How Dissecting a Muscle Fiber Reveals its Type

Muscle Phys Lab, SFSU 2021

As a trainer, you’ll learn all about the different muscle fiber types and their characteristics.

For example, Type 1 fibers are commonly referred to as endurance fibers. Their primary role is to resist fatigue and maintain muscle contraction for long durations of time. Type 1 fibers typically contain more mitochondria and are smaller in diameter than Type 2 fibers. Additionally, Type 1 fibers will appear reddish with higher levels of blood supply and oxygen, which is necessary to fuel its metabolism.

Type 2 fibers are usually larger and produce greater muscle force. Additionally, the rate of contraction is exponentially faster than Type 1 fibers. These muscle fibers are recruited during explosive, heavy, and high-intensity movements. While producing great amounts of force, they are highly fatigable.

Muscle fiber types vary within each muscle cell. No muscle is comprised solely of one or another, but instead a combination of the two (and intermediate fibers too, such as Type 2a and Type 2b, etc). This is necessary to allow all muscles to produce a variety of muscle fiber contractions depending on the task as well as the environment in which the movement is being utilized.

Extensive research has been conducted in an attempt to isolate fibers to better understand the relationship between exercise adaptation and muscle characteristics. The most common way to identify muscle fibers is through microscopy dyeing, a process that involves first dissecting a bundle of muscle fibers to isolate a single fiber. Then, the fiber is washed and rinsed with actin solution, dyed with a specific photon-sensitive dye, and placed under a specific microscope to identify its properties. This arduous process can take a very long time, so researchers will typically create hundreds of samples and freeze them for research at a further date.

At San Francisco State University, I have been working in the muscle physiology lab where we have been practicing muscle fiber isolation. The muscle fibers were donated from a subject’s Vastus Lateralis (VL) muscle, one of the 4 muscles of the quadriceps. This muscle is particularly superficial, making the biopsy fairly noninvasive. We then dissected individual fibers from the bundle of muscles and dyed them for further inspection under a light-sensitive Zeiss Confocal microscope.

The final process has not yet been completed. After developing ~ 100 slides, data will be collected and analyzed before being published.

This type of work is very common in muscle physiology labs to provide research-based evidence that certain exercise protocols produce specific adaptions. Hypertrophy, for instance, is commonly seen in Type 2a and Type 2x fibers from high intensity, heavy resistance training, while Type 1 fibers respond to periods of prolonged contraction with lower resistances.

Exercise adaptation is a response to the demands imposed by the workout. This principle is commonly known as Specific Adaptation to Imposed Demands (S.A.I.D. principle). Research has consistently backed this theory by demonstrating that a VL sample taken before and after a specific training regime will show muscle fiber changes as a direct response to the exercises included.

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Mathias Sorensen