Estimating lumbar spine loading when using back-support exoskeletons in lifting tasks.

Low-back pain (LBP) continues as the leading cause of work-related musculoskeletal disorders, and the high LBP burden is attributed largely to physical risk factors prevalent in manual material handling tasks. Industrial back-support exoskeletons (BSEs) are a promising ergonomic intervention to help control/prevent exposures to such risk factors. While earlier research has demonstrated beneficial effects of BSEs in terms of reductions in superficial back muscle activity, limited evidence is available regarding the impacts of these devices on spine loads. We evaluated the effects of two passive BSEs (BackX™ AC and Laevo™ V2.5) on lumbosacral compression and shear forces during repetitive lifting using an optimization-based model. Eighteen participants (gender-balanced) completed four minutes of repetitive lifting in nine different conditions, involving symmetric and asymmetric postures when using the BSEs (along with no BSE as a control condition). Using both BSEs reduced estimated peak compression and anteroposterior shear forces (by ∼8-15%). Such reductions, however, were task-specific and depended on the BSE design. Laevo™ use reduced mediolateral shear forces during asymmetric lifting (by ∼35%). We also found that reductions in composite measures of trunk muscle activity may not correspond well with changes in spine forces when using a BSE. These results can help guide the proper selection and application of BSEs during repetitive lifting tasks. Future work is recommended to explore the viability of different biomechanical models to assess changes in spine mechanical loads when using BSEs and whether reasonable estimates would be obtained using such models.

A novel approach to quantify the assistive torque profiles generated by passive back-support exoskeletons.

Industrial exoskeletons are a promising ergonomic intervention to reduce the risk of work-related musculoskeletal disorders by providing external physical support to workers. Passive exoskeletons, having no power supplies, are of particular interest given their predominance in the commercial market. Understanding the mechanical behavior of the torque generation mechanisms embedded in passive exoskeletons is, however, essential to determine the efficacy of these devices in reducing physical loads (e.g., in manual material handling tasks). We introduce a novel approach using a computerized dynamometer to quantify the assistive torque profiles of two passive back-support exoskeletons (BSEs) at different support settings and in both static and dynamic conditions. The feasibility of this approach was examined using both human subjects and a mannequin. Clear differences in assistive torque magnitudes were evident between the two BSEs, and both devices generated more assistive torques during trunk/hip flexion than extension. Assistive torques obtained from human subjects were often within similar ranges as those from the mannequin, though values were more comparable over a narrow range of flexion/extension angles due to practical limitations with the dynamometer and human subjects. Characterizing exoskeleton assistive torque profiles can help in better understanding how to select a torque profile for given task requirements and user anthropometry, and aid in predicting the potential impacts of exoskeleton use by incorporating measured torque profiles in a musculoskeletal modeling system. Future work is recommended to assess this approach for other occupational exoskeletons.

Effects of Back-Support Exoskeleton Use on Lower Limb Joint Kinematics and Kinetics During Level Walking.

We assessed the effects of using a passive back-support exoskeleton (BSE) on lower limb joint kinematics and kinetics during level walking. Twenty young, healthy participants completed level walking trials while wearing a BSE (backXTM) with three different levels of hip-extension support torque (i.e., no torque, low, and high) and in a control condition (no-BSE). When hip extension torques were required for gait-initial 0-10% and final 75-100% of the gait cycle-the BSE with high supportive torque provided ~ 10 Nm of external hip extension torque at each hip, resulting in beneficial changes in participants' gait patterns. Specifically, there was a ~ 10% reduction in muscle-generated hip extension torque and ~ 15-20% reduction in extensor power. During the stance-swing transition, however, BSE use produced undesirable changes in lower limb kinematics (e.g., 5-20% increase in ankle joint velocity) and kinetics (e.g., ~ 10% increase in hip flexor, knee extensor, and ankle plantarflexor powers). These latter changes likely stemmed from the need to increase mechanical energy for propelling the leg into the swing phase. BSE use may thus increase the metabolic cost of walking. Whether such use also leads to muscle fatigue and/or postural instability in long-distance walking needs to be confirmed in future work.

Effects of back-support exoskeleton use on trunk neuromuscular control during repetitive lifting: A dynamical systems analysis.

