Human activity recognition (HAR) is essential to many context-aware applications in mobile and ubiquitous computing. A human's physical activity can be decomposed into a sequence of simple actions or body movements, corresponding to what we denote as mid-level features. Such mid-level features ("leg up," 'leg down," "leg still,"...), which we contrast to high-level activities ("walking," "sitting,"...) and low-level features (raw sensor readings), can be developed manually. While proven to be effective, this manua…