Mujeres ministras en Democracia (1975-2015)

In the late 19th century, French anatomist Louis Antoine Ranvier observed that some muscle in rabbits differed in colour and that the redder muscle contracted at a slower rate than the muscle with a more pale coloration (Needham, 1926). From this simple observation, the basis of skeletal muscle fibre type study was formed and the understanding that muscle was a heterogeneous tissue was established. Just shortly after Ranvier’s experiments, Grutzner (cited in Needham, 1926) proposed that all muscle contained a mixture of two different fibres (red and white) and that the response of a muscle to stimulation (whether fast or slow) depended on the proportion of these two fibres within the muscle. These early in-vivo studies were done with animals, until the advancement of surgical techniques made it possible to take samples of human muscle by biopsy (Zierath & Hawley, 2004). In the 1960’s, the needle biopsy technique was re-introduced to the field of physiology (Bergström & Hultman, 1966), enabling physiologists to obtain human muscle tissue while subjects were awake. Using this procedure, samples of human muscle became more readily available, allowing further development in the study of fibre-type.

In the late 1960’s and early 1970’s, studies by Edstrom and Kugelberg (Edström & Kugelberg, 1968), as well as Burke et al. (Burke, Levine, Zajac Iii, Tsairis, & Engel, 1971) showed a relationship between physiological parameters and histochemistry within motor units. These studies paved the way for histological and biochemical analysis of muscle samples, leading to current classifications based on histochemical, biochemical, morphological or physiological characteristics (Scott, Stevens, & Binder-Macleod, 2001).

Since these early studies, many techniques have been developed to classify fibre type, making the interpretation of fibre type data more difficult, as classifications from different techniques do not always agree (Staron, 1997). At present, the most common way of identifying muscle fibres in humans is by dividing them into different categories according to the myosin heavy chain (MHC) isoform found within the fibre. Because myosin isoform content in a given muscle is quite stable and relatively unaffected by contraction, it has been identified as a suitable basis for fibre-type classification (Astrand, Rodahl, Dahl, & Stromme, 2003). Furthermore, measurement of MHC isoforms is probably the best method of fibre typing currently available, because MHC isoforms are identified within single muscle fibres by electrophoresis, meaning the analysis is quantitative.

Another way muscle fibres are often identified is through histochemical staining to measure ATPase activity. The activity of ATPase, the enzyme responsible for breaking down ATP, is known to be associated with myosin isoform content (Billeter, Heizmann, Howald, & Jenny, 1981) and the speed of muscle shortening (Barany, 1967). When histochemically analyzed, muscle fibres with high ATPase activity are identified as type II muscle fibres, while those fibres with low ATPase activity are identified as type I fibres. Differing pH sensitivities within type II fibres can then be used to separate type II fibres into two subgroups; type IIA and type IIX (also commonly known as IIB) (Brooke & Kaiser, 1970 - citation only; Rosen, 1969). The problem with measuring ATPase activity with histochemical staining is that it is based on qualitative analysis of the different staining intensities, so that any given fibre could be classified differently by different researchers (Scott, Stevens, & Binder-Macleod, 2001).

Presently there are three common fibre type divisions in humans based on MHC content; type I – slow oxidative, type IIA – fast oxidative and type IIX– fast glycolytic. There are

limitations in trying to define muscle fibre types however, as functional properties vary within groups of muscle fibres that have the same fibre type designation (Astrand, Rodahl, Dahl, & Stromme, 2003). This is largely due to the ‘plasticity’ of muscle fibres; their ability to transition from fast to slower fibre types and vice versa, in response to certain stimulus which will be discussed later. Because of this ‘plasticity’, many sub-categories of fibre type, those that are transitioning between ‘pure’ fibre types, and therefore express two MHC isoforms, have been identified within human muscle (Pette, 2001). However, this review will only deal with the 3 most commonly used classifications of fibre type, identified above.

