Evaluation of a Lower Leg Support Exoskeleton on Floor and Below Hip Height Panel Work

Abstract

Objective

The aim of this study is to determine the effectiveness of using a leg support exoskeleton (legX) in different modes on simulated work tasks which emulate real-world job tasks.

Background

Prolonged kneeling and squatting tasks increase the risk of work-related musculoskeletal disorders at the knee in industrial occupations.

Methods

We evaluated legX capable of spring assistance throughout one’s range of motion and/or locking support at a fixed angular position. Participants performed a dynamic panel task, alternating between hip and knee height, and a sustained floor level task with and without the exoskeleton. The exoskeleton was evaluated in spring mode, locking mode, and spring + locking mode for the panel task and only in locking mode for the floor task. The participants’ (N = 15) muscle activity was recorded for the right lumbar erector spinae, thoracic erector spinae, tibialis anterior, rectus femoris, semitendinosus, and lateral gastrocnemius.

Results

Significant reduction of the rectus femoris activity was observed with the exoskeleton (median reduction: 22%–56% and peak reduction: 12%–48% for the panel task and median reduction: 57% and peak reduction:34% during the floor task).

Conclusion

legX significantly reduces rectus femoris activity during squatted static (floor) and dynamic (panel) work and may reduce pain and discomfort associated with squatting and potentially reduce the risk of developing knee disorders. Dynamic tasks benefit from both locking modes and spring assistance, the greatest benefit occurring with a combination of the two.

Application

These results show that the legX can be beneficial to activities such as electrical panel work, grinding, sanding of larger surfaces, and concrete laying.

Keywords

wearable devices assistive technologies musculoskeletal system electromyography robotics

Objective

The objective of this study is to evaluate the impact of spring assistance and full body weight support provided by a leg support exoskeleton on the muscle activity at the wearer’s quadriceps, calf, and trunk muscles during a dynamic task and a static task which simulate real-world work tasks.

Background

In 2015, musculoskeletal disorders had the highest occurrence rate for work-related injuries resulting in days absent from work. Knee injuries, which account for 8.75% of the total injuries, resulted in a median of 16 missed days from work (Bureau of Labor Statistics, 2019), which is the second highest of all body regions. There is evidence that prolonged kneeling (Kivimäki et al., 1992) and squatting (Albers & Estill, 2007) can increase the risk of various knee-related disorders such as osteoarthritis (Ditchen et al., 2015; Felson, 2004; Seidler et al., 2008), worsened cartilage morphology at the patellofemoral and medial tibiofemoral joints (Amin et al., 2008), meniscal injury (Baker et al., 2003), and bursitis (Reid et al., 2010). Andersen et al. (2007) found that squatting for greater than 5 min/hr was associated with lower leg pain and discomfort at the knee. Baker et al. (2003) under policy implications state “Work involving regular kneeling is associated with an increased risk of knee pain and meniscectomy.” The National Research Council (US) Steering Committee for the Workshop on Work-Related Musculoskeletal Injuries: The Research Base (1999) discusses how sustained postural muscle activity can create significant musculoskeletal problems and how redesigning workplaces to reduce average muscle activity can lead to reduced sick leave. Exoskeletons have been suggested as a potential method to reduce exposure to activities and postures that increase the risk of knee disorders (Reid et al., 2010).

Exoskeleton technology can be designed to augment a user by providing an external torque to the joints of the user. This has the potential to reduce the user’s own muscle activity as they are now being assisted by the exoskeleton. Thus exoskeleton technology is being considered in various work settings (Kim et al., 2019; Reid et al., 2017; Upasani et al., 2019) as it has the potential to reduce physical fatigue and the risk of WMSDs through a reduction in muscle activity. The majority of exoskeleton studies have been performed on the use of back support and shoulder support exoskeletons. These studies have shown that back and shoulder support exoskeletons can reduce muscle activity and increase endurance time while performing simulated work tasks (Bosch et al., 2016; Kim et al., 2018; Lotz et al., 2009; Van Engelhoven et al., 2019). Some studies have also been performed in work settings to evaluate back support and shoulder support exoskeletons. These suggest that the exoskeletons can reduce discomfort in the lower back and reduce risk factors associated with shoulder injuries (Hensel & Keil, 2019; Smets, 2019).

