Muscle Fiber Types


In this course Professor of Physiology at the University of Melbourne, Mark Hargreaves helps us explore muscle fiber types, by 2 main categories: slow-twitch and fast-twitch muscle fibers.


Exercise Physiology | Muscle Contraction | Muscle Fibers | Muscle Adaptations | Exercise Fuels | CHO Metabolism | Fat Metabolism | Oxygen Uptake | Cardiovascular Exercise | Respiratory Responses | VO2 Max | Temperature Regulation | Heat | Fluid Balance | Fatigue | Sprinting | Endurance | Genes | Practical Case Example


Muscle Physiology 


One aspect of muscle physiology that’s received considerable attention since the late ’60s and early ’70s when investigators started sampling pieces of human muscle using the biopsy technique, was the classification of muscle fiber types, and their relevance to exercise performance. The muscle biopsy technique was introduced by some Scandinavian researchers to exercise physiology research in the late ’60s. It involves taking a small piece of muscle using a specialized needle under local anesthesia. In this case, you can see the needle inserted into a muscle of the lower leg to extract a small piece of muscle. And you can see in this picture here, the muscle is removed from the needle. This piece of muscle can be mounted in special chemicals and analyzed for histochemistry. It can be frozen and later analyzed by chemically or using Immuno blocks to characterize the protein, concentrations of various key enzymes. This technique is widely used in exercise physiology research today to obtain pieces of muscle as part of research programs examining a whole range of aspects of exercise effects on muscle.

Sections 


  1. Muscle Fiber Types
  2. Human Muscle Fiber Type – Histochemistry
  3. Human Muscle Fiber Type – Immunoblotting
  4. Human Muscle Fiber Type Characteristics
  5. Muscle Fiber Recruitment During Exercise
  6. Muscle Fiber Types in Athletes

Muscle Fiber Types – Slow & Fast Twitch 


This exercise physiology education lecture module defines and explores muscle fiber types and characteristics, by two main categories, fast and slow-twitch type muscle fibers. Type 1 muscle fibers are slow-twitch and type 2 muscle fibers are fast-twitch. The lecture continues to explore how muscle fibers are classified and studied, fuel source changes in muscle during exercise, muscle recruitment during exercise, and muscle fiber types in athletes. The lecture closes with the question, “Are athletes born or are they made?”

Human Muscle Fiber Type – Histochemistry


In relation to muscle fiber types, there are two main categories, slow twitch, and fast-twitch. The fast-twitch fibers, in human muscle at least, can be further subdivided into two subcategories. And we’ll come back to that in a moment. The traditional way of typing these muscles is to mount a piece of muscle obtained from a muscle biopsy in a special medium, to then cut in a cryostat and then to stain for an important enzyme in muscle the Myosin ATPase. The activity of this enzyme is deferentially sensitive to acid or alkali depending on what fiber type it’s in. If you pre-incubated the muscle section in acids the type II or these fast-twitch fibers move the Myosin ATPase activity so that when you’re staining for that you see a dark stain. So, on the left-hand side of this with the acid pre-incubation and staining for Myosin ATPase, you can see the type I fibers of slow-twitch fibers. If you reverse the pre-incubation and do that in alkali and then stain for Myosin ATPase in the next section, you can see the reversal of the staining intensity. The fast-twitch fibers now stain darkly and the slow-twitch fibers have a less intense stain. And this was the way in which muscle fiber type distributions were measured and estimated in human samples for many years.

Human Muscle Fiber Type – Immunoblotting 


In recent years, with the development of antibodies to specific proteins in muscle. Another approach is to characterize muscle fibers using an immunoblotting approach with specific antibodies. In this image, we have five individual single fibers dissected from a muscle sample obtained from a muscle biopsy. They’re then blotted with antibodies to either the type I myosin heavy chain, or the type II myosin heavy chain. And you can see these three fibers have the type myosin heavy chain, so they are I fibers. And these two fibers have the myosin heavy chain II protein and they’re type II or fast-twitch fibers. You can also see the fiber type differences in another important protein that’s involved in muscle contraction, Troponin C. Troponin C is the protein that interacts with calcium and the acting in myosin filaments within a muscle. And you can see that there are fast and slow isoforms of this protein which are expressed in the appropriate fiber type manner.

Human Muscle Fiber Type Characteristics


If we characterize the fiber types that are found in human skeletal muscle as I said there are two main types, type I and type II and type II can be subdivided into 2a and 2x. It’s important to note that the differences between individual fibers in human skeletal muscle are much less than you see in rodents or other experimental animals that are being studied, where there are much larger extremes and bigger differences between the individual fiber types. The table here summarizes the differences in characteristics between these various fiber types. The alternate names for type I, you can see, are often referred to as the slow oxidative or the slow-twitch, the type 2a, the fast oxidative glycolytic, or type fast-twitch A. And in the past, the 2x fibers have often been referred to as fast-twitch B. In most limb skeletal muscles in a human, there is no 2b myosin heavy chain. It’s found in some animals and in some of the very fast contracting muscles of the eye. But in most limb muscles what used to be known as 2b is really the 2x myosin heavy chain isoform. You can see differences in the mechanical properties of these fiber types, the force and power output is higher in the type II fibers.

