Genetic factors influence the rate and quantity of muscle growth for an individual throughout his or her resistance training program. According to Vladimir M. Zatsiorsky and William J. Kraemer, authors of "Science and Practice of Strength Training," muscle growth is influenced by muscle fiber type, fat distribution, hormone levels and the quality and duration of your exercise program. Although you cannot change your genetic makeup, you can design a muscle-building routine that will be the most successful for your genetic ability to develop enhanced muscle strength and size.
Muscle Fiber Ratio
Muscles contain an assortment of fast-twitch and slow-twitch fibers. Although both fiber types can hypertrophy, another word for increase in size, fast-twitch fibers possess a greater capacity for growth. Genetics influence fast-twitch fiber distribution within each muscle group. For example, a person may have a greater ratio of fast-twitch to slow-twitch fibers in his lower body, enabling greater and faster leg-muscle growth, explain Zatsiorsky and Kraemer.
Although you can change physical appearance with diet and exercise, your genetically determined body type regulates pre-exercise-program body composition and fitness outcomes. According to the American Council on Exercise, mesomorphs are naturally muscular and hypertrophy quickly, endomorphs possess a round shape with greater fat distribution and ectomorphs are thin and linear. Although endomorphs may require fat loss for visible muscle definition and ectomorphs may gain less mass than a person with a mesomorphic body, all individuals benefit from strength training.
Anabolic, or muscle building, hormones regulate hypertrophy, according to Zatsiorsky and Kraemer. Although resistance training increases anabolic hormone circulation, women naturally posses lower levels than men. For example, a typical man possesses 10 to 20 times more testosterone than a typical woman. The amounts of human growth hormone and insulin-like growth factors – two additional anabolic hormones – also vary between individuals, and are influenced by training status and genetic predisposition.
Weightlifting for Hypertrophy
Do not let genetic misconceptions deter you from seeking the benefits of weightlifting. Proper resistance training produces hypertrophy in disease-free muscle, regardless of genetics. The American College of Sports Medicine prescribes one to three sets of eight to 12 repetitions, using 70- to 80-percent-maximal-ability loads for hypertrophic outcomes. Support muscular balance by utilizing all major muscle groups. Beginners should maintain one to two sessions per week, for two months, before increasing weightlifting frequency, suggests the ACSM. As always, consult a doctor before starting a weight-training program.