(CNN) – Fitness tracker technology, like Fitbit or Mio, continue to advance. A new study in the annals of internal medicine is looking at just how accurate the devices are.

Fitness trackers are a great way to monitor activity levels, but the probably aren’t as accurate as medical equipment, according to a small study out of the University of Wisconsin.

Researchers compared readings from four different commercial fitness trackers with readings from electrocardiographs, or ECG’s.

ECG’s are used in doctors’ offices and hospitals to measure heart rate. The fitness trackers proved fairly accurate when users were at rest, one measured within five beats per minute of the ECG reading.

However, with all of the devices tested, as activity increased, accuracy dropped. The monitors were off by a range of 20 to 40 beats per minute at high activity levels, compared with corresponding ECG measurements.

It should be noted that the survey was very small, there were only 40 participants. Critics argue that fitness trackers are not performing up to their advertised promises. However, supporters note that fitness trackers are not medical devices, and do help boost health generally by making people more aware of their activity level throughout the day.