Thousands of people are now hanging themselves daily, upside down, with gravity-inversion equipment--those metal and foam boots Richard Gere snapped on for a heels-over-head workout in the 1980 movie "American Gigolo."

But while the bat-like behavior claims numerous devotees--including, reportedly, violinist Yehudi Menuhin and body builder Arnold Schwarzenegger--at least a few hangers have been injured after taking the law of gravity into their own hands.

Chevy Chase internist Charles Humphreys treated a patient who fell onto his shoulder after failing to fasten the boots properly. "He had just come out of the sauna and was feeling a little lightheaded," says Humphreys. "He experienced about three months of post-trauma vertigo."

The Food and Drug Administration's single user complaint came from a woman who slipped out of her boots--also after fastening them improperly. The Consumer Product Safety Commission reports no complaints about inversion equipment.

But as hanging moves from fad to trend (the leading manufacturer reports selling more than 350,000 pair of boots at about $80 a pair), some physicians are voicing alarm about the potential for harm from inversion itself.

In a letter to the New England Journal of Medicine, St. Paul internist David W. Plocher said two of his patients developed small hemorrhages in the skin around their eyes after hanging.

"I tried it myself and felt some pressure around my eyes," says Plocher. "I wondered what happens physiologically." He recently completed an "extensive study of all sorts of effects of inversion," for which he is awaiting acceptance from "a major professional journal."

Stanford University physiologist William Haskell, incoming president of the American College of Sports Medicine, says he is "aware of several people with low back pain who say use of inversion equipment aggravated the problem." However, he also points out that a "respected sports-medicine clinic" in his area promotes gravity inversion among selected patients.

"My patients who use (inversion equipment) say it makes them feel good," says Washington orthopedic surgeon David Johnson, a consultant to the President's Council on Physical Fitness and Sports.

"I would imagine hanging could really be a boon to those with lung disease to help bring up secretions, or for people with scoliosis where one of the traditional treatments is hanging from a bar. In the old days, however, you hung rightside up."

The one thing doctors do agree on is that hanging is not for everyone.

"People with hypertension, or who have had recent surgery, or who are scared to death at the idea of being upside down should only use it under the guidance of a physician," cautions Dr. Robert Martin II, a specialist in physical medicine and rehabilitation and son of the Pasadena orthopedic surgeon and one-time gymnast who created contemporary inversion boots 15 years ago for back-pain sufferers. (The idea of hanging it all upside down seems to date back as far as Hippocrates.)

The Martin company, Gravity Guidance, Inc., is the largest manufacturer with sales of more than $12 million last year--up from less than $200,000 in 1979--and anticipated sales of $35 million this year. Since "American Gigolo" popularized hanging, several dozen companies have come out with other gravity gizmos.

The Martin doctors (father and two sons) claim that several minutes of daily inversion reverses the ill effects of gravity's constant downward pull on muscles, joints and organs. Many of their thousands of patients have physical problems--ranging from bypass operations to severe arthritis--"and we've never had an accident or difficulty," says Dr. Martin II.

Regarding Plocher's concern about hemorrhages around the eye, he contends that "many of those people were on high doses of aspirin. Hanging can feel strange at first, until your body is conditioned." The small hemorrhages, he says, are "a temporary thing that goes away. It's not indicative of any more severe problem."

To the "quite a few patients who want to know what I think about it," orthopedic surgeon Robert Nirschl of the Virginia Sports Medicine Institute in Arlington replies: "Like any new thing it's still in the first stage of enthusiasm. Then the bumps start to show. Finally you get to a middle ground where the positive and negative is defined. I'm all for waiting until someone else figures out the bugs."

Adds Washington neurosurgeon Arthur Hustead, who examined the executive who fell from his boots: "It wouldn't hurt to place thick, heavy padding below you." Soaking Up Success:

"Unbelieveable" is how biomedical engineer Bruce W. Vorhauer describes response to his contraceptive sponge, which won marketing approval last month from the Food and Drug Administration.

"Some of the mail is from people who say they want to hop on a plane and fly out to get it," says the 41-year-old president of VLI Corp. of Costa Mesa, Calif. "Half is from people who want to buy stock in the company and the rest want to work for us."

The nonprescription, disposable, spermicide-soaked sponges will be available in 11 western states in July and nationwide this fall for $1 each under the brand name "Today." Once inserted into the vagina, the sponge provides protection up to 24 hours, through repeated acts of coitus. Tests show it to be about 85 percent effective in preventing pregnancy--about the same as the diaphragm or spermicide alone, notes the FDA, but less than the 98 percent rate of the birth-control pill and the 95 percent rate of the IUD.

Unlike the sea sponges soaked in citrus juice used more than 3,000 years ago by Egyptian women to prevent pregnancy, "the sophisticated material we have today is very pure and very clean," says Vorhauer. The polyurethane is "similar to that used in artificial heart valves and arteries."

Vorhauer got the idea for the device in 1975 when employed by a hospital-supply manufacturer. "There was so much dissatisfaction with existing birth-control methods. I was amazed at how little new biomedical application had been done."

When his supervisors refused to pick up on the concept, he quit his job and tried several other manufacturers. When the sponge was met with still more rejections, he formed his own company.

"It's nice to be vindicated," admits Vorhauer, who--like "most of the men at my company"--has had a vasectomy.