الفهرس 
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Abstract
Improvement of growth and feed efficiency resulted in significant gains in body weight traits of economical importance, which reduced production costs. As for the consumer, such result might have an impact on the nutritional quality and cost of products. The use of new tools in genetic selection process to improve product yield and quality might have a great impact in animal production.
Skeletal muscle represents about 40% of body mass and is used not only for locomotion but also acts as a reservoir of amino acids that can be called upon in times of physiological stress or disease. Skeletal muscle as meat in livestock species also serves as an important source of dietary protein and micronutrients. The functional units of differentiated muscle are the muscle fibres, which are long, cylindrical multinucleated cells.
In poultry meat production, it has been shown that muscle fibre properties play a key role in meat quantity and quality. Muscle mass in adults is determined by both myofiber number and size. Increases in muscle generally occur by both an increase in myofiber diameter (hypertrophy) number (hyperplasia).
It is well known that muscle fibre number, size, and fibre-type composition are closely related to each other. Selecting poultry for fast growth rates resulted in changes of the muscle structure e.g. by increasing fibre diameters. Consequences of this intensive hypertrophic growth are higher incidences of deep pectoral myopathies, leg disorders or vesicular alterations of the breast muscle surface.
However, in order to reduce the incidence of the described pathological findings the reduction of the fibre size might be useful. But this alteration would negatively influence the breast and leg muscle yields. A better alternative would be the increase of the total muscle fibre number.