There is no need to jostle for space with angry pedestrians on this bicycle. Welcome to indoor cycling or spinning, a popular mass workout activity.

This indoor group exercise is strenuous. After a 50-minute workout, participants are drenched - as if they had cycled through a thunderstorm.

"It is one of the sweatiest workouts and you can bum up to about 500 calories within a 50-minute session, said Ms Sophie Lamaletie, 44, a leading fitness consultant for Spinning Singapore 2010, an indoor cycling event at the end of this month.

In indoor cycling, riders led by an instructor pedal on stationary exercise bicycles to the accompaniment of music.

This form of exercise, which began here in 2005, now has more than 7,000 regulars.

Classes are held at some 30 locations including gyms like Fitness First, California Fitness and True Fitness.

Indoor cycling was founded in 1987 by South-African cyclist Jonathan Goldberg, who was training for a competition.

As Ms Lamaletie noted, a lot of sweat is produced during such a workout.

"The link between exercise and sweating is that as we exercise, we produce heat and sweating is one way to cool the body," explained Dr Jason Chia, a consultant sports physician at the Singapore Sports Medicine Centre and Changi Sports Medicine Centre.

However, the number of calories burnt is dependent on how hard you exercise, said Dr Chia.

"Depending on your body weight and your running pace, you can bum a similar number of calories in the equivalent time through running on a treadmill," he said.

A typical indoor cycling class starts with a warm up, after which the instructor takes participants through a series of exercises that simulate different terrain profiles such as uphill, downhill and flat.

The bicycles are equipped with a resistance dial to allow the rider to control the terrain incline. Also, even though everyone may be riding at the same speed a rider can modify the intensity to fit his own ability.

The exercises are generally targeted at improving cardiovascular endurance, fat metabolism and overall well-being.

Ms Lamaletie said indoor cycling is one of the few aerobic activities that people of all fitness levels can do together.

Because of the resistance dial, indoor cycling can be low-impact. It is even suitable for pregnant women and those who have just recovered from an injury, provided they have medical clearance from a doctor, she said.

In her case, she was cycling till she was seven months pregnant.

Because stopping during the workout can lead to participants feeling faint, the bicycles can provide the momentum to continue pedalling at a slower pace if necessary.

Ms Lamaletie said that indoor cycling is also good for athletes as a form of cross training. Swimmers, say, can use indoor cycling for cardiovascular fitness.

 If you are new to indoor cycling, do take a bottle of water along to keep yourself hydrated.

The ideal attire would be a dri-fit T-shirt that does not absorb sweat and exercise pants that stop above the knee so knee movement is not restricted.

As for footwear, a harder and less flexible pair of trainers would be ideal. This will give the ball of your foot more protection as you pedal.