
while the rest are carrying around a bazooka. Both of these have a different
amount of “killing power” (yeah, that’s the term I’m using just because it
sounds cool). The M16 is like our slow twitch muscle fibers You have a good
amount of ammo (high endurance), but each individual shot has a fairly low
killing power (low force). While on the other hand, the bazooka gets one shot
(low endurance) and you’re out! The killing power could take out a tank (high
force) so it’s worth the trade off.
This combination of firepower in the platoon makes it well-rounded and ready
for anything that comes its way, just like in our muscle with its combination of
slow and fast twitch fibers. If all you’re trying to do is pick a book off of a
table, your body is just going to call on the slower ones to pick it up. Going
for that new max in the deadlift is another story, and the faster ones will get
recruited.
Slow-twich M16 motor unit
Fast-twitch bazooka motor unit
This brings up the size principle. Recruitment will always start at the
smaller, low force motor units, and as more strength is needed, the body will
start recruiting the stronger ones. This will always be the recruitment pattern
with high force motor units not being recruited until the lower force ones are.
The same thing happens if the opposing enemy force just sent half a dozen enemy
soldiers. Those slow twitch M16’s can take care of that. But wait a second. Now
there are a couple tanks coming too. Time to recruit those fast twitch bazookas
to handle that.
Now, if there were 200 tanks along with enemy soldiers coming over the hill,
it would be too much to contend with. Some could be taken out, but after awhile,
the bazookas would be out of ammo and the M16’s wouldn’t stand a chance. The
same applies when you try a 100-lb PR. Even though you have your slow twitch and
fast twitch fibers recruited, all the force that you’re applying can’t overcome
the raw weight. The total recruitment and “killing force” that you’re trying to
use to lift that weight just isn’t enough.
The same thing happens when you’re trying to lift a near max weight multiple
times. The slow twitch fibers are recruited and then the fast twitch, but due to
the low endurance capability of them, they “run out of ammo” and the set comes
to an end whether you like it or not. Lowering the weight allows more reps to be
done. However, the lower weight and force needed to lift it, the less the fast
twitch is used and the more you rely on slow twitch. That’s not what we want to
do if we want to get stronger.
So, now that we have the basics of motor unit recruitment and the size
principle down, how do we get stronger from this? First, understand that total
motor unit recruitment can’t be done voluntarily. If you have total recruitment
with no golgi tendon inhibition, it’s likely you would rip the muscle tendon
right off the bone. Next, motor unit recruitment in beginning trainers is fairly
low, commonly thought to be no more than half. Lucky for us, training enhances
this.
Going back to our platoon analogy, a beginner trainee is like a beginner
platoon that is still learning how to work together as a team. There are a
hundred soldiers—fifty of the slow twitch M16 fibers and 50 of the fast twitch
bazooka ones. The general yells, “fire!” and what happens? Well, only twenty
M16’s and bazooka’s shoot off while the rest are picking their nose, looking up
at the clouds, and trying to see how long they can stand on one foot. This is
much less than the total “killing force” they are capable of and illustrates the
beginning trainee’s muscle recruitment and strength. Not much at all.
With continued lifting, the fibers become better at becoming recruited and
strength will increase. This can happen without any increase in the size of the
muscle. All that is happening is more of the muscle fibers that are already
present are now being used. This is one of the factors strength athletes use to
stay in their weight class. Instead of increasing muscle size to get stronger,
the neural system is focused on.
So we return now to our war hardened platoon after years of combat. All that
experience has paid off and they are able to demonstrate the full force they are
capable of. The general doesn’t even need to finish yelling “fire” at this
point, and the soldiers are able to unleash a fury of M16 and bazooka fire
against their enemies that would make Rambo proud. The trainee follows suit.
After years of experience under the bar, he can now demonstrate a high level of
motor unit recruitment, lifting a bar-bending weight that seems unlikely for his
size.