While few in number, the studies that have been performed on leg/knee support exoskeletons have shown some promise in reducing muscle activity and discomfort. A passive knee exoskeleton was shown to reduce the quadriceps activity during cycling (Chaichaowarat et al., 2018). Lerner et al. (2017) showed a reduction in knee extensor moment in individuals with crouch gait due to Cerebral Palsy when using a knee exoskeleton. A passive knee exoskeleton mechanism has shown to reduce the peak root-mean-square averages of surface electromyography signals of the knee extensor muscles by 30%–40% during squatting (Ranaweera et al., 2018). A subjective evaluation of a passive leg support exoskeleton (“noonee”) showed that it reduced discomfort during a simulated assembly task (Luger et al., 2019). As seen with back support and shoulder support exoskeletons, an exoskeleton producing a supporting torque at the knee may be able to reduce muscle activity of the muscles around the knee joint during squatting postures. The reduced muscle forces should similarly reduce the compressive loading at the knee joint. It is thus hypothesized that use of leg support exoskeletons may reduce pain and discomfort associated with squatting and potentially reduce the risk of developing knee disorders such as knee osteoarthritis or meniscal injury.

The legX (suitX, 2019) and noonee (NOONEE, n.d.) are two commercially available leg support exoskeletons. While the noonee is capable of locking in place to provide the wearer with full body weight support, it does not provide the wearer with dynamic assistance. The legX provides two modes: a (dynamic) spring assistance mode and a locked mode. In the spring assist mode, the exoskeleton stores energy during knee flexion to cradle the person while lowering to a squatted position and releases that energy during knee extension to augment the muscles while rising to a standing position. The legX exoskeleton also has two different levels of support in the spring assist mode, where different amount of supporting force is provided to the user. The level of spring assistance can be adjusted to accommodate for preference or the weight of the worker. During the locked mode, when the user reaches a predetermined angle (110° of knee flexion for this trial) the user is effectively in a seated position and the upper body weight is supported by the exoskeleton. In both the spring assist and locking modes, the supportive force is applied along the buttocks and shins of the users and transferred to the ground. The locking mode is beneficial for static tasks, while the spring assistance mode is beneficial for dynamic tasks where the work height varies.

The goal of this paper is to evaluate various modes of a leg support exoskeleton (legX) on simulated workplace tasks, which strain the knee. Dynamic work tasks involve repetitive motions or frequent transition between multiple working heights. The dynamic panel work task simulated here requires the same real-world motion associated with tasks like electrical panel work and grinding or sanding of larger surfaces (Figure 1). A “static” work task was also simulated, requiring sustained exertion at a singular work height similar to ground tasks such as cutting rebar and concrete laying (Figure 2). Here we present a quantitative evaluation of a leg support exoskeleton during simulated squatted work tasks. For both dynamic and static tasks, we evaluated the impact of using the various modes of the leg support exoskeleton on the muscle activity at the wearers’ quadriceps and calf and trunk muscles. It was hypothesized that the exoskeleton will reduce the strain on the quadriceps, the primary knee extensor, and have little to no impact on the other muscle groups.

Figure 1

Examples of real work variable height tasks that require the same postures as the panel work task described.

Figure 2

Example of real work fixed height task that requires the same postures as the ground work task described.

Methods

Participants

Fifteen participants (11 males and 4 females) were recruited for participation in this within-subjects study of cross-over design, where the participants’ data with and without the exoskeleton were compared to themselves. Participants were instrumented with surface electromyography electrodes (discussed in more detail later). All trial data from three participants (one male, two females) were discarded as the EMG electrodes kept losing skin contact with the participant, due to a failure of the adhesive caused by excessive sweating. All male participants, on average, weighed 71 ± 10 kg (min of 57 kg and max of 91 kg) and measured 1.73 ± 0.07 m (min of 1.52 m and max of 1.82 m) in height. All female participants on average weighed 58 ± 4 kg (min of 53 kg and max of 61 kg) and measured 1.63 ± 0.07 m (min of 1.56 m and max of 1.73 m) in height. This research was approved by the Institutional Review Board at E&I Review Services. Informed consent was obtained from each participant.

Measures

Participants were instrumented with pairs of pregelled Ag/AgCl surface electrodes of 1 cm diameter and an inter-electrode distance of 2 cm (TeleMyo Mini DTS, Noraxon MyoMuscle, Scottsdale, AZ, USA) to record muscle activation of the right lumbar erector spinae (LES), right thoracic erector spinae (TES), right tibialis anterior (TA), right rectus femoris (RF), right semitendinosus (ST), and right lateral gastrocnemius (LG). The erector spinae were chosen for analysis to view the effect of the leg exoskeleton on the lower back. The descent into full squat is characterized by an increase in TA, vastus lateralis, and RF activity. EMG activity of the vastus lateralis parallels that of the RF during the full squat (Robertson et al., 2008). A parallel relationship is also seen between the RF and the vastus lateralis and vastus medialis under isometric conditions within the middle range of contraction intensities (20%–70% maximum voluntary contraction [MVC]; Pincivero & Coelho, 2000). As such the RF activity was measured and the change in RF activity was assumed to be similar to the vastus lateralis activity. The ST and gastrocnemius were used to assess the knee flexor muscle activity.

