High-tech fitness and activity trackers all share one thing: an accelerometer. Here’s how they work — and don’t. Accelerometers record changes in motion relative to the device: up and down, side to side, and front to back. This little device contains a three-axis accelerometer. When still, it records only the force of gravity. When moved, it records acceleration. The needs of guided missiles and spaceflight first advanced accelerometer technology. Later use in cars to deploy airbags drove the cost down — and now they’re used to rotate smartphone screens. They now cost so little they’re being added to almost everything, even forks and toothbrushes and dogs and cats. Today’s most popular human activity trackers are worn on the wrist and use accelerometers to measure motion. Proprietary software algorithms analyze that motion and estimate the number of steps you’ve taken, how many calories you’ve burned and how active you are in general. To count steps while walking, an activity tracker looks for periodic motion with acceleration above a certain threshold. Activity trackers record your activity, but they also record your inactivity — the amount of time you’ve been sitting around. Even if it’s sitting without a chair. Wall-sits or yoga are much harder than sitting at a desk but an activity tracker generally can’t tell the difference. The accelerometer measures only motion, not exertion — no matter how hard you’re straining yourself or how heavy the deadlift is. The smartphone apps for different activity trackers will let you put in exercises manually to get a more accurate calorie-burn estimate. Future devices may even help you perfect–But right now, the algorithms are far from perfect. Time for a snack. The motion from eating chips looks more vigorous than the bicep curls. The algorithms are improving though. And any movement, even household chores, can be beneficial to physical activity. Unfortunately, today’s wrist-worn accelerometers are still calibrated for steps and can’t tell when you’re cycling. But that shouldn’t stop us from enjoying a ride. Activity trackers still can’t understand every different exercise and move — but they do a pretty good job at seeing a person’s overall daily activity level.