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Exoskeleton

Exoskeletons: The Good, the Bad and the Ugly

Aug. 7, 2020
Exoskeletons have a huge potential for improving human performance and reducing workplace injuries, but do the benefits outweigh the risks?

Over the past several years, occupational exoskeletons have evolved from concept to reality. They have become widely available and are used across a range of industries. Their primary function is to increase human performance and, according to most vendors, they effectively reduce musculoskeletal disorders (MSDs), improve worker stamina, improve quality of work while increasing productivity, and keep the workplace healthier overall. Are there any unintended consequences? Word on the street is there aren’t any, and it might seem illogical not to implement exoskeletons.

Unfortunately, most of this information is coming straight from vendors, delivered in a consistent and positive message to most of the safety professionals who seek it. These devices do have the potential to deliver many benefits, but researchers have found that, for all the hype, there are just as many unintended consequences.

Until now, there hasn’t been a standardized process or set of guidelines for evaluating end-user requirements and exoskeleton specificity. When the job, or in this case, the exoskeleton, doesn’t fit the worker, MSDs can occur. Fortunately, researchers have developed a new process with standardized guidelines for testing occupational exoskeletons under job-specific operating conditions.

For any given use-case, the process considers four support categories (Hand, Shoulder, Trunk, Arm), as well as mode of actuation (Active, Quasi-Passive and Quasi-Active), and if force sensors, inertial measurement units (IMUs), strain gauges, and/or dry EMGS are used to detect user movement or intention of movement. The test process involves four stages: identifying the best exoskeleton for the selected high-risk use-case, laboratory validation, pilot-testing and in-line testing. It can assess how closely the features of an exoskeleton address the user requirements for the intended use-case.

Currently, evaluation criteria include impact, appropriation, utility, usability, and safety of the selected device on the end user. Designing to this protocol has shown to improve user muscle activity, range of motion, and productivity during simulated, use-specific tasks.

What Are Occupational Exoskeletons

Occupational exoskeletons are mechanical structures that, when worn by humans, aim to increase human physical performance and decrease musculoskeletal disorders. This performance boost is delivered either actively or passively; active exoskeletons use actuators (machine components responsible for physical movement) to drive the system, and passive exoskeletons harness human movement to energize the system. The actuators for active devices include electric motors, levers, and pneumatic or hydraulic technologies, while passive devices use materials, springs and dampers to redirect kinetic energy. Both active and passive systems are available in joint-specific, body region-specific, or full-body variations.

Exoskeletons can be soft or hard. Soft exoskeletons are referred to as exosuits, which augment human performance by helping muscles move body segments. Although they add to user strength, the body’s natural skeleton is still responsible for absorbing the increased forces associated with the strength enhancements. As such, exosuits place more load on the bones while reducing load on the muscles.

Hard exoskeletons include external rigid frames that act independently to absorb forces and move objects. Rather than assisting natural muscle activity, hard exoskeletons act as an entirely separate structure that utilizes the external frame to absorb and transfer forces and limit movement and/or activation of their targeted body regions.

Today, occupational exoskeletons are being designed to prevent lower back pain and shoulder tendon damage, specifically by limiting the muscle activity in either the back or the shoulders. Previous research has found that exoskeletons reduce localized muscle activity, but with potential for harm. Because the entire musculoskeletal system is a linked chain, reducing the work of one muscle or structure means transferring that work to a different muscle or structure. Depending on the movement, this can affect muscles responsible for the same movement, or cause an entirely different set of muscles to respond when accomplishing the same task in a different way, all of which may introduce new risks to the system.

Low-Back Industrial Exoskeletons

Currently, there are several studies that have looked at how low-back exoskeletons affect the body, with an emphasis on muscle activity/effort, muscle fatigue, and spinal loading during tasks involving stooping, dynamic lifting, and static load-bearing activities.

One study that examined the effects of passive exoskeletons on dynamic lifting and static trunk bending found that passive devices, which assist with dynamic lifting, reduce muscle activity by 10% to 40%, with reductions to spinal loading of 23% to 29%, and reduced overall muscle fatigue (de Looze et al., 2015). It also found increased muscle activity of leg joints, reduced range of motion, reduced natural motion, increased discomfort due to contact pressure, increased energy expenditure (metabolic cost), and reduced task performance. The overall results are mixed, and long-term effects are still not clear.

Passive Upper-Extremity Industrial Exoskeletons

Passive upper-extremity industrial exoskeletons have been another focus over the past several years. They support heavy tools in construction, petrochemical and large-product manufacturing environments, and support the upper extremities during static overhead work. These devices are smaller than low-back exoskeletons and typically involve a single joint (the shoulder), with unilateral (single-side) and bilateral (both-sides) models. So far, research with regard to these devices has looked at their effects on spinal loading (force experienced by the lower back), posture, peak back muscle activity, and some elements of the kinetic chain.

