Devices on exercise machines that monitor heart rate, distance covered and calories burned are frequently inaccurate and the margin of error is often 20%, according to experts.

Dr Emma Ross, an exercise physiologist at Brunel University, warned that inaccurate devices can be harmful. "Heart patients and pregnant women are often told by doctors to closely monitor heart rates during exercise," she said. "If someone has a blood pressure problem or heart problem and the pulse meter is inaccurate that could be dangerous."

Roger Fielding, an exercise physiologist, said the devices use calculations that assume, wrongly, that everyone burns calories in the same way. For instance, a 57kg (9st) woman with 20% body fat will burn calories more efficiently than a woman the same weight with 30% body fat.

He added: "There is often a 20% difference in the speed the machine tells you it is going, and the speed it is actually moving. This can create a 20% mismatch in the number of calories being burned."

Professor David Bassett, an exercise physiologist, questioned whether some manufacturers programme their machines to overestimate the calories burned.

"The readings can be so inaccurate that you have to ask yourself why," he said. "It's obvious that if a machine tells someone they are burning lots of calories, that person is more likely to use that piece of equipment next time. Exaggerating the number of calories burned, could be regarded as a form of insidious advertising."