Biomechanical consequences of using passive and active back-support exoskeletons during different manual handling tasks.

INTRODUCTION: Previous studies have observed that the use of passive and active occupational back-support exoskeletons could reduce the stress on the lower back muscles during manual handling tasks. However, the magnitude of the benefits varied across studies. The specific protocol of each study, in terms of characteristics of the exoskeleton used (technology, design ) and the task performed (static, dynamic, symmetrical and asymmetrical), as well as the sex of the participants (men and/or women), could explain these variations. The objective of the present study was therefore to evaluate, for both men and women, the consequences of using different back-support exoskeletons during various manual handling tasks on back muscle activity and trunk kinematics. METHODS: Fifteen men and fourteen women performed three standardized manual handling task of a 15kg load (a static task, a symmetric lifting task and an asymmetric lifting task). Four exoskeleton conditions were tested: without equipment (CON), with a passive exoskeleton (P-EXO), and with two active exoskeletons (A-EXO1 and A-EXO2). Electromyographic activity of bilateral lower trapezius (TZ), latissimus dorsi (LD), erector spinae (ES), gluteus maximus (GM) and biceps femoris (BF) muscles was recorded. Trunk kinematics (hip + spine) was evaluated in order to provide average thoracic (C7-T8), lumbar (T8-L5) and hip (L5-thigh) angles. RESULTS: The use of P-EXO decreased the activity of some muscles (LD, GM and BF) from -12 to -27% (p<0.01) compared to without equipment, mostly during the static task. The two active exoskeletons (A-EXO1 and A-EXO2) reduced the muscle activity of all muscles from -7 to -62% (p<0.01) compared to the control condition and from -10 to -52% (p<0.005) compared to P-EXO, and this was true for all tasks. Some kinematic changes were observed only with the use of active exoskeletons (p<0.02). A statistical interaction between sex and exoskeleton was only observed in few rare conditions. DISCUSSION & CONCLUSION: Occupational back-support exoskeletons can reduce the muscle activity of the ES muscles, but also of other muscles (TZ, LD, GM and BF), compared to without equipment. However, these reductions were modulated by the exoskeleton technology (passive vs. active) and design (weight, anthropomorphism), and the task performed (static or dynamic). Our results also showed that the active exoskeletons could modify the trunk kinematics. Finally, there was no consistent difference in exoskeleton benefits between men and women.