Passive lower-limb exoskeletons in industry: A review of “Chairless Chairs” for reducing fatigue and musculoskeletal strain

This systematic review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 criteria. In accordance with these guidelines, it is noted that a formal protocol for this review was not prospectively registered in a public database prior to commencement (Page et al., 2021). The final databases search was executed on 20 June 2025. A combination of the identified keywords was created using the Scopus, ScienceDirect, PubMed, and manual search. Table 1 shows the databases utilized and corresponding search terms.

The screening process was undertaken by two independent reviewers. After the main reviewer had completed the initial search and screening, a second reviewer independently confirmed the final selection of 17 articles and the extracted data. Any disagreements regarding inclusion were resolved by consensus. The most current studies published in the last 10 years were included, including research articles written in English and articles for which a full, free-text translation was available in English. Articles that were not fully accessible were not obtained. Only research papers on lower-limb exoskeleton were included in the study.

A total of 87 papers were identified (74 from 3 databases and 13 from manual searching). After screening the titles and abstracts, 25 records remained, as 62 were eliminated for not meeting the inclusion requirements. Duplicate articles were then eliminated from the 25 remaining entries after filtering the full-text articles for research publications released within the previous 10 years. Eight records were eliminated as a result of this approach, leaving 17 articles. The final 17 publications were chosen for data collection after one last check for duplication.

The methodological quality of the 17 included studies was evaluated in order to calculate the risk of bias. Due to the wide range of study methods employed, including quasi-experimental laboratory studies and randomized controlled trials, the JBI Critical Appraisal Tool was used (Figure 1).

Figure 1

PRISMA flow diagram

The commercial lower-limb exoskeletons covered in this section, along with their features, composition, and performance results are shown in Table 2.

The 2014 launch of the NOONEE popularized the chairless chair. This, in turn, led to the development of similar commercial products, such as the LegX, H-CEX, and Archelis. Nonetheless, back in 2008, Honda R&D developed an alternative bodyweight support device for assembly workers.

The NOONEE wearable chair has the capability to adjust to the user’s height and offers a variety of seating positions. It has a comfortable seat and uses a hydraulic system to support body weight. NOONEE uses lightweight materials, such as carbon fibre and light metals to reduce its weight. Its control system blocks the hydraulic actuator and stops joint rotation by regulating an electronically controlled valve. This valve responds to the initial sitting angle, which is detected by an angle sensor at the joint of the lower and upper links. According to the developers, human research has shown that the NOONEE can reduce strain on the lower extremities by up to 64% (Luger et al., 2019).

The LegX was designed to support industrial workers in squat postures. This design varies from previously presented designs because of the laterally placed support structure. Furthermore, LegX has two working modes that can be switched with an active control system. In the dynamic mode, the device uses compression springs to assist sitting and standing by storing and releasing energy. In the static mode, the wearer can lock the device in three specific locations where it transmits the weight of the wearer to the ground. LegX has shown a 56% reduction in muscle activity during experimental evaluations (Pillai et al., 2020).

The H-CEX, a lightweight wearable device patented by Hyundai Motor Company in 2020, uses a four-bar linkage that deploys passively with knee flexion. An integrated elastic component assists dynamic movements like standing and sitting. This device enhances trunk stability due to its broad support base and offers three angle settings for multi-point sitting assistance (Hyundai, 2018). The developer claims that H-CEX can reduce lower body muscle activity by 30.59–84.08% (Kong et al., 2021). Positive feedback on comfort using H-CEX was obtained when interviewed.

The Archelis wearable was created to support surgeons during long procedures by providing seating for crouching and shifting positions in crowded operating rooms where traditional chairs are impractical due to cables. The Archelis is a passive device lacking electrical components to prevent interference with medical equipment. Its main objective is to prevent fatigue and encourage prolonged concentration in surgeons during surgeries. Study shows that the Archelis dramatically reduces iliopsoas muscle activation (Kawahira et al., 2018). It can sustain a full squat when required, although its ergonomic shape allows for only one locked sitting posture (Matsuzaki et al., 2019).

The Honda bodyweight support system, which was developed for Honda’s automotive employees, stands out from other designs because it does not have a static looking mechanism. This device uses active actuators to assist with walking, climbing, and squatting. The inter-leg seat continuously pushes upward, which lowers the user’s effective body weight on their feet. Two motors behind the wearer drive the actuation and foot pressure sensors continually detect force and modulate motor power to reduce foot strain (Ikeuchi et al., 2009). The test showed that the device reduced average muscular activity by 18% and energy consumption by 11%.

