Sperm motility is the sperm's ability to move. If movement is slow, not in a straight line, or both, the sperm have difficulty invading the cervical mucous or penetrating the hard outer shell of the egg.
If 60% or more of sperm have normal motility, then the sperm is at least average in quality. If less than 40% of sperm are able to move in a straight line, the condition is considered abnormal. Sperm that move sluggishly may also have genetic or other defects that render them incapable of fertilizing the egg.
An important 2001 study identified a protein in the tail of the sperm called CatSper, which might play a central role in the ability of the sperm to swim and penetrate the egg.