Back-support exoskeletons (BSEs) are a promising ergonomic intervention to mitigate the risk of occupational low back pain. Although growing evidence points to the beneficial effects of BSEs, specifically in reducing low-back physical demands, there is limited understanding of potential unintended consequences of BSE use on neuromuscular control of the trunk during manual material handling (MMH). We quantified the effects of two passive BSEs (BackX™ AC and Laevo™ V2.5) on trunk dynamic stability and movement coordination during a repetitive lifting task. Eighteen participants (gender-balanced) completed four minutes of repetitive lifting in nine different conditions, involving symmetric and asymmetric postures when using the BSEs (along with no BSE as a control condition). Maximum Lyapunov exponents (short-term: λmax-s; long-term: λmax-l) and Floquet multipliers (FMmax) were respectively calculated to quantify the local dynamic and orbital stability of thorax and pelvis trajectories. Thorax-pelvis segmental coordination was also quantified using the continuous relative phase. Wearing the Laevo™ significantly increased λmax-s for the pelvis (by ~ 8%) and FMmax for the thorax and pelvis (by ~ 5-10%). Use of either BSE decreased the in-phase coordination pattern for the thorax-pelvis coupling (by ~ 15%). These results suggest that BSE use can compromise neuromuscular control of the trunk, and caution should thus be used in selecting a suitable BSE for use in a given MMH task. Future work is needed, however, to assess the generalizability of different BSE design approaches in terms of unintended short-term and long-term effects on trunk neuromuscular control.

Assessing the potential for "undesired" effects of passive back-support exoskeleton use during a simulated manual assembly task: Muscle activity, posture, balance, discomfort, and usability.

Back-support exoskeletons (BSEs) are wearable systems designed to reduce physical demands on the back, but which could have undesired effects beyond this design intention. Participants (n = 18) used two commercial BSEs to complete a brief (~15-20 s) simulated manual assembly task in varying conditions, with outcome measures that included: working posture, activity levels in "secondary" muscle groups (shoulders and thighs), perceived balance, discomfort, and usability. Using a BSE led to small and inconsistent changes in working postures (e.g., < ~14° change in lumbar flexion), muscular activity in the secondary muscle groups (<±2% of maximum voluntary isometric contractions), or perceived balance. Limitations in movement were reported for both BSEs, however, along with moderate levels of discomfort. Task-specific responses were evident for all outcome measures, though these depended on the specific BSE used and differed between genders in many cases. Future work should focus on interactions between a given user, BSE design, and task conditions.

Biomechanical assessment of two back-support exoskeletons in symmetric and asymmetric repetitive lifting with moderate postural demands.

Two passive back-support exoskeleton (BSE) designs were assessed in terms of muscular activity, energy expenditure, joint kinematics, and subjective responses. Eighteen participants (gender-balanced) completed repetitive lifting tasks in nine different conditions, involving symmetric and asymmetric postures and using two BSEs (along with no BSE as a control condition). Wearing both BSEs significantly reduced peak levels of trunk extensor muscle activity (by ~9-20%) and reduced energy expenditure (by ~8-14%). Such reductions, though, were more pronounced in the symmetric conditions and differed between the two BSEs tested. Participants reported lower perceived exertion using either BSE yet raised concerns regarding localized discomfort. Minimal changes in lifting behaviors were evident when using either BSE, and use of both BSEs led to generally positive usability ratings. While these results are promising regarding the occupational use of BSEs, future work is recommended to consider inter-individual differences to accommodate diverse user needs and preferences.

Effects of Two Passive Back-Support Exoskeletons on Muscle Activity, Energy Expenditure, and Subjective Assessments During Repetitive Lifting.

OBJECTIVE: The aim of this study was to explore the efficacy of two different passive back-support exoskeleton (BSE) designs during repetitive lifting in different postures. BACKGROUND: Although BSEs have been proposed as a potential intervention for reducing physical demands, limited information is available about the impacts of different exoskeleton designs in diverse work scenarios. METHOD: Eighteen participants (gender-balanced) performed lab-based simulations of repetitive lifting tasks. These tasks were performed in 12 different conditions, involving two BSEs and a control condition, two levels of lifting symmetry (symmetric and asymmetric), and two postures (standing and kneeling). Outcome measures described muscle activity and energy expenditure, along with perceived discomfort, balance, and usability. RESULTS: Using both BSEs significantly reduced peak activity of the trunk extensor muscles (by ~10%-28%) and energy expenditure (by ~4%-13%) in all conditions tested. Such reductions, though, were task dependent and differed between the two BSEs. In most of the tested conditions, using BSEs positively affected subjective responses regarding perceived exertion and usability. CONCLUSION: Our results suggest that the beneficial effects of a BSE are task specific and depend on the specific BSE design approach. More work is needed, though, to better characterize this task specificity and to determine the generalizability of BSE effects on objective and subjective outcomes for a wider range of conditions and users. APPLICATION: Our results provide new evidence to guide the selection and application of passive BSE designs in diverse lifting tasks.

