## 2 Materials and Methods
### Experimental Procedure  
Ten healthy participants (5F and 5M), with a mean age of 36.70 \(\pm\) 7.09 years, a height of 1.69 \(\pm\) 0.08 m, a weight of 70.35 \(\pm\) 13.53 kg, and a body mass index (BMI) of 24.45 \(\pm\) 3.30 kg/m\({}^{2}\) were enrolled in the study. All participants were not taking part in any clinical medication trials and had no history of back pain, upper or lower limb or trunk surgery, neurological or orthopedic diseases, or problems with the vestibular system. Participants provided written informed consent for the study, which followed the Helsinki Declaration and was approved by the local ethics committee (Comitato Etico 'LAZIO 2', N.0078009/2021) after it received a thorough description of the experimental process. There was no mention of the expected results to prevent potential bias.  
Before measurements started, the optoelectronic system was calibrated, obtaining a spatial accuracy of 0.2 mm in the x, y, and z dimensions. After marker and IMU placement, the MVN system was calibrated for each participant, using the 'N-pose and walk' technique before starting recordings. To record, MVN Analyse (version 2018.0. 0) was utilized. Furthermore, participants underwent a training session to become familiar with the assessment procedures. This allowed all subjects to perform the lifting tasks correctly.  
The participants were asked to perform a manual material lifting task, on the sagittal plane without trunk rotation, of a plastic crate with handles, using both hands, according to the RNLE. A representative subject who performed the lifting task (Figure 1A) and a graphical representation of the task and experimental setup (Figure 1B) is depicted in Figure 1. After every lift, the load was repositioned at the starting point by two operators. Each lifting task was composed of two subtasks that had the same load but different geometry. For every minute of the experiment, the subject performed first all the liftings of a subtask and then the ones of the other, and the order of the subtasks was randomized. Table 1 shows, for each task, the reference values of the load weight (L), the horizontal (H) and vertical (V) locations, the vertical travel distance (D), the asymmetry angle (A), the lifting frequency (F), and the corresponding reference values of the multipliers. The hand-to-object coupling was defined as 'good' for all lifting tasks. The values of the single-task lifting index (\(STLI\)) and frequency-independent lifting index (\(FILI\)) are also reported [2]. The lifting tasks are designed to obtain reference CLI values of 0.5, 1.5 and 2.5. Each participant was required to perform all three tasks, each lasting 4 min, performing their subtask at the described frequencies. A metronome was utilized to trigger the lifting frequency: each time the acoustic signal was received, the participants raised the load to the set height. These acoustic signals administered to the subjects to impose the desired lifting frequency did not imply specific problems, as the imposed rhythms were not such as to induce execution errors. In detail, for example, during task 1, in each minute, the subjects lifted the load 3 times under the conditions of subtask A (F = 3, see Table 1) and 2 times under the conditions of subtask B. A rest period of 5 min was used between tasks. The lifting tasks and subtasks were assigned to each participant in a random order.