4.1.1. User Activity Categorization

Considering the periodic pattern of the walk, we categorized daily activities into two groups: periodic activities and non-periodic activities. Additionally, according to the similarity among these activities, we categorized them into four classes: walk, walk-like activities, walk-related activities and walk-unrelated activities. We show these in Figure 2. Walk-like activities are an extension of walking, which includes going upstairs and downstairs, and could be used in some undemanding systems. Walk-related activities contain those that might be misclassified by algorithms in prior studies, such as running, riding a bicycle and brushing teeth.

**Figure 2.** Hierarchical walk activity definition.

In fact, it is quite easy to distinguish walking and walk-like activities from walk-related and walk-unrelated activities. Therefore, we largely simplified the analysis and data collection of walk-related and walk-unrelated in the remainder of the article.
