Muscle Fibre Types

The nature versus nurture argument is prevalent in sport. In Weightlifting, elite athletes are often considered to have a genetic predisposition for speed and strength. However there is no conclusive evidence of this.

Coaches and athletes in Weightlifting are understandably interested in the anatomy and function of muscle tissue. Our greatest desire is that such knowledge will enable us to improve our training methodology and explain why it is that individuals differ so considerably in athletic ability. In our search for knowledge, it is inevitable that we will question, at some time or other, how the world’s best athletes in Weightlifting can develop such incredible strength and power. Is it that such athletes are simply freaks of nature possessing a composition of muscle tissue that is significantly different to the ordinary man in the street? Or is it that such athletic prowess is merely the product of an exceptional training environment? Many people believe that high performance in sport requires both factors, a genetic predisposition and an extent of training that exhausts every possibility to nurture the athlete’s talent. This nature versus nurture paradigm is a constant source of argument and debate and has stimulated a great deal of research.

For the spectator watching the Olympic 100m final, genetic predisposition seems perfectly obvious. Not since 1980 has an athlete of Caucasian ethnicity won this event, and indeed the overwhelming majority of finalists have African ancestry. Nowadays we also witness a great deal of success in Weightlifting among Asian nations, particularly the Chinese, and for many people this adds weight to beliefs about the role played by genetic factors in sporting success. But in science, the topic is not at all settled. For example, the importance of genetic factors in determining muscle strength has been gauged by researchers to range anywhere from 0% to 97% (Huygens et al., 2004).

Nevertheless, there is general support for the notion that genetic factors do play a major role in determining success in sport at the highest level. As an example, more than a billion people of European ancestry have inherited a complete deficiency of the protein α-actinin-3 which forms part of the contractile apparatus in muscle cells (MacArthur & North, 2007). The deficiency of this protein has been associated with the inhibition of fast twitch muscle fibre which would seem particularly unhelpful in power sports such as Weightlifting. However, studies involving athletes in Australia and Finland have found that it is exceptionally rare for elite level power athletes to have a deficiency of α-actinin-3 (North et al, 1999; MacArthur & North, 2007). Thus a degree of ‘genetic good luck’ is in play.

One of the most widely held beliefs is that the world’s best Weightlifters must possess a higher proportion of fast twitch muscle fibre. Before taking a look at whether this proposition might be true, some explanation of the difference between slow twitch and fast twitch muscle fibre is needed at this point.

One of the earliest papers that summarised the differences between fast and slow twitch muscle fibres, and used the terminology “Type I/Type II”, was published in 1960 by Dubowitcz and Pearse. In the more than 50 years since, it has been generally accepted that there are three (3) distinct muscle fibre types as summarised by Table 1.

Table 1: Types of Muscle Fibre
Type	Twitch Speed	Twitch Force	Energy Pathway	Fatigability	Mitochondria
I	Slow	Small	Oxidative	Low	Many
IIa	Fast	Intermediate	Oxidative-Glycolytic	Low	Many
IIx	Fast	Large	Glycolytic	High	Few

Adapted from MacIntosh, Gardner, McComes: Skeletal Muscle: Form and Function

Type I fibres produce less contractile force than Type II but are much more resistant to fatigue and therefore play a more prominent role in endurance activity. The greater fatigue resistance of Type I fibres is due to a high capacity to utilise the oxidative (aeroboic) energy pathway.

It should be noted that the classification of muscle fibres in the literature often confusingly refers to the subdivision of Type II fibres into Type IIa and Type IIb (as opposed to Type IIa and Type IIx). This confusion arises from the two different methods for testing for fibre type. In general however, Type IIx is preferred rather than Type IIb. Furthermore, the literature often refers to “hybrids” of these basic types. Such hybrids may exist as a result of imperfect classification techniques, or as a manifestation of cellular transformation from one fibre type to another (Schroeder, Rosser & Kim, 2014) or simply the various stages of degeneration/regeneration of muscle tissue.

In Weightlifting, it is a popular belief that champions must be genetically endowed with a higher proportion of Type II (fast twitch) muscle fibre as opposed to Type 1 (slow twitch). A study by Fry and colleagues (2003) examined and compared muscle biopsies (vastus lateralis) of male USA Weightlifters who had qualified for national championships with untrained male college students. Their finding was that the distribution of Type I /Type II muscle fibres among elite Weightlifters was similar to untrained persons. It is questionable therefore that the genetic gift of the Weightlifter is that they are born with a higher proportion of Type II (Fast Twitch) muscles fibres. What is clearly evident, however, is that there are significant differences between power and endurance athletes in the Type I/Type II distribution ratio (see Table 2). Therefore it can be said that Weightlifters will have a higher proportion of Type II (fast twitch) muscle fibres than endurance athletes.

