## 2 Materials and Methods
### Analysis  
Marker trajectories were filtered using a Butterworth 4 th-order 5 Hz lowpass filter. The three markers taped to the box enabled us to compute the position of the box center. Lower and higher peak values of box center vertical positions were used to split the lifting task into upward and downward phases. By means of a cubic interpolation, each phase was time-normalized into 100 data points. Right hip and right knee joint angles were calculated in three dimensions using a custom MATLAB program following the recommendations of [PERSON] et al. (1995) [34] and [PERSON] et al. (2002) [31]. Maximum knee flexion and hip flexion angles were computed for each cycle. Back curvature was assessed in the sagittal plane using S1, L4, L2, T12, T2, and C7 markers. Three segments (C7-T2, T12-L2, L4-S1) were used to calculate lumbar flexion (T12-L2-L4-S1) and thoracic flexion (C7-T2-T12-L2), as previously carried out with inertial measurement units [9]. Maximum lumbar and thoracic flexion angles were computed for each cycle. As movement smoothness can be representative of the subject's expertise [35], the smoothness of the right wrist was evaluated using the resultant speed of the marker placed on the right radius styloid process during the upward phase. The resultant speed of the marker was used as the input for the spectral arc length function proposed by [PERSON] et al. (2015) [20]. It evaluates the spectral arc-length metric that uses a movement speed profile's Fourier magnitude spectrum to quantify movement smoothness. The output smoothness value was negative, and smoother motion was represented by a value closer to zero.  
Center of pressure data were filtered using a Butterworth 4 th order lowpass filter at 10 Hz. The CoP trajectory was perturbed when the box touched the ground between each cycle. Therefore, data on CoP displacement were analyzed only when the box was off the ground (during both upward and downward phases). Confidence ellipse area (95%) and anteroposterior postural oscillation amplitude (difference between the maximum value and the minimum value of CoP position on the anteroposterior axis) were calculated to evaluate postural stability, as it can be influenced by the use of an exoskeleton [11].  
EMG data were filtered using a bandpass 30-450 Hz Butterworth 4 th order filter. The root mean squared (RMS) values were then calculated for each muscle and for each cycle. The mean value of the 10 cycles was computed for each muscle and for each condition and expressed relative to the muscle activity recorded during a reference task. This reference task consisted of a forward jump as far as possible [36] and was performed prior to the familiarization protocol at the beginning of each session. The jump performance during the first session was marked in order to be reached again during subsequent sessions. Average RMS values over two broad jump attempts were used to normalize each muscle activity.  
For hip and knee angles, back curvature, smoothness, confidence ellipse, postural oscillation amplitude, and EMG RMS, the mean value over 10 cycles was calculated.  
An overall motor control performance was assessed during PRECI, as was the time needed to execute the task, and each error accounted for an additional 2 s lapse. The mean performance value was calculated over three attempts. The timed performance and the number of errors were assessed using an Arduino(r) Uno board and a custom Arduino(r) program.