## 1 Introduction  
Quantifying the loads stressing the lumbar spine during manual lifting activities is crucial for assessing the risk of work-related low back pain [1, 2]. Spinal loads are usually identified as the compressive and shear forces acting on one of the lumbar intervertebral disc centers. The earliest approach for the determination of such forces through indirect methods is the planar linked segment model (LSM) proposed by [PERSON] in 1969 and still currently used for the analysis of static symmetric lifting activities [3]. Briefly, conventional Newtonian mechanics are sequentially applied to each body segment starting at one end of the chain (i.e., the hand) where external loads can be either known or measured. Assuming that the forces of the anatomical structures crossing the joint can be reduced to one moment and one resultant force acting at the joint center, intersegmental forces and moments are calculated at each joint of the chain to the intervertebral disc center of interest (L5/S1). Shear and compressive forces are, then, computed as the components along the S1 vertebra-fixed reference frame of the intersegmental force acting on the intervertebral disc center plus the contribution of the back extensor muscle force and of the force created by the abdominal pressure along the compressive axis only (i.e., the axis normal to the vertebral plate). The inclination of the S1 vertebral plate is computed by adding the inclination of the disc in upright posture (40\({}^{\circ}\)) to the sacral rotation, which is, in turn, predicted as a function of the measured trunk and knee angles [4]. Abdominal pressure is estimated as a function of the hip torque and the hip angle [5] and converted into a force by assuming a given diaphragm area [3]. Finally, the back extensor muscle force is estimated by dividing the intersegmental moment by the muscle moment arm, assuming that the back extensor muscles are represented by a single muscle equivalent and that the intersegmental moment can be attributed solely to muscle forces [6]. The back extensor muscle moment arm is retrieved from cadaveric studies, and its value, with respect to a neutral trunk orientation, is assumed to be constant regardless of the degree of trunk flexion. A detailed representation and implementation of [PERSON]'s LSM model can be found in his book [7]. [PERSON]'s musculoskeletal modelling has known limitations affecting the accuracy of the predicted spinal loads [6; 8; 9; 10]. Most of these limitations have been overcome by more detailed and even subject-specific musculoskeletal models controlled by either static or dynamic optimization and fed with three-dimensional (3D) camera-based motion capture data [11; 12]. Despite these limitations, [PERSON]'s model is still widely used in occupational ergonomics for its affordable measurement technique (the planar positions of the joints of the LSM can be easily retrieved from an image), ease of use and the fact it is implemented in several industry-oriented commercially available software packages [9; 13; 14]. In fact, 3D motion capture is not easily accessible to everyone for its cost, space requirements and expertise related to its use. Moreover, 3D motion capture is hard to use on a routine basis, which is what is needed in industrial settings for risk management. Therefore, it is worthwhile to conduct more research to enhance, where possible, the model proposed by [PERSON]. Among the limitations of this approach, the fixed moment arm of the back-extensor muscles is surely a critical point, as it has been shown that applying the lever arm obtained in an upright stance to a flexed position leads to a significant underestimation and overestimation of the compression and shear forces, respectively [15]. The orientation of the muscle's line of action (crucial for projecting the muscle force on the vertebral body properly), assumed to be parallel to the axis of compression and characterized by a constant value, has even worse effects [15].