No matter how hard you work out, sooner or later you will hit your body's genetic limit for muscle mass. This limit varies per individual and declines with age. With a proper bodybuilding routine, however, you can ensure that you maximize muscle growth within these limits. Genetics also influences how easily you are able to lose excess body fat.
Satellite Cells Grow Your Muscles
Your muscles grow when satellite cells that surround your muscles contribute their nuclei to your muscle cells, thereby stimulating them to produce genetic material that signals your muscles to grow. Individuals with high genetic bodybuilding potential have more satellite cells surrounding their muscles, and their bodies produce even more satellite cells in response to the stress of bodybuilding. As a consequence of this genetic variation, people vary widely in their ability to respond to bodybuilding. Some people exhibit virtually no response to bodybuilding, although most people respond to some degree.
The Role of Muscle Fiber Length
The maximum size of your muscles is partly determined by the length of your muscle fibers. A bicep, for example, is attached at two points along the upper arm. If the top of your bicep is attached to your arm nearer to your shoulder than most people, and the bottom is attached nearer to your elbow than most people, your bodybuilding potential is high, at least with respect to your biceps.
Muscles Affect Metabolism
Your body fat percentage is affected by your basal metabolic rate, which is largely determined by genetics. Your basal metabolic rate is the rate at which your body burns calories simply keeping you alive — maintaining your body temperature, expanding your lungs and pumping your blood. Ironically, the more efficiently your body burns calories, the easier it is to gain weight because your body burns fewer calories to perform the same amount of work. You can marginally increase your basal metabolic rate by building muscle, because it takes more energy to maintain a pound of muscle than a pound of fat.
Working Within Your Limits
Many factors determine the effectiveness of your workout. These include the weights you use, the number of repetitions you perform, the frequency of your workouts and the number of different exercises you perform. Since everyone's body responds slightly differently to various stimuli, change these variables and note the results until you have a workout that is tailor-made for your body's genetic response.