During a race such as a marathon, the skeletal muscle cells in your legs use a lot of energy to continually contract and power your muscles. What organelle is most likely abundant in skeletal muscle cells?

The most likely abundant organelle in skeletal muscle cells are know as myofibrils which is a contractile apparatus mostly containing the contractile proteins actin and myosin with the regulatory proteins troponin and tropomyosin.

In addition the skeletal muscle cell cytoplasm contains numerous mitochondria, as well as abundant glycogen to provide energy. The contractile elements of skeletal muscle cells (myofibrils) are thin cylindrical structures 1-2 µm in diameter.

Explanation:

Skeletal muscle fibers, or muscle cells, are formed from the fusion of developmental myoblasts in a process known as myogenesis. Muscle fibers are cylindrical and have more than one nucleus. They also have multiple mitochondria to meet energy needs. Muscle fibers are in turn composed of myofibrils.

The myofibril is a long cylindrical organelle found in skeletal muscle cells formed by two transverse filament systems: the thick and thin filaments. The thin filament is composed primarily of actin; it is tethered at one end to the Z-disk, and it interdigitates with the thick filaments. Thaey slide past one another, producing a contraction that changes both the length and the shape of the cell to produce force and motion.

The skeletal muscle vigorous activity and unusually high energy requirement is powered by organelles known as mitochondria which resist the force of gravity, generate heat by shivering, pump blood, move substances through hollow organs and produce strong bodily movements.