During exercise training programmes muscle mass can increase. This is called muscle hypertrophy or cardiac hypertrophy (increase of heart mass). The increase of single muscle fibre causes this growth. If the work load is high i.e. heavy weights there is an increase of pressure and stress on the muscle, therefore the muscle can adapt to this. Muscle fibres become thicker because they have adapted by increasing the size and number of contractile proteins, e.g myosin and actin, within myofibrils .
- ↑ Hernadez. R, Kravitz. L, The Mystery of Skeletal Muscle Hypertrophy, available at http://www.unm.edu/~lkravitz/Article%20folder/hypertrophy.html