## 2 Materials and Methods
### Experimental Procedures  
Each subject lifted a crate in the sagittal plane (without trunk rotation) with both hands at three different risk levels determined according to the NIOSH method alone and in team with another subject, as detailed in ref. [6], Figure 1.  
Table 1 shows the values of the experimental setup parameters that contribute to determining the risk level given by the NIOSH lifting index (LI) both in one- and two-person team lifting. To ensure that all NIOSH parameters were effectively controlled, and risk levels were correct, the positions of the feet for the various tasks, as well as the positioning of the box, were marked on the ground with tape so that the horizontal distance (H) between the center of the malleoli and the center of the load was actually (and for all subjects) 60 and 63 cm, for tasks A, B, and C, respectively. Furthermore, the maximum height to which the weight had to be lifted was indicated with a three-level rod, resulting in vertical displacements (D) of 40, 54, and 100 cm for jobs A, B, and C, respectively. Finally, the initial height of the load center (V) from the ground was controlled using a support surface to ensure that it was exactly 10 cm for tasks A and C and 31 cm for task B. Each participant performed 3 repetitions of each risk condition for both one- and two-person team lifting, so to have a total of 18 trials. The different liftings were executed at random across the three risk conditions and one- and two-person team lifting to avoid bias. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Task**} & **LC** & **H** & **(cm)** & **HM** & **V** & **V** & **D** & **DM** & **A** (C)** & **AM** & **F** & **(lift/min)** & **FM** & **C** & **CM** & **L** & **RWL** & **RWL** & **RWL** & **LI** & **Llr** \\ \hline A & 23 & 63 & \(-\)0.40 & 10 & \(-\)0.81 & 40 & \(-\)0.93 & 0 & 1 & \(\leq\)2 & 1 & good & 1 & 7 & 6.88 & 4.59 & 1.02 & 0.51 \\ B & 23 & 60 & \(-\)0.42 & 31 & \(-\)0.868 & 54 & \(-\)0.90 & 0 & 1 & \(\leq\)2 & 1 & good & 1 & 15 & 7.51 & 5.03 & 2.00 & 0.99 \\ C & 23 & 60 & \(-\)0.42 & 10 & \(-\)0.805 & 100 & \(-\)0.87 & 0 & 1 & \(\leq\)2 & 1 & good & 1 & 20 & 6.67 & 4.47 & 3.00 & 1.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: This table reports for each lifting task the 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 hand-to-object coupling (C) and the corresponding values of the multipliers and recommended weight limit (RWL) for one-person and two-person team lifting (RWL and RWL, respectively). LC was defined as 23 kg in RNLE. The value of LI for one-person lifting (LI) and for two-person team lifting (LI\({}_{\text{T}}\)) were also reported. Figure 1: This figure displays the experimental setup: (**A**) one-person and (**B**) two-person-team lifting. The picture depicts how the load’s horizontal distance (H), and vertical displacement (D) were controlled to meet the risk levels identified according to the NIOSH method, (lifting index, LI). Before starting the measurements, a global reference system was defined by executing a calibration procedure according to [48] with a mean spatial accuracy of 0.2 mm. The movement of one spherical marker covered with aluminum powder reflective material was detected at a sampling frequency of 340 Hz by using an optoelectronic motion analysis system (SMART-DX 6000 System, BTS, Milan, Italy) with eight infrared cameras. The marker was placed over the right anterior vertex of the load (a plastic rate). Surface electromyography (sEMG) has been recorded with a 16-channel Wi-Fi transmission surface electromyograph (Mini Wave Infinity, Cometa, Milan, Italy) with a 2000 Hz sampling frequency.