If you feel like your FitBit doesn't give you enough credit, you might be onto something. According to new research, several popular fitness trackers underestimate just how hard we work.For the study , published in May in PLOS One, researchers had 22 participants test four fitness trackers with the ability to measure your heart rate. The trackers included the Apple Watch, FitBit Charge HR, Mio Alpha, and the Samsung Gear S. Each participant came to the lab for initial measurements (e.g. height, weight, and oxygen intake) and then returned two more times for hour-long testing sessions.The testing involved running on a treadmill and indoor cycling interspersed with periods of rest. While each participant had his or her performance measured with one of the fitness trackers, the researchers also monitored heart rate and energy expenditure (calories burned) using lab equipment. Throughout the testing, each participant ended up wearing every device.Results showed that contrary to some previous findings , all the devices were pretty accurate when it came to heart rate. But the trackers weren't so good at estimating how much energy you were putting out — with up to a 43% error rate.It turned out that they actually underestimated how hard the participants were working. Although the Apple Watch was the most accurate at estimating heart rate, it was the least accurate at calculating energy expenditure. On the other hand, the FitBit was least accurate at heart rate and the Samsung Gear S was the most accurate for calories. However, no tracker was consistently more or less accurate than the others overall.If your tracker is reminding you to stay active and motivating you to keep going, that's a good thing — even if it's not an exact measurement of what you're up to. And as this study shows, you might even be working harder than you think.