Approximately one billion people worldwide are homozygous for a stop codon polymorphism in the ACTN3 gene (R577X) which results in complete deficiency of the fast fibre muscle protein α-actinin-3. ACTN3 genotype is associated with human athletic performance and α-actinin-3 deficient mice [Actn3 knockout (KO) mice] have a shift in the properties of fast muscle fibres towards slower fibre properties, with increased activity of multiple enzymes in the aerobic metabolic pathway and slower contractile properties. α-Actinins have been shown to interact with a number of muscle proteins including the key metabolic regulator glycogen phosphorylase (GPh). In this study, we demonstrated a link between α-actinin-3 and glycogen metabolism which may underlie the metabolic changes seen in the KO mouse. Actn3 KO mice have higher muscle glycogen content and a 50% reduction in the activity of GPh. The reduction in enzyme activity is accompanied by altered post-translational modification of GPh, suggesting that α-actinin-3 regulates GPh activity by altering its level of phosphorylation. We propose that the changes in glycogen metabolism underlie the downstream metabolic consequences of α-actinin-3 deficiency. Finally, as GPh has been shown to regulate calcium handling, we examined calcium handling in KO mouse primary mouse myoblasts and find changes that may explain the slower contractile properties previously observed in these mice. We propose that the alteration in GPh activity in the absence of α-actinin-3 is a fundamental mechanistic link in the association between ACTN3 genotype and human performance.