# Squat Lifting Imposes Higher Peak Joint and Muscle Loading Compared to Stoop Lifting
## 2 Materials and Methods  
Ten healthy participants (7 men; age: 31.5 years (\(\pm\) 12.7), body mass index (BMI): 22.19 kg/m\({}^{2}\) (\(\pm\)3.98)) participated in this study. Participants were excluded when suffering from musculoskeletal disorders or were experiencing pain that could affect activities of daily living. All participants provided written informed consent prior to the start of the study, and all study procedures were approved by the local ethics committee (Universitair Ziekenhuis Leuven, S61611). Participants executed two standard lifting techniques: Stoop and squat. The two lifting techniques were verbally explained, and a small demo was performed before the actual measurements. During stoop lifting, participants were instructed to fully extend their knees and incline their trunk to lift the box from the ground to the upright position; the squat was performed with fully flexed knees, a heel lift-off, and the trunk close to erect (Figure 1) [8]. Participants had to lift a standard weight of ten kilograms as well as a weight equal to 40% of the arm lifting strength test (ALST) [23], performed before the start of the measurement. Ten infrared cameras (100 Hz, VICON, Oxford Metrics, Oxford, UK) captured 3D-marker trajectories during the lifting tasks. Retroflective markers were placed on the participants according to the plug-in gait marker model [24] extended with a spine marker model [25], and the single markers on the upper and lower arms and legs were replaced by three marker clusters. Two ground-embedded force plates synchronously recorded the ground reaction forces (1000 Hz, AMTI, Watertown, MA, USA). In addition, the unilateral (right side) muscle activation of 9 muscles was recorded, (more specifically, m. rectus femoris, m. biceps femoris, m. trapezius descendens, m. deltoideus anterior, m. deltoideus medius, m. triceps brachii, m. biceps brachii, m. erector spinae illicocostalis, and m. erector spinae longissimus) using surface electromyography (1000 Hz, Cometa, Bareggio, Italy) according to the Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM)-guidelines. Measured data were processed using a standard musculoskeletal modelling workflow implemented in OpenSim 3.3 [26] to calculate joint angles and moments. Three validated musculoskeletal models were combined: A modified model of the fully articulated thoracolumbar spine and rib cage model [27], a modified model of an upper extremity model [28], and the gait2354 lower extremity model [29]. To simplify the model, the joints between the five lumbar vertebrae were described as 3 degrees of freedom joints, and the thorax was defined as one rigid segment linked as a weld joint with L1. In addition, the kinematics of L4-L5 and L2-L3 were defined using a coupled coordinate function using the joint angle of L5-S1 and L3-L4 as an independent variable. The weight of the box was accounted for by adding half of the weight of the box to the mass of metacarpal III on the right and left hand. Firstly, the generic model was scaled to the subjects' anthropometry based on the weight and the marker positions measured during a calibration trial. Secondly, joint angles (Appendix A) were calculated using inverse kinematics [30]. Thirdly joint moments were calculated using the Inverse Dynamics Tool implemented in OpenSim and normalized to body weight to account for differences in body mass. Joint power was then calculated based on the angular velocity and the normalized joint moment. Lastly, the moment impulse was calculated as the area under the normalized moment curve  
Figure 1: (**a**) Squat lifting and (**b**) stoop lifting. to estimate the cumulative loading over the whole lifting or lowering phase. Raw EMG signals were band pass filtered (20-400 Hz) and full-wave rectified. Then, the rectified signal was filtered with a 6 Hz low pass filter and normalized to the maximal muscle activity obtained during maximal voluntary contractions (MVC) measured during isolated tests before the lifting tests.