If you’ve spent time around fitness people before, then you’ve probably heard the word
“activated”. It just means that you’re contracting a specific muscle. So someone might tell you to
“activate your core during this squat”, or “activate your glutes during a step up”.
If you wanted to check for yourself whether a muscle was activated or not,
you might poke the muscle of interest. If it’s firm, it’s probably contracted and thus activated.
But there are all kinds of situations where you’d need way more detail about which muscles you’re
contracting. Maybe you’re a physical therapist designing a new exercise and want a more nuanced
idea of which muscles it works. Or maybe you want to study something more precise like how
a mouse will affect someone’s wrist muscles. And that’s where electromyography comes in, or EMG.
It’s a technology that uses the electrical activity of muscles to tell us what muscles
are activated at any given time. It’s like how an ECG measures the electrical activity of the heart.
EMG measures the electrical activity of muscles.
So today, we’re going to look at some of the ways that clinicians and scientists can tell
that muscles are activated, and how we can use EMG studies in the real world.
Hello and welcome. If you’re new here,
my name is Patrick and this channel is all about anatomy and physiology.
Alright, from the get go, EMGs are overkill for your average fitness gym. The old look and poke method is
good enough if you need something quick and dirty. Here in the US, licensed physical therapists are
allowed to perform EMG tests and might use it in certain circumstances like testing how quickly
someone’s nerves conduct electrical signals. But if they’re just checking muscle activation
during exercise, they have plenty of other manual skills and low-tech tools in their tool belts . So
while they use the results from EMG studies, the EMGs themselves are usually conducted in
the academic or neurological settings. At its core, electromyography measures
electricity. Specifically, the electricity going from our nerves to our muscles.
Those measurements are compiled and recorded into a readout called an electromyogram.
See, when you’re at rest, your muscles don’t have much electrical current to measure,
but once you decide to contract a muscle, an electrical impulse starts at your brain, travels
through the central and peripheral nerves, and ends at a motor unit, which is made up of a motor
neuron and the muscle it innervates. This is where your nervous system meets your muscular system.
And the more forcefully you try to contract a muscle, like when you lift a heavy weight,
the more of these motor units you’ll recruit, which is what you’ll see on the electromyogram.
EMGs typically have time on the X axis and amplitude on the Y axis. The amplitude of
the wave matches up with activation of the muscle. So the graph will look like a flat line when the
muscle is at rest, a big wave when the muscle is flexing hard, and everything in between.
But different types of EMGs measure this electrical activity in different ways.
The most common type that you might see is a surface EMG.
It uses adhesive electrodes on the skin that are connected to a monitor to measure
the difference in voltage between the two electrodes on the skin. This style is more
appropriate on superficial muscles like this one on someone’s hypothenar mass.
There are also inserted EMGs which use a needle or wire stuck into the skin. That lets the user
analyze deeper or more delicate muscles that might be overpowered by the bulkier muscles during
surface EMGs. These things are mostly used for neurological studies, and sometimes in PT clinics.
One field that uses EMG extensively is the field of biomechanics. To show you how they use it,
I’m gonna walk you through a study which I’ve linked down in the description along
with all my other sources if you want to follow along. Here we go.
Researchers from Norway published an article in the Journal of Sports Science and Medicine in
2020 comparing two classic chest exercises: the barbell bench press and dumbbell chest flies.
From a biomechanics perspective, these are super different movements. The chest
fly is a single joint exercise while the bench press is a multi joint exercise.
And importantly for this study, the chest fly places a different resistance on the pecs in
different phases of the movement thanks to the changing arm position. The resistance is lowest
when the weights are straight above the lifter, and is highest when the weights are out to the
sides. It has to do with torque and moment arms — topics we’ll save for another video.
But that variable resistance made the researchers curious — which exercise leads to greater
activation of the pecs? Maybe they’d see a big, steady activation of the pec major during the
bench press but one really big spike in activation during the lowest part of the chest fly movement.
An EMG study is perfect for this kind of thing. Just hook
up electrodes and compare the graphs. So they recruited 17 men in their 20s,
all of which had years of weight training experience. These were fit guys.
The researchers hooked electrodes to four muscles: the pec major,
anterior fibers of the deltoid, the triceps brachii and biceps brachii, then they had the
participants do 6 repetitions of both exercises. As expected they found that bench press led to
higher activation of the pecs, deltoids, and triceps throughout the entirety of the motion.
Meanwhile, the pec fly saw a much higher activation of the biceps than the chest press
since the bicep needs to maintain elbow position against gravity in the fly but not the bench.
So that answered one of their questions — bench press stresses the pec muscles the most. But they
did something clever. During the 5th repetition of each exercise, they took snapshots of muscle
activation during the upper, middle, and lower phase of each motion for each muscle.
They also compared the ascending and descending parts of the motion.
So Throughout the entire exercise,
the bench press led to a 16% greater mean activation of the pec major over the fly.
When we broke it down by phase though, the bench press elicited more pec activation than the fly
in the upper, middle, and lower phases of the descending portion,.
But in the ascending portion of the movement, the bench and the fly were mostly
similar — the biggest difference was greater pec activation in the upper phase during the bench.
That makes sense since that’s when the resistance is lowest in the chest fly.
All that taken together, the authors concluded that people should train with bench press instead
of chest flies if they’re trying to train at the highest mechanical stress possible.
That doesn’t mean that chest flies are garbage and nobody should use them. An EMG study like this one
is meant to be just one piece of information you can use when designing exercise programs.
Now, my background is in kinesiology and physiology,
but EMG technology is being used in so many cool, and downright futuristic applications.
My Youtube buddy Milan is literally studying EMGs for his degree right now,
and his university is doing some of that cutting edge research. If you want an even
more in-depth explanation of how EMGs work plus some applications I’d never heard of before,
go check out the video he did on his channel. I’ll link to it up here and in the description.
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