At first, the sight of 20 red-faced women lying on the damp grass of a central London park and juggling newborn babies is quite worrying.

But this is "power pramming," a new craze taking off in Britain in which new mothers are encouraged to use their offspring and the inevitable baggage that comes with them as exercise aids.

Mothers exercise with their babies at a park in London. (Reuters)

Fitness expert Liz Stuart is immune to comments by bemused passers-by as she cajoles her students into doing another 20 deep squats or fourth set of bicep curls using their precious babies as weights.

"Engage your pelvic floor and give your bum muscles a good squeeze as you come up," she cries. "You can use your babies as resistance for this one."

Stuart a qualified post-natal exercise instructor set up "power pramming" after the birth of her son because she was keen to get back to her pre-pregnancy shape and wanted a way of keeping him with her and meeting other new mothers while she worked out.

"I invited the girls from the local mother and baby group to get fit with me," she said.

"We started off... with our prams and I gave instructions to everyone for various exercises. We really worked hard. Within a few weeks we had a real routine going and the number of mums joining in just grew and grew."

Now she runs classes in parks across London and is franchising her exercise concept throughout the country and the world.

Stuart's programme starts out with a power-walk or jog across the park with the babies tucked up in the prams and enjoying a high-speed ride. "Imagine you are on a mission to get home and feed the baby!" yells Stuart as she strides off ahead off the class.

The mothers then complete a series of circuit-training based exercises. They use their babies as weights for shoulder lifts and bicep curls, and lie them on their backs on the grass and kiss them each time they dip on a press-up.

At any point, mums can drop out for a few minutes to feed or comfort a wailing baby.

"Older babies seem to find powerpramming fascinating and hilarious," said Stuart. "The younger ones tend to find the movement makes them sleepy and they often stay in their prams throughout the workout."

Rebecca Maxwell-Hyslop, whose twins Ben and Alice are just 11-weeks-old, is a new convert to "power pramming."

"It's such a fabulous idea," she says, breathless but happy after her hour-long workout.

"If I had to go to the gym and put the twins in a creche, it would cost me a fortune. This way, we all get some fresh air, I get some exercise and my babies have slept all the way through."