BUFFALO, N.Y. (WIVB) — How well does your fitness tracker work? Millions of us count on them to track our every move.

The newer ones are supposed to measure heart rate, but a new class-action lawsuit claims the tracking technology does not do as well as Fitbit’s maker says.

One of the attorneys who filed the lawsuit says the device does not provide meaningful heart rate measurements.

The suit claims the defect can be dangerous because people may be using it to plan their workouts. Wrong numbers may make people push themselves too far.

Lawyers updated their complaint late last week to include their own data. They tested 43 healthy adults, comparing their Fitbit heart rate versus an EKG.

They found the devices were off by 20 beats per minute on average.

The company has already responded to this, questioning the validity of the test. Fitbit representatives said they tested the pure pulse technology for three years before it hit the market and they keep working to improve it.