Physical Therapy Korea

Low back pain remains the most prevalent work-related disability, accounting for approximately 40% of all musculoskeletal disorders [1]. Work-related lower back pain is directly attributed to various physical risk factors such as overexertion, abnormal work postures, and prolonged or repetitive bending and lifting [2]. Previous studies have indicated that mechanical spinal loading is a significant risk factor in moderate-to-severe cases [3,4]. Despite ongoing mechanization and automation, 30% of workers still need to lift heavy loads for at least a quarter of their working hours. Because lifting-assistive devices are often not used owing to their limitations [5], the focus has shifted to devices that are worn directly on the user’s back and waist for support. Wearable passive back-support exoskeletons have been developed to reduce mechanical loading on the spine [6,7].

Recently, wearable passive back-support exoskeletons have attracted attention as devices that support the posture of workers, enhance their physical capabilities, and reduce physical risk factors [8]. Previous studies have indicated that Wearable Passive Back Support Exoskeletons reduce trunk muscle activity and energy expenditure during repetitive lifting tasks [9]. A wearable passive back-support exoskeleton is designed to support or strengthen the back and hip muscles, potentially reducing the physical burden on the lower back during both static and dynamic tasks associated with non-neutral trunk postures [6]. These studies consistently reported potential benefits, including a 54% reduction in the maximum lumbar muscle activity during symmetrical lifting [9]. Research on Wearable Passive Back Support Exoskeleton using biomechanical assistive apparel has demonstrated a significant decrease (23%–43%) in erector spinae muscle activity and a substantial reduction in lower back pain during sustained forward bending tas [10]. Several wearable passive back-support exoskeletons have already been evaluated, showing an effect of reducing back muscle activity measured by electromyography (EMG), ranging from 10% to 40% [11,12]. Previous studies have demonstrated a significant effect of the device on EMG activity during lumbar flexion, particularly during full flexion. The elongated connective tissue resulting from flexion and extension generates the necessary extension moment, leading to a decrease in EMG activity in the back muscles [13,14].

The material factors (such as load, distribution, size, stability, shape, handles, etc.) are critical elements influencing operations within logistics tasks. Previous studies have indicated that handling heavier loads increases spine loading, with lifting from the floor having the most significant impact on spine loading [15]. Other previous studies have also researched the impact of lifting height and mass on spine load and reported that both height and mass influence spine load [16]. This indicates the significance of both lifting height and load weight in spinal loading during logistics operations. However, studies typically assessed spinal loading during work at or above waist height [17]. Although there have been studies on the effect of wearable devices on the maximal muscle activity of the back during object lifting, research on how the height below the waist level at which objects are lifted affects the maximal muscle activity of the back during lifting tasks involving weighted objects is limited.

Therefore, this study aimed to investigate the effect of a wearable passive back-support exoskeleton on the maximal activity of the thoracolumbar muscles when performing lifting tasks at different heights.

1. Participants

The sample size for the research was calculated using the G*Power program ver. 3.1.9.7 (Kiel University; two-tailed, effect size dz: 0.8, α error probability: 0.05, power [1-β error probability]: 0.80), which determined that 15 research participants were needed. Selected participants included 20 healthy adult males (age: 23.55 ± 1.56 years; height: 174.65 ± 4.19 cm; weight: 71.50 ± 10.07 kg) without musculoskeletal pain within the past 6 months. Individuals who had experienced lower back pain in the preceding 6 months were excluded [17,18]. Prior to conducting the study, all participants were provided with an overview of the experimental procedures and safety measures. All participants signed an informed consent form before participating in the study. This study was approved by the Cheongju University Ethics Committee (IRB no. 1041107-202302-HR-070-01).

2. Equipment

EMG (Telemyo Direct Transmission System, Noraxon) was used to measure the maximum activity of the thoracolumbar muscles. The EMG data was analyzed using the Noraxon MR3.12.70 myoMUSCLE software program (Noraxon). To minimize resistance, the skin was first cleaned with an alcohol swab, and EMG pads were attached to both sides of the thoracic and lumbar erector spinae muscles [19]. After attaching the EMG pads, the maximum voluntary isometric contractions (MVIC) of each muscle were measured for 5 seconds using the middle 3 seconds after excluding the initial and final seconds. The MVIC measurement method involved subjects flexing their trunk at a 90-degree angle and exerting maximum force to ensure optimal engagement. This procedure was repeated five times to enhance the accuracy. Additionally, when lifting objects at different heights, the peak muscle activities of the thoracolumbar muscles with and without the wearable passive back-support exoskeleton were compared. The sampling rate for the EMG signal was set at 1,000 Hz, with a frequency band of 20–500 Hz and a 60 Hz notch filter applied [20]. The extracted EMG signals were normalized to the percentage of maximum voluntary isometric contractions (%MVIC). Muscle peak times were determined by finding the global maximum of the processed, smoothed signals [21].

