This started as stupid idea. It was influenced by someone's project in Coursera course (Data Analysis and Statistical Inference), when I did volunteer evaluation. The other student analyzed his own data from Fitbit Flex . I almost forgot on it. Then later on in another Coursera course (Getting and Cleaning Data from Data Science Specialization) we did data processing in R and this data contained also information from accelerometer and some experiment with couple of human subjects (see Human Activity Recognition Using Smartphones Data Set).
Then continued when I was discussing my dissertation thesis topic with my supervisor and we came to the experiment with fitness bands and sentiment written into twitter.