ONCE the most popular way to get fit was through sport and outdoor exercise. Then came the aerobics age and the man-in-the-street moved into the domain of the body builder with gymnasiums suddenly becoming second home to the flabby and unfit of the world. This interest in health and fitness peaked as people became more aware of the environment. ''Green'' and lean was the thing to be. The next step was to move training equipment into the home, and so compact, versatile units were developed. These were generally lighter than the heavy duty systems found in the gym. Meanwhile, people were becoming more health conscious. This trend is reflected in various displays at the home and electrical appliance expos with everything from saunas to rowing machines, spas to calorie counters, and even a semi-conductor toothbrush! Names such as ''Jacuzzi'' and ''Biofit'' are two which will feature on the Kung Sheung stand. The Jacuzzi brothers invented the whirlpool bath back in 1968 and Jacuzzi is now the world leader in whirlpool baths, with 250 international patents. Baths, for one or two or more, raised contoured backs, built-in waterfalls, accessory areas, optional faucets for a more conventional-style bath, water rainbow lights, armrests, finger-touch activator - these are features found in some of the more than 10 Jacuzzi designs. But just as the Jacuzzi changed the bathtub forever, so the J-Dream revolutionised the shower with hydrotherapeutic jets. These feature powerful steam nozzles, and hi-tech shower heads with a range of sprays from fine mist to full flood. The largest in the J-Dream family has an integral seat and a built-in closet to warm your robe and towel. There is even a compact design for corner spaces. Yet another scene-stealer is the Tylo herbal sauna. Scatter fresh or dried herbs in the heater's fragrance dispenser and relax as the scent permeates your private world. For a thorough work-out before your sauna, the Kung Sheung people are also experts on Biofit systems. The USA Star Trac multi-user treadmill has pre-programmed running elevation terrains with the ability to customise speed and running times. A digital display gives immediate feedback on the runner's progress. Rowers can opt for the American Proform set-up, with options for both skiing and rowing. Or there is the Preference HRT exercise bike which monitors and displays the heart rate with a wireless ECG transmitter. Meanwhile, at the Fitness Concept booth, fitness is highlighted with the USA Precor Ergo Logic step machine. Ergo Logic software is a sophisticated package which records personal information for up to four people, including weight, work level and best performance on a variety of courses. On a less energetic scale, the Health Co introduces four unusual products. A semi-conductor toothbrush carries the promise of ''removing plaque and stains, killing germs and preventing decay, gum disease and bad breath''. It has no batteries and is said to be good for 15 years.