Spinal load is one of the largest contributors to the development of lower back pain, and in 2018, Kim et al. found that wearing upper-extremity exoskeletons reduced compressive forces to the lower back by 19%, and shear forces by 30%. A similar 2018 study by Weston et al. found that, on the contrary, upper-extremity exoskeletons increased compressive forces by 38% to 57%, and increased shear forces by 30%.

The impact of upper-extremity industrial exoskeletons on worker stamina and health can affect quality of work, and has been measured using muscle activity, muscle fatigue and perceived discomfort.

● Muscle activity, a physiological measurement of effort, has consistently been found to be elevated in all body regions other than the intended target, the shoulder. So far, researchers have found that upper-extremity industrial exoskeletons reduce muscle activity in the shoulders by 36% to 73% (Kim et al., Part I, 2018; Rashedi et al., 2014; Theural et al., 2018) and in the (back of) arm (a contributing region to shoulder movement) by 40% (Rashedi et al., 2014). This isn’t necessarily a good thing, however, and upper-extremity exoskeletons also show increases in muscle activity in the (back of) arm by 95% to 116% (Thueural et al., 2018), increased low-back muscle activity and demand by 31% to 120% (Rashedi et al., 2014; Theural et al., 2018; Weston et al., 2018), increased abdominal muscle activity by 42% to 66% (Weston et al., 2018), and increased (front) lower leg muscle activity (Theural et al., 2018). Not surprisingly, increases in foot muscle activity have also been seen (Theural et al., 2018). More recently, Alabdulkarim and Nussbaum (2019) found that unilateral upper-extremity industrial exoskeletons uniquely increase peak back muscle activity.

●Fatigue is another contributor to musculoskeletal disorder development. A tired or fatigued muscle won’t work as hard, and alternative muscles not normally used must be recruited to accomplish the same task. This results in both postural changes and added strain on other muscles. Rashedi et al. found reduced localized muscle fatigue in the upper arms and shoulders, with no change in the lower back (2014).

● Perceived discomfort measures how hard an individual feels he or she is working; it is used to measure the physical activity intensity level. Ideally, upper-extremity exoskeletons would reduce perceived discomfort in all body areas, indicating potential benefit to non-targeted areas from a user standpoint. Research has shown reduced perceived physical discomfort in the forearms (Kim et al., Part I, 2018) with no effect on the neck, shoulders, upper arm, upper back, lower back, or legs. Additionally, decreases have been reported in the upper arms by 54% to 57% and the shoulders by 34% to 45% (Rashedi et al., 2014). Rashedi et al. also reported increases in perceived physical discomfort by 24% to 48%.

Another important indicator of usability is preference for use; without worker buy-in, these expensive devices may end up as permanent wall decorations. At $4,000 to $7,000 a piece, equipping an entire workforce could be costly. When asked about their preference for wearing exoskeletons, a few users found them beneficial and would continue wearing them (Rashedi et al., 2014), while most said they did not perceive any improvement (Theural et al., 2018). Some also found the devices uncomfortable, unresponsive, or loose (Rashedi et al., 2014; Weston et al., 2018), and preferred not to use them (Weston et al., 2018).

Other significant changes that have been found with upper-extremity industrial exoskeleton use are postural and metabolic. Energy expenditure and cardiovascular effort are indicators of the energy required of the user when wearing a device. Although one aim of exoskeletons is to reduce workload on the user, upper-extremity exoskeletons currently increase energy expenditure and cardiovascular effort by 14% during use (Theural et al., 2018, and Young and Ferris, 2017).

Irregular postural control can happen when the nervous system encounters unexpected forces (externally or internally) and exoskeletons contribute a substantial amount of weight to the upper body, sometimes on one side, creating a constant state of imbalance. Consistent with this, postural instability and strain have been found to increase (Kim et al., Part II, 2018, and Theural et al., 2018), along with changes to postural control (increased magnitude and velocity of postural oscillations) while using upper-extremity exoskeletons (Kim et al., 2018). The hard, rigid structures of hard exoskeletons also constrain movement, which can be measured as kinematic strain. This is reflected through fewer torso movements (Rashedi et al., 2014) and reduced shoulder range of motion by 10% (Kim et al., Part II, 2018).