Video was also obtained of the task being performed.

Procedures

The legX exoskeleton was adjusted to fit each participant according to the manufacturer’s instructions. The exoskeleton has two primary hard adjustments to accommodate the participants’ below-knee shank length and the above-the-knee thigh length. The shank length adjustment ensures that when the supported posture is assumed, the load is transferred to the ground and not to the user. Additionally, this adjustment positions the location of the knee joint relative to the ground. The thigh length is adjusted to position a “soft seat” under the user’s buttocks. When the user is squatting, the “soft seat” imparts a force on the user to support their upper body weight. The structure of the exoskeleton transfers the load from the soft seat to the ground, by-passing the user’s knee joint. Figure 3 shows the shank link, thigh link, knee joint, and soft seat of the exoskeleton on a person.

Figure 3

legX has two hard adjustments to accommodate the user’s shank length (adjust the shank link) and to accommodate the user’s thigh length (adjust the thigh link). The legX is designed to have the exoskeleton knee joint aligned to the user’s knee joint. A soft seat is positioned on the user’s buttocks and is where the legX applies a supporting force on the user when the user is in a squatted position.

The legX exoskeleton has two different levels of support in the spring assist mode, where different amount of supporting force is provided to the user. At the beginning of each study, the participants’ exoskeleton fit was determined and they were allowed to use the exoskeleton for 10 min and self-select a support level. Ten participants chose the high support level while five participants chose the low support level. The participants were then instrumented with electrodes to ensure that the exoskeleton straps and hardware would not come into contact with the electrodes during testing. Once this was determined, the exoskeleton settings were recorded and the exoskeleton was doffed in order to obtain a reference signal for the MVC for each respective muscle group.

Two sets of tasks were performed in random order:

Panel work: The participant is asked to hold a drill and aim it at a target positioned at waist height and a second target at knee height (Figure 4). The participants were asked to move between the two markers at a rate of 6 s per action. This process was repeated for 30 s. Four task conditions were performed in random order. The conditions were one without the exoskeleton, one with the exoskeleton in spring assist mode at a self-selected strength, one with the exoskeleton in the lock mode (110° of knee flexion), and one with the exoskeleton in simultaneous lock mode and spring mode.

Figure 4

Experimental setup for panel work. A target is placed at hip height and knee height of each participant. The participant is asked to alternate placing the drill between the two points. The participants’ posture is not controlled.

Sustained ground work: The participant is asked to move a rod between three markers on the floor placed at different distances (Figure 5). Markers were placed 1 ft apart with the middle marker also being placed 1 ft forward. Participants self-selected their starting location for performing the task. Task conditions without the exoskeleton and with the exoskeleton in locking mode (110° of knee flexion) were performed in random order.

Figure 5

Experimental setup for ground work. Participants were asked to place a rod in the center of three targets placed 1 ft apart. The middle target is also placed 1 ft forward. Participants’ position relative to the targets was not controlled.

Each task and condition combination was performed twice and the average of the two trials is presented. However, for one participant a single trial was performed in the spring-only mode during panel work, for another participant a single trial was performed in the locking mode during panel work, and for another participant a single trial was performed in the spring + locking mode during panel work. These three single trials were included in the results presented. Participants’ posture was not controlled, as it was hypothesized that participants may not default to a squatted posture to perform the task and may choose to use higher risk stooping postures. This would result in a better simulation of real-world working conditions.

It should also be noted that in equivalent work applications where these postures are assumed, such as electrical panel work and grinding and floor work, the tool/payload weight can vary significantly. However, the worker’s upper body weight is typically much larger than the tool weight and the majority of the load being offsetted by the exoskeleton is due to the weight of the user.

Data Analysis

Electromyography data were rectified, smoothed with a root-mean-square algorithm (100 ms window), and normalized as a percentage of each participant’s MVC. The median muscle activity and peak muscle activity of each muscle at or below 50% and 90% of the time, respectively, are determined using an amplitude distribution probability function (ADPF). Henceforth, the median percent MVC data are referred to as ADPF50 and the peak percent MVC data are referred to as ADPF90. Additionally, a t-test is performed on the ADPF50 and ADPF90 data to obtain a p-value. Due to the limited number of female participants in the study, the data were not analyzed according to gender.

