Παράθεση:
"What happens when exercise stops? Do the additional 2a fibers then convert back to 2x? The answer is yes, but not in the precise manner that might be expected. To study this issue, we took muscle samples (biopsies) from the vastus lateralis muscle of nine young, sedentary Danish men. We then had the subjects conduct heavy resistance training, aimed mainly at their quadriceps muscle, for three months, ending with another muscle biopsy. Then the subjects abruptly stopped the resistance training and returned and their sedentary lifestyle, before being biopsied for a third and final time after a three-month period of inactivity (corresponding to their behavior prior to entering the training).
As expected, the relative amount of the fast myosin 2x isoform in their vastus lateralis muscle was reduced from an average of 9 percent to about 2 percent in the resistance-training period. We then expected that the relative amount of the 2x isoform would simply return to the pretraining level of 9 percent during the period of inactivity. Much to our surprise, the relative amount of myosin 2x reached an average value of 18 percent three months into the detraining. We did not continue the biopsies after the three-month period, but we strongly suspect that the myosin 2x did eventually return to its initial value of about 9 percent some months later.
We do not yet have a good explanation for the overshoot phenomenon of the expression of the fast myosin 2x isoform. Nevertheless, we can draw some conclusions that can have useful applications. For instance, if sprinters want to boost the relative amount of the fastest fibers in their muscles, the best strategy would be to start by removing those that they already have and then slow down the training and wait for the fastest fibers to return twofold! Thus, sprinters would be well advised to provide in their schedule for a period of reduced training, or tapering, leading up to a major competition. In fact, many sprinters have settled on such a regimen simply through experience, without understanding the underlying physiology.
Conversion between the two fast fiber types, 2a and 2x, is a natural consequence of training and detraining. But what about conversion between the slow and fast fibers types 1 and 2? Here the results have been somewhat murkier. Many experiments performed over the past couple of decades found no evidence that slow fibers can be converted to fast, and vice versa. But in the early 1990s we did get an indication that a rigorous exercise regimen could convert slow fibers to fast 2a fibers.
Our subjects were very elite sprinters, whom we studied during a three-month period in which they combined heavy resistance training with short-interval running (these are the foundation exercises in a sprinters yearly training cycle). At around the same time, Mona Esbornsson and her CO-workers at the Karolinska Institute in Stockholm reported similar findings in a study involving a dozen subjects who were not elite athletes. These results suggest that a program of vigorous weight training supplemented with other forms of anaerobic exercise converts not only type 2x fibers to 2a but also type 1 fibers to 2a.
If a certain type of exertion can convert some type 1 fibers to 2a, we might naturally wonder if some other kind could convert 2a to 1. It may be possible, but so far no lengthy human training study has unambiguously demonstrated such a shift. True, star endurance athletes such as long-distance runners and swimmers, cyclists and cross-country skiers generally have remarkably high proportions-up to 95 percent, as mentioned earlier-of the slow type 1 fibers in their major muscle groups, such as the legs. Yet at present we do not know whether these athletes were born with such a high percentage of type 1 fibers and gravitated toward sports that take advantage of their unusual inborn trait or whether they very gradually increased the proportion of type 1 fibers in their muscles as they trained over a period of many months or years. WE do know that if fast type 2a fibers can be converted to type 1, the time required for the conversion is quite long in comparison with the time for the shift from 2x to 2a.
It may be that great marathon runners are literally born different from other people. Sprinters, too, might be congenitally unusual: in contrast with long-distance runners, they of course would benefit from a relatively small percentage of type 1 fibers. Still, a would-be sprinter with too many type 1 fibers need not give up. Researchers have found that hypertrophy from resistance training enlarges type 2 fibers twice as much as it does type 1 fibers. Thus, weight training can increase the cross-sectional area of the muscle covered by fast fibers without changing the relative ratio between the number of slow and fast fibers in the muscle. Moreover, it is the relative cross-sectional area of the fast and slow fibers that determines the functional characteristics of the entire muscle. The more area covered by fast fibers, the faster the overall muscle will be."
the most important thing is type II fibers, the difference between type IIA and type IIB isn't that great... there is also a chart that shows type IIA and type IIB levels of elite sprinters... they have 80% type II and about 40/40 type IIA/type IIB.
What really should be avoided if you want to be fast/explosive is high volume aerobic activity as it can convert type II to Type I...