# Toward a Symbolic AI Approach to the WHO/ACSM Physical Activity & Sedentary Behavior Guidelines
## 2 Materials and Methods
### Symbolic AI Approach: From Analysis to Design
#### 2.3.2 Ontological Analysis  
We aim at understanding what we need to represent in terms of the entities of our domain. To this end, as reported in Figure 1 (the step labeled as (b)), we performed an ontological analysis of the Physical Activity & Sedentary Behaviour Guidelines (WHO and ACSM). To exemplify, we show in Figure 2 a typical example of WHO/ACSM recommendation. In general, guidelines associate a recommendation to a target group of people. In this view, we identify two main components: target audience and recommendation. The target audience defines a group of people in terms of a set of shared characteristics--in the example, having an age between 18 and 64 years old. These definitions refer to properties that change over time, such as age or conditions (e.g., pregnancy or postpregnancy). In other words, these guidelines define transient rather than static groups (e.g., by ethnicity or gender). The recommendation describes a physical activity as a set of characteristics (see Figure 3) involving modality, intensity, frequency, and duration. The WHO guidelines do not provide a list but specify modality, in the example, as an \"aerobic\" activity, as a class of exercises. In this regard, the use of the guidelines requires filling a gap in terms of mapping the specific activity observed to the relevant modality--e.g., fast-walking to \"aerobic\", yoga to \"muscle-strengtening\" activities. The guidelines also specify a level of intensity (e.g., \"moderate\" or \"vigorous\" intensity), which provides a qualitative evaluation of the effort spent by a person on the activity. Specifically, WHO defines intensity in terms of multipliers of energy consumption at rest--e.g., low-intensity is when an activity consumption is less than three times resting. Figure 2: Example of WHO recommendation [3]. Concerning frequency, WHO guidelines refer to a minimum number of sessions per week (e.g., three times or once a week). In this view, the duration (e.g., 150-300 min) is cumulative of all sessions in a week and expressed in the guidelines as a requirement for a minimum number of minutes per week or a range (with a maximum). It is worth noticing that the WHO definition of duration is not a single value plus/minus a variation, but a range of equally valid durations, depending on the intensity. It is also worth pointing out that the WHO guidelines do not specify ontological and contingent constraints related to the combination of modality, intensity, frequency, and duration. For instance, an aerobic activity has a minimum duration and therefore, implies an upper bound to the frequency, in terms of maximum number of aerobic activities of minimum duration, which can be carried out in the specified time period. Similarly, the frequency, as minimum number of sessions per week, implies a lower bound to their duration. Duration can be unbounded but, relying on external knowledge, the implementation of the guidelines may include an explicit upper bound. Different from fitness ontologies, the WHO does not qualify the settings of the exercise such as the elevation, peace, or temperature of the exercise location. Indeed, a fitness ontology may focus on exercising a specific muscle, whereas the WHO guidelines are concerned with the overall effect on the person. In this view, WHO guidelines provide a constraint-centred description of an optimal weekly regime. Another level of flexibility concerns the possibility to combine different physical activities. However, WHO does not specify a metric to evaluate combinations. Hence, the realization of the guidelines into a scheduler system also requires the inclusion of additional sources of domain knowledge. Finally, getting inspired also by the Neon methodology [38], in Table 1, we describe the HeLiFit specification in terms of its purpose, scope, implementation language, target users, and intended use.