In nonrunners, the IIa/IIx hybrid fibers correlated with exercise volume (r = −0.72, P < 0.05), and, in this case, the exponential curve fitting yielded a better fit (R2 = 0.82), whereas the I/IIa hybrids showed no linear or exponential relationship.
Proportions of MHC within hybrid fibers.
The proportion of IIa/IIx hybrid fibers (percentage of total fibers dissected) expressing predominantly MHC IIa (greater than 50%) was higher in nonrunners compared with runners (P < 0.05), with no difference between the two groups in the percentage of these hybrids with predominantly MHC IIx (Table 2). Similarly, runners and nonrunners did not differ for predominance of either MHC IIa or MHC I in the I/IIa hybrid fibers when the hybrid fibers were categorized simply into two subgroups. However, when the hybrid fibers were identified according to five categories of relative proportions of the slower of the two MHC isoforms within given hybrid fibers (1-20, 21-40, 41-60, 61-80, or 81-99%), trends emerged.
The histograms in Figure 5 illustrate the frequency distributions for the slower of the two MHC isoforms expressed in the two hybrid fiber populations (i.e., MHC I or MHC IIa proportions for I/IIa or IIa/IIx hybrid fibers, respectively), with the runners subdivided according to exercise pattern. The distribution of MHC IIa proportion in the IIa/IIx hybrid fibers is relatively normal in the runners training < 65 km·wk−1 (the same runners who preferred racing distances < 8 km) (Fig. 5A). However, in the runners with a longer training and racing distance, no fibers were found that expressed more than 60% MHC IIa. In contrast, when considering the influence of training pattern on the distribution of MHC I within the I/IIa hybrid fibers, a skewed distribution pattern was observed for both groups of runners (Fig. 5B): the frequency distribution was skewed in favor of a greater proportion of MHC I in the hybrids of those runners with training volume > 70 km·wk−1, but it was skewed in the opposite direction for those training < 65 km·wk−1.
This study addressed a variety of issues: i) whether training volume would account for the proportion of hybrid fibers in a group of athletes all participating in competitive distance running but with varying habitual training and in a nonhomogenous group of subjects either sedentary or recreationally active in a variety of sports; ii) whether a variable that influences both the volume of training and the speed of running would correlate better or worse with the proportions of hybrid fibers in runners; iii) whether the relationships would be more significant for the proportion of IIa/IIx than the I/IIa hybrid fibers; iv) whether the selection of subject cohorts with a range of training volumes could shed light on how much training might promote the formation of I/IIa hybrid fibers, which are less well understood than the IIa/IIx hybrid fibers; and v) whether quantification of the relative amounts of the different MHC isoforms within each hybrid fiber could illuminate whether hybrid fibers tend to be in transition or are clustered midway between two isoforms.
Significant differences between runners and nonrunners were found for the mean proportions of IIx fibers and IIa/IIx hybrid fibers, confirming existing knowledge. The literature provides ample evidence that regular exercise training reduces the proportion of type IIx fibers (4,11,21,29). Several studies have indicated that the proportion of IIa/IIx hybrid fibers is decreased in trained athletes and decreases in previously sedentary individuals with a training intervention (4,13,15,21,29). A main finding of the current study was that in both populations, exercise volume (kilometers per week and hours per week) was significantly related to IIa/IIx hybrid fiber proportions, despite the fact that one group trained systematically in a single sport for competition, whereas the other group had individuals who did not exercise or who exercised only for recreational reasons. In both populations, a higher volume of exercise resulted in fewer IIa/IIx hybrid fibers. The current study design allows for some refinement of the existing conclusion that increased exercise volume decreases the proportion of IIa/IIx hybrid fibers. Our data indicate specifically that training > 70 km·wk−1 consistently resulted in < 7% IIa/IIx hybrid fibers in distance runners. An application of this finding could be that training volume should not exceed 70 km·wk−1 if the velocity and power provided by these fibers are required for superior performance in shorter, middle-distance running events.
Previous data suggest that high proportions of IIa/IIx hybrid fibers (>15%) are mainly attributable to inactivity, resulting from paralysis, weightlessness, and detraining (2,5,6,17), whereas Harber et al. (13) found none of these hybrid fibers in gastrocnemius of endurance-trained track athletes racing between 3000 and 10,000 m. However, Andersen et al. (4) report that well-trained sprinters (100 m) had 12.9 ± 5.0% IIa/IIx hybrid fibers in their vastus lateralis, and our data suggest that even with training distances between 35 and 65 km·wk−1, these hybrids are in evidence in endurance runners (14.4 ± 4.6%, Fig. 2). We also show, for the first time, that taking into account the preferred speed of racing (i.e., PRDA) improves the prediction of how many IIa/IIx hybrid fibers will exist in vastus lateralis. This finding may support the concept of fine tuning skeletal muscle functional capacity through multiple different fiber types expressing differing proportions of MHC isoforms to accommodate both endurance and high power demands. When the runners were subdivided into two groups, the IIa/IIx hybrid fibers seemed to cluster predominantly midway between the two pure types, not tending toward a full conversion to type IIa in those runners with higher running speeds and relatively shorter training distances (Fig. 5A). In this subgroup, these fast hybrid fibers might not be simply transitional.
