High above the Potomac, a slender woman hangs on to a cliff. The toes of one foot press hard against a small nub of rock no larger than a quarter. The other foot dangles in air. Most of her weight is supported by three fingers of her right hand curled over a tiny ledge, while her left hand gropes for a better hold. Forty feet below, the river courses.

The woman trembles slightly. Her strength is waning. She strains to reach the the tiny bump of rock that will be her hold, inches away from her outstretched fingers. Part desperation, part calculation, she lunges. But her fingers barely graze the rock as her heart leaps for her throat. "FALLING!" she yells as she peels off the rock into space.

But she doesn't fall.

Instead, she swings wildly a few times like a pendulum and then dangles gently in midair from a brightly colored climbing rope that is supported or "belayed" by her climbing partner on the ground. "Awright!!, Good try . . ." her partner yells. "Nice Peter Pan!" laughs another climber. "Whew," she mutters while being lowered to the ground, "I just couldn't get the crux move."

Every day, rock climbers come with their guidebooks, ropes and soft-soled climbing shoes to scale the sheer walls of Mathers Gorge, a spectacular narrow canyon below Great Falls, and to Carter Rock, another popular site on the Maryland side of the river. Traditionally a sport with limited appeal, rock climbing has been growing in popularity in recent years, gradually shedding its public image as a macho, dare-devilish endeavor. Climbing magazine puts the number of serious climbers in the U.S. between 12,000 and 200,000. Perhaps 30 percent or more are women. They may not have the upper-body strength of men, but they have the advantage of smaller fingers and, often, superior flexibility and balance.

The key is not brute strength but the ratio of strength to weight. Better to be elastic, aggressive, and have a a fine sense of timing. A tough climb is like a chess match: you are always thinking one move ahead. You also learn to feel comfortable hanging by one hand, 70 feet off the ground.

An afternoon on the cliffs: Great for building strength in forearms, shoulders, legs and hands. Veterans say the varied motions and demands on different muscle groups during good climb build a combination of flexibility and strength that lessens the chance of injury.

Training might include weight lifting (for strength, not bulk), running (for endurance) and perhaps a variety of specialized climbing exercises like finger hangs and finger pull-ups.

Most climbing here is top-roped: the climbing rope is anchored securely to a tree or boulder at the top of the cliff. The climber is tied to one end, while the belayer holds the other end, ready to take up slack and arrest any fall. Higher cliffs are usually lead-climbed: The lead climber places "protection" along the route as he climbs -- an assortment of metal devices (chocks, nuts, wedges, cams, hexes, Friends, Wallnuts, Rock N' Rollers, Sliders, Quickies) that are jammed into faults and cracks in the rock. Climbing is safe as long as you don't climb too far above the last piece of protection.

Paths up the rock are rated by difficulty from 5.0 (easiest) to 5.14 (hardest) as the climb gets steeper with fewer and smaller holds. Rock climbing isn't particularly dangerous. With proper climbing techniques and equipment, the chances of a fatal fall or serious accident are virtually nil.

Most climber ailments are the ordinary kind: muscle strain, tendonitis, shoulder pain and torn finger tendons. Many climbers just keep a huge bottle of aspirin on their desks.

Skip Brown is a Washington Post photographer.