Key result
Women with the ACTN3 X/X genotype had lower knee extensor concentric peak torque (89.5 vs. 99.3 Nm, p<0.05) and fat free mass than R-allele carriers, which mostly explained the strength differences.
Why the study?
Does the ACTN3 X/X genotype influence muscle strength and fat free mass in men and women across the adult age span?
Cross-Sectional (n=803)
Does the ACTN3 X/X genotype influence muscle strength and fat free mass in men and women across the adult age span?
The ACTN3 X/X genotype is associated with lower knee extensor strength and fat free mass in women, but not in men, across the adult age span.
Answers are grounded in this paper and cite their sources.
X/X genotype should not guide training or sarcopenia care; leaves open sex-specific effects for prospective validation.
Alpha-actinin-3 is a structural protein of the sarcomeric Z-line that is specifically expressed in type 2 muscle fibers and is important for anchoring actin and playing a regulatory function in coordinating muscle fiber contraction. Alpha-actinin-3 is absent in individuals with homozygosity of a common stop codon (X-allele of the R577X polymorphism), in the ACTN3 gene. The ACTN3 R577X polymorphism has been associated with elite athletic performance, and recently associated with muscle strength differences in the general population. PURPOSE: We sought to determine the association of ACTN3 R577X with muscle strength and mass phenotypes in men and women across the adult age span. METHODS: Eight hundred three (n=803) adult volunteers (416 men and 387 women) aged 22-90 years were genotyped for ACTN3 R577X using standard techniques. Knee extensor (KE) concentric and eccentric peak torque values were determined using isokinetic dynamometry and fat free mass (FFM) was determined by dual-energy X-ray absorptiometry. RESULTS: Sample sizes by genotype group were: X/X=132, X/R+R/R=671 (men: X/X=77, X/R+R/R=339, women: X/X=55, X/R+R/R=332). Women deficient in alpha-actinin-3 (X/X) displayed lower KE concentric peak torque (30°/sec: 89.5 ± 3.5 vs. 99.3 ± 1.4 Nm, p<0.05; 180°/sec: 60.3 ± 2.6 vs. 67.0 ± 1.0 Nm, p<0.05) and KE eccentric peak torque (30°/sec: 122.8 ± 5.7 vs. 137.0 ± 2.2 Nm, p<0.05; 180°/sec: 121.8 ± 5.8 vs. 138.5 ± 2.2 Nm, p<0.05) than R-allele carriers. Women X/X carriers also displayed lower levels of FFM (38.9 ± 0.5 vs. 40.1 ± 0.2 kg, p<0.05) compared to R-allele carriers. When these FFM differences were covaried in the KE torque analyses, only KE concentric peak torque at 30°/sec remained significant. CONCLUSION: Our results indicate that the absence of alpha-actinin-3 (i.e., ACTN3 X/X genotype) influences KE strength and FFM in women but not men. The strength differences observed between genotype groups in women are mostly explained by genotype-related differences in FFM. Supported by intramural NIH-NIA funds, AG21500 and AG22791.
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Walsh et al. (2008) conducted a cross-sectional in Healthy adults (n=803). ACTN3 X/X genotype vs. ACTN3 R-allele carriers (X/R + R/R) was evaluated on Knee extensor concentric peak torque at 30°/sec in women (p=<0.05). Women with the ACTN3 X/X genotype had lower knee extensor concentric peak torque (89.5 vs. 99.3 Nm, p<0.05) and fat free mass than R-allele carriers, which mostly explained the strength differences.
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