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Sensor data stream from the micro-electro-mechanical systems (MEMS) sensors within a smartphone. The accelerometer, gyroscope and magnetometer produce signals along three orthogonally mounted axes: x (blue), y (red) and z (green). The vertical line (magenta) which is present in all of the sensor data streams denotes the time point corresponding to an older adult walking up a staircase whilst a smartphone is placed in the pocket of their pants (image on the right).
