## 2 Materials and Methods
### Procedures
#### 2.3.3. Wobble-Board Hand Exercises  
A computerized wobble board (Balance Board WSP, GSJ Service, Rome, Italy; 40 cm diameter with a half plastic sphere 6 cm in height and 20 cm in width; maximal tilt angle = 20\({}^{\circ}\)) containing a tri-axial accelerometer (Photiget Spatial 0/0/3 Basic 1041, Phidgets Inc. 2016, Calgary, AB, Canada) internally calibrated and able to measure \(\pm\)8 g (\(\pm\)78 m\(\cdot\)s\({}^{-2}\)) per axis and both dynamic acceleration (change in velocity) and static acceleration (gravity vector) was used to perform the hand exercises. Tilt angle data were transferred to a computer via a USB cable at 200 Hz via proprietary software (Software WSP version 1.0.0.1, GSJ Service, Rome, Italy) and displayed on the monitor (1920 \(\times\) 1080 resolution screen). Participants were asked to focus on the motion marker (diameter = 6 mm) and to keep it inside the target zone (diameter = 6.5 cm) for as long as possible. The target zone stopped in the middle of the screen represents the stability area at a 0\({}^{\circ}\) tilt angle measured by the tri-axial accelerometer. The size of the motion marker and target zone were standard for all the participants and exercises. The wobble-board software (Software WSP version 1.0.0.1, G.S.J. Service S.r.l., Rome, Italy) user interface also displayed a countdown of 5 s before starting every exercise [43; 44]. For all wobble-board hand exercises, the goal was to keep the motion marker within the target zone for as long as possible during the exercise period. The performance counter becomes active when the motion marker is in the target zone for at least one second (the integer is the only number recorded, not second fractions). The target zone displayed different motion patterns to simulate everyday motion tasks, and for all patterns, the target zone moved on the laptop for 15 s, and the details are presented below.  
For Clockwise and Counterclockwise, starting from the top of the screen, the target zone started to move in a clockwise/counterclockwise motion, with the full lap taking 15 s. The displacement of the motion marker was allowed from the circular movement of the hand around the longitudinal axis.  
For Antero-posterior and Medial-lateral, starting from the middle of the screen (a 0\({}^{\circ}\) tilt angle), the target zone moved to the top (antero-posterior) or to the left (medio-lateral) in 0.008 s and then started to move in a vertical direction (antero-posterior) or a horizontal direction (medio-lateral). The displacement of the motion marker was allowed from the movement of the hand around the frontal (antero-posterior) or sagittal (medio-lateral) axis.  
Figure 2: Tri-axial accelerometer attached to the hand using the customized elastic belt (**a**). Starting (**b**) and recording (**c**) positions of the hand during physiological hand tremor measurement.  
Participants were in a standing position, with the working limb placed at 90\({}^{\circ}\) on the wobble board and the contralateral one (the limb not performing the hand exercise) resting on the same side (Figure 3a). The wobble board was placed on a table and the monitor was at eye level 2 m in front of the participant (Figure 3b). The wobble-board hand exercise included 1 min of familiarization with the unstable platform for each motion pattern, with 1 min of sitting recovery in between. After familiarization, the central phase included four 15 s trials for each motion pattern (clockwise, counterclockwise, antero-posterior, and medial-lateral) and for each limb (dominant and non-dominant) in a randomized order.