In nonrunners, there were fewer subjects participating in large volumes of exercise, but the data indicate that possibly 10 h or more of exercise per week would be required to clearly reduce IIa/IIx hybrid fiber proportions in athletes participating in sports with less continuous effort than distance running. This finding may be influenced by either the intensity or the duration of the rest periods of discontinuous-type exercise, as previously suggested by Campos et al. (11). The current study did not attempt to quantify, directly or indirectly, the exercise intensity or rest periods of nonrunners. Other authors have shown an increase in type IIa/IIx hybrid fibers as a result of high-power output training using either sprint cycling or resistance training as modalities (1,11). It is possible that the higher percentage of IIa/IIx hybrid fibers in nonrunners was also, in part, a function of the required power output in the chosen sports or the daily physical activities.
An important finding in the group of runners was that it was not only the type IIa/IIx hybrid fibers that correlated with training volume and PRDA, but also the I/IIa hybrid fibers (Fig. 4). Previous findings in human subjects after bed rest provide compelling evidence that a lack of physical exercise results in the expression of high proportions of I/IIa hybrid fibers (3,26). In contrast, Andersen et al. (4) report that well-trained sprinters (100 m) had only 0.2 ± 0.2% I/IIa hybrid fibers in biopsies taken from the vastus lateralis muscle after a competitive season. Holden (14) has concluded that little or no evidence currently exists to confirm that type IIa or IIx fibers convert to type I as a result of training. In the current study, the relationship between the proportion of pure slow-twitch muscle fibers and training distance was weak (P < 0.06), but the I/IIa hybrid fiber data (along with the previously published data on highly trained endurance skiers who had approximately 36% I/IIa hybrid fiber composition in the vastus lateralis muscle (15)) indicate that higher volumes of endurance training increase the existence of I/IIa hybrid fibers. In endurance runners who are training consistently at speeds suitable for races around 10 km (and in cross-country skiers), it is likely that both type I and type IIa fibers are recruited. With little or no recovery during a training session, there may be an increased duration of exposure of the intramuscular milieu to the signals promoting the expression of MHC type I, even in type IIa fibers, thus promoting the existence of more type I/IIa hybrid fibers than might be found in marathon runners training at lower speeds or sprinters who include long rest periods. In contrast, the inconsistent and intermittent activity patterns of the nonrunners in our study may explain the wide variety of type I/IIa hybrid expression, which could not be related statistically to training volume.
Caiozzo et al. (10) have suggested that the variety and continuum of hybrid fibers observed in rodents in response to various physiological conditions that affect MHC expression may lead to a spectrum of functional capacities, including small and finely tuned steps. Our data on the MHC distribution within the type IIa/IIx hybrid fibers in the runners training < 65 km·wk−1 suggest that a similar observation and conclusion are possible in human subjects. However, the proportions of MHC I within the type I/IIa hybrid fibers exhibit a skewed distribution, with more of these hybrid fibers containing higher MHC I proportions in those runners doing greater volumes of training (Fig. 5B). Therefore, the conversion of individual fibers in response to running training may not result in a well-distributed functional continuum for these hybrids. Also, Trappe et al.'s (25) data indicate that the response of pure type I fibers to marathon training is to improve maximal velocity of contraction without any change in MHC expression. Taken together with our data, this may be interpreted to mean that the significance of a greater expression of type I MHC may not be related solely to mechanics but, rather, to other concomitant changes such as smaller diffusion distances, altered metabolic profiles, improved economy, and resistance to fatigue.
In the current study, the proportions of pure fiber types did not correlate with either training volume or intensity. Therefore, although our data show that the proportions of hybrid fibers containing the IIa and IIx MHC isoforms, or a combination of fast and slow MHC isoforms, are both strongly influenced by recent training patterns, we suggest that the proportions of pure fiber types are also influenced by additional factors, most likely a longer training history.
This study is the first to show a linear decrease in IIa/IIx hybrid fibers as exercise volume increases in runners, but an exponential decrease with increasing preferred racing distance. This finding led specifically to the conclusion that in endurance runners, the proportion of IIa/IIx hybrid fibers is influenced by training volume and is further modulated by running speeds typically associated with the preferred racing distance. Furthermore, training distances below 65 km·wk−1 did not appreciably decrease the proportion of IIa/IIx hybrid fibers, despite the endurance nature of the training, whereas training distances above 70 km·wk−1 were clearly associated with a low proportion of the fast-twitch hybrid fibers. This is also the first study to show a positive linear relationship between I/IIa hybrid fibers and increasing training volume in runners and greater proportions of MHC I within the I/IIa hybrid fibers in those runners training more than 70 km·wk−1.
This study was supported by the Harry Crossley grant (K. H. Myburgh) and the South African National Research Foundation and the Swedish International Development Agency's bilateral program (K. H. Myburgh and B. Essen-Gustavsson). We would like to thank the Swedish Institute for a scholarship awarded to Tertius Kohn during a study period in Sweden. A special thank you goes to Bengt Saltin and Jesper Andersen for their financial and technical support during the study period of Dr. Kohn at the Copenhagen Muscle Research Centre, Denmark. The authors want to sincerely thank the subjects who participated in this study.
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Keywords:©2007The American College of Sports Medicine
SINGLE FIBERS; ELECTROPHORESIS; MYOSIN HEAVY CHAIN; TRAINING VOLUME