Characteristics of muscle-cellular adaptation to intense physical loads.

We studied the influence of a 5-weeks training program, mainly with intense work-loads in form of explosive power, on the skeletal muscle system in 7 male sports students. Muscle biopsy specimens from m. vastus lateralis were analysed histochemically, biochemically and by means of electron microscopy. After the training of several weeks, a clear hypertrophy of FT-fibres (24%, p less than 0.05) and ST-fibres (20%, p less than 0.05) became evident. The fibre composition was not affected by the training program. The increase of mean cross-sectional areas of muscle fibres was not connected with any shifting of volume proportions of cellular compartments (volume density of mitochondria, myofibrils and sarcoplasm) at ultrastructural level. Only in the peripheric fibre region mitochondrial volume density increased from 5.9 +/- 2.5 to 8.3 +/- 1.2% in tendency. Contrary to endurance athletes and untrained subjects, power athletes show higher values of sarcoplasmic volume density in their muscle fibres. Results will be discussed in connection with specific demands of explosive powerful muscular performances.