Table 3 lists the various research models and their features, as well as the main findings of the assessment.

The semi-active exoskeleton has three operating modes: follow mode, elastic support mode, and rigid support mode. In rigid support mode, the exoskeleton supports the wearer’s weight like a wearable chair, allowing for easy adjustment of the knee flexion angle between 0 and 135° when squatting or semi-squatting. The elastic support mode uses a torsion spring to offer support for tasks that require frequent adjustments in squatting height. Follow mode efficiently reduces muscle fatigue without obstructing mobility, enabling the exoskeleton to move passively with the wearer’s legs (Wang et al., 2021).

The SIAT exoskeleton is a wearable chair that allows employees to squat for extended periods of time. It helps users stand up by storing energy from the squatting motion in torsion springs, which are then released. The device supports the user’s weight with a mechanical ratchet that locks the exoskeleton in place. Its lightweight construction is the result of optimizing its dimensions. Studies have shown that the SIAT exoskeleton reduces muscular activity by 44.8–71.5% and has a user comfort rating of 70% or higher (Yan et al., 2021).

The single-stand prototype weighed 3 kg, could endure a maximum compression force of 1,090 kg, and was made from hardened steel, nylon, polyurethane foam, and an aluminium alloy. According to the muscle contraction investigation, when wearing the prototype, the contraction of the lower limb muscles was significantly reduced (P-value < 0.05). Additionally, the study found that contact pressure beneath the thighs was eliminated. A usability test revealed that the prototype’s SUS score was 79.5, surpassing the 67.3 score of the double-stand commercial exoskeleton (Halim et al., 2024).

The chairless chair concept forms the basis of several posture assistance technologies. For instance, passive weight-support exoskeleton with a posterior linkage system can provide three locking settings (Zhu et al., 2018). To facilitate posture shifts, this device often uses a linear spring actuation system. The HUST-EC exoskeleton also has a three-bar linkage to disperse load (Han et al., 2019). Both the passive-weight support exoskeleton and HUST-EC use slot slider mechanisms to lock the device. Both prototypes have shown a notable 80% decrease in muscular activity.

The linkage supporting the wearer’s shank is driven by an active linear actuator system in the ChairX (Wijegunawardana et al., 2019). In the desired sitting position, a mechanical ratchet lock stabilizes the thigh at the knee. With seven different seating positions, the device can accommodate forward and backward leg inclinations. Its fibre-reinforced plastic pads provide sufficient surface area for the shank and thigh, ensuring comfort. Additionally, ChairX can include passive or active actuators to assist with posture adjustments. Research indicates that ChairX lowers root mean square (RMS) sEMG by about 20%.

Most basic posture-assistance devices use gas shocks to assist with dynamic sitting and standing. They are attached to the back of the leg. To provide static support, they also integrate advanced locking mechanisms. The chairless chair uses joint stoppers (Siddha et al., 2018), while the SimpChair (Allias et al., 2015) and CCES (Ramachandran et al., 2018) employ telescopic link extensions for locking. However, these early prototypes are often large and heavy.

The knee assist robotic exoskeleton (Noh et al., 2016) helps people who are seated by using a four-bar mechanism that locks into place when it reaches its unique position. An absolute encoder directs an electric motor to retract the linkage to allow for mobility. A passive gas spring provides dynamic support during standing and sitting. A force sensor on the gas spring distinguishes between sitting and walking modes. Tests on prototypes show that it reduces muscular activation.

Another important category of posture-assistive equipment includes seats that support body weight. A well-known commercial example is the Honda bodyweight assist system. These designs usually use powered joints either active or passive to provide upward support. The passive weight-support LEE is an example of an exoskeleton that uses passive joints to mimic the compliant behaviour of the human knee (Lee and Wang, 2015).

Soft gait presents a comparable structure that uses powered pneumatic actuation. It provides passive support through linear springs aligned with the actuator, and foot pressure feedback controls its operation. During effectiveness evaluations, soft gait demonstrated an approximate 83% reduction in foot reaction force (Hong et al., 2015).

Despite the encouraging results, it is important to acknowledge the review’s limitations. First, according to the formal quality evaluation conducted with the JBI Critical Appraisal Tool, there was a great deal of variation in the study designs, outcome measures (such as different EMG sensor locations) and experimental settings. It is difficult to draw firm conclusions about the long-term applicability in the real world because most of the included studies were short-term laboratory tests with relatively small sample sizes. Furthermore, while EMG-based results are useful for measuring muscle fatigue, long-term productivity and health statistics from real-world industrial deployment are also required to ensure broad effectiveness.