Table 2: Muscle Fibre Type Distribution in Vastus Lateralis
Fibre Type	Type I and hybrids	Type IIa, IIx and hybrids	Source
Hammer Throwers	40%	60%	Terzis et al. (2010)
Weightlifters	48%	52%	Fry et al. (2003)
Cyclists	57%	43%	Costill et al. (1976)
Triathletes	63%	37%	Wilmore, Costill & Kenney (2008)
Long Distance Runners	67%	33%	Tesch & Karlsson (1985)
Orienteers	68%	32%	Jansson & Kaijser (1977)
Non-Athletes	44%	56%	Fry et al. (2003)
Non-Athletes	47%	53%	Wilmore, Costill & Kenney (2008)

While the proportion of Type II fibres in Weightlifters may not significantly differ from untrained individuals, there have been consistent findings of a decrease in Type IIx fibres and a corresponding increase in Type IIa fibres in individuals who engage in resistance training (Hather, Baldwin and Dudley, 1993; Williamson et al., 1991; Carroll et al., 1998; Williamson et al, 2001; Fry et al., 2003; Goruljov & Rumjanceva cited in Smrkolj & Škof, 2013;).  It is controversial to say that resistance training causes Type IIx fibres to convert to Type IIa but this altered ratio of Type IIa/Type IIx muscle fibres occurs in individuals irrespective of the nature of weight-training (Adam et al., 1993). Furthermore, “interconversions” between type IIa and IIx are well recognised in the literature (Smrkolj & Škof, 2013).

In addition, to an increase in proportion of Type IIa muscle fibres, the literature also recognises that there is a significant increase in their cross-sectional area, or hypertrophy of individual muscle fibres. This increase in cross-sectional area could be explained by an

Table 3: Cross-sectional area (μM2)
	Weightlifters	Untrained
Type I	6326	5488
Type IIA	8735	7024
Type IIB	2561	6234
Source: Fry et al. (2003)

increase in the contractile proteins, sarcoplasm, connective tissue or a combination of all of these (Wilmore, Costill & Kenney, 2008). Table 3 compares the cross-sectional area of muscle fibres in Weightlifters and untrained individuals and interestingly Type IIB (Type IIx) fibres are significantly smaller in size in Weightlifters than untrained individuals.

The notion that Weightlifting training increases the size and proportion of Type IIa fibres (medium twitch force, fatigue resistant) at the expense of Type IIx fibres (high twitch force, highly fatigable) seems counter-intuitive in the case of Weightlifters and raises many questions.  It is the very nature of the sport of Weightlifting that athletes should specialise in one-off maximal force contractions. Therefore why does the higher contractile force Type IIx fibre become practically non-existent? This question has no definitive answer at the moment but there are theories. One theory is that the transformation from IIx to IIa fibres might be due to requirement for Weightlifters to focus on power production rather than absolute strength, and that Type IIa fibres have more force development capability (Carroll et al., 1998). Another theory is that Type IIa may have a lower activation threshold which may be important in the recruitment of muscle fibres (Henneman cited in Carroll et al., 1998).

But one possible answer seems yet to be tested. If Type IIa fibres are more fatigue resistant is the Weightlifter’s preoccupation with sets of multiple repetitions to blame? Much of the training of Weightlifters focuses on sets of 3 repetitions and at various times of the year 5 repetitions on exercises such as squats is not uncommon. On the otherhand, single efforts at the highest intensities are a very small proportion of overall training.

Lastly, experiments have clearly demonstrated that muscle fibres change in characteristics when innervated by a different motor neuron. If a motor nerve to a fast twitch muscle fibre is transplanted to a slow twitch muscle fibre, the muscle fibre is transformed and takes on characteristics of a fast twitch muscle fibre. Similarly, a fast twitch muscle fibre will transform to a slow twitch muscle fibre if a motor nerve is transplanted that was formerly connected to a slow twitch muscle fibre (Buller, Eccles & Eccles, 1960). These transformations show strong evidence of the nervous system as a controlling agent of muscle contraction.