Fibre Type Distribution in Human Muscles

Brief Review - by Robert Colling

Contents

• Introduction
• Fast Fibres
• Slow Fibres
• Fibre Subtypes
• Fibre Identification
• Distribution of Fibre Types
• Fibre Formation
• Muscle Response to Training
• Immobilisation
• Aging
• Conclusion
• Short Answer Review Questions
• References

Introduction

The classification of human muscle fibre types can be seen as multifactorial . To simplify matters these fibres will first be discussed in terms of contractile ability.

Fast Fibres

Firstly, fast twitch fibres are anatomically pale, have fast generation and relaxation of tension. They are also able to generate large levels of tension, however, have low endurance capabilities. The fast twitch fibres are serviced by large diameter nerve axons allowing fast nerve conduction velocities with high frequencies of action potentials.

Histochemically, the fast twitch fibres have a high level of myosin ATPase, a rapid calcium release with uptake of sarcoplasmic retinaculum. Myosin ATPase is used to split the ATP to ADP with Pi to provide instantaneous energy for muscle contraction.

The fast twitch fibre is rich in glycolytic enzymes to allow instantaneous release of intramuscular glycogen. The fast twitch fibres, therefore are able to activate at a rate of 2-3 times that of slow twitch fibres. As a result, these fibres are best suited to fast forceful muscle contractions involved in short term sprint activities (Thompson, 1994).

Slow Fibres

Conversely the slow twitch fibres are dark in appearance and have a slow generation with relaxation of tension. The slow twitch fibres generate lower levels of tension however, are able to maintain this for a greater duration. They are serviced by smaller diameter nerve axons with slower conduction velocities and stimulation frequencies.

The levels of myosin ATPase are low, with the glycolytic capacity less developed than for fast twitch fibres, and the levels of oxidative enzymes are high. Accompanying the high oxidative capacity, is a high concentration of mitochondrial enzymes with myosin. These high levels of mitochondria and myosin give the slow twitch fibres their characteristic red appearance (Thompson,1994).

As the slow twitch fibre is capable of producing a lower tension contraction over a large period of time then the slow twitch fibres are activated at maximal levels during aerobic activities such as sustained running or swimming.

Fibre Subtypes

Historically researchers have termed the slow twitch fibres Type I and fast twitch fibres Type II.

Further to this, subdivisions of Type II fibres have been described. These are IIa or a fibre capable of fast speed contraction with a moderately developed capacity for aerobic, with anaerobic energy transfer.. These are fast-oxidative glycolytic or speed endurance fibres. The second subdivision (Type IIb) has the greatest anaerobic potential with the true fast-glycolytic or speed fibre (Thompson, 1994). The third subdivision or Type IIc is often seen in developing, reinnnervating or transforming muscle fibres (Cole, 1996).

Fibre Identification

In order to correctly distinguish the Type I and Type II fibres and more importantly the Types IIa and IIb fibres, several researchers have used a histochemical analysis of myosin-ATPase, as the contraction characteristics are closely related to this (Howald, 1987; Jakobsson, Grimby & Edstrom,1992; Johnson, Polgar, Weightman & Appleton, 1972; Alway, Grumbt, Gonyea & Stray-Gundersen, 1989; Tesch & Karlsson, 1985.)

Pre-incubation of the muscle tissue samples under different acid and alkaline conditions, allows for subdivision of Type II fibres. In general fast myosins are inactivated at acid pH 4.3 and 4.6 whilst slow myosins are inactivated at alkaline pH of 9.4 (Jones & Round, 1990) to 10.7 (Howald, 1982). Other commonly used histochemical tests include staining for mitochondrial enzyme activity for Type I fibres and glycolytic activity for Type II fibres (Howald, 1973; Gollnick, Armstrong, Saubert, Piehl & Saltin, 1972)

In terms of contractibility the fast muscle fibres develop force and relax more rapidly than do the slow muscle fibres. For instance, the faster extensor digitorum longus when stimulated at frequencies of 10Hz and 20Hz reacts quicker than the slower soleus muscle. The reaction speed of the extensor digitorum longus is such as to allow relaxation and tension to fall to baseline before activation of the next impulse. In the soleus however, the next impulse is activated before complete relaxation, resulting in a superimposed, almost tetanic contraction (Jones & Round, 1990).

