Yes — ACTN3 affects muscle growth by determining your fast-twitch versus slow-twitch fiber ratio, which shapes how aggressively your muscle fibers hypertrophy. The R577X variant produces three genotypes (RR, RX, XX) with meaningfully different hypertrophy profiles. The Precision Peptide Genetic Test analyzes ACTN3 as one of 15 Muscle Growth insights across 14 pathways, 49 peptides, and 150+ genetic insights.