The wearable passive back-support exoskeleton used was Back-X (model AC, US Bionics Inc.). When in use, the waist apparatus was adjusted to fit the waist using strap, and after fastening the two straps together, the inner hinge with the chest and thigh support devices was adjusted for height before detaching. In terms of operation, when rectangular switches located on the back portion of the worn Back-X are pushed downwards (on), the chest-side device provides support. Conversely, when the switches are raised (off), they do not perform their functions. A feature of Back-X is its dual-mode switches, which allow for either strong support in Instant Mode or assisting mobility in Standard Mode, depending on the user’s needs. In this study, the Instant Mode was employed.

3. Procedure

EMG was used to measure the MVIC of the thoracolumbar erector spinae muscles. The handle of the box was positioned at a height of 30 cm, and a box 55 cm wide and 35 cm high was selected. The distance between the foot and box was set to 10 cm, and the lifting posture was defined as bending the waist while keeping both knees straight.

The participants underwent a familiarization process in which they performed lifting and lowering motions of a 10 kg weight from heights of 30, 40, and 50 cm above the ground [22]. This was performed with and without the wearable passive back-support exoskeleton. The MVIC of the thoracolumbar erector spinae was measured (Figure 1).

Figure 1. Repetitive lifting and lowering in three experimental conditions. (A) 30 cm, (B) 40 cm, and (C) 50 cm.

A 3-minute rest period was provided before measurements at different heights to reduce fatigue in the lumbar erector spinae muscles. The sequence of tasks was randomized [10]. A flowchart outlining the procedure is presented in Figure 2.

Figure 2. Procedure flow chart. EMG, electromyography.

4. Data Processing

The data collected in this study were analyzed using IBM SPSS software Window (ver. 22.0, IBM Co.). To verify whether the measured data followed a normal distribution, a Shapiro–Wilk test was conducted, confirming that the data exhibited a normal distribution. The difference in the MVIC of the thoracolumbar muscles with and without wearing the Wearable Passive Back Support Exoskeleton was assessed using a paired t-test at heights of 30, 40, and 50 cm from the ground while lifting a 10 kg weight. Statistical significance was set at p < 0.05 was set for statistical significance testing.

This study aimed to investigate the influence of a Wearable Passive Back Support Exoskeleton on the MVIC of the thoracolumbar muscles when lifting objects from different heights. The results are as follows:

1. Change in MVIC According to the Wearable Passive Back Support Exoskeleton at a Ground Height of 30 cm

The results of the MVIC measurements with and without the back-support exoskeleton at a ground height of 30 cm while lifting a 10 kg weight indicated a significant decrease in all muscles (Table 1).

Table 1 . Changes according to the presence or absence of a Wearable Passive Back Support Exoskeleton at a height of 30 cm from the ground.

Variable	Wearable lumbar orthosis		p-value
	Without suit X	With suit X
Lt. TES (%MVIC)	126.0 ± 48.66	102.20 ± 41.48	0.001*
Rt. TES (%MVIC)	114.36 ± 37.36	93.79 ± 31	0.001*
Lt. LES (%MVIC)	124.76 ± 39.82	101.70 ± 35.93	0.001*
Rt. LES (%MVIC)	143.20 ± 53.62	113.6 ± 47.67	0.001*

Values are presented as mean ± standard deviation. Lt., left; Rt., right; TES, thoracic erector spinae; LES, lumbar erector spinae; %MVIC, percentage of maximum voluntary isometric contractions. *p < 0.05..

2. Change in MVIC According to the Wearable Passive Back Support Exoskeleton at a Ground Height of 40 cm

The results of the MVIC measurements with and without the back-support exoskeleton at a ground height of 40 cm while lifting a 10 kg weight indicated a significant decrease in all muscles (Table 2).

Table 2 . Changes according to the presence or absence of a Wearable Passive Back Support Exoskeleton at a height of 40 cm from the ground.

Variable	Wearable lumbar orthosis		p-value
	Without suit X	With suit X
Lt. TES (%MVIC)	118.13 ± 41.48	96.08 ± 35.84	0.001*
Rt. TES (%MVIC)	116.68 ± 37.26	94.56 ± 37.53	0.003*
Lt. LES (%MVIC)	131.83 ± 53.56	106.50 ± 46.50	0.006*
Rt. LES (%MVIC)	150.28 ± 53.50	118.37 ± 44.55	0.001*

Values are presented as mean ± standard deviation. Lt., left; Rt., right; TES, thoracic erector spinae; LES, lumbar erector spinae; %MVIC, percentage of maximum voluntary isometric contractions. *p < 0.05..

3. Change in MVIC According to the Wearable Passive Back Support Exoskeleton at a Height of 50 cm From the Ground

The results of the MVIC measurements with and without wearing the back-supporting exoskeleton at a ground height of 50 cm while lifting a 10 kg weight indicated a significant decrease in all muscles (Table 3).

Table 3 . Changes according to the presence or absence of a Wearable Passive Back Support Exoskeleton at a height of 50 cm from the ground.

