# Squat Lifting Imposes Higher Peak Joint and Muscle Loading Compared to Stoop Lifting
## 1 Introduction  
Annually, more than 40% of all employees in Europe suffer from work-related musculoskeletal disorders (WMSDs), such as low back, neck or shoulder pain, despite ongoing efforts to improve the working conditions [1]. These WMSDs result from excessive physical workloads due to repetitive movements (61%), non-neutral posture (44%) and heavy material handling (31%) [1]. They are responsible for the absence of 3%-6% of the working time, causing an average cost of 2.5% of the gross domestic product across Europe [2]. Of all these WMSDs, 52% are due to overloading during lifting tasks, of which 65% affect the back [3]. It is assumed that by reducing excessive physical workloads, the amount of WMSDs can be reduced, as can the concomitant costs. For this, ergonomic guidelines, such as the ISO standards, can be used. The ISO standards, more specifically ISO standard 11228 Part 1, advises a reference mass for two handed lifting under ideal conditions. Ideal conditions are characterized by having a symmetric standing, one's trunk in a neutral position, a horizontal distance to the object center of mass less than 25 cm, and a grip height of less than 25 cm above knuckle height,thereby allowing a firm grip. Lifting duration should not exceed 1 h per day, with a frequency less or equal than 0.2 lifts per minute. In addition, favorable environmental conditions are assumed, i.e., not too warm/cold and humid. Under ideal conditions, the reference loads are 25 and 15 kg for 95% of men and women, respectively. It is assumed that when the lift situation is in accordance with the above-mentioned guidelines, the lifting technique is more ergonomic and thus reduces the risk of developing WMSDs. The action mechanism potentially reduces the compressive and shear forces between the vertebas. This is important, as high compressive forces on the vertebral discs have been related to low back injury [4]. Two standard lifting techniques, the stoop and the squat, are well-described in literature [5; 6; 7; 8; 9; 10; 11]. In general, squatting is advised to lift heavy objects, as this technique is thought to result in lower disc compression and shear forces compared to the stoop. On the other hand, stoop lifting should be restricted to lifting light objects because stoop lifting causes higher disc compression and shear forces compared to the squat [8]. The high disc compression observed during stoop lifting results from a high flexion moment around the LSS1 joint, causing high muscle forces delivered by the back extensor muscles (i.e., m. erector spinae) to overcome the external flexion moment. Indeed, it is known that the muscle forces of the extensor muscle are the main contributors to the compression of the vertebral joints [6]. To estimate joint moments, shear and compression forces, different biomechanical models (like static models [12], electromyography (EMG)-assisted models [6; 13; 14; 15], finite element models [9; 12], 2D dynamic linked segment models in combination with an EMG-assisted models [14] and a full-body musculoskeletal OpenSim model [16]) have been used. These studies found lower moments in the lower back with lower disc compression and shear forces during squat lifting compared to the stoop, and they thereby advised using the squat technique for lifting. However, most studies have only focused on the LSS1 joint in isolation and have neglected loading on the other joints within the human body, which are also likely to be affected by lifting techniques. Squatting results in lower lumbar extensor moments, but to our knowledge, the effect on the shoulder or other joints has not yet been investigated during these lifting techniques. This is highly relevant given the comparable prevalence of shoulder and low back injuries in industry [1]. When muscle loading associated with different lifting techniques has been studied based on EMG, most studies [8; 17; 18; 19; 20] have only focused on a limited number of muscles that span the fifth lumbar vertebrae joint (i.e., m. erector spinae illicostalis).