Final Thoughts

Occupational exoskeletons have significant potential, but they still have a long way to go. In their current state, these devices reduce localized muscle activity by putting the load elsewhere in the musculoskeletal system. Depending on the device, the user and the environment, this mechanism of action exposes other areas to unfamiliar and potentially harmful forces. Additionally, unnatural movements and forced prolonged postures may lead to long-term injury if a nerve is compressed or a tendon is ruptured. We’re also still unaware of long-term effects of regular exoskeleton use that may occur due to months or years of reduced muscle activity; if the body responds to chronic movement constraints like wrist braces or back belts, there is potential for muscle disuse atrophy (shrinking), deconditioning and structural weakness.

As tempting as it may be to implement occupational exoskeletons in the workforce, there is not enough yet known to give the green light. At their current phase of the 2017 Gartner Hype Cycle, exoskeletons are in the “peak of inflated expectations” phase, which makes sense with so many benefits yet so little existing research. These devices have an extraordinarily large potential for improving human performance and reducing musculoskeletal disorders in the future, but for now, the benefits don’t outweigh the risks.

Blake McGowan, CPE is the director of research insights at VelocityEHS | Humantech, a provider of EHS software solutions, and Brandon Beltzman, AEP is an ergonomist at VelocityEHS | Humantech.

References

Alabdulkarim, S., & Nussbaum, M. A. (2019). Influences of different exoskeleton designs and tool mass on physical demands and performance in a simulated overhead drilling task. Applied Ergonomics,74,55–66. doi:10.1016/j.apergo.2018.08.00

Bosch T, van Eck J, Knitel K, de Looze M. (2016). The effects of a passive exoskeletons on muscle activity, discomfort and endurance time in forward bending work. Appl Ergon. 2016 May;54:212-7.

de Looze MP, Bosch T, Krause F, Stadler KS, O'Sullivan LW. (2016). Exoskeletons for industrial application and their potential effects on physical work load. Ergonomics. 2016 May;59(5):671-81.

Kim S, Nussbaum MA, Mokhlespour Esfahani MI, Alemi MM, Alabdulkarim S, Rashedi E. (2018). Assessing the influence of a passive, upper extremity exoskeletal vest for tasks requiring arm elevation: Part I - "Expected" effects on discomfort, shoulder muscle activity, and work task performance. Appl Ergon. 2018 Mar 7. pii: S0003-6870(18)30059-0. doi: 10.1016/j.apergo.2018.02.025.

Kim S, Nussbaum MA, Mokhlespour Esfahani MI, Alemi MM, Jia B, Rashedi E. (2018). Assessing the influence of a passive, upper extremity exoskeletal vest for tasks requiring arm elevation: Part II – “Unexpected” effects on shoulder motion, balance, and spine loading. Appl Ergon. 2018 Mar 7. pii: S0003-6870(18)30058-9. doi: 10.1016/j.apergo.2018.02.024.

Marras WS. (2012). The complex spine: the multidimensional system of causal pathways for low-back disorders. Hum Factors. 2012 Dec;54(6):881-9.

Masood J, Dacal‐Nieto A, Alonso‐Ramos V, Fontano MI, Voilqué A, Bou J. Industrial Wearable Exoskeletons and Exosuits Assessment Process. In: Carrozza M, Micera S, Pons J, eds. Wearable Robotics: Challenges and Trends. WeRob 2018. Biosystems & Biorobotics. 22. Cham: Springer; 2019.

McGowan B. (2018). Industrial Exoskeletons: What You’re Not Hearing. Retrieved from https://ohsonline.com/Articles/2018/10/01/Industrial-Exoskeletons-What-Youre-Not-Hearing.aspx

National Research Council & Institute for Medicine. (2001). Musculoskeletal disorders and the workplace: Low back and upper extremity. Washington, DC: National Academy of Sciences, National Research Council, National Academy Press.

Rashedi E, Kim S, Nussbaum MA, Agnew MJ. (2014) Ergonomic evaluation of a wearable assistive device for overhead work. Ergonomics. 2014;57(12):1864-74.

Theurel J, Desbrosses K, Roux T, Savescu A. (2018). Physiological consequences of using an upper limb exoskeleton during manual handling tasks. Appl Ergon. 2018 Feb;67:211-217.

Theurel J, Desbrosses K (2019). Occupational Exoskeletons: Overview of Their Benefits and Limitations in Preventing Work-Related Musculoskeletal Disorders, IISE Transactions on Occupational Ergonomics and Human Factors, 7:3-4, 264-280, DOI:10.1080/24725838.2019.1638331.

Weston EB, Alizadeh M, Knapik GG, Wang X, Marras WS. (2018). Biomechanical evaluation of exoskeleton use on loading of the lumbar spine. Appl Ergon. 2018 Apr;68:101-108.

Young AJ, Ferris DP. (2017). State of the Art and Future Directions for Lower Limb Robotic Exoskeletons. IEEE Trans Neural Syst Rehabil Eng. 2017 Feb;25(2):171-182.

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