Results

Muscle activity and p-values comparing the no exoskeleton condition with the exoskeleton conditions for the LES, TES, TA, RF, ST, and LG for the panel work (Figures 6 and 7, Tables 1 and 2) and ground work (Figures 8 and 9, Table 3) are presented below.

Figure 6

ADPF50 muscle activity: panel work. The asterisk indicates significance when compared to the no exoskeleton condition. ADPF = amplitude distribution probability function.

Figure 7

ADPF90 muscle activity: panel work. The asterisk indicates significance when compared to the no exoskeleton condition. ADPF = amplitude distribution probability function.

Table 1

p-Values from Paired t-Test Comparing Median (ADPF50) Percent MVC Data for the No Exoskeleton Condition to the Exoskeleton Condition in Various Modes for Panel Work

Exoskeleton Mode	RF	ST	LG	TA	LES	TES
Spring	.015	.496	.666	.279	.486	.132
Locking	.009	.135	.447	.238	.143	.145
Spring + locked	.000	.452	.490	.285	.054	.144

Notes. Values in bold indicate statistical significance when compared to the no exoskeleton condition. ADPF = amplitude distribution probability function; LES = lumbar erector spinae; LG = lateral gastrocnemius; MVC = maximum voluntary contraction; RF = rectus femoris; ST = semitendinosus; TA = tibialis anterior; TES = thoracic erector spinae.

Table 2

p-Values from Paired t-Test Comparing Peak (ADPF90) Percent MVC Data for the No Exoskeleton Condition to the Exoskeleton Condition in Various Modes for Panel Work

Exoskeleton Mode	RF	ST	LG	TA	LES	TES
Spring	.345	.217	.631	.659	.218	.103
Locking	.004	.070	.655	.341	.170	.247
Spring + locked	.000	.075	.510	.508	.085	.591

Figure 8

ADPF50 muscle activity: ground work. The asterisk indicates significance when compared to the no exoskeleton condition. ADPF = amplitude distribution probability function.

Figure 9

ADPF90 muscle activity: ground work. The asterisk indicates significance when compared to the no exoskeleton condition. ADPF = amplitude distribution probability function.

Table 3

p-Values from Paired t-Test Comparing No Exoskeleton Condition to Exoskeleton Condition for Ground Work

	RF	ST	LG	TA	LES	TES
ADPF50	.033	.208	.295	.151	.505	.016
ADPF90	.087	.311	.852	.588	.068	.214

Discussion

For the dynamic panel work task, a statistically significant reduction in median muscle activity is observed in the RF with a reduction of 22%, 36%, and 56% when the exoskeleton is used in the spring mode, locked mode, and spring + locked mode, respectively. As this task required work at variable heights, most participants performed both flexion and extension at the knee to raise and lower between the two points of interest during the task and sat with their full weight on the device for the lower target. This is most likely why the condition with both the locked mode and the spring mode showed a greater reduction than other modes. Statistically significant reduction in peak EMG activity is also observed in the locked mode and the spring + locked mode in the ST.

The median EMG activity of the TA, ST, and LG does not appear to be affected by the use of the exoskeleton. A nonstatistically significant reduction in muscle activity in the LES and the TES is observed when using the exoskeleton. The effect of the leg exoskeleton on reducing the muscle activity in the participant’s back should be investigated further.

Since the participants’ posture was not controlled during this trial, participants did not have the same posture even when using the exoskeleton. For instance, one participant remained in the lower seat position when the exoskeleton was in locking position, that is, this participant would not raise and lower his body for each target, but remained in the seated position. This participant showed substantially lower muscle activity during the locked mode than any other mode. This suggests that even for an otherwise dynamic task of variable target height, a locked exoskeleton position may exist where the user may remain statically seated. Another participant, whose data were discarded due to electrode adhesion, stooped rather than squatted to perform the task without the exoskeleton and performed the task in the seated position in both the locked mode and spring + locked mode tests with the exoskeleton. Squat lifting is biomechanically more favorable than stoop lifting (Hwang et al., 2009), and thus for some tasks a leg supporting exoskeleton may improve one’s posture and may provide benefits beyond reducing strain at the knee. For example, due to the high demand on one’s quadriceps a worker may often stoop instead of adopting the more beneficial squatted posture for sustained tasks performed at a work surface that is below eye level. With the assistance of an exoskeleton, the worker may more readily adopt a squatted posture due to the exoskeleton’s assistance, which may help reduce the strain not only on the knee but also on the back by preventing a stooped posture. Further investigation of the effect of the preferred posture for various tasks is required.

