Active exoskeletons are potentially more effective and versatile than passive ones, but designing them poses a number of additional challenges. An important open challenge in the field is associated to the assistive strategy, by which the actuation forces are modulated to the user's needs during the physical activity. This paper addresses this challenge on an active exoskeleton prototype aimed at reducing compressive low-back loads, associated to risk of musculoskeletal injury during manual material handling (i.e., repeatedly lifting objects). An analysis of the biomechanics of the physical task reveals two key factors that determine low-back loads. For each factor, a suitable control strategy for the exoskeleton is implemented. The first strategy is based on user posture and modulates the assistance to support the wearer's own upper body. The second one adapts to the mass of the lifted object and is a practical implementation of electromyographic control. A third strategy is devised as a generalized combination of the first two. With these strategies, the proposed exoskeleton can quickly adjust to different task conditions (which makes it versatile compared to using multiple, task-specific, devices) as well as to individual preference (which promotes user acceptance). Additionally, the presented implementation is potentially applicable to more powerful exoskeletons, capable of generating larger forces. The different strategies are implemented on the exoskeleton and tested on 11 participants in an experiment reproducing the lifting task. The resulting data highlights that the strategies modulate the assistance as intended by design, i.e., they effectively adjust the commanded assistive torque during operation based on user posture and external mass. The experiment also provides evidence of significant reduction in muscular activity at the lumbar spine (around 30%) associated to using the exoskeleton. The reduction is well in line with previous literature and may be associated to lower risk of injury.

OCCUPATIONAL APPLICATIONS Many new occupational back-support exoskeletons have been developed in the past few years both as research prototypes and as commercial products. These devices are intended to reduce the risk of lowerback pain and injury for workers in various possible application sectors, including assembly in automotive and aerospace, logistics, construction, healthcare, and agriculture. This article describes the technologies adopted for back-support exoskeletons and discusses their advantages and drawbacks. Such an overview is intended to promote a common understanding and to encourage discussion among different stakeholders such as developers, ergonomics practitioners, customers, and workers. TECHNICAL ABSTRACT Background: The large prevalence and risk of occupational lower-back pain and injury associated with manual material handling activities has raised interest in novel technical solutions. Wearable back-support exoskeletons promise to improve ergonomics by reducing the loading on the lumbar spine. Purpose: Since many new prototypes and products are being developed, this article presents an up-to-date overview of the different technologies. By discussing the corresponding advantages and drawbacks, the objective is to promote awareness and communication among developers, ergonomics practitioners, customers, and factory workers. Methods: The state-of-the-art is presented with a focus on three technological aspects: (i) the actuators generating assistive forces/torques, with a main distinction between passive and active devices; (ii) the structures and physical attachments that transfer those forces/torques to the user, with structures being soft, rigid, or a combination of the two; and (iii) the control strategies employed (i.e., how devices adjust assistive forces/torques to accommodate different activities and parameters). Discussion: The choice of actuation technology may determine the applicability of a device to different scenarios. Passive exoskeletons appear more suitable for tasks requiring relatively light assistance and little dynamic movements. By contrast, heavier and more dynamic tasks will justify the use of more complex active exoskeletons. While onboard battery power is increasingly present on active exoskeletons, the tradeoff between power autonomy and additional battery mass will probably depend on the

Occupational back-support exoskeletons are becoming a more and more common solution to mitigate work-related lower-back pain associated with lifting activities. In addition to lifting, there are many other tasks performed by workers, such as carrying, pushing, and pulling, that might benefit from the use of an exoskeleton. In this work, the impact that carrying has on lower-back loading compared to lifting and the need to select different assistive strategies based on the performed task are presented. This latter need is studied by using a control strategy that commands for constant torques. The results of the experimental campaign conducted on 9 subjects suggest that such a control strategy is beneficial for the back muscles (up to 12% reduction in overall lumbar activity), but constrains the legs (around 10% reduction in hip and knee ranges of motion). Task recognition and the design of specific controllers can be exploited by active and, partially, passive exoskeletons to enhance their versatility, i.e., the ability to adapt to different requirements.