One of the most studied genes related to sport is the ACTN3 gene, which encodes the α-actinin-3 protein expressed specifically in fast glycolytic muscle fibers. A common polymorphism of this gene is R577X, which results in a change of codon for arginine (R) to termination codon (X). The aim of the thesis is to review the existing literature describing the influence of the ACTN3 R577X null polymorphism on various aspects of physical activity. The absence of α-actinin-3 has phenotypic consequences such as altered structural properties, metabolism and signaling pathways of fast-twitch muscle fibers, altered glycogen content in muscles, different Ca2+ kinetics in connection with the sarcoplasmic reticulum and reduced bone mineral density. Homozygous carriers of the mutation (genotype XX) have a smaller diameter of fast-twitch muscle fibers, larger stores of muscle glycogen and reduced bone mass. Moreover aerobic metabolism dominates in their fast-twitch muscle fibers. Because of that athletes with the XX genotype should have greater muscular endurance, while athletes with the RR genotype should have greater muscular strength and explosiveness. Research has also shown that X allele affects individual's increased susceptibility to endurance training, faster regeneration after sports activity and ensures better physical fitness of the elderly. However, X allele is also associated with increased severity and frequency of muscle, joint and bone injuries caused by exercise and the poor health of the elderly.
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