When sporting enthusiasts discuss in-line skates -- popularly known as Rollerblades -- they talk as if they've just found the perfect workout machine: They like how fast in-lines ride, how easy they are to control, how good they look, and how well they provide an alternative to high-impact sports like jogging. When retailers discuss in-line skates, which cost anywhere from $90 to $250, they talk as if they've just found the ultimate hot product. They say that they can't keep enough in-line skates on the shelves. "{In-line skates} can aptly be described as a phenomenon," says Jim McDowell, owner of Rip City Skates, a Santa Monica shop that caters to Greater L.A.'s most serious skaters. "I get probably eight to 20 calls each day asking about them." "Since Christmas, we've continued to break our own records," says Mary Horwath, public relations director for Rollerblade, Inc., the small Minneapolis company that's captured 75 percent of the in-line skate market and whose name has become almost synonymous with in-line skating. "Our sales are 300 percent higher than last year." Roller skates were invented in the early 18th century in Holland. Ice-skating was popular at that time and the anonymous inventor of roller skates hoped to enjoy the sport during the summer. But because these and later skates used fixed axles, they could move only in a straight line and were difficult to master. The first roller skate manufacturer himself was once severely injured while showing off at a London high-society party. James L. Plimpton of Massachusetts solved the problem of such "straight-running" skates in 1863. Plimpton's "guidable parlor skates" had an India rubber ball placed between each roller assembly and the foot plate. This allowed users to skate in curves. Plimpton soon constructed a number of roller rinks across the country and, hoping to promote roller skating as a morally high-minded activity, he marketed the sport to the wealthy and genteel. Seized by a general fitness craze after the Civil War, many Americans became obsessed with Plimpton's invention. Roller skating's initial success was also due in part to its popularity among women, according to Harvey Green, professor of history at the University of Rochester and vice-president of Rochester's Strong Museum. Roller skating allowed "definite possibilities for courtship," says Green. It provided both close physical contact and a liberating sense of motion to women confined by restrictive Victorian norms. After Plimpton's patent expired in 1880, his former colleague Samuel Winslow began making his own skates. In alliance with businessman Horace Bigelow, who lowered roller rink admission charges, Winslow helped market roller skating to the general public. Spurred by the 1884 patent of the ball-bearing wheel, roller skating soon became a more democratic activity. It achieved a permanent place in American culture during the 1930s, when it provided cheap entertainment for a poor nation looking for diversions. Given a boost by Fred Astaire and Ginger Rogers' 1937 movie "Shall We Dance," in which the two stars dance on roller skates, it also continued to be associated in part with courtship and romance. The invention of the polyurethane wheel in 1973 eventually allowed for serious outdoor skating. Made of resilient, smooth-riding plastic, the new wheels increased speed, reduced noise and let skaters glide over road and sidewalk cracks with relative safety. This technological development was largely responsible for roller skating's furious growth in the late 1970s. In-line skates have existed in various forms since the first invention of roller skates -- Giacomo Meyerbeer used a variation to simulate ice skating in his opera "Le Prophete" in 1849 -- but they were even more difficult to master than early roller skates and so were never really popular. Rollerblade's in-lines entered the market in 1980. Featuring optimized wheel and frame design and a boot fashioned from a supportive polyurethane shell, the skates were faster, lighter and more easy to control than regular roller skates. They also were relatively easy to ride. Rollerblade first directed its product to off-season players of the National Hockey League and to Nordic and Alpine skiers, who used them for summer training. Some of the same muscles used for ice-skating and skiing are exercised when skating with in-lines, so they give serious athletes year-round workouts. In the last two years, however, Rollerblade has undertaken an aggressive and successful campaign to increase its public visibility and to tap the recreational market. Using a tour group of professional skaters to give nation-wide demonstrations, issuing a self-consciously funky in-line skate music video, and undertaking promotional tie-ins with Golden Grahams cereal, Mott's applesauce, Quaker Oats and Hi-C fruit drinks, Rollerblade hopes to expand its market both above and below its normal 18-35 age range. Users all over the country are riding in-lines for a wide range of individual applications -- for physical fitness, transportation, artistic expression, even games of street hockey. Such consumer interest stems in part from the rise of low-impact sports and athletic cross-training. Many fitness buffs who once were runners or involved in original aerobics programs now are interested in exercise schedules that use fewer hopping and skipping movements, according to Daniel Kosich, program director for Jane Fonda Workout in Los Angeles. They also want regimens that combine diverse athletic activities. "Many people have found that doing low-impact movements is tremendously beneficial," says Kosich. "It slows the pace of exercise and is much more efficient. {And} cross-training physiologically helps train the body in a slightly different fashion. The body's muscles are trained in a broad range." "The skating motion is certainly a desirable low-impact activity," says James Garrick, director of the Center for Sports Medicine at St. Francis Memorial Hospital in San Francisco. "{And} aerobic dance brought about a change. The rabid runner is less commonly seen {nowadays}." With these and other endorsements, in-lines have seized those who are sophisticated in their fitness concerns. With its emphasis on fitness, individual expression and the quick conquering of distance, roller skating may be quintessentially the American sport. Now, with Rollerblade's space-age look, in-lines combine these cultural dispositions with an American penchant for technological utopianism. Because they resonate so deeply with these aspects of American cultural history, our contemporary forms of fitness training, and our new acceptance of synthetic materials -- and simply because they are an improvement on older forms of roller skating -- in-lines should continue their phenomenal growth well into the future. They may just transform America into Henry Ford's dream -- "a nation on wheels."