When looking at the muscle twitch, and distinguishing the fibre type, four aspects must be accounted for.These are; the shape of the twitch, the relaxation from an isometric tetanus, the fusion frequency and fatiguability.The fast twitch muscle has an earlier peak and more rapid relaxation than when compared to slower twitch fibres. The fusion frequency is higher in fast twitch muscles, as is the rate of fatiguability (Jones & Round, 1990).

At this point ti would be prudent to mention the relationship between the structure and function in terms of muscle fibre type. It is accepted that the basic contractile elements of the muscle are actin and myosin. Muscle contraction is due to activation of actin-myosin crossbridges. This is said to be due to the binding of calcium to a protein complex consisting of troponins and tropomyosin, which are already bound to the actin (Schaub 1978 cited in Howald 1982).

The key element in muscle contraction is myosin. It is during contraction and force production that the myosin catalyzes hydrolysis of ATP into ADP and P1, with free energy (Howald, 1982).

Each myosin filament contains 300-400 myosin molecules and each molecule (140nM in length) consists of two heavy chains and four light chains. The heavy chains are entwined to form a tail and neck, and at one end, form a globular head position. The myosin heads, which form the crossbridges to actin, contain the light chains, as well as the ATPase activity.

Within human muscle fibres there are different heavy and light chain patterns for both types I and II (Howald, 1982). There is a vast difference in heavy chain peptide fragments, when comparing types I and II fibres. However, the fewer differences in the heavy chain peptide map, when comparing types IIa and IIB, is due to the respective types I and II having different amino acid sequences, and are therefore presumably products of different genes (Howald, 1982; Alway et al, 1989).

The differentiation of fibres can be seen in the arrangement of "fast" light chain and "slow" heavy chains. Since the positioning of the light chains is close to the site of myosin ATPase, this not only has a role in the cross-bridge interaction with actin, but also the speed of contraction. Analysis of Type IIc found various amounts of both fast and slow myosin, forming a "transitional" fibre, or one which allows transformation from Type I to Type II (Thompson, 1994; Howald, 1982).

Further to this each Type I fibre was found to have small quantities of "fast" myosin and Type II fibre were found to have small amounts of "slow" myosin.

Distribution of Fibre Types

Given that each muscle has a specific function, then it would seem likely that the composition of the fibre type within the muscle would have a bearing on the nature of this function (Johnson et al, 1973; Vudjonvich 1995; Tesch and Karlsson 1985; Jakobsson et al, 1992).

Keeping this in mind then by using the percentage of slow twitch or type I fibres per muscle then one can determine its role as either tonic or phasic. For instance, those muscles with the greater percentage of Type I fibres can be considered to be tonic muscles. These are muscles associated with the maintenance of posture, or stability during activity, such as standing, walking, throwing etc. A prime example of this is the soleus muscle, containing 87.7% type I fibres (Johnson et al, 1972 ). When compared to the triceps muscle with a type I % of 32.6%, it would seem apparent that the soleus has a greater role as a postural or tonic muscle. This is indeed the case with the majority of people.

Another case in point, is that of the Vastus Medialis Obliquus (VMO), and the Vastus Lateralis (VL).The VMO has a type I fibre % of 52.1%, compared to the VL with values ranging from 39.5% (Gollnick et al, 1972) to 51.4% (Kuzon, Rosenblatt, Huebel, Leatt, Plyey & McKee, 1990). This is clinically significant in that, Jenny McConnell (1986) illustrated that the VMO is "the only dynamic medial stabilizer of the knee". This being the case it would seem reasonable to accept the level of type I distribution within the VMO as 52.1%. However, given the scenario that an individual has an atrophy of her VMO, due to disuse, then this would increase the lateral drift of the patella (McConnell, 1986). One would also expect the percentage of type I fibres to decrease (Jansson, Sjodin & Tesch, 1978). As a result a large part of the McConnell's regime (1986), is aimed at restoring normal levels, and strength of type I fibres, thereby restoring normal painfree function (McConnell, 1986).

