8 Fibre Types

8.1 Introduction

8.2 Red and White Muscle

Red and white muscle is a concept widely used by both meat traders and consumers.  An obvious example is the comparison of breast meat with leg meat in chickens.  The breast meat is white.  The leg meat is red.  Another example is seen in whole cross sections through a cured ham.  The outer muscles are less pigmented than deep muscles around the bones. Although not as clearly different as in the chicken, the less pigmented superficial ham muscles are called white muscles and the heavily pigmented deep ham muscles are called red muscles. Finally, we often use the term white meat for all types of chicken and veal, in contrast to red meats such as beef and lamb. A little confusing, eh? Keep in mind these two points.

  1. Each of our meat animals has some degree of physiological differentiation among muscles.  Fast-contracting muscles are needed for locomotion.  Slow-contracting muscles are needed for respiration, chewing and maintaining posture.
  2. There are differences among our species of meat animals in their overall degree of muscle pigmentation. Beef is darker than lamb, lamb is darker than pork, and pork is darker than chicken or turkey.
Consider these two points together.  We will only SEE differences between red and white muscles when the overall degree of species pigmentation is medium (pork) or low (chicken). Physiological differentiation certainly exists in beef - there are both fast- and slow-contracting muscles - but we cannot easily SEE the differences.

The final point - IT IS THE MYOFIBRES WHICH ARE PHYSIOLOGICALLY DIFFERENTIATED.  Nearly all muscles in our meat animals have a balance of fast- and slow-contracting myofibres. If most myofibres in a muscle are fast-contracting, then the overall contraction speed of the muscle is fast (a white muscle).  If most myofibres in a muscle are slow-contracting, then the overall contraction speed of the muscle is slow (a red muscle).

8.3 Myoglobin


The image above shows a thin slice of pork illuminated from below and viewed with a low-power microscope. A fasciculus fills most of the image. Individual myofibres are clearly visible.  The central myofibres are heavily pigmented with myoglobin.  The surrounding myofibres have very little myoglobin. This is a white muscle.  Most myofibres have a low concentration of myoglobin.  If we attempted the same demonstration with white chicken breast meat - we would see no heavily pigmented myofibres. If we attempted the same demonstration with any beef muscle - we would see all myofibres heavily pigmented.

Myoglobin is very soluble and is located inside myofibres in living animals.  But fluid leaks from myofibres in meat, and so myoglobin colours the juices lost from meat in the supermarket.

Haemoglobin is the red pigment within red blood cells (erythrocytes).  Meat animals are exsanguinated at slaughter. THERE IS NO HAEMOGLOBIN IN MEAT.  The fluid leaking from meat does not contain any blood.  The fluid leaking from meat does not contain any haemoglobin.  The only traces of haemoglobin we sometimes find in meat or meat juices come from a few erythrocytes remaining in capillaries. The wiggly blue lines in the image below are capillaries running on the surfaces of myofibres in beef (they have been stained with methylene blue dye).

8.4 Fast and Slow Fibres

 Historically, it appeared  the relationship between fast- and slow-contracting myofibres in animals was quite simple.  The subject really got started in 1873 with the great French histologist,  Ranvier (who gave his name to the nodes of Ranvier along myelinated axons).  It became accepted  fast myofibres were usually white (low myoglobin), while slow myofibres were usually red (high myoglobin). When redness was found to be due to myoglobin, and myoglobin was found to be correlated with aerobic metabolism, this explained the relationship between redness and speed of contraction. The pale or white myofibres with a low aerobic potential were found to be well endowed with glycolytic enzymes enabling them to obtain energy rapidly by the incomplete oxidation of glycogen. This explained why white myofibres soon became fatigued once their glycogen stores were depleted and why they had to wait for the removal of lactate by the circulatory system.

 At the extremes of the range in physiological differentiation (fast white myofibres versus slow red myofibres) these discoveries are still valid. The problem, as we see it now, is there are also myofibres with a fast contraction speed and a dual energy supply. In other words, some fast myofibres have both aerobic and anaerobic capabilities.

The discovery of these myofibres with dual energy supply coincided in a most confusing way with a growing awarenesss slow red myofibres in meat animals and poultry were rather different from those of frogs and other animals used in biomedical research. It is difficult to write a research report on myofibre types without giving them names. Unfortunately, everybody seemed to use different names, and the numbers of myofibre types recognized tended to be a function of the number of  techniques used to identify them.

Cutting a long story short,  most researchers recognize three main types of myofibres (each with multiple names).

8.5 Histochemical reactions

ATPase reaction.

