Most fitness trackers are good at measuring heart rate but poor at measuring calories burned, a study suggests. As a result, people should be cautious about using them to judge what to eat, Stanford University scientists said. The study recommended that companies release data showing how their devices work out measurements. The accuracy of seven wrist devices were tested while 60 volunteers were asked to walk, run and cycle. Researchers found that six out of seven of the fitness devices were good at estimating the heart rate of the person wearing it, with an error rate under 5%. They were the Apple Watch, Fitbit Surge, Basis Peak, Microsoft Band, PulseOn and MIP Alpha 2 – but the Samsung Gear S2 had the highest error rate of 6.8%. However, when it came to keeping track of energy used during exercise, the five devices that performed this function was all a long way out. Not one of the devices had an error rate below 20% – and some, such as the PulseOn, were much more inaccurate, the US research team found. Dr Euan Ashley, co-author of the study from the department of cardiovascular medicine at Stanford University, said the public should be aware of the strengths and limitations of fitness devices worn on the wrist.