As our good friend Webster tells us, motility is something's capability to move. Sperm motility specifically refers to the ability of a sperm to swim. After semen is ejaculated into the vagina, it still needs to swim through the uterus and the fallopian tubes, to reach and fertilize the egg, in order for conception to occur. Thus, sperm motility is one of the most important factors affecting fertility. A low sperm count can be compensated for by good motility.
As I'm sure you're all aware, a sperm is a small cell with a bulbous head and a longer tail. It swims by whipping this tail around in a spiral motion, which pushes it ahead through a liquid. If you didn't know this, you're not watching nearly enough Simpsons.
The problem is that not all sperm do this. Some of the sperm might not be moving at all. And some of the sperm will be swimming around in circles, or in a curved path. In order to reach the egg, sperm pretty much need to exhibit what the scientists call progressive motility. It has to be travelling in roughly the same direction, spiraling, and in a rather rapid fashion. If it isn't going fast enough, or is curving too much, there is virtually no chance that the sperm will reach the egg before it dies. They can last between 1 and 2 days.
Motility is generally measured simply as the percentage of sperm cells that are moving. In a human male, having up to half of the sperm non-motile is considered normal. Of those sperm that are moving, another half can be reasonably expected to be exhibiting non-progressive motility. So, if any less than 25% of the sperm is moving quickly in a straight line, you will have decreased fertility.
Acceptable motility levels vary from species to species. Dogs are expected to have up to 70% motility, whereas cattle can have as low as 30% and still be normal.
Sperm motility will be reduced after extended periods of sexual inactivity. This will, however, only affect the first few ejaculations, after which motility levels will be restored.
Drug abuse, alcohol abuse, and excessive caffeine consumptions have been proven to reduce sperm motility. As well, people living in areas with elevated levels of pollution will experience decreased motility, as well as decreased sperm count.
Temperature has an impact upon motility. The reason that the testicles hang in a vulnerable sack, instead of being stored safely inside the abdomen, like a woman's ovaries, is that sperm need to stay cooler than the body. They hang low, and keep a few degrees cooler than body temperature. Anything that negates this effect can decrease sperm motility. Wearing tight underwear, use of hot tubs, saunas, and even waterbeds can all have a negative impact upon sperm motility, and thus on fertility. So, if you're attempting to breed, switch to boxers.
Intake of vitamin C, and zinc have also been shown to improve sperm motility.
One method of treating low sperm motility is intrauterine insemination, where the sperm is inserted directly into the uterus, thus giving it a head start. If this doesn't work, there is also in vitro fertilization, where an egg is taken from the female's body, and fertilized by sperm in a test tube. The fertilized egg is then implanted back into the woman, where hopefully it attaches to the wall of the womb. This technique pretty much eliminates the need for motility.
Sources:
diet-and-health.net. "Sperm Motility," Nutrition for a Living Planet. <www.diet-and-health.net/Diseases/SpermMotility.html> (January 2, 2005).
Charles Lindemann. "Sperm Facts," Mechanisms of Sperm Motility. December 15, 2003. <www2.oakland.edu/biology/lindemann/spermfacts.htm> (January 2, 2005).
Strong Fertility and Reproductive Science Center. "Infertility in Men," Strong Health. <www.stronghealth.com/services/womenshealth/ivf/infoseekers/infertilitymen.cfm> (January 2, 2005).
Melissa Rouge. "Sperm Motility," Collection and Evaluation of Semen. January 20, 2003. <arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/semeneval/motility.html> (January 2, 2005).