Biomechanical Evaluation of Passive Back-Support Exoskeletons in a Precision Manual Assembly Task: "Expected" Effects on Trunk Muscle Activity, Perceived Exertion, and Task Performance.

OBJECTIVE: To assess the efficacy of two different passive back-support exoskeleton (BSE) designs, in terms of trunk muscle activity, perceived low-back exertion, and task performance. BACKGROUND: BSEs have the potential to be an effective intervention for reducing low-back physical demands, yet little is known about the impacts of different designs in work scenarios requiring varying degrees of symmetric and asymmetric trunk bending during manual assembly tasks. METHOD: Eighteen participants (gender balanced) completed lab-based simulations of a precision manual assembly task using a "grooved pegboard." This was done in 26 different conditions (20 unsupported; 6 supported, via a chair), which differed in vertical height, horizontal distance, and orientation. RESULTS: Using both BSEs reduced metrics of trunk muscle activity in many task conditions (≤47% reductions when using BackX™ and ≤24% reductions when using Laevo™). Such reductions, though, were more pronounced in the conditions closer to the mid-sagittal plane and differed between the two BSEs tested. Minimal effects on task completion times or ratings of perceived exertion were found for both BSEs. CONCLUSION: Our findings suggest that using passive BSEs can be beneficial for quasi-static manual assembly tasks, yet their beneficial effects can be task specific and specific to BSE design approaches. Further work is needed, though, to better characterize this task specificity and to assess the generalizability of different BSE design approaches in terms of physical demands, perceived exertion, and task performance. APPLICATION: These results can help guide the choice and application of passive BSE designs for diverse work scenarios involving nonneutral trunk postures.

The influence of external load configuration on trunk biomechanics and spinal loading during sudden loading.

Sudden loading is a major risk factor for work-related lower back injuries among occupations involving manual material handling (MMH). The current study explored the effects of external weight configuration on trunk biomechanics and trunk rotational stiffness in the sagittal plane during sudden loading. Fifteen asymptomatic volunteers experienced sudden loadings using the same magnitude of weight (9 kg) with two different configurations (medially- or laterally-distributed) at three levels of height (low, middle and high). Results of this study showed that the medially distributed weight resulted in a significantly higher peak L5/S1 joint compression force (2861 N vs. 2694 N) and trunk rotational stiffness (2413 Nm/rad vs. 1785 Nm/rad) compared to the laterally distributed weight. It was concluded that when experiencing sudden loading, a more laterally distributed weight could increase the load's resistance to physical perturbations and alleviate spinal loading during sudden loading events. Practitioner summary: Increased trunk rotational stiffness and peak L5/S1 joint compression force were observed when undergoing a sudden load release of a medially distributed load compared to a laterally distributed load revealing a less stable hand load condition due to the reduced moment of inertia. The laterally distributed load could increase the load's resistance to physical perturbations and mitigate spinal loading during sudden loading events.

Effects of the weight configuration of hand load on trunk musculature during static weight holding.

The performance of manual material handling tasks is one major cause of lower back injuries. In the current study, we investigated the influence of the weight configuration of hand loads on trunk muscle activities and the associated spinal stability. Thirteen volunteers each performed static weight-holding tasks using two different 9 kg weight bars (with medial and lateral weight configurations) at two levels of height (low and high) and one fixed horizontal distance (which resulted in constant spinal joint moment across conditions). Results of the current study demonstrated that holding the laterally distributed load significantly reduced activation levels of lumbar and abdominal muscles by 9-13% as compared with holding the medially distributed load. We believe such an effect is due to an elevated rotational moment of inertia when the weight of the load is laterally distributed. These findings suggest that during the design and assessment of manual material handling tasks, such as lifting and carrying, the weight configuration of the hand load should be considered. Practitioner summary: Elevated trunk muscle activities were found when holding a medially distributed load vs. a laterally distributed load (with an equivalent external moment to the spine), indicating a reduced spinal stability due to the reduced rotational moment of inertia. The configuration of the hand load should be considered when evaluating manual material handling tasks.