Key points from article :
Activity trackers that count steps did not improve the chances of losing weight in 2 year study.
500 overweight volunteers who were asked to diet and take more exercise.
The half with a fitness tracker had lost 2kg less weight by the end of the trial.
Drop off in the usage data as the study went on.
Certain people may be more likely than others to benefit from using the technology.
The study was published in the Journal of the American Medical Association (JAMA).