In contrast, the endurance capacity or the fatigability is the endurance capacity is high, the fatigability is less in the type I fibers. This relates in part to the mitochondrial differences and the supply of blood. So the type I fibers are more dependent on the oxidative energy system. The fiber areas, the type II fibers tend to be bigger than the type I fibers. If you look at the distribution roughly in an untrained Vastus Lateralis, a muscle of the thigh. It’s usually about 50/50. Fast-twitch, slow twitch. Then, obviously, there are individual differences.

Muscle Fiber Recruitment During Exercise


The functional significance of these different fiber types plays out when you look at how they’re utilized during exercise. If we look at how muscle fibers are recruited to contribute to the increased force requirements as we move from light exercise to moderate-intensity exercise to very intense exercise. This is really governed by what we know as the size principle. Those motor neurons, that innovate, that are the smallest, and innovate the slow-twitch fibers, are activated first. So at the lower exercise intensities, we have primary lines on the slow-twitch fibers. As we increase the exercise intensity and the force requirements of the activity we increase the recruitment of slow-twitch fibers so that more of those fibers are activated. But then we also increase the recruitment of the 2a fibers initially. As we approach maximal levels, we start to recruit the 2x fibers whereas in this graph they’re designated as the 2b fibers.

One of the early ways that were utilized to assess the recruitment of fiber types, was to measure the glycogen levels. This was done using histochemistry. So, you would characterize the fibers, according to their myosin ATPase, using their acid and alkaline pre-incubation, and then you would stain the fiber with a chemical known as PAS, which would stain the glycogen content. Then you could measure that at various times during exercise. You can see here the change in glycogen within these individual fiber types. You can also note that the numbers of fibers that were counted and what should be readily apparent are how labor-intensive this was in the early days. Fortunately, in more recent times there are computer software packages that scan images and make this a much more efficient task. But what you can see from this rather complex slide is that as you move from low to medium to high, you start to rely more on the greater proportion of the slow-twitch fibers and more of the fast-twitch fibers. So this, in human exercise physiology, was the method used for many years to see which muscle fibers were recruited during different types of exercise, different modalities, different intensities, different durations, there’s a considerable literature that looks at muscle fiber recruitment during exercise.

Muscle Fiber Types in Athletes


Another interesting aspect of muscle fiber types is their relevance to sports performance. Not surprisingly, the suggestion is that endurance athletes who have to exercise for prolonged periods at relatively lower intensities, and resist fatigue, might have more slow-twitch fibers. In contrast, sprinters who have to generate very high forces and only have to do so for a relatively short period might have more fast-twitch fibers. There’s been a lot of debate as to whether fiber types can be transitioned, changed as a result of different types of exercise training. If you look at the studies that have looked cross-sectionally at the fiber type characteristics of various athletic groups, we do see differences between groups of athletes.

Here is a muscle section obtained from the leg muscle, the lower leg muscle of Frank Shorter, who was a very successful marathon runner, an Olympic Games gold and silver medalist in the marathon. He was studied in the midst ’70s by a group of researchers in the United States led by Professor David Costill. David has generously supplied this picture which shows the cross-section of front shoulder muscle pre-incubated in acid so that the slow-twitch fibers stain dark, and yes you can see here, it’s readily apparent, that Frank Shorter’s muscle had a lot of slow-twitch fibers.

If you look at fiber type distribution in various athletic groups, and this has been done by a number of investigators in the early to mid-’70s, you can see this fiber type distribution. In untrained subjects, roughly 50% or so of slow-twitch fibers. This is higher in endurance athletes but lower in the sprinters. So, the sprinters have many more fast-twitch fibers, and the endurance athletes have many more slow-twitch fibers. One of two questions to be asked is, Are these athletes born this way and therefore have a natural predisposition for these various athletic disciplines? Or as a result of training, have there been adaptations and transformations within a muscle that explain this? It’s probably a bit of both, although it’s thought there is a large genetic component to fiber type distribution. Having said that, prolonged endurance-type exercise has been shown to involve some fiber type switching and transition from 2a fibers towards type I fibers certainly in metabolic characteristics if not in terms of their contractile characteristics.

An interesting aspect and an interesting study involved looking at different types of athletes in different events. And what they did here was they looked at the slow-twitch fiber in leg muscles, the Vastus Lateralis, and compared it with an arm muscle, the deltoid. And you can see in runners, they had relatively greater numbers of slow-twitch fibers in their legs. On the other hand, kayakers, who heavily involve their upper limb muscles including the deltoid, had relatively more slow-twitch fibers in their deltoid muscles. So this might suggest that over time, there are adaptations within various muscle groups that are specifically recruited during specific training, which might induce adaptation in fiber types. Equally, you might argue that the success of this particular athlete either in kayaking or in running was a consequence of their natural fiber type composition. In our last module in this course, we’re going to examine some of the aspects of genes in athletic performance and ponder, are athletes born or are they made? And one of the questions that apply to is clearly muscle fiber type.[3].


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    Muscle Fiber Types was last modified: October 12th, 2019 by Derek Curtice