Table 1.1: The percentages of type I fibres in the Trunk

Muscle	% Type I fibres	Function
Deltoid	57.1	Tonic
Erector Spinae	56.4	Tonic
Supraspinatus	59.3	Tonic
Frontalis	64.1	Tonic
Trapezius	53.7	Tonic
Latissmus Dorsi	50.5	Tonic
Gluteus Maximus	52.4	Tonic
Infraspinatus	45.3	Phasic
Rectus Abdominus	46.1	Phasic
Temporalis	46.4	Phasic
Orbicularis Oculi	12.8	Phasic
Rhomboid Trunk 44.6 Phasic	44.6	Phasic
SCM Trunk 35.2 Phasic	35.2	Phasic

Table 1.2: The percentages of type I fibres in the Upper Limb

Muscle	% Type I fibres	Function
Abd. Poll. Long.	63.0	Tonic
Add. Poll.	80.4	Tonic
1st Dors. Inter.	57.4	Tonic
Abd. Dig. Min.	51.8	Tonic
Biceps Brachii	46.5	Phasic
Ext. Dig.	47.3	Phasic
Ext. Dig. Brev.	47.3	Phasic
Flexor Digitorum Prof.	47.3	Phasic
Brachioradialis	39.8	Phasic
Triceps	32.6	Phasic

Table 1.3: The percentages of type I fibres in the Lower Limb

Muscle	% Type I fibres	Function
Add. Magnus	58.2	Tonic
Biceps Femoris	66.9	Tonic
Peroneus Longus	62.6	Tonic
Soleus	87.7	Tonic
Tibialis Anterior	73.0	Tonic
Vastus Medialis Oblique	52.1	Tonic
Sartorius	49.6	Phasic
Vastus Lateralis	42.3	Phasic
Gastrocnemius (lateral head)	50.5	Tonic
Gastrocnemius (medial head)	43.5	Phasic
Rectus Femoris	35.4	Phasic

Orbicularis Oculi and Soleus muscles contain the lowest and highest percentages of type I fibres, with 12.8% and 87.7% respectively.

Table 2: The percentages of type I fibres with reference to sport. (Johnson et al, 1972; Kuzon et al, 1990; Gollnick et al, 1972)

Sport	% Type I fibres in Vastus Lateralis	Nature (UL vs LL)
Cyclist	61.4	Sustained LL
Canoeist	61.4	Sustained LL
Runner	58.9	Sustained LL
Swimmer	57.7	Sustained UL
Weightlifter	46.1	Power UL/LL
Orienteer	68.8	Sustained LL
Sprinter	26.0	Power LL
Soccer Player	52.9	Sustained/Power LL
Untrained	42.1	Nil

On closer inspection, these figures are as would be expected. The sprinter, who requires very powerful, very fast muscle contractions, particularly of the lower limb has, as expected a low percentage of type I fibres (26.0%). The orienteer, on the other hand requiring excellent endurance has a type I percentage of 68.8%. The same can be said for the cyclist, canoeist, and runner.

One point of interest is that the weightlifter and the untrained groups have similar type I percentages (46.1% & 42.1%). The average percentage for area of the Type I fibres for the weightlifter group was 23.5%, compared to 32.7% for the untrained group (Gollnick et al, 1972). This preferential hypertrophy of the type II fibres will be discussed at a later stage.

Also of interest is the soccer players. This is the only group sampled, to be in a sport of true sustained nature with bursts of short sharp sprints. Not surprisingly then, their percentage for type I fibres was approximately 50% (actual 52.9%)( Kuzon et al, 1990).

Table 3: The percentages of type I fibres with reference to fibre depth (Johnson et al, 1972)

Muscle	% Type I fibres
	Deep	Superficial
Adductor Magnus	63.6	53.5
Deltoid	61.0	53.3
Soleus	89.0	86.4
Tibialis Anterior	73.4	72.7
Vastus Medialis Oblique	61.5	43.7
Biceps Brachii	50.6	42.3
Erector Spinae	58.4	54.3
Gastrocnemius (lateral head)	50.3	50.8
Vastus Lateralis Oblique	46.9	37.7
Rectus Femoris	42.0	29.5

What is most striking about this table is that almost every value for the fibres has a higher proportion of type I fibres. One explanation for this is that, functionally the superficial fibres have an origin at a greater distance from the insertion point, when compared to the deep fibres of the same muscle. Thus as the muscle contracts and shortens, the superficial fibres must work at a faster rate of shortening, as opposed to the deep fibres, to cover the same distance (Ranson & Strauss, 1997). As a result an equal tension is taken throughout the deep and superficial fibres.

