Mitochondria
The mitochondria are also called the power plants of the cell. In them ATP is formed, which serves as an energy supply for the cells and without which the bodily functions cannot take place. In cells with high energy consumption, there are correspondingly many mitochondria; the number of mitochondria adapts to the energy consumption of the cell.
After starting a running training, the mitochondria first become larger and later you can find more of them in the muscle cells of the legs.