While not statistically significant, a small increase in the peak EMG activity of the TA is observed when comparing the no exoskeleton condition to the exoskeleton conditions. In the case for the panel work task, the TA activity increased during the transition from one work level to another. Higher spikes are observed when the user sits into the locked mode seated position. This may be a result of user needing to rebalance after transitioning from a moving state to a sitting state or may relate to the user’s familiarity with the exoskeleton. It is worth noting that when looking at the data individually, 3 of the 12 participants showed a decrease in the peak muscle activity of the TA when using the exoskeleton in all of the exoskeleton modes (spring, locked, spring + locked) while 4 showed an increase in peak muscle activity of the TA when using the exoskeleton in all of the exoskeleton modes. Of the remaining five participants, different participants showed either an increase or a decrease in the peak muscle activity of the TA in the different exoskeleton modes without a clear trend appearing, with two of five participants showing a decrease in activity during locking mode, two of five participants showing a decrease in activity during the spring mode, and four of five participants showing a decrease in activity during the spring + locking mode when compared to the no exoskeleton condition. Future studies should investigate TA activation and balance with respect to proficiency with the exoskeleton to isolate any training-related effects.

The ground work task occurs at a fixed height, as no such dynamic motion is required from the lower body. Statistically significant reduction of 56% and 16% in average median muscle activity is observed in the rectus femoris and the TES, respectively, when the exoskeleton is in the locked mode. While not statistically significant the median activity of the other muscles appears to decrease during the exoskeleton use.

Increase in the peak average muscle activity of the TA is also observed for ground work. Data acquisition for this task began from a standing position transitioning to a seated position before the main task of moving between the targets began. It is observed that during the transition from standing to seated position, the activity in the TA spikes. As mentioned above this may be the result of the user needing to rebalance after transitioning from a moving state to a sitting state. Data during seated to standing transition state was not captured. All participants showed this small increase in peak muscle activity of the TA. Further investigation of the causes of this increase is required.

Due to the limited number of female participants, a meaningful comparison of the differences between the male and female participants is difficult. However, female participants appeared to show a significantly larger decrease in the RF activity as compared to the male participants for both panel work and ground work. For example, during panel work women showed a median muscle activation reduction of 25%, 69%, and 75% in the spring mode, locked mode, and spring + locked mode, respectively. The male participants showed a median muscle activation reduction of 21%, 27%, and 51% in spring mode, locked mode, and spring + locked mode, respectively. Overall women showed a greater reduction in the median muscle activity during the panel work in all muscles in almost all modes when compared to male participants. Male participants showed a slightly higher reduction in the median muscle activity in the TES than the female participants in the spring mode. It should be noted that the p-value for the female data did not show any significance. This data suggests gender-based differences may occur during the use of the leg support exoskeleton. Thus further study is needed to assess the differences between male and female participants.

As the assumption about the linear relationship between the RF and the vastus lateralis is more accurate between 20% and 70% MVC and we observe the RF activity in a lower region of MVC, evaluating the effect of the exoskeleton on the vastus lateralis independently is required. Further study is also needed to assess the effect of these devices over longer durations of time.

Conclusion

Leg support exoskeletons, such as the legX, significantly reduce muscle activity of the RF during simulated squatted static and dynamic work which mimics panel work, or large grinding work, or concrete laying work. The simultaneous use of spring assistance and a locked mode assistance can reduce muscle activity more than using only spring assistance or locked mode for the simulated dynamic task. By reducing the required muscle effort from the knee extensor muscles, a reduction in joint loading at the knee joint is expected. This has the potential to reduce knee discomfort and pain in occupational settings and may have the potential to reduce the risk of developing knee disorders such as knee osteoarthritis and meniscal injury. The effect of the exoskeleton may differ based on gender and further study is required on this subject.

Key Points

Exoskeleton technology has the potential to reduce the risk of pain, discomfort, and WMSDs by reducing the load and strain on workers’ muscles.

Leg support exoskeletons significantly reduced muscle activity of the RF during squatted static and dynamic work.

The simultaneous use of spring assistance and a locked mode can reduce muscle activity more than each of the assistance types independently for the studied dynamic task.

Footnotes

Acknowledgments

The authors would like to thank Zoe Moskowitz for her work on this project.

ORCID iDs

Minerva V. Pillai

Homayoon Kazerooni

Author Biographies

Minerva V. Pillai is the vice president of engineering at US Bionics dba suitX. She received her PhD in mechanical engineering from the University of California Berkeley in 2014.

Logan Van Englehoven is the senior mechanical engineer at US Bionics dba suitX. He received his PhD in mechanical engineering from the University of California Berkeley in 2018.

Homayoon Kazerooni is a professor of mechanical engineering at the University of California, Berkeley. He received his PhD in mechanical engineering from the Massachusetts Institute of Technology.

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