The influence of variations in muscle fibre

[warsaw]

Physiological Evolution of the
Rowing Athlete: a 25-year Study
Authors: Fritz Hagerman and Kumika Toma (USA)
Introduction
It is well known that a 2000-mcompetitive rowing effort requires a substantial
aerobic energycontribution (75-80%). Although anaerobic energysources are
only responsible for 20-25% of the total energyfor this effort, theyare no less
important as they are needed most at critical periods of the race: at the start and
again during the sprint at the finish. Optimal energyproduction is dependent on
several complex and interdependent physiological responses which are often
described as utilizationand transportation. Utilization of stored energy in
skeletal muscle fibers begins with the use of high energy phosphate compounds
(ATP and Phosphocreatine) followed rapidlybythe metabolismof intracellular
carbohydrate (glycogen), fat, and protein which, when combined with oxygen,
will provide the energy to sustain exercise. Transportation is represented bya
series of important processes using such functions as neurological, cardiovascular,
respiratory,and hematological, all working together to deliver oxygen and
nutrients to the working muscle and stimulate its contraction.
Several aerobic sports and/or events are dependent on a similar energycomponent
relationship as rowing, however, athletes in these sports and/or events are not
required to generate as much muscular power as rowers and all involve individual
participation. A crew is thus onlyas strong as its weakest member. The
excessive energydemands of rowing and the emphasis on high muscular power
output seemto attract several geneticallygifted and hard working athletes whose
skeletal muscle fiber composition and respiratory- cardiovascular delivery
systems show extreme adaptations. These adaptations are represented bythe
development of someveryunique physical and physiological qualities which
have continued to evolve and improve as indicated bythe comparative data that
are presented in this paper. All of the comparisons represent observations of U.S.
rowers over a 25-year period and include over 2000 subjects.



Skeletal Muscle Qualities
Because of the emphasis on both muscular power and cardiorespiratoryendurance
it was originallyhypothesized that the dominant muscle fiber in rowing muscles
would be the normally larger fast IIA or Fast-Twitch-Oxidative fiber. However,
to our surprise most of the fibers turned out to be of the Slow-Twitch or Type I
variety (similar to a marathon runner's muscle) but unusuallylarge in diameter.
Average percentages of 70-75%for Type I fibers have been consistentlyobserved
in the vastus lateralis muscle of rowersas opposed to 20-25%Type IIA fibers and
almost no Type IIB or Fast-Twitch-Glycolytic fibers represented (see Figure 1).




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Weightliftiers&Powerlifters



www.researchgate.net/publication/8966942_Muscle_fiber_characteristics_and_performance_correlates_of_male_Olympic-style_weightlifters/file/32bfe50eea5ec4a8b5.pdf
www.luzimarteixeira.com.br/wp-content/uploads/2011/04/tipo-de-fibra03.pdf

[warsaw]

ENZYME-IMMUNOHISTOCHEMICAL ASPECTS OF MUSCLE
FIBER TYPE CLASSIFICATION IN MAMMALS
Gregor Fazarinc
Summary: Skeletal muscles are the most abundant and adaptable tissue in mammalians. They are composed of heterogeneous muscle fibers, in which distinct sets of structural proteins and metabolic enzymes are expressed. The percentages
of different muscle fibers in the muscle define its morphological and functional characteristics. In this review, we summarize
enzyme-immunohistochemical techniques to present muscle fiber type characteristics and their diversity in somatic skeletal
muscles of various animal species. The principal methods to define myofiber properties on the tissue sections are based
on the immunohistochemical determination of myosin heavy chain (MHC) isoforms and the myosin ATPase and metabolic
enzyme histochemistry Four MHC isoforms (-I, -IIa, -IIx and -IIb) have been detected in somatic skeletal muscles of small
mammals. Fibers that co-express more than one MHC isoform simultaneously are labeled as hybrid myofibers and are indicators of muscle fiber transition. The maximal shortening velocity of MHC fibers increases in the following order: -I < -IIa
< -IIx < -IIb. On the basis of the myosin ATPase activity myofibers have been classified as types I, IIA, IIB and IIC. Type IIC
fibers represent an intermediate type between MHC-I and type MHC-IIa fibers. Most large mammals do not possess fastest
MHC-IIb isoform, although some recent studies in pigs and llamas have shown the existence of all three fast MHC isoforms
in their skeletal muscles. Additional MHC isoforms are present transitorily during development, and in some highly functionally specialized muscles such as extraocular, laryngeal and masticatory muscles (MHC-extraocular, MHC-m). Embryonic
and neonatal MHC isoforms are expressed during muscle development and regeneration. Slow MHC-I myofibers show
high oxidative capacity, whereas fast MHC-II myofibers revealed entire spectrum of metabolic enzymes activity with large
overlaps between contractile fiber types. Combining the contractile classification with metabolic enzymes activity, myofibers can be basically defined as slow-twitch oxidative (SO), fast-twitch oxido-glycolytic (FOG) and fast-twitch glycolytic (FG).
In most cases enzyme and immunohistochemical techniques are not fully interchangeable, which makes combination of
different techniques necessary to get a reliable classification of myofibers.
Correspondence between myofiber
classification systems
It is well documented that maximal shortening
velocity of muscle fibers expressing homologous
MHC isoforms greatly decreases with increasing
body size (61). Such functional diversity of the homologous fast MHC isoforms between species is
likely related to different structural characteristics.
This is probably one reason why MHC antibodies,
which are usually raised against rat isoforms, can
have diverse reactivity with homologous MHC in
large mammals and why myosin ATPase histochemistry protocols must be adjusted to each animal species. Because of some important differences in the
reactivity of MHC antibodies between species (Table
1), a set of different antibodies is usually used to
avoid myofiber misclassification.



