Skeletal muscle growth in response to training is determined by genetics, and a rare version of the MSTN gene, which encodes the muscle-limiting protein myostatin, is associated with much greater muscle mass and strength. Your FitnessGenes result tells you which version of this gene you have and how this affects muscle building in response to training.
We look at two genetic variations that scientists have identified in the MSTN gene, that we call MSTN (the K153R variation) and MSTNRARE. They code for the same gene and protein, but are found at different locations in the gene, and thereby have different effects on the myostatin protein.
Over two centuries ago, in 1808, a very special type of cattle called the Belgian Blue was first described. This is an extremely lean, hyper-sculpted, ‘double’ muscled breed. The breed has been preserved until now, but it was unknown up until 1997 that a mutation of the MSTN gene was the cause for their extreme physique.
Scientists that discovered the myostatin protein showed that by knocking out the MSTN gene in mice and preventing its normal functioning, that it resulted in a much more muscular and heavier body weight (2-3 times) compared to normal animals. It was found that this extreme muscle mass was a result of both muscle cell hyperplasia (higher number of muscle cells) and hypertrophy (larger muscle cells).
Thus, we now know that myostatin is a protein that inhibits (negatively regulating) muscle mass, and prevents muscle from over-growing.
Source: 1000 Genome Project. Global averages for both sexes