The mid-region of a sperm cell is rich in mitochondria and generates the tail movement necessary for forward progression and swimming speed.

“A mid region-- that mid region has a bunch of things related to cell motility and to the forward progression of the cells. First of all, it is chock a block full of mitochondria. In fact, if you were to look just behind the head of the sperm, what you'd see is that it is completely surrounded by mitochondria. There are mitochondria elsewhere in the cell, but most of them are contained in this mid region compartment just behind the head of the sperm. And that thick region is where the tail movement of the sperm, the flagellation back and forth, is actually generated from.”