Evidence for ACTN3 as a Speed Gene in Isolated Human Muscle Fibers

PURPOSE: To examine the effect of α-actinin-3 deficiency due to homozygosity for the ACTN3 577X-allele on contractile and morphological properties of fast muscle fibers in non-athletic young men. METHODS: A biopsy was taken from the vastus lateralis of 4 RR and 4 XX individuals to test for differences in morphologic and contractile properties of single muscle fibers. The cross-sectional area of the fiber and muscle fiber composition was determined using standard immunohistochemistry analyses. Skinned single muscle fibers were subjected to active tests to determine peak normalized force (P(0)), maximal unloading velocity (V(0)) and peak power. A passive stretch test was performed to calculate Young’s Modulus and hysteresis to assess fiber visco-elasticity. RESULTS: No differences were found in muscle fiber composition. The cross-sectional area of type II(a) and II(x) fibers was larger in RR compared to XX individuals (P<0.001). P(0) was similar in both groups over all fiber types. A higher V(0) was observed in type II(a) fibers of RR genotypes (P<0.001) but not in type I fibers. The visco-elasticity as determined by Young’s Modulus and hysteresis was unaffected by fiber type or genotype. CONCLUSION: The greater V(0) and the larger fast fiber CSA in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions.