3.1. Biomechanics of musculoskeletal load

The biomechanics of musculoskeletal load is the study of physical stresses acting on and created within the human body and their effects on tissues and fluids (Hansen et al., Reference Hansen, De Zee, Rasmussen, Andersen, Wong and Simonsen2006). The loads imposed on tissues, such as muscles may arouse physiological and psychological results, such as pain or discomfort. During voluntary or involuntary exertions and movements, muscles are essential for transmitting stresses through tendons, ligaments, and bones (Johnson et al., Reference Johnson, Bogduk, Nowitzke and House1994b). Biomechanical models are frequently employed to estimate loading and its impact, because they impose issues while performing duties in the workspace, activities, and daily routines (Pitzen et al., Reference Pitzen, Geisler, Matthis and Feldges2002; Arjmand et al., Reference Arjmand, Gagnon, Plamondon, Shirazi-Adl and Larivière2009). Numerous internal and external factors, including body posture; exertions, forces; motions; environmental factors like thermal or vibrational energy transmission; and individual traits like anthropometry, strength, agility, and dexterity, all have an impact on biomechanical loading (Radwin and Lavenderand, Reference Radwin and Lavender1999). Therefore, comprehending how these factors interact with the musculoskeletal system to affect internal loads on tissues, fluids, or muscles is important for enhancing performance, avoiding injuries, and supporting musculoskeletal health (Roman-Liu, Reference Roman-Liu2013).

3.1.1. Upper limb

Comprising the shoulder, elbow, wrist, and fingers, the upper limb is a highly dynamic system essential for a wide range of functions of gross and fine motor tasks through coordinated muscle, tendon, ligament, and bone interactions (Betts et al., Reference Betts, Young, Wise and DeSaix2013; Pristerà et al., Reference Pristerà, Gallo, Fregola and Merola2020). Each joint has specialized roles contributing to motor movement (Figure 2a). While the shoulder covers a vast area of motion, and the elbow performs as a solid lever, the wrist, and the fingers enable subtle and fine manipulations. Balanced muscle activation, efficient force transmission, and joint stability are essential for optimizing performance and reducing injury risk in the upper limbs (Georgarakis et al., Reference Georgarakis, Xiloyannis, Wolf and Riener2022).

Figure 2.

Musculoskeletal structure of the human body. (a) Arm bionics design with degree of freedom (Gong et al., Reference Gong, He, Wang and Yu2019). (b) Hand segment model with rotational axis (Mirakhorlo et al., Reference Mirakhorlo, Visser, Goislard De Monsabert, Van Der Helm, Maas and Veeger2016). (c) Musculoskeletal load simulation on the human spine (Meszaros-Beller et al., Reference Meszaros-Beller, Hammer, Riede, Pivonka, Little and Schmitt2023). (d) Lower limbs joint model with degree of freedom (Ren et al., Reference Ren, Liu and Chen2020).

The shoulder consists of four distinct joints (glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic) that involve three bones: the clavicle (collarbone); scapula (shoulder blade); and humerus (upper arm bone) (Engín, Reference Engín1980). The most mobile of the four joints, the glenohumeral joint, is essential to the upper body’s kinetic chain and allows for flexion, extension, abduction, adduction, internal/medial rotation, external/lateral rotation, and circumduction, three degree of freedom (DOF) in total (McLaren, Reference McLaren2023; Physiopedia, 2024b). it is crucial to acknowledge that the glenohumeral joint’s instantaneous COR shifts with upper limb motions and to consider it the dynamic COR while designing the shoulder mechanism of an exoskeleton (Stienen et al., Reference Stienen, Hekman, Van Der Helm and Van Der Kooij2009; Prinold et al., Reference Prinold, Masjedi, Johnson and Bull2013). The elbow joint comprises the humeroradial and humeroulnar joints with 2 DOF, forming a synovial complex (Card and Lowe, Reference Card and Lowe2024). The elbow enables forearm extension, flexion, supination, and pronation, with muscles like the biceps brachii, brachialis, brachioradialis, triceps brachii, and anconeus facilitating movement and stabilization (Card and Lowe, Reference Card and Lowe2024). The wrist joint is a condyloid synovial joint that allows different movements: flexion/extension and adduction/abduction (Andrews and Youm, Reference Andrews and Youm1979). Wrist movements happen around an immediate COR and involve several muscle groups: flexors, extensors, and abductors/adductors, coordinating to ensure wrist dexterity (Neu et al., Reference Neu, Crisco and Wolfe2001; Bigale, Reference Bigale2024). The wrist joints are often susceptible to several injuries, including sprains, fractures, tendonitis, and dislocations (van der Sluis and Dekker, Reference van der Sluis and Dekker2015).

Fingers are composed of phalanges and interconnected joints that play a vital role in fine motor skills and tasks requiring high precision. Each finger, except the thumb, has three phalanges connected by interphalangeal joints, enabling flexion/extension movements (Mirakhorlo et al., Reference Mirakhorlo, Visser, Goislard De Monsabert, Van Der Helm, Maas and Veeger2016). The metacarpophalangeal joints, where the fingers merge with the hand, additionally allow abduction/adduction to enhance the versatility of complex movements (Figure 2b). Fingers can deal with the forces necessary for daily tasks, such as grasping, typing, or manipulating small objects, which are often replicated in robotic grippers (Li et al., Reference Li, Tang, Chakan and Kaz2008). The flexor and extensor muscles, located on the forearm and the palm and connected to the finger phalanges by tendons, mainly coordinate to ensure precise control and stability (Schieber, Reference Schieber1995; Nimbarte et al., Reference Nimbarte, Kaz and Li2008). In addition, several intrinsic hand muscles, such as the hypothenar, interosseous, and lumbrical muscles, are located inside the hand. These muscles work together to provide fine motor control for accurate finger movements (Dominguez, Reference Dominguez2018; Jones, Reference Jones2023). Rehabilitation often aims to improve strength, dexterity, and joint mobility through specialized tasks with assistive devices (Klug et al., Reference Klug, Hessinger, Koka and Kupnik2019).

3.2. Muscle function and overuse

Muscle function is fundamental to human body movements and activities, which the muscle fibers coordinate to activate as the basic contractile units (Jennings et al., Reference Jennings, Cianciolo, Wilkie and Kendziorski2020) (Figure 3a). Muscle fiber contraction is the primary process to generate force and initiate joint movements. According to the sliding filament theory, two types of filaments within each fiber – actin and myosin – slide and get closer to the received neural signal, thereby shortening the muscle fiber (Rye et al., Reference Rye, Wise, Jurukovski, DeSaix, Choi and Avissar2016) (Figure 3b). The collective shortening of muscle fibers results in the contraction of the whole muscle, which generates joint movements. For example, the long head of the biceps starts from the scapula (the shoulder blade), crosses the elbow joint, and reaches the forearm. When the biceps contract, the forearm is pulled toward the shoulder on the radius, triggering the elbow joint to flex (Betts et al., Reference Betts, Young, Wise and DeSaix2013) (Figure 3c). On the other hand, the triceps on the opposite side of the upper arm relax moderately during the elbow flexion to allow the biceps to function optimally and prevent excessive and unstable elbow movement (Seiber et al., Reference Seiber, Gupta, McGarry, Safran and Lee2009). While the muscle shortens during the concentric contractions, the muscles lengthen or maintain the same length during eccentric and isometric contractions. These contraction capabilities of muscles are vital in joint stability, protection, and body movement efficiency.

Figure 3.

Muscle elements and contraction. (a) Structure of muscles (Jennings et al., Reference Jennings, Cianciolo, Wilkie and Kendziorski2020). (b) Muscle contraction mechanism (Rye et al., Reference Rye, Wise, Jurukovski, DeSaix, Choi and Avissar2016). (c) Muscle contraction initiating the joint movement (Betts et al., Reference Betts, Young, Wise and DeSaix2013).

