Muscle Fiber Types

Muscle fibers

Table of Contents

Are you a better distance runner or a sprinter? Have you ever wondered why is that so?
 
The answer is simple: muscle fibers.
 
A single skeletal muscle is composed of many thousands to millions of long, narrow muscle fibers. These muscle fibers are composed of functional units called sarcomeres. Within each sarcomere are the myofibrillar proteins myosin (the thick filament) and actin (the thin filament). The interaction of these 2 myofibrillar proteins allows muscles to contract.
Muscle fibers

Muscle Fiber Types

There are three types of skeletal muscle cells:
Fiber Type
Contraction Speed
Time To Peak Power
Fatigue
Color
Type I (slow twitch)
Slow
100 milliseconds
Slowly
Red
Type IIA (fast twitch oxidative fibres)
Fast
50 milliseconds
Fast
Red
Type IIB (fast twitch glycolytic fibres)
Very Fast
25 milliseconds
Fast
White
  1. Type I fibers are characterized by low force/power/speed production and high endurance, The slow twitch muscle fibers are more efficient at using oxygen to generate more adenosine triphosphate (ATP) fuel for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.
  2. Type IIB fibers are characterized by high force/power/speed production and low endurance. These fast twitch fibers use anaerobic metabolism to create energy and are the “classic” fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a faster rate of fatigue and can’t last as long before it needs rest. These fibers are the ones that are most involved when we perform a maximum repetition.
  3. Type IIA fibers fall in between the two. These fast-twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of type I and type IIB muscle fibers.
This range of muscle fiber types allows for the wide variety of capabilities that human muscles display. On average, people have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.

Motor Units

Muscle fibers are organized into motor units grouped within each muscle. A motor unit is simply a bundle or grouping of muscle fibers. When you want to move, the brain nearly instantaneously sends a signal or impulse through the spinal cord that reaches the motor unit. The impulse then tells that particular motor unit to contract it’s fibers. 
 
The body recruits the lower threshold motor units first (slow-twitch), followed by the higher threshold motor units (fast-twitch) and continues to recruit and fire motor units until you’ve applied enough force to do whatever it is you’re trying to do regarding movement. When you are lifting something extremely heavy or applying a lot of force your body will contract practically all the available motor units for that particular muscle.
Type I muscle motor units contract less forcefully and a little slower then type II motor units and they reach peak power slower. This is why you can sit and eat all day or play Playstation all day and never get tired!
 
The type II motor units are capable of greater levels of absolute force than type I and also fatigue a lot quicker. Type IIA and IIB are capable of roughly the same amount of peak force, but the IIA fibers take longer to reach their peak power in comparison to type IIB.
 
Fast twitch fibers don’t like high volumes or long durations of work. They don’t even like a high frequency of work. If we go back to our ancestral roots, fast twitch IIB fibers were used only in times of stress situations. These would include running away from a predator, fighting, chasing food, or other brief explosive muscle action. They were only active for a few minutes per day at most. Since they weren’t used often the body had no real need to sacrifice them for a more efficient fiber. Sedentary people are the same way and have more fast twitch IIB muscle than athletes as the use of their fibers is limited and there is no need for their bodies to make more efficient adaptations.

Changing size or fiber type composition

Muscle fibers can adapt to changing demands by changing size or fiber type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient’s force development or endurance. There is evidence that muscle fibers not only change in size in response to demands, but they can also convert from one type to another. This plasticity in contractile and metabolic properties in response to training and rehabilitation allows for adaptation to different functional demands.
 
Fiber conversions between type IIB and type IIA are the most common, but type I to type II conversions are possible in cases of severe deconditioning or spinal cord injury.
 
Less evidence exists for the conversion of type II to type I fibers with training or rehabilitation, because only studies that use denervated muscle that is chronically activated with electrical stimulation have consistently demonstrated that such a conversion is possible.
 
Changes in the muscle fiber types are also responsible for some of the loss of function associated with deconditioning.
Some of the loss of muscle performance (decreased force production) due to aging does not appear to be only due to the conversion of muscle fibers from one type to another, but largely due to a selective atrophy of certain populations of muscle fiber types. With aging, there is a progressive loss of muscle mass and maximal oxygen uptake, leading to a reduction in muscle performance and presumably some of the loss of function (decreased ability to perform activities of daily living) seen in elderly people. Age-related loss of muscle mass results primarily from a decrease in the total number of both type I and type II fibers and, secondarily, from a preferential atrophy of type II fibers. Atrophy of type II fibers leads to a larger proportion of slow type muscle mass in aged muscle, as evidenced by slower contraction and relaxation times in older muscle.
 
Fortunately, physical therapy interventions can affect muscle fiber types leading to improvements in muscle performance. Physical therapy interventions can be broadly divided into those designed to increase the patient’s resistance to fatigue and those designed to increase the patient’s force production.
 
Evidence is lacking to demonstrate that type II fibers convert to type I with endurance training, although there does appear to be an increase in the mixed type I and IIA fiber populations. Researchers have found that type I fibers become faster with endurance exercise and slower with deconditioning.
 
High-intensity resistance training (high-load–low-repetition training) results in changes in fiber type similar to those seen with endurance training, although muscle hypertrophy also plays an essential role in producing strength gains. Initial increases in force production with high-intensity resistance training programs are largely mediated by neural factors, rather than visible hypertrophy of muscle fibers, in adults with no pathology or impairments. Even so, changes in muscle proteins, do begin after a few workouts, but visible hypertrophy of muscle fibers is not evident until training is conducted over a longer period of time (>8 weeks).
 
