#29: How Do Muscles Contract? Type I and Type II Muscle Fibers

#29: How Do Muscles Contract? Type I and Type II Muscle Fibers

When our muscles contract, our bodies move through space and time. When our muscles contract, our bodies are kept warm. When our muscles contract, our bodies are pumped with more nutrient rich blood. When our muscles contract, life happens!

The PowerDot Smart Muscle Stimulator makes life happen by using Neuromuscular Electrical Stimulation (NMES) to recruit your muscles to contract.

The Anatomy of Muscle

Let’s take a look at the anatomy of skeletal and how it all works. At a macroscopic level, our muscles are just dense bundles of fibrous tissue wrapped in connective tissue called fascia. But when we take a deeper look at the anatomy of our muscles we notice that our muscles have layers (Like Shrek… Ogres are like onions, they have layers).

Starting at the outermost layer, the entire muscle is surrounded by epimysium (if you see -mysium we’re talking fascia). Going a layer deeper we notice that the entire muscle is comprised of fascicles that are wrapped in perimysium. Going deeper yet again, the fascicles contain several muscle fibers or muscle cells wrapped in endomysium. And last but not least, at the most microscopic level, our muscle fibers contain myofibrils which is where contraction happens.

How Muscles Contract: Excitation Contraction Coupling and the Sliding Filament Theory

Josh Bridges workout

The myofibril is where our contractile proteins are located, actin and myosin. These contractile protein filaments make-up our sarcomere which is known as the basic contractile component of skeletal muscle. Because without a sarcomere, or any actin and myosin, no contraction will occur.

But we also need to remember that there is more to contraction than just the elements that make-up our skeletal muscle.

The brain and spinal cord are going to regulate the signals that trigger a contraction. Just like without a sign or signal from the coach, no play happens on the field...the same goes for muscle contraction.

An alpha-motor neuron in the spinal cord is going to send an electrical signal called an action potential down a chain called an axon to the muscle fibers. The alpha-motor neuron and all the muscle fibers it innervates is called a “motor unit”. Because we never just contract one muscle fiber.

So, the signal travels down the axon where it reaches the site of communication between the alpha-motor neuron and the muscle fiber known as the neuromuscular junction.

A neurotransmitter, acetylcholine, now crosses the synaptic cleft and binds to receptors on the muscle cell which continues the electrical signal (action potential) in muscle fiber. This electrical signal causes calcium ions to be released from the sarcoplasmic reticulum (starting to see how a muscle fiber is a muscle cell?), resulting in actin and myosin being bound together with myosin cocked back like a pitcher ready to deliver a pitch.

To cause contraction, or sliding of the (contractile) filaments, energy is released from Adenosine Triphosphate (ATP) and the myosin pulls on the actin which shortens the entire muscle. Myosin will then release and cock back to attach to another actin. This will happen over and over again. The speed at which myosin attaches and pulls on each actin over and over again is called the cycling rate.

So, without the excitation of the electrical signal, there will be no contraction or sliding filaments.

Type I and Type II Muscle Fibers

All muscles have the same anatomy and go through the same sequence of receiving a signal resulting in a contraction. But all of our muscles are comprised of Type I and Type II fibers. These muscle fibers are your “slow twitch” Type I fibers and “Fast Twitch” Type II fibers.

Yes, no muscle is just a Type I or Type II muscle. And actually, our lower body muscles have a higher percentage of Type I fibers and our upper body muscles have a high percentage of type II fibers.

This makes sense because our Type I fiber are more fatigue resistant where our Type II fibers are more powerful. Back in the day, we use to walk long distances. For instance, Roman soldiers had to march about 30 kilometers in a day (not many of us walk on our hands for long distances). So our lower body was designed to fight fatigue while our upper body was designed to be more forceful and powerful.

So what makes Type I fibers slow twitch and Type II fibers fast twitch? It all goes back to what we just talked about above.

