Types of Skeletal Muscles : Slow Twitch vs Fast Twitch

Ever wondered while curling the dumbbells, what kind of muscle was getting deployed to aid that movement? Lay aside bicep curls, how many of us verily knew that even muscles could be of different types? The overall classification isn't just based on a single parameter but a bunch of characteristics (size, color, speed, force, energy holding capacity) pertaining to muscle fiber. Keep reading and today we shall concern ourselves with our muscles in particular. 

The Muscular System : Muscle Types

The muscular system has been long interpreted as a combination of 3 different types of muscles.

• Cardiac Muscles : Involuntary muscle that comprises of your heart.
• Smooth Muscles : Involuntary muscle that builds up the organ system excluding the heart.
• Skeletal Muscles : Voluntarily controlled muscle distributed across rest of the body.

For this discussion we shall limit ourselves to the skeletal muscles and look at their composition in detail. 

Types of Skeletal Muscle Fibers

Thousands of parallel thread like structures called myofibrils together form a muscle cell. A group of such muscle cells forms a muscle fiber. We shall pick up our discussion from here once again. We pause here to have a look at how these fascicles or muscle fibers are broadly classified into 2 categories :

• Slow Twitch Muscle Fiber or Type 1 Muscle Fiber
• Fast  Twitch Muscle Fiber or Type 2 Muscle Fiber
• Type 2-A Muscle Fiber
• Type 2-X Muscle Fiber

I hope the tabular resemblance of the respective differences was self explanatory. This comparison can be better understood by the Henneman's size principle of motor unit recruitment. Don't worry this isn't something as perplexing as it looks to be. To understand this better let me start with a facile example. Consider a construction contractor hiring 10 laborers for a particular assignment. 

Say, he needs a water container to be shifted to the construction site. What would he do ideally? You guessed it! He would order laborer A to get this done.

What if the size of the water container was even bigger. Laborer A and B together could do this. As simple as that, isn't it? 

I was just wondering what would happen if they needed to move a container equivalent to the size of a water tanker. Eventually, the builder would recruit A,B,C,D,E and maybe F to get the work done. 

He still has more options left in case the container gets heavier but what's the learning ultimately? Try relating this to your muscle system, when you curl a 20 Lbs dumbbell, you recruit considerably lesser number of muscle fibers of the arm than when you curl a 30 Lbs dumbbell.

Let's catch back to the point where we switched the discussion. We were talking about muscle fibers or fascicles. We haven't yet found out what makes your muscles move or say contract. To be able to understand what I am about to tell you, you need to get acquainted with two new terminologies first. 

• Motor Neuron : A nerve cell located in the spinal cord that supplies nerve connections to the                              skeletal muscles.
• Motor Unit : A connection between a motor neuron and the muscle fibers it connects to.

You should also know that since there are lesser number of motor neurons in our body than the number of muscle fibers, one motor neuron takes up the responsibility to connect with multiple muscle fibers together at the same time.  

The All or None Rule of Motor Unit Contraction

According to this rule, there is no partial communication between a motor neuron and its motor units. If a motor unit contracts, all the associated muscle fibers get recruited. Take an example. If you lift a 15 Lbs dumbbell, your motor neuron will generate a signal for the activation of one or more motor units depending on the energy needed to match the threshold. Lets say motor units A and B get recruited to get this done. On the contrary, if you lift a 2.5 Lbs dumbbell, a smaller motor unit will be recruited. This time maybe a motor unit C which is less powerful than A and B should suffice . If you thought your body would recruit a portion of motor unit A and get the work done, unluckily you would be wrong. Partial recruitment of a motor unit is impractical. Relate this to the contractor laborer example I mentioned earlier. How do we apply the all or none rule to the laborers? Just by putting a constraint that no laborer more powerful can be recruited until some lesser powerful laborer capable of doing the work is available at hand.

This takes us to the Hanneman's size principle.

The Henneman's Size Principle

Not all motor units can be of the same size. Imagine all the laborers holding the same capability and potential. Not practical, is it? How do we decide the size of any motor unit? Well, actually we don't decide that. It depends on the number of muscle fibers that particular motor unit is attached to. When lifting weights, our motor neuron triggers signal for the activation of motor units. You increase the weight, the motor neuron compensates the force required by recruiting more and more motor units until enough force is developed. But there's a limit to this. Upon having recruited all the motor units if you still keep increasing the load, what would happen? Nothing! Perhaps you could figure this out yourself by pondering upon why you can't curl the weight you dead-lift with. 

What do we conclude from this discussion? What I could figure out is that smaller the size of the motor unit, lower would be the threshold for its activation i.e slow twitch muscle fibers connected to smaller motor neurons would fire up right before the fast twitch muscle fibers that are connected to bigger motor units. So, can we not say that there's an order (Slow Twitch -> Fast Twitch) in which motor units or muscle fibers get recruited? Thankfully, even the Henneman's size principle states the same and I quote "Under load, motor units are recruited from smallest to largest. In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers." Moreover, this minimizes the amount of fatigue an organism experiences by using fatigue-resistant muscle fibers first and only using fatigable fibers when high forces are needed.

For those of my readers who would like to know what does the term 'Twitch' actually mean, a twitch is as simple as a sudden contraction and release of muscle fibers. Before finishing this off, I would take another example about smaller and larger motor units to ensure that the information being shared here gets conveyed fully. 

Our eyelids require rapid and fine movement but little strength and hence would have smaller motor units, each connected to an average of 3 to 4 slow twitch muscle fibers. On the contrary our arms have larger motor units connected to several hundreds of muscle fibers as they require more energy to operate. Can you now form your own opinion on what kind of muscle fibers in majority do your arms consist of?