Model-Based Biomechanical Exoskeleton Concept Optimization for a Representative Lifting Task in Logistics

Abstract

Occupational exoskeletons are a promising solution to prevent work-related musculoskeletal disorders (WMSDs). However, there are no established systems that support heavy lifting to shoulder height. Thus, this work presents a model-based analysis of heavy lifting activities and subsequent exoskeleton concept optimization. Six motion sequences were captured in the laboratory for three subjects and analyzed in multibody simulations with respect to muscle activities (MAs) and joint forces (JFs). The most strenuous sequence was selected and utilized in further simulations of a human model connected to 32 exoskeleton concept variants. Six simulated concepts were compared concerning occurring JFs and MAs as well as interaction loads in the exoskeleton arm interfaces. Symmetric uplifting of a 21 kg box from hip to shoulder height was identified as the most strenuous motion sequence with highly loaded arms, shoulders, and back. Six concept variants reduced mean JFs (spine: >70%, glenohumeral joint: >69%) and MAs (back: >63%, shoulder: >59% in five concepts). Parasitic loads in the arm bracing varied strongly among variants. An exoskeleton design was identified that effectively supports heavy lifting, combining high musculoskeletal relief and low parasitic loads. The applied workflow can help developers in the optimization of exoskeletons.

Keywords: AnyBody Modeling System; activities above shoulder height; assistive systems; ergonomics; exoskeleton; heavy lifting; logistics; manual work; multibody simulation; musculoskeletal modeling.

Figure 1

(a) Subject marker positions for optical motion markers. (b) Replicated loading platform with attached markers, highlighted with black circles. (c) Box A with attached markers and handle on top. (d) Box B with attached markers and two lateral handles.

Figure 2

Subject performing motion sequences T1–T6 in the motion laboratory. From left to right: carrying box A (T1), lifting (T2, red arrow) and lowering (T3, green arrow) box A, carrying box B (T4), lifting (T5, red arrow) and lowering (T6, green arrow) box B.

Figure 3

Model of the exoskeleton, including the hip belt (I, grey) and back plate (II, grey), the load-bearing structures (III, violet) between the hip belt and torque-generating joint (IV), the upper arm bracing (VI, pink), as well as the proximal (red) and distal (cyan) parts of a structure (V) connecting the torque-generating joint and arm bracing. Black squares show the three possible joint positions per side. In the respective magnifications (black rectangles) the joint variants are illustrated: The hip connection (between grey and violet structures) can include a revolute joint (1 (a)), universal joints (1 (b), first axis: vertical axis or 1 (c), first axis: vertical axis)) or a spherical joint (1 (d)). The parts (red and cyan) of the structure between the torque-generating joint and arm bracing can be connected rigidly (2 (a)) or via a prismatic joint (2 (b)). The connection between the distal part of the structure and the arm bracing can be fixed (3 (a)), include two types of revolute joint (3 (b) and (c)) or a universal joint (3 (d), first axis: vertical axis). Grey blocks, cylinders, and spheres symbolize the respective joints. Additional axes (dotted lines) and black arrows indicate the possible axes of translation (2 (b)) or rotation.

Figure 4

(Left): Two different angles from the exoskeleton connected to the human model in the AnyBody Modeling System^TM (AMS). The exoskeleton (grey) is connected with the human body model with AnyKinExtraDrivers (positions pictured with green spheres) in the kinematic analysis and with predictive contact elements (PCE, grey transparent cylinders) in the inverse dynamic analysis. They are positioned at the arms (16 elements), hip (10 elements), and back (one element). Arrows show exemplary positions of PCE at the sides, front, and back of the hip belt, the back, and the arm bracings. Blue lines show normal and friction forces currently acting on the PCE. Torques generated by the torque-generating joints and spring forces between the back plate and torque-generating joints are depicted with red arrows. (Right): Arm and Arm bracing (transparent) with coordinate system defined in its center with three axes: the distoproximal axis (along the arm longitudinal axis), the craniocaudal axis (points to the bottom when shoulder flexion is 90°) and the lateromedial axis (points to the sagittal plane when shoulder abduction is zero). The position of the AnyKinExtraDrivers is in the center of the coordinate system (green sphere). Positions of the PCE are illustrated by eight black coordinate systems around the arm bracing.

Figure 5

(Left): Resultant forces in the glenohumeral joints (averaged between left and right of one subject) over the motion sequence T5 without (black) and with exoskeleton concept variants (colored). (Right): Compression forces between L4 and L5 over the motion sequence T5 without (black) and with exoskeleton concept variants (colored).

Figure 6

Mean forces and torques in the arm interface with standard deviations for six concepts averaged over left and right arm bracing and the motion sequence T5. The diagrams show absolute torques around the craniocaudal (green) and distoproximal (orange) axis and absolute forces in the direction of the lateromedial (blue) and distoproximal (orange) axis. For craniocaudal forces and lateromedial torques averages of signed values are shown.

Figure 7

The six final exoskeleton variants are depicted with their respective joints (symbolized by grey blocks and cylinders). Corresponding DOF (axes of translation or rotation) are depicted with black arrows and dotted lines.