Why are There Different Fibre Types?

Brief Review - by Cathy Stewart

Contents

• Learning Objectives
• Background Knowledge
  • Skeletal Muscle Fibres
  • The Motor Unit of a skeletal muscle
  • Identification of different fibre types
  • Characteristics of skeletal muscle fibre types
  • Classification of skeletal muscle fibre types
• New Knowledge
  • Other fibre types identified
  • Force-Velocity relationship and fibre types
  • Power-Velocity relationships and fibre types
  • Recruitment of fibre types - the "size principle" (Henneman)
  • Motor unit recruitment during eccentric muscle action
• Clinical Implications
• Summary
• Short Answer Review Questions
• References

Learning Objectives

• To know the major fibre types in skeletal muscle and how they are identified and classified.
• To understand the different structural, physiological, functional, neural and isoform characteristics of skeletal muscle fibre types.
• To understand the relationships between force and velocity and power and velocity for FT and ST muscle fibres and the relevance to different athletes.
• To understand the size principle theoryof motor units for concentric and eccentric muscle action.
• To apply this knowledge clinically.

Background Knowledge

Skeletal Muscle Fibres

Skeletal muscles are ensheathed in a connective tissue wrapping of epimysium. They are composed of a number of muscle fibres which when grouped together become a fasiculus. The fasiculus is covered in perimysium and the individual muscle fibres are surrounded by endomysium.

Microscopically, the muscle fibre is made up of a functional unit named the sarcomere (defined as the unit between two Z-lines), which contains a number of myofilaments. The main myofilaments are the contractile proteins myosin and actin. Myosin is composed of 2 heavy chains (MHC) and 4 light chains (MLC) and in turn both of these chains are composed of different isoforms.

In an average muscle fibre there are 4500 sarcomeres and a total of 16 billion thick (myosin) and 64 billion thin (actin) filaments (McArdle, Katch & Katch, 1996, Howald, 1982).

The Motor Unit of Skeletal Muscle

A motor unit is defined as being an alpha motor neuron (with its cell body is located in the ventral root of the spinal cord) and its associated muscle fibres. The motor neurons' axons branch many times, with each small terminal branch innervating a single muscle fibre (Lieber, 1992).

Depending on the muscles' function a motor unit may contain between 10 and 1700 muscle fibres (Bird, 1992), but usually between 300 and 500 fibres (McArdle et al, 1996). Motor unit size is reflected in the number of muscle fibres innervated as well as the diameters of both the neuronal cell body and axon (Albert, 1995). Fibres from the same motor unit are always of the same type (eg. fast-twitch or slow-twitch), but they can be scattered throughout the muscle (Howald, 1982).

Identification of different Fibre Types

Historically, different muscle types in animals were first identified by Lorenzini in the 17th century and classified as red and white muscle. In 1870, Ranvier reported red muscles contracted more slowly than white muscles. In the years following, skeletal muscle fibres were found to display different anatomical, physiological, histochemical and biochemical properties.

In 1955, a technique of histochemical staining for myosin ATPase (ATP enzyme) was outlined by Padykula and Herman. Subsequently this technique was used to separate type I and type II fibres and with further investigation more subdivisions of type II were identified - type IIA and IIB (Thompson, 1994).

Characterisics of skeletal muscle fibre type

The close relationship between structure and function is demonstrated in the different characteristics of muscle fibre types. Type I or slow-twitch (ST) muscle fibres are smaller in size, have a lower recruitment threshold and slower conduction velocities which match their functional task of postural maintenance and other movements which are not critical of speed. Type II or fast-twitch (FT) fibres are larger in size and have faster conduction velocities which enable them to contain more contractile proteins for faster and more forceful muscle contractions.

The other structural aspects of the FT and ST fibres are seen to support their different metabolic functions. ST fibres have higher mitochondrial and capillary density in support of aerobic metabolism, whereas FT fibres are structurally suited to anaerobic metabolism. The latter is also displayed in the high myosin-ATPase and glycolytic enzyme activity.

Functionally, FT fibres reach peak tension twice as fast as ST fibres, which matches up with the metabolic and structural characteristics discussed above. ST fibres are more energy efficient however, which means they produce more isometric force per unit of energy used and are therefore relatively resistant to fatigue. The elastic component of ST fibres is reduced due to their higher collagen content, although this doesn't interfere with function due to their slower contraction time. However, FT fibres have high elasticity to enable initiation of rapid, forceful contractions without undue hinderance.