We’ve all heard the phrase, “bigger, faster, stronger”. Well the same rings true for our muscle fibers. Type II alpha-motor neurons and muscle fibers are bigger, faster, and stronger than Type I fibers. Type II fibers also break down ATP faster, which increases the cycling rate of contraction. Lastly, calcium is released from the sarcoplasmic reticulum faster in Type II fibers, which coupled with the faster ATP breakdown, makes the contraction even faster.

Type II fibers demonstrate physiologically to be used for strength and power whereas our Type I fiber, though produce less force, are more fatigue resistant and have more stamina.

So, how does Type I and Type II fiber recruitment work? This has to do with what exercise scientists call the “Size Principle” and “Principle of Orderly Recruitment”.

The Size Principle tells us that motor units (the alpha-motor neuron and all the muscle fibers it innervates) are recruited from smallest to largest. Since we know that Type II fibers are bigger, faster, and stronger, we can conclude that Type I fibers will be activated first, followed by Type II fibers. So, there is an order to fiber type recruitment.

We can further classify our motor units into “low threshold” and “high threshold” categories. Motor units for Type I fibers are classified as “low threshold” motor units meaning they have a lower activation threshold making it easier for them to fire. All the while, motor units for Type II fibers can be further classified as “high threshold” motor units as they have a higher activation threshold that must be reached in order for them to activate.

Let’s look at an example putting together the concepts of the Size Principle and Principle of Orderly Recruitment with low and high activation thresholds.

It would not be very efficient of your body to use your big and strong Type II fibers to pick-up our cell phones. Cell phones are light and require little amounts of force, so your lower threshold Type I fibers will be activated. However, if you are trying to deadlift 500 pounds (which takes a lot of force), your body will first start to recruit your Type I fibers and quickly realize that you need to use your big, high force output, Type II fibers.

Type I = Fatigue Resistant

Type II = More Forceful Contraction!

How NMES Stimulates and Trains Both Fiber Types

The PowerDot NMES Smart Technology device is designed to specifically target both Type I and Type II fibers.

Josh Bridges using PowerDot

How does it do this?

It basically acts as an alpha-motor neuron. Remember, the alpha-motor neuron sends an electrical signal from the spinal cord to the muscles. Your PowerDot is able to send a similar signal like that which results in muscle contraction.

But there are way more benefits with NMES!

During a typical voluntary muscle contraction, the Size Principle is in effect, in which our body will recruit smaller and slower motor units and then if needed will activate our larger faster motor units. This is not the case with NMES.

NMES takes a “nonselective approach”, activating both fiber types at the same time! It has also been proposed that artificial-activation of skeletal muscle using NMES may actually do a reversal of the Size Principle and activate larger motor units first. All this meaning that both Type I and Type II fibers are activated while your PowerDot is on.

This adds an additional stimulus to your training! Typically, we perform endurance training to improve the function of our Type I fibers and heavy resistance training to improve the function of our Type II fibers. It is possible to see adaptations to both fiber types when adding NMES to your training.

NMES was added to an Elite Weightlifter’s training program and within just two-weeks, major gains were made in the athlete’s snatch, clean and jerk, and front squat. For instance, his front squat increased by 20 kg within the first week of adding NMES. On top of that, his Type I fiber size increased. So you can see where adding NMES is beneficial for both fiber types.

Researchers have also explored how adding NMES 3 days per week for 6-weeks improves quadriceps strength. What they did though was stimulate one leg with NMES during training and the other leg acted as a control, meaning one leg received NMES and the other leg did not. So, these participants started with similar strength in both legs and by the end of the 6-weeks, the leg that received NMES increased in strength by 24% and the control leg by only 10%.

Physiologically, how does this work?

The cause behind the increase in muscle size and strength may be due to both low and high-frequency NMES upregulating a key anabolic signaling pathway. Don’t worry, this is an all natural anabolic signaling pathway that promotes muscle repair and growth thus in turn leading to improvements in muscle size and strength.

Take advantage of the scientifically designed programs already created for you in your PowerDot App to make the most of your training activating both fibers types which may help to augment and improve both endurance and strength performance thus enhancing overall human performance.



Joshua D. Dexheimer, PhD, CSCS, USAW, PES

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