The contractile organelles of a skeletal muscle fiber are thread-like structures called

The contractile organelles of a skeletal muscle fiber are thread-like structures called myofibrils.

Myofibrils are long protein filaments within the muscle cell (myocytes) responsible for contraction. Actin and myosin are myofibrils that slide past one another and thus change the length and the shape of muscle cell.

Myofibrils contain contractile proteins, regulatory proteins

and structural proteins and a huge amount of mitochondria around them.