Inverse optimal control results for motions of workers with LBP

It is a widespread assumption that motions of humans and animals are carried out in an optimal way due to evolution, learning and training. The behavior under certain circumstances such as pain or fatigue can also be considered as the result of an optimization process. Since most of these optimizations are unintentional and unconscious, the specific optimization criteria underlying a motion are usually not known.

In order to identify suitable optimality criteria for describing the lifting motions of people with and without low-back pain, we performed inverse optimal control computations.

For this purpose we carried out a biomechanical study on people with and without low-back pain performing different lifting techniques including stoop- and squat-lifts and lifts with body rotation as well as different range of motion trials. The motions were recorded using a motion capture system.

We created mathematical models of the subjects with virtual markers attached. We fitted marker positions and the orientation of grouped markers of all recordings and subjects to obtain joint angle trajectories which we analyzed an compared. We observed that the range of motion of the subjects was very individual. We also had subjects without low-back pain with a more limited range of motion than subjects with low-back pain. In addition, we found that performing squats tend to be more difficult for people with low-back pain.

We set up an inverse optimal control problem to test a set of optimality criteria including minimization of joint torque, mechanical work, joint velocity and joint angle. We performed the computations for the stoop-lift of four different subjects and compared the results.

We could reproduce the movements quite well for the subjects. Most deviation was observed for the arms and the head. We got similar results regarding the optimality criteria for all tested subjects, so there could be no significant difference identified between subjects with and without low-back pain performing stoop-lifts.

We plan to test further optimality criteria and lifting techniques and present the results soon.

Wearability of the passive exoskeleton

In spite of the remarkable advances in the field of exoskeletons, lack of wearability and ease-of-use is still an important technological barrier that often impedes the acceptability of exoskeletons among the healthy individuals. To overcome this barrier in the design of SPEXOR exoskeleton, we developed a special combination of joints that compensate the misalignment between the joints of the exoskeleton and the joints of the user while at the same time the exoskeleton unloads the lower back by redirecting the unwanted forces and moments from the lumbo-sacral spinal region to the less harmful and more stable location on the pelvis, thighs and chest. The result of our design is drastically improved range of motion which can represent a significant increase in acceptability of this technology:

Evaluation of the passive exoskeleton

We performed a comprehensive evaluation of the new version of our passive SPEXOR exoskeleton, assessing the effect of wearing the exoskeleton on biomechanical loading of the spine, functionality, user satisfaction and self-efficacy. Recruiting employees from load-handling professions we tested participants with and without low back pain. We concluded that we reached beyond the state of the art by high versatility due to implementation of a clutch, higher support levels (50 Nm) and a limitation of lumbar flexion in static bending, leading to reduced loading of the spine during lifting and forward bend positions. In addition, we solved previously observed limitations of benchmarking devices, such as interference with tasks, discomfort and restricted range of motion. Both, healthy people and employees with low back pain can benefit from wearing the device. Design improvements that can still be considered are higher support to increase the effect on spine loading and higher user satisfaction by improving general comfort.

Design optimization for active spinal exoskeleton

Researchers at UHEI performed various optimization studies on the design of active spinal exoskeletons for the support of lifting motions which serve as an input for the upcoming work in the project.

  • For an exoskeleton model with rigid segments and motors at the hip and lumbar joints, we determined motions and corresponding human and exoskeleton torques that correspond to the smallest effort for the human, also respecting limits on the actuators.
  • For an exoskeleton model with rigid segments and motors with parallel springs at the hip and lumbar joints, we reconstructed recorded individual lifting motions of different subjects without exoskeletons. Various objective functions were evaluated that lead to different distributions of the dynamic efforts between the muscular torques of the human and the passive and active torques of the exoskeleton. Contact forces between the human and exoskeleton were limited to ergonomically reasonable values.
  • We also considered an exoskeleton model with a flexible beam element in the back and springs in the hip joint, corresponding to the current passive SPEXOR prototype. In addition to these passive elements, parallel actuators in the hip joints have been considered. The results show potential reductions in the human torques by the introduction of the active components.

Extending the spinal orthotic module with a passive self-aligning hip module

A misalignment compensation mechanism for the hip was developed to increase the comfort of the exoskeleton and to hinder movements as little as possible. A purely passive torque source at hip level generates the support needed to unload the lower back.

Passive spinal orthosis with viscoelastic element

Continuus carbon fiber beams which generate support and allow for a large range of motion are used as a back interface. Combined with the torque source at the hip, they generate the torque which reduces the strain on the lower back.

Biomechanical requirements for active version of spinal exoskeleton

We assessed full body kinematics and spine load components in natural lifting, to find kinematic and support pattern requirements for the SPEXOR actuated exoskeleton. Furthermore, using a benchmark actuated exoskeleton, we investigated how specific actuation control modes interact with subject behavior, and how this affects spine loading.

Biomechanical requirements for passive spinal exoskeleton are defined

We assessed to what extent a benchmark exoskeleton reduces spine loading and changes body kinematics during static trunk bending and during lifting. Additionally, we investigated how it affects perceived comfort, effort, performance, and metabolic energy consumption during functional activities such as walking and stair climbing. Based on these data, biomechanical requirements for the spexor passive exoskeleton were refined.

Monitoring system runs with full body model

The SPEXOR monitoring system is now linked with the full-body dynamical model providing insight into the musculoskeletal stress parameters. Further optimization for real-time processing and feedback are being addressed.

Controller for passive spinal exoskeleton

The controller for engagement and disengagement of the hip spring is developed. It is based on the probabilistic model of the human motion that classifies whether the user requires the support of the exoskeleton or the exoskeleton should remain disengaged to allow free motion.