Triathletes are not the only people who go in for the classic endurance sports of swimming, running and cycling. Swimming in particular is considered good for your health. It is also good for those who want to lose weight.

"Swimming after handball, water polo and ice hockey is the sport that expends the most energy," noted Roland Baartz, a sports scientist at Hamburg University. Regular swimming diminishes the spare tyre around the waist and puts far less strain on ligaments and joints than do land-based sports like running.

Swimming also exercises muscle groups that you are normally unaware of.

If burning calories is your main goal when you swim, steer clear of overheated pools. The colder the water, the more energy your body requires to maintain its normal operating temperature. And so it burns fat.

"The key in swimming is technique," remarked Ulrich Ossenberg, who has run a swimming school in the German town of Schmallenberg for 25 years. A person who is running can place one foot ahead of the other any way he likes, and still arrive at a finish line 40 kilometres away, he pointed out.

When swimming, however, "it's important that your movements are bio-mechanically correct," Ossenberg said.

The breaststroke is not wholly advisable from an orthopaedic standpoint, especially for people who perform it improperly by keeping the head bent back out of the water instead of submerging it and propelling the body forward underwater in a flat, stretched position. Stiff neck muscles can be the price of keeping a hairdo in place.

Another problem is the breaststroke's "frog kick," which can strain the knee joint.

"The knee joint is a hinge joint and ill-suited to the breaststroke," Baartz said. "That's why a lot of people twist their hips and don't move their legs synchronously." Baartz said that nearly one in two people made this mistake when swimming the breaststroke. Recreational swimmers should make sure to move both legs simultaneously.

"The backstroke is the most relaxed swimming style," Ossenberg said. As in "the jellyfish" or "dead man's float," the body lies on the water, breathing is easy, and movement can be reduced nearly to swimming in place.

By "backstroke" Ossenberg did not mean the competitive swimming style that is swum on the back. "That's the back crawl," he said. "The backstroke is like the breaststroke: The arms are spread outward and then drawn in again."

The fastest swimming style is the front crawl, which is nearly universally swum in freestyle competitions. Both arms and legs move alternately. Breaths are usually taken by turning the head to the side after after every cycle of movement. Sometimes breaths are taken alternately from the left and right sides, with the face remaining underwater for a full arm cycle.

According to Baartz, many recreational swimmers overuse their legs when doing the front crawl. "Most of them get out of breath because they're ceaselessly wriggling their legs," he said. "The legs have big muscle groups and have to be supplied with a lot of energy, but they account for no more than 20 per cent of the propulsion in a front crawl."

Hence swimmers should rely on arm strength when doing the front crawl - and the back crawl too.

Swimming instructor Annette Gasper, who runs a company in Darmstadt called Total Training Europe/Swimpower.de and helps recreational swimmers optimise their technique, is an expert on the perfect front-crawl style.

"Effective swimming boils down to three criteria: posture in the water, breathing and how to generate as much propulsion as possible," she said.

"For the front crawl, the body should lie flat as a board in the water," Gasper explained. "The less frontal resistance you offer the water, the faster you can go."

Gasper said that many recreational swimmers, fearful of becoming disoriented, lifted their head out of the water. This "ruined their posture" because their body then tilted, their feet sank, and effective propulsion became impossible.

To effectively generate propulsion during the front crawl, the forearms must form a paddle that moves downward in front of the belly until the thumb reaches thigh-level. The elbow is then lifted slightly and the arm stretched forward out of the water.

Under no circumstances should a stretched arm move downward underwater like a mill wheel.

"If you push down a stretched arm, you lift the upper body out of the water," Gasper warned.