## 5 Discussion
### Challenges  
The careful analysis of the articles included in the review highlights several challenges, which are reported below. The identification of these challenges can help design and conduct future studies related to digital biomarkers for motor symptoms assessment in PwPD. **Participant Selection:** Most studies focused on the diagnosis of PD, alone or in combination with other objectives (e.g., estimation of disease severity, treatment conditions). The proper evaluation of digital tools for computerized early diagnosis should be conducted on newly diagnosed PwPD. However, some studies have not reported any measurement of duration and/or severity of disease, which makes it complicated to assess the potential of the proposed solution. When reported, the average duration of the disease was in the range of 3.5-13 years and the H&Y stage was mostly equal to or greater than 2. This means that the recruited PwPD were mostly at an advanced stage of the disease, when motor symptoms were fully visible. This raises doubts about the usefulness of the solutions developed and their real potential for early diagnosis. It is essential to define appropriate standardized criteria that provide guidelines for the recruitment process and the reporting of participant characteristics. For example, in order to enable a proper evaluation of the PD, at least disease duration, H&Y and MDS-UPDRS scores must be reported. Strict inclusion criteria with regard to disease duration (e.g., less than 2) and H&Y stage (e.g., less than or equal to 1) must be adopted when evaluating digital systems for early diagnosis. Additionally, regarding the number of participants, the sample size significantly varies. Two clear trends are observed, 14 studies with a number of PwPD between 5 and 42, and 8 works with more than 80 participants (3 studies with more than 1000). Furthermore, studies tend to have a higher number of HC than PwPD (of the articles that are clearly specified, 77% of the total participants are HC). It would be interesting for a correct definition of DB-MS-PD that these are defined from a significant number of PwPD registries, close to 50% of the total. In addition, another factor that should be taken into account is the gender balance of the participants, as, for HC and PwPD, the percentage of male participants is high compared to female participants (71% HC, 65% PD). It could be interesting to try to define a minimum number of patients or healthy participants and age or gender distribution for a biomarker to be proposed as such, as it would allow us to know that it has been tested with a relevant number of people. **Reporting of methods:** Related to the activities and tasks performed, in some cases, the activity carried out is very detailed, while in others it is described in a general way. For example, distance travelled, gait trajectories to be followed, and speeds are indicated in a limited number of studies. Similarly, the addressed tasks are sometimes referred as MDS-UPDRS tasks, without precise indication of the specific UPDRS item. A similar phenomenon is observed in the description of the positioning of wearable devices on the body. Some studies explicitly describe the position of the device through photographs or diagrams, while others merely refer to general anatomical regions, such as the upper limb or lower limb. A possible option would be to use a diagram in the shape of a human body to identify the location of each of the devices on the body. Several articles [67; 73; 74; 77] have already used this approach and it is an effective way to identify the positions discussed in detail. **Tasks performed:** Gait and finger tapping have been found to be the most studied activities for monitoring and understanding the evolution of PD severity and motor symptoms. In the reviewed studies, most PD participants were at the same stage of the disease and performing the movements in a fairly similar way, which allows for a classification of each stage easily.