Make a fist. If you are a man, chances are your thumb is clenched over four curled fingers. Powerful.

If you are a woman, your thumb is probably inside the curled fingers or alongside the index finger. Not so powerful, says West Point Physical education instructor Susan G. Peterson, who has developed a self-defense program for women based on what she sees as basic cultural and physiological differences between the sexes.

"The male musculature is different from a woman's," says Peterson, 34, a native Californian and the first woman to hold her job in military academy's 175 years. "Men have more developed upper-body strength while women generally have developed co-ordination, balance and strength in the lower body."

Other differences:

Men have broader shoulders and stronger arm and back muscles. Women can build muscle tone, but not bulging muscles because they lack the hormone testosterone. Women have smaller hearts and lungs and fewer red blood cells, giving them less capacity for endurance. For these reasons Peterson stresses techniques that do not require a great deal of endurance.

"The best weapon any woman has is her feet," she says. "Kick the attacker in the shins or stomp on his instep with the bottom of the foot, not the toe, and kick low so the attacker can't grab your leg."

She also, along with other self-defense instructors, advises women ot take advantage of men's vulnerability in attacks to the groin.

A woman's ability to defend herself aggressively - such as clawing at the eyes, another one of her tips - can be a psychological as well as physical problem, admits Peterson. "But if you're in a rape situation, depending on the violence of the attack, hopefully you could respond."

Peterson devised her first self-defense course for women in 1968 after several rapes on the University of Illinois campus - where she was a graduate student - spurred college women into asking for help. She joined the military academy in 1976 to develop a self-defense program for West Point's first class or female cadets. Her husband is also a West Point physical education instructor specializing in boxing.

Peterson's techniques are presented in a new book, "Self Defense for Women the West Point Way," published by Simon and Schuster (256 pages, $9.95). CAPTION: Picture, Susan Peterson; by Harry Naltchayan - The Washington Post