A titin, encoded by a TTN gene, is a third most abundant sarcomere component. Although, this myofilament plays a wide range of key roles in muscle tissue such as structural, developmental, mechanical, and regulatory functions, it is a usually missed aspect of the muscle properties formation. At first, the TTN gene variants was described in development of skeletal and cardiac muscle diseases. Recently, the gene is also considered a very promising genetic marker for sport performance which may underlie differences in the potential to be an elite athlete. The aim of the present study is to provide the comprehensive update of the titin protein and the TTN gene variants role in formation of skeletal and cardiac muscle properties. We review function and structure of the protein, the gene, and the isoforms, as well as molecular mechanisms, disease-causing mutations, associated phenotypes, and their implications for human health, physical performance, adaptive changes of muscles in response to training, and injury risk.