A method to quantify the reduction of back and hip muscle fatigue of lift-support exoskeletons

Abstract

Cumulative back muscle fatigue plays a role in the occurrence of low-back injuries in occupations that require repetitive lifting of heavy loads and working in forward leaning postures. Lift-support exoskeletons have the potential to reduce back and hip muscle activity, thereby delaying the onset of fatigue in these muscles. Therefore, exoskeletons are being considered a potentially important tool to further reduce workload-related injuries. However, today no standards have been established on how to benchmark the support level of lift-support exoskeletons. This work proposes an experimental protocol to quantify the support level of a lift-support exoskeletons on instant changes in muscle activity and fatigue development while maintaining a static forward leaning posture. It then applies the protocol to experimentally assess the effect of the support provided by a commercially available lift-support exoskeleton, the LiftSuit 2.0 (Auxivo AG, Schwerzenbach, Switzerland), on the user. In a sample of 14 participants, the amplitude of the muscle activity of the back muscles and hip muscles () was significantly reduced. Wearing the exoskeleton significantly reduced the amount of fatigue developed during the task (). Changes in muscle fatigue can be objectively recorded and correlated with relevant changes for exoskeleton users: the time a task can be performed and perceived low-back fatigue. Thus, including such measures of fatigue in standardized benchmarking procedures will help quantify the benefits of exoskeletons for occupational use.

Keywords: biomechanics; exoskeletons; exosuits; industry.

Figure 1.

(a) The passive lift-support exoskeleton used in this study is the Auxivo LiftSuit v2.0. (b) During static forward leaning the load of the body, the external load, the muscles, and the exoskeleton create opposing moments around the center of rotation of the hip (in green). The moments generated by the weight of the upper body and the external load (in red) are counteracted by the muscles that create torque around the hip and the exoskeleton (in black). Dotted lines represent moment arms. (c) In this study, we measured the erector spinae at lumbar and thoracic level, the quadratus lumborum, and the gluteus maximus.

Figure 2.

(a) Participants held a box of 20% body weight in a 45° forward leaning position. (b) Real-time feedback of trunk angle was provided on a screen in the line of sight of the participant.

Figure 3.

(a) Root mean square of the muscle activity of each muscle group. (b) Slope of the change in median frequency over time for each muscle group. Individual participants (grey lines), as well as the sample average (n = 14, black line), are plotted. Stars indicate statistically significant differences based on paired samples t-test analysis: *p < .05, **p < .01.

Figure 4.

Change in median frequency (MDF) over time for one participant. The MDF of the EMG signal was calculated for non-overlapping 1 s time windows (dots). A linear regression line was calculated for each condition: NoExo (grey) and Exo (blue).