A frozen section of muscle is exposed to ATP solution and ATPase within myofibres cleaves off the phosphate. But the phosphate is invisible and tends to move around. First we stop the reaction product moving by precipitating the phosphate with cobalt, then we blacken the cobalt salt by converting it to a sulphide. If that is all we do, all the myofibres will go black, because they all have ATPase. So first of all, before starting the reactions described above, we pre-incubate the frozen sections of meat in solutions (such as acetic acid, formaldehyde, etc) to inactivate the isoenzyme in either the fast or slow-contracting myofibres. Then we see differences between myofibres, as above. This is a section of pork. The fast-contracting myofibres are located peripherally in their fasciculi. Therefore, we are looking at ATPase activity in fast-contracting myofibres which was stable when the section was pre-incubated with formaldehyde.  The unstained myofibres are slow-contracting fibres whose ATPase was inactivated by the formaldehdye.  Incubation with acetic acid instead of formaldehdye would have produced the opposite staining pattern.

In the example above, 1 is a white myofibre (alpha-W or Type II white), 2 is an intermediate myofibre (alpha-R or Type II red)  and 3 is a red myofibre (Beta-R or Type I). The section on the left was pre-incubated with acetic acid.  The section on the right was pre-incubated with fomaldehyde. How did I separate myofibres 2 and 3 which both have the same ATPase reaction?  Answer - using the SDH reaction on a serial slice.

SDH reaction.

 SDH  (succinate dehydrogenase) is an enzyme specific to mitochondria. Each little granule of diformazan (the reaction product of nitroblue tetrazolium) indicates the location of mitochondria. This is a section of pork.  The aerobic myofibres are grouped centrally within their fasciculi surrounded by anaerobic myofibres.  Note how SDH is concentrated under the cell  membrane around the outside of the myofibre.

Phosphorylase reaction.

Phosphorylase is the first enzyme involved in glycogenolysis. It normally breaks down glycogen, but we can make it run backwards to make new glycogen (amylose) stainable with iodine  (the reaction works best if there is some natural glycogen present in the myofibre to start the reaction). Thus, absence of a phosphorylase reaction does not automatically mean that there is no phosphorylase present!

Stain for triglyceride - Sudan Black B.

Sudan black has stained the lipid droplets inside aerobic myofibres in this slice of pork, and it has also stained a large triangle of intramuscular (marbling) adipose cells between the muscle fasciculi.

8.6 Aerobic versus anaerobic features

Many of the cellular features associated with aerobic and anaerobic metabolism in myofibres are fairly straightforward.
Aerobic myofibres are:-

Quantitatively, however, the range from aerobic to anaerobic metabolism is usually a continuous variable and is seldom broken into discontinuous steps.

From which we may deduce two points: Thus, to some researchers, the histochemical categorization of myofibres by any method, including myofibrillar ATPase, is merely a useful, but artificial subdivision of a continuously variable range.  Myofibres may undergo a continual alteration throughout life as an adaptation to changing functional demands, and "fibre type" merely reflects the constitution of a myofibre at any particular time. From an agricultural viewpoint, this is particularly interesting since it suggests the existence of some degree of genetic or developmental plasticity in the fibre type continuum. In meat animals, this might be a vital link in relating muscle growth to meat quality.

8.7 Intracellular differentiation

Physiological differentiation may vary intracellularly across individual myofibres, at least as far as aerobic metabolism is concerned. But as far as is known at present, factors relating to contraction speed are fairly uniform within individual myofibres. Aerobic metabolism, as indicated by the distribution of mitochondria, may be graduated radially so that the subsarcolemmal region (the outer part of a myofibre) has a high level of aerobic metabolism while the central axis has a low level. Mitochondria from peripheral and axial regions of the myofibre may differ in their biochemical characteristics, and. proportional mitochondrial volume and maximal rate of oxygen consumption are linearly related among different muscle regions.

The subsarcolemmal concentration of mitochondria in some types of myofibres may be related to the supply of oxygen arriving in capillaries on the surface of the myofibre. Mitochondria are larger in red myofibres than in intermediate or white myofibres and, in red myofibres, they may form thick longitudinal columns between the myofibrils. The arterial and venous elements of muscle capillaries tend to occur in an alternating manner along the length of the myofibre, with longer arterial segments of capillaries in white muscle relative to red muscle.

This image shows the results of computer mapping of the SDH deposits in a myofibre. Dark blue shows high SDH, and light blue shows low SDH (and cyan is medium). Once on the computer, these data can be used for studying the radial gradients of SDH activity in different types of meat animals. 

Further information

Structure and Development of Meat Animals and Poultry.  Chapter 6.


The microgaphs above are from my own research from 1970 to 1990. The last image, computer mapping of diformazan, dates from the early 1980s.  This was one of the first colour screens easily available (Intecolor 3650).  The pixels were controlled with 8-bit assembler.