Physical exercise is necessary for optimal health, the prevention of chronic diseases, and the avoidance of premature mortality. According to the 2018 physical activity guidelines for Americans, a balance of moderate and rigorous intensity physical activity, as well as muscle-strengthening activities involving the major muscle groups, is recommended. A meta-analysis published in PLOS ONE by Cambridge University researchers identified 13 potential genes linked to fitness results in previously untrained adults. Genetic factors were responsible for 72 percent of the variation in the strength training group's results. In the aerobic (44 percent) and anaerobic power groups, genetic variables had a smaller impact on the outcomes (10 percent). More research is needed to establish the precise roles of fitness genes and how to effectively tailor exercise instruction to individual genetic profiles. Cardiovascular fitness, muscle strength, and anaerobic power are the three components needed to evaluate health-related fitness. Cardiovascular fitness, also known as cardiorespiratory fitness, refers to how well the respiratory and circulatory systems provide oxygen to the skeletal muscle during physical activity. The maximum oxygen uptake (VO2) test determines the maximum oxygen consumption capacity of the body throughout a high-intensity activity, such as treadmill exercise. "Environment is a major factor for trainability," Dr. Bernd Wolfarth, professor in the Department of Sports Medicine at Humboldt University, Berlin, says in a session at the 22nd Annual Congress of the European College of Sports Science. "We know that about 25–40 percent of the variability of phenotype comes from genes, and the other 60–75 percent comes from environmental effects." Candidate genes may be able to predict successful responses to specific types of exercise training. These genes may have an impact on the body's energy processes, metabolism, storage, and cell proliferation. Following these findings, researchers from Anglia Ruskin University's Cambridge Centre for Sport and Exercise Sciences conducted a meta-analysis to discover the exact versions, or alleles, of candidate genes linked to the exercise response in untrained people. Strength, anaerobic power, and cardiovascular fitness were all measured by the team. From each parent, an individual inherits one allele of each gene. If both alleles are the same, the individual is homozygous for the gene; if the two alleles are different, the individual is heterozygous for the gene.