2.6 Muscle Fibre type and Diabetes
2.6.5 Fibre type proportion: Genetically or environmentally determined?
When observing a variety of sporting and athletic events at the elite level, it is difficult not to notice an ethnic bias towards success in particular events. It is common to see an African- American running the 100m sprint at the Olympics, while Caucasians usually find success in long-distance events (Cited in Suminski, Mattern, & Devor, 2002). On the other hand it is not uncommon to see an African Marathon runner take a gold medal, or a European dominate the power lifting. It is often remarked that some ethnicities are genetically predisposed to excel in different athletic events (Hunter, 1998). Alternatively factors like socio-economic status or exposure to certain sports may also explain these ethnic biases.
From their classic studies which investigated fibre type proportion in monozygous and dizygous twins, Komi et al. (Komi et al., 1977) were the first to provide evidence that fibre type proportion is mostly determined by genetic influences. In accordance with these findings, Ama et al. showed that a cohort of Black men had a lower percentage of type-1 fibres when compared to Caucasian men (Ama et al., 1986). However, the differences in fibre type proportion between the two ethnic groups in this study, was only slightly larger than the sampling error which occurs when using the needle biopsy to collect muscle tissue. However, a study in the US found that obese African-American woman had a lower percentage of type- 1 fibres than obese Caucasian women (Tanner et al., 2002), while others have showed in a cohort of sub-elite endurance runners, that African men had lower proportions of type I muscle fibre than European men (Kohn, Essen-Gustavsson, & Myburgh, 2007). Others challenge this idea however, suggesting that environmental factors explain a large part of the variation (J. A. Simoneau & Bouchard, 1995).
Simoneau and Bouchard (J. A. Simoneau & Bouchard, 1995) suggest that fibre type proportion can be partly explained by inherited factors (45%) and partly by environmental factors (40%). Elite athletes involved in endurance exercise appear to have a high proportion of slow-twitch fibres, while sprinters have more fast-twitch fibres (Costill et al., 1976). While this seems to imply a fibre-type change with training, selection into events by individuals possessing a natural endowment can occur (P. D. Gollnick, Armstrong, Saubert, Piehl, & Saltin, 1972), which may in part explain the ethnic grouping observed in certain athletic events.
Interestingly, studies have shown that muscle fibre proportions can change with training. Adams et al. (Adams, Hather, Baldwin, & Dudley, 1993) found that resistance training led to
a decrease in type IIB fibres and an increase in type IIA fibres in a cohort of healthy men. Staron et al. (Staron et al., 1991) also saw an increase of type IIA fibres and a decrease in type IIB fibres after resistance training in a cohort of women that had detrained previous to the study. Both of these studies also showed that detraining causes the conversion in reverse (from IIA to IIB). Similar to resistance training, it appears that the shift from IIB to IIA occurs with endurance training also (Schantz, Billeter, Henriksson, & Jansson, 1982). A reduction in the proportion of type I fibres is seen in the denervated muscle of paraplegics, but can be reversed with muscle contraction by way of electrical stimulation (Martin, Stein, Hoeppner, & Reid, 1992; Rochester et al., 1995). All of these findings suggest that chronic contractile activity induces a conversion from faster, fatigable muscle fibres, to slower, fatigue resistant fibres. Detraining or denervation on the other hand, induces the conversion in the opposite direction, from slow to fast fibres. Nevertheless, it appears that in normal individuals fibre type conversion in response to training or detraining, is limited to the conversion from IIA to IIX and vice versa (P. Andersen & Henriksson, 1977; Houmard et al., 1993). While, studies in rats have revealed that electrical stimulation within denervated fast-twitch muscle fibres, can ‘transform’ these fibres into slow-twitch muscle (Windisch, Gundersen, Szabolcs, Gruber, & Lømo, 1998), no conclusive research exists to show conversion from type IIA to type I fibres in humans (Daugaard & Richter, 2001). Some even propose that exercise increases the oxidative capacity of muscle fibres rather than actually changing fibre type composition (P. D. Gollnick, Armstrong, Saubert, Piehl, & Saltin, 1972).
2.6.6 Summary
In summary, though it was previously thought that fibre type was solely genetically determined, it has more recently been shown that conversion between fibre types does exist,
proportion also exist, but caution must be used in interpreting the results of these studies as confounding factors that also relate to ethnicity, such as socioeconomic status, may explain part of the variation. It appears that chronic exercise induces a switch from faster, less insulin sensitive muscle to slower, more insulin sensitive fibres. Inactivity on the other hand, causes a shift from slower to faster muscle fibres. If this is so, the physiological benefits of exercise would be two-fold for those at risk of insulin resistance; first, from reductions in lipid content and improved oxidative capacity of individual muscle fibres, and second from an increased proportion of the slower, more insulin sensitive fibre types. Genetics, rather than determining a set fibre type proportion, may instead provide the limits of fibre type conversion in response to stimulus. Because of the invasive nature of the muscle biopsy, research on fibre type proportion has been limited in humans. Less invasive procedures, which measure muscle contractile properties externally, have produced results that are closely related to the fibre type proportions obtained through biopsy (E. Suter, Herzog, Sokolosky, Wiley, & Macintosh, 1993). However, it appears that these methods have not been utilized as a substitute to the biopsy technique in research, or to identify those at risk of insulin sensitivity. Exercise induced improvements in glucose tolerance are specific to the fibre types recruited, thus developing a non-invasive means to measure fibre type proportion could be valuable in prescription of fibre-type specific exercise.