THE PHRASE "One small step for man, one giant leap for mankind" came to mind this week. It was not the launch of the spectacular space shuttle Columbia that prompted the recollection, but the sight of a beautiful 22-year-old Ohio woman walking. The event is miraculous and full of promise because the woman, Nan Davis, a student at Wright State University in Dayton, has been paralyzed from the ribs down for the last four years.

Miss Davis was severely injured in an automobile accident on the night she graduated from high school. For 3 1/2 years, her recovery was slow, and her paralysis was diagnosed as permanent. Because of her spinal-cord injury, the messages her brain was sending to her legs were not received, and she could not voluntarily move the lower two-thirds of her body.

Five months ago, Miss Davis began to work with Dr. Jerrold Petrofsky, director of the biomedical engineering lab at Wayne State, who has developed a computer that can be programmed to stimulate muscle function. Electrodes were taped to Miss Davis' skin over the appropriate muscles in her legs, and the computer sent carefully timed electric impulses that caused those muscles to move and her legs to walk.

This new therapy is in its infancy, and Dr. Petrofsky is firm in cautioning that widespread use of the technique is still years away. One problem is that muscles -- including the heart -- atrophy in persons who have been paralyzed. They must be strengthened gradually before standing is attempted. Soon the computer will be made smaller and easily portable -- perhaps the size of a purse. And eventually, Dr. Petrofsky predicts, the entire apparatus, except for a computer the size of a calculator, will be implanted and as undetectable as heart pacemakers are today.

Nan Davis' face, as she struggled to walk this week, reflected no doubts or apprehension about her pioneering role, just wonder and joy as she got up out of her wheelchair and walked six steps away from it. Her walk is the beginning of hope for many young people whose lives have been devastated by spinal-cord injuries. The rest of us are simply awed by this triumph of medicine and technology.