Rate coding
Rate coding is the second factor in strength production in muscle. Where
recruitment is more about how many fibers are recruited, rate coding is more
about “turning them on” and the firing rate of the motor unit. While it seems
like when you’re lifting a weight, your muscle flexes and turns on the entire
time, that’s not really the case. There is a certain amount of the force against
the weight when you’re lifting it, which is greater than the bar weight, and
then when you lower it, the force has to be slightly less than bar weight.
Think of doing a curl. If your muscle just turned on full tilt, your muscle
would flex and bring up the bar. The only way to bring it down would be to turn
off the muscle and it would fall uncontrolled. That’s not what happens though.
There is a gradual application and relaxation of force, which allows for a
smooth movement and even allows for holding the weight steady. (Not so smooth
and steady if you’re the frat boy grunting and swinging the bar during curls
with ten pounds on the bar while your frat buddy is telling you how sweet your
‘guns’ will look in the club that night.) You can control how fast or slow the
bar goes. This different force being applied by your muscle is due to motor unit
recruitment (talked about above) but is mostly due to rate coding.
Motor unit on!
With a light switch, you have an on or off position. “On” turns on the light
(your motor unit), and “off” turns off the light (as well as your motor unit).
But to get that difference of force from your motor units, we need to think of
it more like a strobe light that has numerous pulses. Fewer strobe light pulses
means less light going out and less force applied by a motor unit. Lifting a
lighter weight gives less “pulses,” lower rate coding, and a lighter application
of force. Lifting that light weight faster results in higher rate coding, just
like lifting a heavier weight will. So with those max attempt lifts, our “strobe
light” is flashing so quickly it almost looks like a light switch that is turned
on, shining blinding light as that new PR is reached.
Synchronization
Not a lot for this one. First off, synchronization basically means “working
at the same time.” In our case, it’s motor units that are working at the same
time. So, remember that motor units control muscle fibers and those muscle
fibers control the force. In trained athletes, motor units can work
synchronously to produce more force. Going back to our platoon analogy, when the
general yells “fire,” this means multiple platoons fire at the same time. So we
have a synchronous multi-platoon “killing force” (I just had to use it again)
unmatched by any singular platoon. Obviously, two platoons have more force than
one, just like two motor units will be able to recruit more muscle fibers and
generate more force than only one. This is the synchronization piece of the
puzzle to strength.
Intermuscular coordination
We just finished up the three factors in intramuscular coordination. The
“intra” factors talk about what is happening inside the muscle and the “inter”
factor is what is happening around the muscle. Any movement isn’t completely
isolated and involves multiple muscle groups. Something like a squat involves
much more than a curl, but even a curl requires stabilization and isometric
action by your body.
This concept can be explained by talking about two different ways to try to
increase your bench press. While benching, there are many muscle groups being
used, but the main ones are the chest, shoulders, and triceps. One training
routine focuses on benching to increase the bench (novel concept, huh!) while
the other routine focuses on cable crossovers, side raises, and triceps
extensions. Both routines are working the muscles involved in the bench. Only
one is working the muscles together that are involved in the bench. This is the
basis of intermuscular coordination—getting all the muscles involved to work
together at the same time. The better they all work together, the smoother and
easier the movement will become. Our platoon of soldiers, along with some air
support and tanks, will be able to overtake the enemy easier than if they were
alone.
So, there you have it. Didn’t realize all of that was actually going on when
you lifted weights, huh? Hopefully, you now realize that there are multiple
factors involved in strength, and I hope you understand a little more about how
they come into play. The most important factor isn’t just the size of a muscle
but how that muscle works. Happy training!
Dustin Kuhl is an office worker by day and weightlifter by night living in
Michigan. He has competed in the USAPL at both the state and national levels.
Judging by this article, he has watched a few too many war movies. He is always
willing to help others who want to learn to achieve their goals in training.
Dustin can be contacted at
dustinkuhl@gmail.com.
Elite Fitness Systems strives to be a recognized leader in the strength
training industry by providing the highest quality strength training products
and services while providing the highest level of customer service in the
industry. For the best training equipment, information, and accessories, visit
us at www.EliteFTS.com.
