Ability of athletes in speed or endurance contests is somehow determined by inherited muscle fiber types.
Muscle fibers are multi-nuclei cells, responsible for muscle contractions. Each muscle is nearly composed of 10,000 to 450,000 muscle fibers.
There are two types of muscle fibers which are detectable on the color.Red muscle fibers (fiber type I or slow-ontracting fibers), due to high storage of blood and high percentage of myoglobin and mitochondria, can be observed in red. They slowly become exhausted and use the glycogen and fat as their fuel. White muscle fibers (type II fibers or fibers with fast contraction) have average blood storage and low levels of myoglobin and mitochondria. They only use glycogen as their fuel, and comparing to red fibers become tired more quickly. However, they are larger than red fibers with stronger contractions.
Red muscle fibers are consistent for long period aerobic activities and white muscle fibers for speed and short term anaerobic activities.
On average, these two types of contractile fibers are present equally in individuals. However, endurance athletes, have a greater percentage of red fibers (fibers with slow contraction) while, sprint athlete's have more white fibers (fast contracting fibers).
Distribution of these fibers in human muscles is also determined by genetic factors. One of the important genes involved in sport genetics is ACTN.(1
ACTN3 is located on chromosome 11q13-q14 and encodes α-actinin-3 protein, which belongs to highly conserved family of α-actinin proteins.
This family contains cytoskeletal proteins and belongs to the Spectrin superfamily; α-actinin-3 is one of the sarcomeric actinin isoforms.
Indeed; there are two genes that encode skeletal-muscle α-actinins; ACTN2 (MIM 102573) and ACTN3 (MIM 102574).
These two proteins have different patterns of expressions, ACTN2 is expressed in all fibers (including all skeletal muscle fibers, cardiac muscles and also brain) whereas expression of ACTN3 is confined to fast (type 2) muscle fibers and produces higher amounts of force in fast movements. Low expression of ACTN3 has reported in brain, as well.
Both of these proteins are components of the Z-line and play an actin anchoring role in the muscle. However, it has been stated that these proteins have also regulatory functions in coordinating myofiber contractions and a role in maintaining muscle cell integrity by linking Dystrophin.(2
, and 5
Genetic analysis of α-actinin-3 gene has showed a polymorphism R577X (rs1815739), which results in premature stop codon in the gene and leads to non functional α-actnin-3 protein.
Although, at first it was considered responsible for some muscular diseases, but more investigation cleared this polymorphism as a common one in normal population.(6
This data, suggested that α-actinin-2 can compensate absence of α–actinin-3 in homozygous people for null allele of ACTN3. However, no up regulation of α-actinin-2 in the absence of α-actinin-3 and highly conservation of α-actinin-3 since its divergence from α-actinin-2 and additionally their different patterns of expression, all together suggests a distinctive role for this protein.(4
Further researches showed that, ACTN3 genotype can be related to the performance in elite and endurance activities.
R577X polymorphism occurs in exon 16, which replaces Arginine by stop codon. Individuals homozygous for R577 have full copy of α-actinin-3 and elite and power sprint athletes show significantly higher frequency of 577R allele. In the other hand, some studies represented that X allele have high level of frequency in endurance athletes.(2
The mutated allele, 577X, has different frequencies in different populations which are shown in
Table 1 Frequencies of 577X and 577R alleles in different populations.2, 8
In ancient times, Iran was presumed in the way of human's migration, since there was no information about the frequency of ACTN3 in Iranian population, it seemed necessary to obtain information about genetic background of Iranian population. The objective of this study was to genotype normal Iranian individuals to determine the prevalence of each allele in our population.