ST fibres are tonically active and are found in higher proportion in the muscles requiring this function, such as postural muscles. In contrast, higher proportions of FT fibres, which are of a phasic nature are found in the muscles used for quick, forceful movements, such as those displayed by a power lifter. (Fox, Bowers & Foss, 1989).

Table 1. Characteristics of Skeletal Muscle Fibre Types (Adapted from Fox et al, 1989, McArdle et al, 1996 and Strauss, 1997)

Characteristics	Fibre Types
Structural Aspects	ST	FTa	FTb
Colour	red	white/red	white
Muscle fibre diameter	small	medium	large
Sarcoplasmic reticulum	less	more	more
Mitochondrial density	high	medium	low
Capillary density	high	medium	low
Myoglobin content	high	medium	low
Energy Substrates	I	IIA	IIB
Phosphocreatine stores	low	high	high
Glycogen stores	low	high	high
Triglyceride stores	high	medium	low
Enzymatic Aspects	SO	FOG	FG
Myosin-ATPase activity	low	high	high
Glycolytic enzyme activity	low	high	high
Oxidative enzyme activity	high	high/medium	low
Functional Aspects
Twitch time	slow (110 ms)	fast	fast (50 ms)
Relaxation time	slow	fast	fast
Force Production	low	intermediate	high
Time to peak force	90-140 ms	40-80 ms
Energy efficiency	high	low	low
Fatigue resistance	low	high	high
Neural Aspects
Motoneuron size	small	larger	largest
MN recruitment threshold	low	high	high
Nerve conduction velocity	slow (60-70 m/s)	faster (80-90 m/s)	fastest
Discharge frequency	10-20 Hz	30-60 Hz
Electrical Activity Pattern	tonic, low frequency	phasic, high frequency

Classification of skeletal muscle fibre types

The procedure involved in muscle fibre typing requires a muscle biopsy to be taken. The sample is then analysed by chemical staining to identify whether various oxidative (succinate dehydrogenase/SDH) and glycolytic (alpha glycerophosphate dehydrogenase/alphaGP) enzymes are present. The level of myosin ATPase (an enzymatic indicator of muscle fibre contraction speed) is also measured. Based on these analyses the fibre types are classified as slow-oxidative (SO), fast-oxidative-glycolytic (FOG) or fast-glycolytic (FG). (Fox et al, 1989 and Lieber, 1992).

When muscle tissue sections are tested in an acid environment (i.e. low pH=4.3), slow-twitch (ST) fibres stain dark (acid stable) and fast-twitch (FT) fibres stain light (acid labile). The opposite occurs in an alkaline environment (i.e. pH=10.4). Fibres are also stained in medium pH (= 4.6) conditions to further distinguish between fibre types. Histochemically the fibres are labelled as type I, IIA and IIB.

Electron micrographs (225,000 magnification) are now also being used to distinguish between the different fibre types through M- and Z-line identification. It is found that ST fibres have wide Z-lines and five strong bands that make up the M-line, whereas at the other end of the spectrum, FTb fibres have narrow Z-lines and only three strong bands in the M-line. Using both of these parameters in combination there is 95 percent success in correctly allocating fibre types (Fox et al, 1989 and Lieber, 1992).

New Knowledge

Other fibre types identified

Type IIC

• The type IIC fibre is an undifferentiated, less specialized muscle fibre. It has been identified during gestation as the primary fibre type until approximately week 31 onwards, when type IIA and type I fibres become more predominant.
• In adulthood, type IIC fibres make up 0-2 percent or no more than 5 percent of the total human fibre population in skeletal muscle (Fox, 1989).
• Type IIC fibres contain a mixture of type I and IIA myosin heavy chain (MHC) isoforms. (Thompson, 1994 and Howald, 1982).

Type IIX

• A further fibre type has been isolated, type IIX, and metabolically seems to be in between the type IIA and IIB fibre. This fibre contains a 4th isoform IIX-MHC which has been recently identified (Thompson, 1994).
• Type IIX fibres have been found in most leg muscles, are especially abundant in the diaphragm, are rich in oxidative enzymes and belong to motor units relatively resistant to fatigue (Schiaffino & Reggiani, 1994).
• Zachazewski, Magee & Quillen (1996) state six fibre types can be distinguished based on staining intensities and pH level, which are type I, IC, IIA, IIB, IIAB and IIC, with a seventh identified IIX, as stated above. No detail was provided on the characteristics associated with type IC and IIAB fibre types.
• These findings seem to indicate there is a whole continuum of muscle fibre types present that deal with the aerobic and anaerobic demands of skeletal muscle.