Fibre Formation

Embryologically skeletal muscle is derived from the mesodermal germ layers in the limb buds, somites and branchial segments (Cole, 1996; Bandy & Dunleavy, 1996 ).

Accumulation of several mesodermal cells form myoblasts which fuse to form myotubes. At this stage formation of actin and myosin begins. At the same time within each newly formed myofibril proliferation of mitochondria and glycogen occurs, without the influence of the neural elements. At this point, the motor neurons send axons to the muscle forming myoneural junctions, innervating the muscle and ultimately affecting its physiological properties, ultrastructure and histochemical properties as well as myofibrillar ATPase activity (Bandy & Dunleavy, 1996).

Early in the foetal development, the predominant muscle fibre type is the undifferentiated fast twitch fibre or Type IIc.With effective neural innervation, and as the muscles of the limbs, head, neck and trunk begin to work, the different fibre types become distinguishable as Types I, IIa and IIb (Cole, 1996). This great change occurs between 26 and 30 weeks of gestation (Fig.1) (Colling-Saltin, 1978). The differentiation of fibres continues after birth (Colling-Saltin, 1978), along with muscle hypertrophy (Bandy & Dunleavy, 1996).

Muscle Response to Training

Hypertrophy

Hypertrophy is the growth in muscle, in response to overload training occurring primarily as an enlargement of the individual fibres (Luithi, Howald, Claasen et al, 1986;Gollnick, Timson, Moore & Reidy, 1981; Frontera, Meredith, O'Reilly & Evans, 1988). As a result the force generated by the muscle can be related to the cross sectional area (Luithi, et al 1986, Gollnick, et al 1981). Studies have shown that in the untrained state the percentages of fibre types are similar to trained subjects, however, the trained subjects exhibited a preferential hypertrophy of the Type II fibres (Macdougall, Sale, Alway ,& Sutton, 1984; Gollnick, et al 1981; Alway , MacDougall, Sale, Sutton & McComas, 1988). The amount of hypertrophy is dependant upon heredity and the amount of training (Thompson, 1994; Hopp, 1993; Staron, Hikida, Hagerman, Dudley & Murray, 1983), and as a result many studies have differing figures for the hypertrophy of the muscle fibres.

For instance, Macdougall et al ( 1976) demonstrated an 11% increase in arm circumference following a 5 month training regime, and Frontera et al (1988) demonstrated a 9.3% increase in the cross sectional area of the quadriceps muscle of the men after 12 weeks of training.This fibre hypertrophy is very exercise specific as shown by Gollnick, Armstrong, Saltin, Saubert, Sembrovich and Shepherd (1973), whereby after a 5 month endurance training programme, the cross sectional area of the Type I fibres was increased by 23%.

Further to this Staron et al (1984), showed that the specificity of sport had an effect on the preferential hypertrophy of fibre types with his study comparing runners and weight lifters to non-exercising controls. Tesch & Karlsson (1985), furthered this by comparing the Deltoid and Vastus Lateralis of runners, kayakers, wrestlers and weightlifters (Fig.2). Not surprisingly the area of Type II fibres in the weightlifters were significantly more hypertrophied in the Vastus Lateralis muscle than the other groups, followed by wrestlers, and surprisingly, kayakers. Runners had the smallest area for both Deltoid and Vastus lateralis muscles and the highest cross sectional area of Type I fibres for the Vastus Lateralis when compared to the Deltoid. The weightlifters were the only group to have equal Type I area for both muscles.

The mechanism behind hypertrophy is due to synthesis and thickening of myofibrils and increase in their number. Further to this, as resistance training increases the number of sarcomeres increase as the synthesis of protein increases, and the breakdown of protein decreases (Bandy & Dunleavy, 1996). This acceleration of the protein synthesis is directly dependant upon the increased tension that the muscle is required to generate during exercise (Bandy & Dunleavy, 1996).

