# Test-Retest Repeatability of Human Gestures in Manipulation Tasks
## 2 Materials and Methods  
### Participants, Experimental Set-Up and Protocol  
Twenty-six healthy participants (fourteen males and twelve females) with no musculoskeletal or neurological diseases were recruited for the experiment. Twenty-three subjects were right-handed; three subjects were left-handed. Their main anthropometric data (mean \(\pm\) standard deviation) are reported in the following: age = 27.7 \(\pm\) 2.3 years, BMI = 22.2 \(\pm\) 2.0 kg/m\({}^{2}\), forearm length = 0.29 \(\pm\) 0.01 m, and upper arm length = 0.34 \(\pm\) 0.01 m. The study was conducted in accordance with the Declaration of Helsinki and informed consent was obtained from all involved subjects. Two wireless MIMUs (OpalTM APDM, Portland, OR, USA) containing tri-axial accelerometer (range \(\pm\) 200 g), gyroscope (range \(\pm\) 2000 deg/s) and magnetometer (range \(\pm\) 8 Gauss) were exploited during the test. As shown in Figure 1a, these units were positioned on participants' right (RFA) and left (LFA) forearms, aligning their x-axes with the longitudinal axes of the corresponding human segments. The communication between MIMUs and a PC was guaranteed via Bluetooth. The proprietary software Motion StudioTM V2R, Version 3 (APDM, Portland, OR, USA) was used to acquire data at 200 Hz. A typical industrial task of pick-and-place was realized through the workstation illustrated in Figure 1b. It was composed of a table, a chair, a box containing 30 golf balls, and four stations: S0, S1, S2 and S3. Due to the heterogeneity of the tested group in terms of anthropometric characteristics, it was necessary to adapt the experimental set-up to each participant. To this purpose, a board with thirty holes (diameter of 6 cm) was realized to choose the more appropriate distances for S0, S1 and S2 stations. Moreover, a second board with one hole (diameter of 6 cm) corresponding to the S3 station was added to the set-up at a fixed height of 30 cm from the table and an adjustable distance from participants. The task was the same proposed in a previous pilot study [11]. In detail, it was composed of 30 pick-and-place gestures identified as normal movements. Each of these gestures consisted of picking a ball at a time from the box and placing it into a specific hole corresponding to one of the four stations. Participants were asked to place the balls in the holes following a lighting sequence of green LEDs positioned near each station. In addition, during the task, sudden visual alarms or acoustic alarms were randomly produced by lighting red LEDs near each station or switching on a sound buzzer, respectively. In both cases, participants were asked to perform an abrupt movement as fast as possible, placing the ball inside the hole corresponding to the activated red LED or vertically extending the arm in case of the sound buzzer [11]. An Arduino Nano microcontroller (Arduino, Italy) with the following characteristics was exploited to generate both the visual and acoustic alarms: processor = ATmega328, clock speed = 16 MHz, and operating voltage = 5 V. The periodic lighting of the green LEDs (every 3 s = 20 bpm) and the random lighting of the red LEDs or of the buzzer were controlled by a code written through an integrated development environment. The Arduino microcontroller was also used to send a voltage trigger of 5 V to Opal sensors for systems synchronization. The experimental protocol involved two different sessions, the test and the retest. Both sessions were performed under the same conditions by all subjects with an intercurrent period between 20 and 65 days (42.2 \(\pm\) 11.2 days) depending on the subject. In detail, two main groups can be identified based on a threshold of 45 days.