To figure out how to program the app, I need to know how an accelerometer works, otherwise I won’t be able to make heads or tails of the data I recieve from it.

What applications can an accelerometer have in digital health?

There’s obviously plenty of use for accelerometers in exercise tracking, not only for use in sport and fitness, but it could be used in a much simpler way, such as measuring arthritis patients activity. From this sort of data, you could infer how much discomfort a patient was in based off of how much activity they were doing compared to ‘normal’.

Another use in digital health could be for detecting sudden falls. For example, elderly patients could have a device that sends an alert to a medical professional if it detects a sudden fall or trip. The same could go for neurological diseases that could cause a sudden fall such as epilepsy or narcolepsy.

Accelerometer data has also been used to measure sleep patterns as movement can indicate how deep someone is sleeping. This could be useful for those struggling to sleep, to see how good the sleep that they are getting is.

There could even be reason to use accelerometers in cars to determine the forces involved in a crash and what likely medical trauma (for example, whiplash) may arise from the forces recorded.

What are some existing apps that make use of accelerometers?

As I mentioned, there are a lot of fitness apps like strava, runkeeper, mapmyrun etc. that record accelerometer data for activities like running and cycling. Sleep as Android would be an example of an app that measure movements during sleep, although I believe apps associated with other wearables such as fitbit or garmin watches also measure this.

If you look outside of digital health apps, there are an enormous amount of apps making use of the accelerometer. Plenty of games and entertainment apps make use of it for things like character movement.

What is the difference between speed, velocity, acceleration and acceleration due to gravity?

Speed is how fast something is moving. The SI unit of speed would be meters per second.

Meters per second just so happens to be the same unit as velocity. There is one crucial difference between velocity and speed though. Speed is scalar, meaning it only refers to one thing, distance over time. Velocity is a vector, not only does it take into account speed, but it also takes into account direction as well . An example of this would be if you went on a run in a straight line away from your house and back. Let’s say you run at a constant speed of 5km/h. Your velocity (relative to the house) when running away from the house would be 5km/h, but on the way back, it would be -5km/h.

Acceleration is the change in velocity (note that this is velocity, not speed because acceleration is also a vector) over time, or the rate of change of velocity, and is measured in meters per second squared. For example, if a car increases it’s velocity from 5m/s to 15m/s in 10 seconds, it’s acceleration over that time would be 10(m/s)/10(s) = 1m/s^2.

An object accelerates if there is a net force acting on it, objects at rest or constant speed do not have a net force. For example, a car moving at a constant speed has no net force because the acceleration is equal to that of air resistance, so the speed stays the same. However, to accelerate, the force moving the car forwards, must be higher than that of any forces working in the opposite direction. From these forces, you could work out the acceleration at any given moment in time.

Acceleration due to gravity refers to the constant acceleration of a falling object. On Earth, gravitational acceleration is 9.81m/s^2, meaning that any object dropped from any height will always fall with an acceleration of 9.81m/s^2 as long as there are not any other forces acting on the object (such as drag). In reality, you don’t always see this constant acceleration due to drag (air resistance), but you would see this effect if you dropped objects in a vacuum.

How is gravity managed with accelerometers?

Accelerometers measure proper acceleration – acceleration of the accelerometer relative to itself. What this means is that when the accelerometer is stationary, it will record acceleration due to Earth’s gravity directly upwards. If we think about acceleration in terms of forces as I mentioned earlier, this is because gravity is exerting an acceleration of 9.81m/s^2 on the accelerometer, therefore if the accelerometer is stationary, it must be exerting an equal and opposite force. So, as it is only measuring acceleration in relation to itself, it must be accelerating 9.81m/s^2 away from the Earth to stay still, and if it was falling, the net acceleration relative to itself would be zero.

What are the x, y and z axes?

x, y and z value represent the three dimensions or planes in which the accelerometer can measure acceleration. In the default smartphones orientation (portrait), the x axis is horizontal and points to the right , the y axis is vertical and points upwards, the z axis points towards/out of the front face of the screen. Movement in the opposite direction of these axes would be represented as negative values.