A common polymorphism (R577X) in the ACTN3 gene results in complete deficiency of alpha-actinin-3 protein in approximately 16% of humans worldwide. The presence of alpha-actinin-3 protein is associated with improved sprint/power performance in athletes and the general population. Despite this, there is evidence that the null genotype XX has been acted on by recent positive selection, likely due to its emerging role in the regulation of muscle metabolism. alpha-Actinin-3 deficiency reduces the activity of glycogen phosphorylase and results in a fundamental shift toward more oxidative pathways of energy utilization.