Fiber types are denoted with capital letters (IIA, IIAB, etc.) whereas MHC is small letters. MHC IId/x has been found in many mammals, was first found in the diaphragm (hence the “d”). Again, sprinters will have a lot of IIa, some I, and likely little else.
Andersen JL ; Klitgaard H ; Saltin B
Affiliation: August Krogh Institute, University of Copenhagen, Denmark.
Title: Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training.
Source: Acta Physiol Scand (Acta physiologica Scandinavica.) 1994 Jun; 151(2): 135-42
Additional Info: ENGLAND
Standard No: ISSN: 0001-6772; 1365-201X; NLM Unique Journal Identifier: 0370362
Abstract: The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of a group of male sprint athletes (n = 6) was analysed, before and after a three months period of intensive strength- and interval-training, using a sensitive gel electrophoretic technique. Significant improvements were observed after training in almost all of a series of performance tests. After training the sprinters revealed a decrease in fibres containing only MHC isoform I (52.0 +/- 3.0% vs. 41.2 +/- 4.7% (mean +/- SE) (P < 0.05)) and an increase in the amount of fibres containing only MHC isoform IIA (34.7 +/- 6.1% vs. 52.3 +/- 3.6% (P < 0.05)). Fibres showing co-existence of MHC isoforms IIA and IIB decreased with training (12.9 +/- 5.0% vs. 5.1 +/- 3.1% (P < 0.05)). Only one out of 1000 fibres analysed contained only MHC isoform IIB. In contrast, a higher amount of type IIB fibres (18.8 +/- 3.6% vs. 10.5 +/- 3.9%, (P < 0.05)) was observed with myofibrillar ATPase histochemistry. The majority of histochemically determined type IIB fibres of sprinters seems therefore to contain both MHC isoforms IIA and IIB. Sprint-training appears to induce an increased expression of MHC isoform IIA in skeletal muscles. This seems related to a bi-directional transformation from both MHC isoforms I and IIB towards MHC isoform IIA.
The point is, whether you are faster or slower as a result of training, porbably has little or nothing to do with your fiber type. Even endurance athletes have a IIb to IIa conversion. Fast or slow, well trained people have little IIb MHC or IIB fibers. Can you tell me where I can find the reference to the slowest IIB being the same speed as the fastest type I? I am interested as I was not aware of that. Bottinelli has shown that IIa and IIb fibers overlap in unloaded shortening velocity, but that overlap is explained (r=.9) by MLC expression.
I am not convinced that fiber type tells you all that much. Percent area may be a good indicator, but we cannot separate out the influence of the nervous system.
I was answering the sceanrio above. Whether fast fibres have converted and are still explosive. Probably yes to the conversion and slowing in speed. The reason I used the Staron article was to make the point that fibre conversions are reversible in adults and so he could regain his speed again if he trained more specifically as a sprinter. Some seem to believe that fibre conversions once intiated are irreversible, which is not the case. This was the point I was trying to make.
There are different types of type ii such as iia, iia/b, iib, iix (iix not yet confirmed in human subjects but in Rats), no one knows precisely which set of subtypes are most important to or abundant in sprinters. I was not trying to say that iib are predominant in trained sprinters just the fact that conversion is reversible and so any specific adaptations made by other training not similar to sprinting can be reversed by proper training. Also the Guassian curve will apply to even fast fibres, what I mean is a percentile of type ii will contract as slowly as the fastest type i fibres; and so it is not correct to say that in-vivo performances do not support the speed contractions of iia and iib, as found in in-vitro. The guassian curve applies even to fast fibres, some will be almost predestrian others as fast as the proverbial off a shovel in in-vivo. So it is fair to say that most likely a sprinters fast fibres are operating in the higher levels of fast fibre speeds and a middle/long distance in the lower percentiles.[/QUOTE]