, 2008). Finally, several reports proved that the D allele was associated with elite power athlete status. On the other hand, an excess of the I allele has been associated with some aspects of endurance performance (Bray et al., 2009). The allele I and II genotypes are related to greater improvements in medium that duration aerobic performance (Cam et al., 2007) as well as being associated with an increase in the endurance and effectiveness of muscles and are also responsible for an increase in the proportion of free fibers (type I muscle fibers) (Macarthur and North, 2005). Nevertheless, some

studies do not confirm these observations (Orysiak et al., 2013). The second most frequently investigated gene in the context of sport is ACTN3, coding the protein α-actinin-3. Alpha-actinins are an ancient family of actin binding proteins (Mills et al., 2001) that play structural and

regulatory roles in cytoskeletal organization. In skeletal muscle, two alpha-actinin proteins (α-actinin-2 and α-actinin-3) comprise an important structural component of the Z disc, where they anchor actin thin filaments, helping to maintain the myofibrillar array. Besides their mechanical role, both sarcomeric alpha-actinins interact with proteins involved in numerous signalling and metabolic pathways (Mills et al., 2001). While α-actinin-2 is expressed in all types of muscle fibers, the expression of α-actinin-3 is almost exclusively restricted to fast, glycolytic type II fibers (Clarkson et al., 2005). In 1999, North et al.

(1999) identified a common polymorphism in ACTN3 R577X (dbSNP rs1815739) that resulted in the absence of α-actinin-3 in more than one billion people worldwide. The first evidence that a mononucleotide difference in DNA sequence was associated with power ability referred to the R577X polymorphism of the ACTN3 gene (Yang et al., 2003), where the translation (C > T) at nucleotide position 1747 in the ACTN3 coding sequence converts an arginine (R) to a stop codon (X) at residue 577 (Mills et al., 2001). This variation creates two different versions of the ACTN3 gene, both of which are common in the general population: the 577R allele is the normal, functional Carfilzomib version of the gene, whereas the 577X allele contains a sequence change that completely prevents the production of functional α-actinin-3 protein (North et al., 1999). The 577R allele and 577RR genotype of the ACTN3 gene are associated with top-level, power-orientated athletic performance in a wide array of ethnic groups (Yang et al., 2003; Niemi et al., 2005; Cięszczyk et al., 2011). Additionally, there is a positive association between the presence of the R allele and the capacity to perform high-power muscle contractions (Clarkson et al., 2005). Furthermore, Vincent et al.