The specificity of MHC-slow antibodies is unambiguous because they revealed type I myofibers in skeletal muscles of all species (25, 48, 49,
60), suggesting that the slow MHC-I isoform is
highly conserved among species. On the opposite,
fast MHC isoforms can show different antigenic
properties between species. Thus, both A4.74 and
SC-71 antibodies are specific to MHC-IIa isoform
of rat, but cross-react with MHC-IIx in human,
dog, pig, goat (26, 48, 60, 62, 63). In bear skeletal muscles both antibodies actually recognize
MHC-IIx and not MHC-IIa isoform (51). This was
confirmed with antibody BF-35, which reveals all
MHC exept IIx in rat (42). Similar problems wer
observed with the antibody F113.15F4, which
recognizes MHC-IIa, -IIx and -IIb in rat, and only
MHC-IIa and -IIx in dog and bear (25, 51). Some
misclassification of myofibers between species
also occurs using myosin ATPase histochemistry.
The staining pattern of fibers depends upon the
lability of myosin ATPase to pH preincubation and
is related to the MHC isoforms content within a
single myofiber. When two isoforms are expressed
in the same myofiber, the staining pattern of the
myosin ATPase is ambiguous and can lead to misclassification of the muscle fiber type, especially
of fast type II fibers. In the past myosin ATPase
based classification led to some contradictory reports on fast fiber sub-types in large mammals.
In some studies of canine muscles only type IIA
and IIC myofibers were found (64, 65, 66, 67). On
the contrary, other authors claimed that type IIB
myofibers are present even in dogs, although they
were slightly less acid-labile than type IIB in other
species (68). The immunohistochemical labeling
of MHC isoforms demonstrated that strongly acid
stable subclass of canine fast fibers, which were
dark after preincubation at pH 4.6 and would thus
correspond to type IIB myofibers of other species,
actually expressed MHC-IIa isoform, and the more
acid labile sub-class, which were named as IIDog
fibers (69, 70) actually corresponded to MHC-x fibers (26). Such integrated use of both myosin ATPase and immunohistochemical labeling of MHC
isoforms demonstrated that type IIB fibers have
been misclassified in numerous previous studies
based upon traditional myosin ATPase histochemistry in other large mammals as well (71). Myosin
ATPase can also lead to fiber misclassification because of partial denaturation of the enzyme. Thus,
the rapid postmortem acidification combined with
increased muscle temperature encountered in
some glycolytic muscles of stress susceptible pigs
can lead to irregular and altered myosin ATPase
staining these PSE (pale, soft and exudative) muscles (Figure 3). In such case, the use of antibodies
against MHC isoforms is a far more reliable technique to type myofibers (72)
Conclusion
Big differences in muscle fiber type composition
exist between muscles and species, and between individuals within species. It is well documented that
skeletal muscles is a highly adaptable tissue which
can be influenced by many intrinsic and extrinsic
factors, such as age, altered neuromuscular activity and mechanical loading. The principal methods
to type myofibers on the tissue cryosection are the
immunohistochemical detection of MHC isoforms
and the myosin ATPase and metabolic enzyme histochemistry. Overall, it must be stressed that immunohistochemical and enzyme histochemical
classifications are not always fully interchangeable
between and even within species, suggesting that
different techniques often have to be combined to
get a reliable myofiber typing.
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