The needle biopsy technique also has many limitations. Within humans, fibre type varies between different muscles as well as within a given muscle (Blomstrand & Ekblom, 1982; J. A. Simoneau, Lortie, Boulay, Thibault, & Bouchard, 2008). Because biopsies are usually taken from only a few sites, this inter and intra-muscular variation is often not accounted for, and muscle samples are falsely considered as representative of the fibre type proportion of the whole muscle (Lexell, Taylor, & Sjostrom, 1985). Blomstrand and Ekblom reported that fibre type percentage varied between two biopsies taken from the same leg by 6% for type I fibres, 4% for type IIA and 5% for type IIX (Blomstrand & Ekblom, 1982). When the samples were taken from both legs this variation increased to 12%, 7% and 7% respectively. Furthermore, it appears that fibre type distribution varies depending on the depth of the muscle sample, with a greater proportion of type II fibres close to the surface and type I fibres in deeper regions of the muscle (Lexell, Henriksson-Larsen, & Sjostrom, 1983). The varying distribution of fibre type within whole muscle raises concern regarding the repeatability of obtaining muscle samples by biopsy, and the biopsy technique’s reliability in obtaining muscle samples to identify those with a certain ‘fibre type proportion’. What’s more, the muscle biopsy

technique is invasive and requires the skill of a trained physician with specialized equipment (Bergström, 1975), making it impractical for many research and clinical settings.

In accordance with early observations of different fibre types having different functional properties (Needham, 1926), more recent studies have shown, using non-invasive techniques, that contractile function of muscle, such as time to peak torque and fatigability measured externally, is related to fibre type proportion in-vivo (Ivy, Withers, Brose, Maxwell, & Costill, 1981; MacIntosh, Herzog, Suter, Wiley, & Sokolosky, 1993; Sadoyama, Masuda, Miyata, & Katsuta, 1988; E. Suter, Herzog, Sokolosky, Wiley, & Macintosh, 1993; Thorstensson & Karlsson, 1976). To the author’s knowledge, these techniques have not yet been used as a surrogate measure of fibre type proportion in larger studies. Nevertheless, the relationships observed between histo/biochemically determined fibre type and functional characteristics, indicate that methods which measure muscle contractile properties externally, may provide a non-invasive way to estimate fibre type proportion in future studies. Therefore, further development of these techniques is required.

One method of modelling muscle contraction which has been used a great deal in the clinical setting is electrical stimulation (ES). ES enables muscle to contract independent of the central nervous system and is therefore useful for maintenance of strength, muscle mass and physical activity levels during muscle immobilization/paralysis (Delitto & Snyder-Mackler, 1990). This technique is useful in a research setting as the magnitude of muscle contraction can be controlled with ES, and psychological factors such as motivation are bypassed. For example, electrical stimulation has been used to study the role of the nervous system in force production. By administering an electrical impulse to a maximal voluntary contraction, and observing that greater force was produced with the addition of electrical stimulation, early

researchers showed that neural factors limited maximal force production rather than the muscle structure (Shield & Zhou, 2004). The most important factor to consider when administering percutaneous ES is the location of electrode on the skin. Placement of electrode pads should ensure that antagonist muscles are not being stimulated at the same time as agonists as incorrect placement of electrodes can alter force measurements (Shield & Zhou, 2004). Nevertheless, apart from error associated with electrode placement, technical error is small, as the administration, the timing and magnitude of the twitch, is controlled electronically.

Although the ES technique has been used extensively in a research setting as a substitute for voluntary exercise, its use in stimulating muscle to measure contractile properties has hardly been employed. The only published study I know of which has employed ES to measure muscle contractile properties is one by Blimkie et al. (Blimkie, Sale, & Bar-Or, 1990). Thus, although validity and limitations have been discussed in regard to the use of electrical stimulation, these discussions have focused on the validity of ES as a tool for simulating exercise rather than creating twitches for measurement of contractile properties. Therefore, the validity of ES for use in this way, and its limitations, has not been addressed.