Myosin Isoforms

• MHCs and MLCs both exist in various isoforms that are differentially distributed in the various fibre types. MHC distribution define the major fibre types (I, IIA, IIB, IIC & IIX).
• MHC isoforms (four) identified in rat skeletal muscle are "slow" (beta-MHC) and "fast" (IIa, IIx, IIb). In type II fibres there is a predominence of fast MHCs and other fast contractile protein isoforms (Strauss, 1997).
• MLC isoforms (three) identified in rat skeletal muscle are "slow" (MLC1s) and "fast" (MLC1f, MLC3f).
• Studies done indicate both MHC and MLC isoforms determine the maximum velocity of shortening of skeletal muscle fibres. (Schiaffino and Reggiani, 1994)

Force-Velocity relationship and fibre types

The force-velocity curve seen in figure 1 (not available) presents the picture that the higher the percentage of FT fibres in a muscle, the greater the torque that can be generated at any given velocity. The converse is also depicted, in that at any given torque the velocity is greater when there is a higher percentage of FT fibres. However, peak torque decreases with increasing velocity of movement, demonstrating that the greatest torque is produced at the slowest speeds regardless of fibre type. These findings indicate that FT fibres are capable of producing greater peak muscular tension and a faster rate of muscular tension, than ST fibres. Therefore, the higher the percentage of FT fibres the more advantageous it will be to power athletes (Fox et al, 1989).

Figure 1 (Figure 5.19 Fox et al, 1989) Figure 2 (Figure 5.21 Fox et al, 1989)

Power-Velocity relationship and fibre types

Figure 2 (not available) depicts the power versus velocity relationship for knee extensors. As seen here, peak power increases exponentially with increasing velocity. An increase in power occurs more at lower speeds and decreases at higher speeds, and at very high speeds may level or decrease. This graph also demonstrates that the peak power generated is greater the higher the percentage distribution of FT fibres, at any given velocity (Fox et al, 1989). These observations lead to the conclusion that power training at slower speeds will enhance power performances and particularly so with the greater percentage of FT muscle fibres present.

Recruitment of fibre types - the "size principle" (Henneman)

A positive correlation exists between the size of the motor unit and the threshold for activation of the unit. Smaller motor units comprised of ST fibres havea lower excitation threshold and therefore will be recruited first. For low-force or endurance-type muscle actions, these smaller units are recruited initially (eg. sustained activities or jogging on level ground), but as force or duration increases (eg. the jogger experiences a hill) more motor units will be recruited and it will be those with larger axons (the FT fibre units). In all-out power performances the three major fibre types are recruited as quickly and completely as possible. Therefore, ST fibres are always recruited first regardless of exercise intensity. The FT fibres are then recruited according to exercise intensity, endurance or with any fatigue that occurs (Fox et al, 1989).

The result of this selective and orderly recruitment of ST and FT motor units is a smooth muscle contraction. The neural control required for modulation of motor unit firing patterns can be seen in the skilled performance of different athletes (McArdle et al, 1996). Zachazewski et al (1996) report recruitment of fibre types for exercise based on intensity, occurs in the following order: I, IC, IIC, IIA, IIAB, IIB. There has been the suggestion in the literature that because FT fibres are not always required for concentric and isometric muscle activity, these fibres may become "unfit" due to lack of training, which may in turn predispose them to injury (Albert, 1995). Further research into this area is needed.

Derecruitment of muscle activity occurs in the reverse order to recruitment. Therefore, as tension decreases (following the state of peak forcewhen all units are active), the motor units that turn off first are those that were most recently recruited (i.e. the larger units or FT units).

Albert (1995) discusses research that reports a few exceptions to the size principle, although these findings are challenged.

1. When there is sensory stimulation of skin, the reverse order of recruitment occurs.
2. When muscles act as synergists, instead of prime movers, reversal of recruitment occurs.

Other research has found there to be a reverse order of recruitment with the application of artificial, electrical stimulation. It was shown that low- intensity shocks to peripheral nerves, only excited large axons, but an increase in stimulus intensity then stimulated smaller axons (Albert, 1995).

Motor unit recruitment during eccentric muscle action

Albert (1995) discusses the work by Nardone and Schieppati. In EMG studies of the triceps surae with eccentric loading, muscle activity changed from soleus to the lateral gastrocnemius (particularly in subjects with relatively slow relaxation rates in the soleus twitch).

