Assessing effects of exoskeleton misalignment on knee joint load during swing using an instrumented leg simulator

Abstract

Background Exoskeletons are working in parallel to the human body and can support human movement by exerting forces through cuffs or straps. They are prone to misalignments caused by simplified joint mechanics and incorrect fit or positioning. Those misalignments are a common safety concern as they can cause undesired interaction forces. However, the exact mechanisms and effects of misalignments on the joint load are not yet known. The aim of this study was therefore to investigate the influence of different directions and magnitudes of exoskeleton misalignment on the internal knee joint forces and torques of an artificial leg. Methods An instrumented leg simulator was used to quantify the changes in knee joint load during the swing phase caused by misalignments of a passive knee brace being manually flexed. This was achieved by an experimenter pulling on a rope attached to the distal end of the knee brace to create a flexion torque. The extension was not actuated but achieved through the weight of the instrumented leg simulator. The investigated types of misalignments are a rotation of the brace around the vertical axis and a translation in anteroposterior as well as proximal/distal direction. Results The amount of misalignment had a significant effect on several directions of knee joint load in the instrumented leg simulator. In general, load on the knee joint increased with increasing misalignment. Specifically, stronger rotational misalignment led to higher forces in mediolateral direction in the knee joint as well as higher ab-/adduction, flexion and internal/external rotation torques. Stronger anteroposterior translational misalignment led to higher mediolateral knee forces as well as higher abduction and flexion/extension torques. Stronger proximal/distal translational misalignment led to higher posterior and tension/compression forces. Conclusions Misalignments of a lower leg exoskeleton can increase internal knee forces and torques during swing to a multiple of those experienced in a well-aligned situation. Despite only taking swing into account, this is supporting the need for carefully considering hazards associated with not only translational but also rotational misalignments during wearable robot development and use. Also, this warrants investigation of misalignment effects in stance, as a target of many exoskeleton applications.

Influence of Varied Load Assistance with Exoskeleton-Type Robotic Device on Gait Rehabilitation in Healthy Adult Men

“…The knee moves through two sets of flexion and extension in a normal gait cycle: the first occurs from initial loading to midstance, and the second occurs from pre-swing to the initial swing [ 18 , 25 ]. A few studies have suggested that some misalignments between the user’s anatomical and exoskeleton joints can cause undesired interaction forces, which in turn reduce comfort and safety [ 26 , 27 , 28 ]. Misalignments have been discussed as a potential cause for lower limb fractures for a powered exoskeleton use [ 29 ].…”

The Effect of Lower Limb Exoskeleton Alignment on Knee Rehabilitation Efficacy

“…Bessler-Etten, et al [13] Investigating the effects of misalignment in different directions and rotationally to find if there is an increased load on the knee joint.…”

“…Kinematics of such systems are analyzed through different motion trajectories ( Figure 1 ) [ 12 ]. Although there are many related studies on moving the knee joint axis over various different subjects [ 13 , 14 , 15 , 16 ], it seems more research is required to develop a comprehensive insight through the sensitivity of exoskeleton design so as to provide a more reliable human-exoskeleton coordination. Table 1 presents a brief summary of similar studies, presenting their characteristics and limitations.…”

A review of the design of load-carrying exoskeletons

“…However, the exoskeleton assisting the ankle would increase the distal inertia inevitably, and a load acting far from the centre of mass would significantly increase the metabolic consumption [53]. To avoid this disadvantage, exoskeletons assisting the knee joint [54][55][56] or the hip joint [12,[57][58][59] was developed which are shown in Figure 2(c) and (d). The primary function of the knee is to support the body weight both in the single-support and double-support phases, and to increase foot clearance, avoiding obstacles in the swing phase by bending [56].…”