sed by movement), typically simultaneously on the vertical (Y), horizontal right-left (X) and horizontal front-back axis (Z). Through actigraphy, we study the frequency, duration, and intensity of physical activity. Figure <a href="activity.html#fig:genea-one-hour">6.2</a> shows one hour of data collected from a wrist-worn GENEActiv accelerometer. As can be seen, three accelerometers (X, Y, Z) were simultaneously providing data. Data were sampled with a frequency of 30 Hertz (Hz; thirty measurements per sec