Biomechanical Analysis of Stoop and Free-Style Squat Lifting and Lowering with a Generic Back-Support Exoskeleton Model

Abstract

Musculoskeletal disorders (MSDs) induced by industrial manual handling tasks are a major issue for workers and companies. As flexible ergonomic solutions, occupational exoskeletons can decrease critically high body stress in situations of awkward postures and motions. Biomechanical models with detailed anthropometrics and motions help us to acquire a comprehension of person- and application-specifics by considering the intended and unintended effects, which is crucial for effective implementation. In the present model-based analysis, a generic back-support exoskeleton model was introduced and applied to the motion data of one male subject performing symmetric and asymmetric dynamic manual handling tasks. Different support modes were implemented with this model, including support profiles typical of passive and active systems and an unconstrained optimal support mode used for reference to compare and quantify their biomechanical effects. The conducted simulations indicate that there is a high potential to decrease the peak compression forces in L4/L5 during the investigated heavy loaded tasks for all motion sequences and exoskeleton support modes (mean reduction of 16.0% without the optimal support mode). In particular, asymmetric motions (mean reduction of 11.9%) can be relieved more than symmetric ones (mean reduction of 8.9%) by the exoskeleton support modes without the optimal assistance. The analysis of metabolic energy consumption indicates a high dependency on lifting techniques for the effectiveness of the exoskeleton support. While the exoskeleton support substantially reduces the metabolic cost for the free-squat motions, a slightly higher energy consumption was found for the symmetric stoop motion technique with the active and optimal support mode.

Keywords: biomechanics; ergonomics; manual handling; musculoskeletal modeling; occupational exoskeletons.

Figure 1

Selection of commercial back-support exoskeletons (BSEs) applying supportive torques aligned with the user’s hips [12,14,17].

Figure 2

Illustrations of measured symmetric and asymmetric motion sequences.

Figure 3

Generic exoskeleton–human model with anatomical directions as the basis for the orientations of the axes of rotation and translation for the interaction forces and moments. The model is illustrated with anatomical directions (a) and implemented interaction forces and moments (b) based on Table 1. Further, the axes of the actuator torques are outlined.

Figure 4

Characteristic torque–angle curves for WPM, SPM (left graph), and all modes, including summed torque for both actuators for a symmetric, free-squat motion with 20 kg over flexion and extension phase (right graph).

Figure 5

Lumbar joint moments for all motion techniques and external weight of 20 kg illustrated together with the human model in sagittal (a,b) and frontal (c,d) plane as mean moments for all five trials over normalized time.

Figure 6

Forces and impulses in L4/L5 joint (± standard deviation (SD)).

Figure 7

Mean peaks over five trials for lumbar extension moments and metabolic costs, both approximated based on the modeling assumptions in the AMS (±SD).

Figure 8

L4/L5 peak compression forces with the different exoskeleton support modes (±SD), compared to the reference as mean without exoskeleton support.

Figure 9

Metabolic costs with the different exoskeleton support modes (±SD), compared to the reference as mean without exoskeleton support.