Although the trends in fiber type conversions are similar for endurance training and resistance training, differences in physiological changes that occur with each type of exercise are also important. Endurance training increases the oxidative capacity of muscle, whereas training to increase force production of sufficient intensity and duration promotes hypertrophy of muscle fibers by increasing the volume of contractile proteins in the fibers.

18 thoughts on “Muscle Fiber Types”

  1. This is great information!! As fit and toned as I tend to be (lean muscle), endurance has always been my weak point. I am working on it though!!

    1. Thank you, Angela! I’m glad you like it. We all have weak points to work on, right?
      Big hugs,
      David

    1. Thank you Joan, for reading, sharing and take time to write this encouraging comment 😃👌
      Big hugs!!

  2. I had to work up to endurance when I was still running, but I kept getting better. Granted I did it everyday. My son did the Lewa Marathon in Kenya years ago and when he ran track in school, he was best at long distance, which made no sense to me before. He is light framed, so it was hard for me to imagine how he could sustain distance, but he did. I would get winded quickly but if I stuck with it, I got stronger as I went along. Very interesting information.

    1. I hope this post helped you to understand why your son is so good running long distance 😃 The Lewa Marathon is very special for a number of reasons, I’m sure he enjoyed it a lot!

      1. Oh, he did! It was a once in a lifetime experience and he was delighted that he out ran some of the natives. It made him feel very good. He said many of the very good runners ran barefoot which impressed him too.

      2. I’m glad he enjoyed the experience. I´d love to visit Kenya (not to run a marathon, I would settle for watching the race, you know? :D ).

      1. I know, I’m surprised too! I’ve even had two levels of Anatomy and Physiology in college, and we never talked about the different kinds of muscle fibers in class! I should ask for my money back :P

  3. Pingback: Challenge is coming - Chape Fitness

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Lower Leg

The gastrocnemius and the soleus form what we know as calf. They are involved in activities such as walking, running, jumping… 

Hamstrings

A hamstring is any one of the three posterior thigh muscles in between the hip and the knee (from medial to lateral: semimembranosus, semitendinosus and biceps femoris). The hamstrings are quite susceptible to injury.

Quadriceps

The Quadriceps Femoris is the knee extensor muscle.  As a group, the quadriceps femoris is crucial in walking, running, jumping and squatting. It´s subdivided into four separate “heads”.

Glutes

The gluteal muscles are a group of three muscles which make up the buttocks: the gluteus maximus, gluteus medius and gluteus minimus. The three muscles originate from the ilium and sacrum and insert on the femur. The functions of the muscles include extension, abduction, external rotation, and internal rotation of the hip joint.

Lower Back

The Erector Spinae is not just one muscle, but a bundle of muscles and tendons. Paired, they run more or less vertically. It extends throughout the lumbar, thoracic and cervical regions, and lies in the groove to the side of the vertebral column.

Obliques

The External Oblique is situated on the lateral and anterior parts of the abdomen. It is broad, thin, and irregularly quadrilateral. It is the largest and the most superficial (outermost) of the three flat muscles of the lateral anterior abdomen. 

Abs

The Rectus Abdominis is the most superficial of the abdominal muscles. It is this muscle which forms the six-pack shape! It is a paired muscle running vertically on each side of the anterior wall of the abdomen. There are two parallel muscles, separated by a midline band of connective tissue called the linea alba.

Pecs

The pectoralis major makes up the bulk of the chest muscles in the male and lies under the breast in the female.

The pectoralis minor is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major. 

Forearm
(Posterior muscles)

The Extensor Digitorum muscle helps in the movements of the wrists and the elbows. It extends the phalanges, then the wrist, and finally the elbow. It acts principally on the proximal phalanges. It tends to separate the fingers as it extends them.

Forearm
(Anterior muscles)

The Pronator teres pronates the forearm, turning the hand posteriorly. If the elbow is flexed to a right angle, then pronator teres will turn the hand so that the palm faces inferiorly. It is assisted in this action by pronator quadratus.

Triceps

The Triceps Brachii muscles  have three muscle heads: Lateral, Medial and Long head. Primarily responsible for the extension of the elbow joint. The lateral head is used for movements requiring occasional high-intensity force, while the medial fascicle enables more precise, low-force movements.

Biceps

The Biceps brachii is  actually two separate bundles of muscles (heads). The two heads of the Biceps vary in length and as a result, are called the Short and the Long Biceps heads.

Infraspinatus

The Infraspinatus muscle is one of the four rotator cuff muscles crossing the shoulder joint and is commonly injured. It is the main external rotator of the shoulder joint.

Deltoids

The Deltoid muscle is the muscle forming the rounded contour of the shoulder. It is divided into three portions, anterior, lateral and posterior, with the fibers having different roles due to their orientation.

Latissimus Dorsi

The latissimus dorsi is the larger, flat, dorsolateral muscle on the trunk, posterior to the arm, and partly covered by the trapezius on its median dorsal region.

Trapezius

The trapezius is a broad, flat and triangular muscle. The muscles on each side form a trapezoid shape. It is the most superficial of all the back muscles.

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