Hyperplasia

Another means hypothesized by which muscle can increase in size during intense training, is by hyperplasia or fibre splitting.It has been suggested that once the fibre has reached maximum hypertrophy any further strength or size gains can come only from formation of two daughter cells through lateral budding, or longitudinal fibre splitting (Gonyea & Erickson, 1976; Gonyea, 1980).

Although work on animal species suggest that hyperplasia is an active component of muscle adaptation to exercise (Gonyea & Erickson, 1976; Gonyea, 1980), little has been shown to support this in a human model.

One study, used insulin-like growth factor to bring about increases in peptide growth factions which precede muscular cell proliferation and differentiation (Devold, Rotwein, Sadow Novakovski & Bechtel, 1990).

Therefore with this and the animal studies it has been suggested that gene alteration due to muscular tension can occur, resulting in the formation of new muscle fibres (Vujnovich, 1995). Comparisons of animal studies to humans, however, are flawed in that animals have a limited capacity for muscle cell hypertrophy (Bandy & Dunleavy, 1996). As a result fibre splitting or hyperplasia may be the only means of coping with muscular overload.

Researchers tend to agree that an increase in cross sectional area of muscles following training is mainly due to hypertrophy (Luithi et al 1986; Gollnick et al, 1981; Frontera et al, 1988; Macdougall et al, 1984, Alway et al, 1988; Staron et al, 1984).

Gender Differences

It has been shown that, preferential hypertrophy occurs in Type II compared to Type I fibres as a result of resistance training (Alway et al, 1988; Macdougall et al, 1984,).In one study comparing biceps of male and female bodybuilders it was found that the relative amount of Type I fibres was comparable, however, the ratio of Type II to Type I fibres in the male group was much higher suggesting greater preferential hypertrophy of Type II fibres in males (Alway et al, 1989).

Another study by Willmore (1974), showed that hypertrophy of muscle in females was less when compared to males. Further to this the percentage increase in strength was slightly greater in females. The speculations behind this, have attributed the amount of hypertrophy in the male, to the 20-30 times more testosterone (Bandy & Dunleavy, 1996). Other factors to consider are the pretraining strength of females, as well as the smaller cross sectional area of muscles, and the presence of subcutaneous fat in the female athlete when compared to her male counterpart (Alway et al, 1989, Bandy & Dunleavy, 1996).

Muscle Fibre Type

Several studies have shown no significant change in the fibre types of human muscle following either endurance resistance training on muscle (Gollnick et al, 1973; Thorstensson, Grimby & Karlsson, 1976) furthering the belief that muscle fibre type distribution is genetically determined. However, the training periods for these studies appeared too short, and of an intensity not high enough to elicit a response. It should be remembered that elite athletes train at extremely high levels for many years. (Howald, 1982).

It has been shown that with a change in training work load from continuous to intermittent paralleled a decrease in the Type I fibres with a proportional increase in Type IIc or transitional fibres (Jansson et al, 1978). Further to this a possible conversion of Type IIb to Type IIa has been demonstrated with intensive endurance training (Andersen & Henriksson, 1977a), through the intermediate high frequency, intermediate fatique resistant Type IIx fibres (Thompson, 1994).

It has also been demonstrated that the Type II fast twitch fibres have been transformed to Type I slow twitch fibres with electrical stimulation of 10Hz (Leiber, 1992). The transformation begins with increases in percent of mitochondria, oxidative enzyme activity, capillaries per square millimetre, total blood flow and consumption. There is an increase in the percentage of fast oxidative glycolytic fibres at the expense of fast glycolytic fibres (Leiber, 1992).

The amount of calcium ATPase decreases, as is evident in the prolonged time to peak twitch tension and relaxation (Essen, Jansson, Henriksson, Taylor & Saltin, 1975). After approximately 4 weeks of continuous stimulation, the myosin light chain profile changes from containing fast fibre light chains to light chains characteristic of slow fibres. Furthering this, the new slow fibre light chain synthesizes heavy chains to be incorporated in the myosin filament. By this time the former fast twitch Type II fibre resembles a slow twitch Type I fibre in every respect (Leiber, 1992).

