The myofibrils can be observed in the figures labeled My. The arrangement of the myofilaments shows up as bands. The actin and myosin are involved in the muscles contracting. The contractile unit of muscles is called a sarcomere. A sarcomere starts and stops at the Z-lines.
Energy is essential to the muscles' ability to contract.
The slow fibers have more mitochondria and less sarcoplasmic reticulum in its cytoplasm than fast muscle fibers, and usually have smaller myofibrils. Slow muscle fibers rely almost exclusively on aerobic metabolism. The many mitochondria are present to handle the large amount of oxidation that is needed to produce ATP since the limiting factor in aerobic metabolism is mitochondria. Since a small sample was taken this is not a good representation of the whole muscle. Only small parts of muscles were observed in the electron micrographs.