Variable	Wearable lumbar orthosis		p-value
	Without suit X	With suit X
Lt. TES (%MVIC)	104.84 ± 34.16	77.50 ± 27.25	0.001*
Rt. TES (%MVIC)	105.40 ± 36.93	77.66 ± 26.99	0.001*
Lt. LES (%MVIC)	123.37 ± 44.72	82.65 ± 28.50	0.001*
Rt. LES (%MVIC)	135.52 ± 52.76	104.78 ± 39.09	0.001*

Values are presented as mean ± standard deviation. Lt., left; Rt., right; TES, thoracic erector spinae; LES, lumbar erector spinae; %MVIC, percentage of maximum voluntary isometric contractions. *p < 0.05..

This study aimed to investigate the effect of a Wearable Passive Back Support Exoskeleton on the MVIC of the thoracolumbar muscles when lifting objects at different heights using EMG analysis. The experimental subjects were instructed to wear the Back-X and then, with knees straight, flex their trunk to lift the box according to its height.

Regardless of the height of the box handle from the ground (30, 40, or 50 cm), wearing the Wearable Passive Back Support Exoskeleton resulted in a significant reduction in the activation of the thoracic and lumbar erector spinae muscles. Similar studies reported a significant decrease in MVIC after wearing a Wearable Passive Back Support Exoskeleton during lifting tasks at different heights. This suggests a potential reduction in the risk of back injury and lumbar spinal muscle fatigue during lifting tasks [23]. Additionally, the Wearable Passive Back Support Exoskeleton does not increase the activity of muscles that are not directly involved in the task [24]. While prolonged wear of the Wearable Passive Back Support Exoskeleton may be uncomfortable and have limitations in widespread use, it has a positive impact on reducing the MVIC of the thoracolumbar muscles and improving both back injury and muscle fatigue [25]. In contrast, a previous study reported side effects, including increased abdominal/leg muscle activity and changes in joint angles [26]. Therefore, when considered comprehensively, the use of a Wearable Passive Back Support Exoskeleton leads to decreased activation of the thoracolumbar erector spinae muscles.

In particular, when lifting a 10 kg weight from a height of 50 cm, there was a significantly greater reduction in MVIC compared to heights of 30 and 40 cm. At heights of 30 and 40 cm, there was a significant decrease in the MVIC of the erector spinae muscles. The thoracic erector spinae muscles showed a reduction of approximately 17.98% to 23.39%, the lumbar erector spinae muscles experienced a decrease of around 18.48% to 26.95%. Conversely, at a height of 50 cm, lifting a 10 kg weight resulted in an even greater reduction in MVIC than that observed at heights of 30 and 40 cm. The thoracic erector spinae muscles showed a significant reduction, ranging from approximately 26.07% to 35.71% with a larger magnitude, whereas the lumbar erector spinae muscles exhibited a reduction of approximately 22.68% to 49.26%. To speculate on the cause of these differences, it is possible that, at a height of 50 cm, there was a decrease in the movement of the hip joint, leading to increased utilization of the back muscles. This could result in a greater reduction in the MVIC when using the Wearable Passive Back Support Exoskeleton. As the anterior pelvic tilt angle increases, vertebral movement during trunk flexion increases [26]. When the trunk bends forward, the posterior parts of the spine and hip joints are elongated. This enhances perceived passive tension, and when perceived passive tension reaches a threshold, the Central Nervous System deactivates the active control element (the erect spine muscles) to conserve energy [9]. The reduction in MVIC at different working heights tends to be most pronounced under conditions close to the vertical plane, depending on the design of the Wearable Passive Back Support Exoskeleton [18]. Consequently, the use of a Wearable Passive Back Support Exoskeleton at specific heights demonstrated its effectiveness. It is recommended that tasks should be performed at an appropriate height when using a Wearable Passive Back Support Exoskeleton.

This study had several limitations. First, the participants were limited to healthy males. The sample size was relatively small, and the study included only male participants, making it difficult to generalize the findings to a broader population. Second, there was limited diversity in the heights considered, making it challenging to generalize the results to all common working scenarios. Third, the study did not account for differences in participants’ heights, conducting research with the same posture and distance for all participants, which introduced inconsistencies in trunk flexion angles. In future research, it is crucial to increase the representation of women, include a broader range of age groups, and expand the sample size to enhance the generalizability of the study. Additionally, measuring changes in trunk flexion angles could enhance the comprehensiveness of this study.

In conclusion, regardless of the height of the box handle, wearing the Wearable Passive Back Support Exoskeleton led to a decrease in the MVIC of the thoracolumbar erector spinae muscles. This effectively reduces the load on the lower back when lifting objects. Therefore, the use of a Wearable Passive Back Support Exoskeleton is recommended for lifting tasks.

Furthermore, the study demonstrated the effectiveness of using a wearable passive back-support exoskeleton at specific heights during tasks.

The authors thank to Eun-cheol Kim, Sun-woo Han, Do-uk Kang, Ji-young Lee, and Min-seo Hong for advice on experimental and statistical analysis.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: HEA, TLY. Data curation: HEA, TLY. Formal analysis: HEA, TLY. Investigation: HEA, TLY. Methodology: HEA, TLY. Project administration: HEA, TLY. Resources: HEA, TLY. Software: HEA, TLY. Supervision: HEA, TLY. Validation: HEA, TLY. Visualization: HEA, TLY. Writing - original draft: HEA, TLY. Writing - review & editing: HEA, TLY.