Capillarisation

It has been discussed that specificity of training results in specific responses, for instance, endurance training will result in a marked increase in oxidative enzymes. Along with this, comes an increase in capillary density (Andersen & Henricksson 1977b).

Klausen, Andersen and Pelle (1981) demonstrated that following an intensive endurance bicycle training for 8 weeks there was considerable proliferation of the capillary network. The number of capillaries per millimetre increased by 22%, and the capillaries per fibre by 20%. The capillary number differed for each fibre with Type I increasing by 24%, Type IIa by 20% and Type IIb by 30%. There was also a 10-15% decrease in the fibre area per capillary. These results were consistent with those demonstrated by Andersen & Henricksson (1977b). Further to this, during a detraining period of 8 weeks these trends were reversed (Klausen et al, 1981).

Enzyme Activity

Periods of endurance training have been demonstrated to increase the activity of oxidative enzymes, Succinate dehydrogenase and cytochrome oxidase, which was accompanied by an increase in the VO2 max (Klausen et al, 1981). Surprisingly an increase in endurance training can also lead to increases in the glycolytic enzyme phosphofructokinase (Gollnick et al, 1973). Conversely, with detraining the levels of both oxidative and glycolytic enzymes decreased.

The amount of change in the enzyme activity is highly dependant on the type of activity with glycolytic enzymes responding best to short bursts of maximal effort and oxidative enzymes to endurance training.

Immobilisation

Atrophy

In opposition to hypertrophy following training, the result of immobilisation is atrophy. The signs of muscle atrophy are decreases in size, muscle strength and endurance (Bandy & Dunleavy, 1996). Atrophy can occur due to pathology, malnutrition, denervation or from disuse.

Following immobilisation, the decrease in cross sectional area and decrease in percentage of type I fibres (Jakobsson, et al, 1992) can lead to abnormal movement patterns, increased stress on joints and increased risk of injury, as well as extracellular fluid accumulation and connective tissue alterations (Bandy & Dunleavy, 1996).

Histologically, animal studies show that atrophy can result from a decrease in the rate of protein synthesis and an increased rate of protein catabolism (Watson, Stein & Booth, 1984). There is specific catabolism of myofibrillar proteins, with Type I myosin being affected in preference to Type II myosin levels (Jakobsson et al, 1992).

Fibre Type

As mentioned before, immobilisation due to disuse causing hypokinesia, will result in greater atrophy of type I fibres than type II fibres. Therefore the muscles with a large amount of type I fibres will be most susceptible to immobilisation. These are the postural or tonic muscles, therefore functional tasks such as standing, will be significantly affected for extended periods of time (Bandy & Dunleavy, 1996).

In contrast, atrophy associated with muscular pathologies i.e., Muscular Dystrophy, result in preferential atrophy of the Type II fibres (Wheeler, 1982).

Aging

The aging process as with nearly every bodily function is extremely variable from person to person (Hopp, 1993; Thompson, 1994). In physical terms, aging is associated with slowed movements, a decrease in muscle strength and force production, and loss or decrease in fine motor control. Although the symptoms of muscular atrophy are evident, decline in muscle strength and decreased independence due to frailty are commonly seen, little is known as to the exact cause.

Several studies have been performed concerning the biochemical changes, distribution of fibre types, muscle atrophy, fibre number and size.

Biochemical

Within the studies there appears to be some agreement. Most show that there is no statistical difference in the glycolytic enzymes such as phosphofructokinase, lactate dehydrogenase in the aging human when compared to the younger counterpart ( Larson & Karlsson 1978, Larson, Sjodin & Karlsson, 1978). Further to this, Essen-Gustavson & Borges (1986), found that although the glycolytic enzymes were lower in Type I when compared to Type II fibres, a comparison of the glycolytic enzymes in the fibres showed no difference between young and older subjects. Larson et al (1978) showed that age did not have an adverse affect on the mitochondrial activity of the fibre.