Figure 3 (Figure 2.1 Albert, 1995) (not available)

The theory is that the nervous system departs from the size principle theory for eccentric actions and preferentially activates larger, fast-twitch units first. This is more pronounced with fast eccentric actions, because the rate of lowering is best controlled by units with rapid relaxation times, namely the FT units.

Muscle cell damage after eccentric exercise is observed to occur in FT (type II) fibres. Work by Brenda Eisenberg (reported in Lieber, 1992) found that the thickness of the Z-line is significantly less in type IIB and IIA fibres compared with type I.

• IIB (FG) = 60nm
• IIA (FOG) = 80nm
• I (SO) = 150nm

The Z-line adheres to the sarcolemma to give stability to the contractile structure and has been identified as the weak link in eccentric contraction induced muscle injury (Lieber, 1992, McArdle et al, 1996). Could the thickness of the Z-line be the reason for this weakness?

Clinical Implications

• ST fibres are always recruited with any level of activity. Therefore, if the aim of an exercise is to target FT fibres specifically, the intensity or duration needs to be increased adequately to initiate recruitment.
• Strength and power training involves the recruitment of FT fibres with the indication that to enhance performance, velocity of movement needs to be low.
• The velocity of eccentric exercise has been reported to influence the recruitment order of motor units. The implication is that FT fibre training may need to be targeted for the specific muscles used in eccentric exercise with sports people to reduce risk of injury.
• The concept that FT fibres may become unfit or untrained due to less recruitment time compared with ST fibres, may be a factor influencing muscle injury.

Summary

Muscle fibres contain myosin and actin contractile filaments. The distribution of MHCs define the major fibre types. A whole continuum of fibre types exist, which include type I (SO/ST), type IIA (FOG/FTa), type IIB (FG/FTb), type IIC (undifferentiated) and type IIX. ST fibres are used for low-intensity or endurance exercise (e.g. postural activities). FTa fibres are recruited for higher intensity or more prolonged exercise. FTb fibres are recruited for all-out force production or as other fibres begin to fatigue.

Recruitment of fibre types according to the size principle theory states ST are always first, followed by FTa, FTb, etc according to exercise intensity, endurance or fatigue. Derecruitment of motor units with muscle activity occurs in the reverse order to recruitment. For fast eccentric muscle action the theory of preferential activation of large FT units has been proposed.

Different fibre types have different functions in skeletal muscle, to enable a muscle to perform with a high degree of efficiency. Theories are proposed to explain the structural aspects displayed in muscle fibres and how they relate to function so that this knowledge can be integrated to enhance muscular performance and decrease risk of injury.

Short Answer Review Questions

1. Name the different types of muscle fibres identified in the literature.
2. What microscopic units differentiate these from each other?
3. Describe the main metabolic, structural, neural and functional differences between the major fibre types.
4. What are the metabolic mechanisms involved in fibre typing using the chaemical staining method?
5. What is the significance of the relationship between force-velocity and power-velocity, and fibre type?
6. How does the "size-principle" affect recruitment patterns in concentric and eccentric muscle?

References

Albert, M. (1995). Eccentric muscle training in sports and orthopaedics (2nd ed.). New York : Churchill Livingstone.

Bird, S.R. (1992). Exercise physiology for health professionals. London : Chapman & Hall.

Fox,E.L., Bowers, R.W. & Foss, M.L. (1989). The physiological basis of physical education and athletes (4th ed.). Iowa : Wm.C. Brown Publishers.

Howald, H. (1982). Training-induced morphological and functional changes in skeletal muscle. International Journal of Sports Medicine, 3, 1-12.

Lieber, R.L. (1992). Skeletal muscle structure and function : Implications for rehabilitation and sports medicine. Baltimore : Williams & Wilkins.

McArdle, W.D., Katch, F.I. & Katch,V.L. (1996). Exercise physiology : Energy, nutrition and human performance (4th ed.). Philadelphia : Lea & Febiger.

Schiaffino, S. & Reggiani, C. (1994). Myosin isoforms in mammalian skeletal muscle. Journal of Applied Physiology, 77, (2), 493-501.

Strauss, G.R (1997). Lecture notes. Curtin University : Perth.

Thompson, L.V., (1994). Efffects of age and training on skeletal muscle physiology and performance. Physical Therapy, 74, (1), 71-81.

Zachazewski, J.E., Magee,D.K. & Quillen, W.S. (1996). Athletic injuries and rehabilitation. Philadelphia : W.B. Saunders.

Exercise Physiology Educational Resources 1997