HSA | FSA CARDS ARE NOW ACCEPTED

Last reviewed: May 12, 2026 Last updated: May 12, 2026

Written by: Jay Hastings , CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance. His work has included scaling healthcare startups, leading CLIA lab integrations, and helping expand consumer access to precision health tools.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

This article is part of the PlexusDx Education Hub — your resource for evidence-based guidance on peptides and GLP-1. Browse all Peptides & GLP-1 education

Two people follow the same program. Same reps, same rest, same protein intake. One adds fifteen pounds of lean mass in a year. The other adds four. The difference almost never comes down to effort — it comes down to genetics. The Precision Peptide Genetic Test analyzes 15 specific genetic insights across your muscle growth pathway — part of a broader panel spanning 14 pathways, 49 peptides, and 150+ genetic insights. This pillar guide walks through what each insight reveals, why each gene matters, and how to translate your results into training, recovery, and protocol decisions that actually fit your biology.

The Five Systems That Shape Muscle Growth

Hypertrophy isn't a single process — it's the output of five genetically regulated systems working in concert. Muscle fiber composition determines what kind of stimulus your body responds to. Growth hormone axis signaling controls how efficiently you recover and synthesize new tissue. The myostatin pathway sets the biological ceiling on how much muscle you're allowed to carry. IGF-1 signaling drives the downstream anabolic response. And recovery-side genetics — inflammation tolerance, vitamin D receptor activity, cardiovascular architecture — determine how fast you can push again. The 15 insights in your Muscle Growth pathway map across all five.

ACTN3 — The Sprint Gene and Fiber Type Architecture

ACTN3 encodes alpha-actinin-3, a structural protein found almost exclusively in fast-twitch (Type II) muscle fibers. Your combination of the R and X alleles — RR, RX, or XX — shapes the ratio of fast-twitch to slow-twitch fibers in your body, which in turn shapes what kind of training produces the fastest visible response. RR genotypes are overrepresented in elite sprinters and power athletes; XX genotypes are overrepresented in elite endurance athletes. Neither genotype is "better" — they require different approaches to programming, progression, and protocol selection.

IGF1 — The Anabolic Signal Amplifier

IGF-1 (insulin-like growth factor 1) is the primary downstream messenger of growth hormone. When GH is released, much of the anabolic action happens through IGF-1 binding to its receptor on muscle cells — driving protein synthesis, satellite cell activation, and tissue repair. Variants in the IGF1 gene influence baseline circulating IGF-1 levels, receptor sensitivity, and how aggressively your body responds to growth hormone axis stimulation. A high-responsive IGF1 profile means growth hormone axis support has more leverage. A less responsive profile means training volume, sleep, and nutrition do more of the heavy lifting.

MSTN — Myostatin and the Growth Ceiling

Myostatin is the biological brake on muscle growth. It's a negative regulator — literally signaling your muscles to stop growing once a certain mass is reached. Variants in MSTN can reduce expression or receptor sensitivity, effectively raising your genetic ceiling for muscle mass. The famous examples are real: documented loss-of-function myostatin mutations have produced dramatically increased muscle development in mice, cattle, and a small number of confirmed human cases. Your MSTN variant profile reveals where your natural ceiling sits and how responsive you are to myostatin pathway modulation.

GHSR — The Ghrelin Receptor Switch

GHSR encodes the growth hormone secretagogue receptor — the binding site that triggers GH release when activated. Variants at GHSR influence how sensitive this receptor is to natural ghrelin signaling and, by extension, how your body responds to GH-releasing pathway support. A less sensitive GHSR variant doesn't block response; it changes the dosing relationship and the magnitude of response you can expect. This insight is particularly relevant when planning growth hormone axis interventions, as GHSR sits at the front end of that entire signaling cascade.

GHR — The Growth Hormone Receptor

Once growth hormone is released, it has to bind a receptor to do anything at all. GHR encodes that receptor. Common variants — including the exon 3 deletion (d3-GHR) — are associated with differences in receptor sensitivity and downstream IGF-1 production. A high-sensitivity GHR variant amplifies the effect of endogenous GH and any growth hormone axis support. A low-sensitivity variant means the same GH signal produces a smaller anabolic response downstream. Combined with your IGF1 and GHSR profiles, GHR completes the growth hormone axis picture at the genetic level.

Beyond the 15: How Muscle Growth Connects to the Full Panel

Muscle growth doesn't live in one pathway. Your 15 Muscle Growth insights intersect with the 9 Tissue Repair insights (how fast you recover between sessions), the 12 Energy Metabolism insights (how efficiently you produce ATP during training), the 17 Longevity insights (sarcopenia resistance across the 50+ decades), and the 6 Inflammation insights (how much recovery load your body can absorb). For a full overview of what the Precision Peptide Genetic Test covers, see our guide to genetic peptide testing. The Peptide Pathways Report synthesizes these cross-pathway findings into a single actionable roadmap.

Genetics as a Guide, Not a Guarantee

Your muscle growth results don't tell you how much muscle you can build. They tell you where your headwinds and tailwinds live. An XX ACTN3 genotype isn't a ceiling on hypertrophy — it's a signal that your programming should emphasize volume and time-under-tension. A less-sensitive GHR variant doesn't mean growth hormone axis peptides can't help — it means your provider should set expectations around a different response curve. The real win is replacing generic training advice with decisions calibrated to your actual biology — the premise of testing before you invest in a muscle protocol.

The Precision Peptide Genetic Test analyzes how your genes influence muscle growth pathways. It does not recommend, prescribe, or determine which peptides you should use. Consult a qualified healthcare provider before beginning any peptide protocol.

Ready to see your muscle growth genetic profile? Take the Precision Peptide Genetic Test

Frequently Asked Questions

How many genetic insights does muscle growth testing cover in the PlexusDx panel?

The Precision Peptide Genetic Test delivers 15 muscle growth insights within a panel of 14 pathways, 49 peptides, and 150+ total insights. Core genes include ACTN3, IGF1, MSTN, GHSR, and GHR — covering fiber type, growth hormone axis signaling, and the myostatin ceiling. Analyzed via Illumina Global Screening Array at CLIA-certified labs.

Can genetic testing tell me which muscle peptides to use?

No — and it shouldn't. PlexusDx analyzes the pathways your 15 insights measure; it doesn't prescribe peptides. Your results reveal how responsive your biology is to growth hormone axis, myostatin, and IGF-1 pathway interventions, informing the conversation you have with a qualified healthcare provider, who then makes protocol decisions based on your complete clinical picture.

What's the difference between ACTN3 and myostatin for muscle growth?

ACTN3 shapes muscle fiber type — the ratio of fast-twitch to slow-twitch fibers that determines what stimulus works best. MSTN (myostatin) sets the genetic ceiling on total muscle mass. They answer different questions: ACTN3 tells you how to train; MSTN tells you how much muscle your biology will tolerate before it signals stop.

This article is part of the PlexusDx Education Hub. Browse all Peptides & GLP-1 education

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance. References are included at the end of the article when scientific, medical, or health-related claims are discussed.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.