In contrast, one study found the oxidative muscle enzyme activity to be about 25% lower in the older subject in comparison to the younger group (Coggan, 1992, cited in Thompson, 1994). One reason given for this is the differing level of physical activity.

In fact, it seems that the major factor influencing glycolytic enzyme capacity and, although investigated to a lesser extent, oxidative enzymes in relation to age, is the level of physical activity (Thompson, 1994).

During aging, the proportion of Type I and Type II fibre types seems to alter (Larsson et al, 1978).

The percentage of Type I fibres taken from the Vastus Lateralis muscle seemed to alter from 39% in 20-29 year old to a higher percentage of 66% in the 60-66 year old. From the third to the seventh decade the proportion of Type II fibre seemed to decrease in a linear manner, however the Type II subtypes remained unaltered. (Larsson & Karlsson, 1978).

Possible reasons for this proliferation of the Type I fibres with aging include splitting or new fibre formation, preferential atrophy or transformation of Type II to Type I through the Type IIc fibre (Larsson, 1978; Larsson et al, 1978).

However, recent studies on pectoralis muscles showed that the Type I fibre percentage remained unchanged with age (Sato et al, 1983, cited in Thompson, 1994).

Many researchers have found that the distribution and percentage of muscle fibre has more to do with the level of physical activity, and heredity rather than the aging process (Larsson, 1978; Larsson et al, 1978; Lexell, Downham &Sjostrom, 1986).

Muscle Atrophy

Muscle atrophy can be expressed in terms of the muscle fibre size and muscle fibre number.

Muscle Fibre Size

The fibres, when at the maximal size (20-30 years of age) (Thompson, 1994), have cross sectional areas, whereby the Type II fibres have areas 15-20% in excess of Type I fibres (Lexell, Taylor & Sjostrom, 1988). During the seventh decade, however, the fibre sizes will be approximately equal (Lexell et al,1986). As age increases, so the size of the Type II fibres, and in particular the Type IIb fibres decreases (Lexell et al, 1988)

Muscle Fibre Number

With many animal species a decrease in fibre number has been shown with aging. (Thompson, 1994). In human studies, using mainly the Vastus Lateralis muscle, the older group were found to have both fibre number and total muscle size to be smaller than their younger counterparts (Larsson et al, 1978; Lexell et al, 1986; Lexell et al, 1988).

Therefore a combined effect of both the decrease in the size of the fibre, and in particular the size of Type IIb fibres, and the number of fibres add to affect the amount of age related muscle atrophy. Once again, this phenonmenon is largely related to heredity and the level of physical activity (Hopp, 1993; Thompson, 1994).

Conclusion

In summary, muscles are dynamic functional units. They function in accordance to their structure, and they also respond by altering their structure according to the functional demands. For instance, a trained muscle will hypertrophy in accordance to its training and an immobilized muscle will atrophy.

It seems that with the right training of muscle Type IIb can be converted to Type IIa (Andersen & Henricksson, 1977a), and Type I can decrease with a relative increase in Type IIc (Jansson et al, 1978).

However, it is impossible to be able to predict performance on the grounds of hypertrophy or percentage of muscle types. Due to the complexity of most functional and sporting activities there are a number of physiological and psychological factors that comprise each performance, therefore muscle fibre type is just one small piece of an extremely complex jigsaw (Howald, 1982).

Short Answer Review Questions

1. Provide values for percentage of type I fibres for Vastus Lateralis, Biceps Femoris, Tibialis Anterior, Soleus, Deltoid, Supraspinatus, Infraspinatus, Erector Spinae, Rectus Abdominus, Vastus Medialus Obliquus, Triceps, and Orbicularis Oculi.
2. Provide average values for percentages of type I fibres in Vastus Lateralis muscles of runners, sprinters, weightlifters, canoeists, cyclists and soccer players.
3. Briefly describe the formation of muscle fibres from conception to birth.
4. Discuss mechanisms of muscle growth in response to training.
5. Describe the effects of electrical stimulation to muscles. How does this differ from training effects?
6. Discuss the effects of aging on the muscle fibre.
7. Discuss the statement that "Sprinters are born and not made", and why this may not be true.

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Exercise Physiology Educational Resources 1997