Muscle fatigue is a common phenomenon that occurs when intense use of muscles is done, leading to a decline in muscle performance. Intensive muscle activity causes a decline in performance, known as (Allen and Westerblad, Reference Allen and Westerblad2001). Individuals often experience this muscle fatigue during activities like exercise, and it can also be present in various pathological conditions affecting the neurological, muscular, and cardiovascular systems (Enoka and Duchateau, Reference Enoka and Duchateau2008). Fatigue can arise due to a number of factors, including impaired neural activation, metabolic disturbances, and depletion of reserved energy. Muscle fatigue not only impairs physical performance but also increases the musculoskeletal injury risks. Muscular stiffness induced by fatigue may impair the capacity of muscles, including the paraspinal, to uphold stability, potentially heightening the likelihood of spinal instability and injury (Granata et al., Reference Granata, Slota and Wilson2004; Mawston, Reference Mawston2010). Comprehending how muscle fatigue affects musculoskeletal stability is crucial for developing efficient training regimes and supportive equipment, such as exoskeletons (Lamers et al., Reference Lamers, Soltys, Scherpereel, Yang and Zelik2020).

Musculoskeletal injuries are frequently associated with heavy physical work, including repetitive lifting and forward-leaning, which causes recurrent load on muscles, tendons, ligaments, or joints (Luttmann et al., Reference Luttmann, Jäger, Griefahn, Caffier and Liebers2003). Repetitive strain without sufficient recovery, sudden high loads, or incorrect movement patterns can lead to overuse problems or acute injuries, such as sprains or strains. According to the US Labor Bureau, musculoskeletal disorders account for about 30% of work-related absences (Statistics, Reference Statistics2016). This issue is common across a variety of fields, including health care, construction, fishing, and agriculture (Smedley et al., Reference Smedley, Egger, Cooper and Coggon1995; Latza et al., Reference Latza, Pfahlberg and Gefeller2002; Holmberg et al., Reference Holmberg, Thelin, Stiernstrom and Svardsudd2003; De Kok et al., Reference De Kok, Vroonhof, Snijders and Isusi2019; Rempel et al., Reference Rempel, Ricker and Feiri2021). For example, soldiers carry bulky loads in their daily tasks, which can easily exceed 31 kg (68.2 lbs) (Army Techniques Publication, 2017). Introducing these external loads will require the bearer to change their gait or posture or both to compensate. As both postural stability and gait mechanics can indicate a soldier’s health, determining how load carriage affects these postural stability and gait mechanics can be essential in preserving a soldier’s well-being. It is well-known that adding an external load to an individual will change both his kinematics (stride length, stride frequency, etc.) and postural stability, as that load will affect the location of the body’s center of mass (Seay, Reference Seay2015). Therefore, forward lean happens to the individual’s body (Attwells et al., Reference Attwells, Birrell, Hooper and Mansfield2006; Birrell and Haslam, Reference Birrell and Haslam2009; Seay et al., Reference Seay, Frykman, Sauer and Gutekunst2014; Nath et al., Reference Nath, Asish, Debi, Chowdhury, Zamora and Muñoz2023a, Reference Nath, Chowdhury and Rhaman2023b). Besides forward trunk lean of the body, several factors, such as spine shape, spinal compression, spinal shearing forces, and thoracic–pelvic rhythm, have been associated with these load carriage (Attwells et al., Reference Attwells, Birrell, Hooper and Mansfield2006; Fowler et al., Reference Fowler, Rodacki and Rodacki2006; Meakin et al., Reference Meakin, Smith, Gilbert and Aspden2008; Majumdar et al., Reference Majumdar, Pal and Majumdar2010).

4.1. Material and fabrication

Throughout history, humans have used knitting, weaving, twisting, and stitching to decorate and warm themselves with textiles made from high-aspect-ratio fibers like cotton, linen, silk, and wool (Gries et al., Reference Gries, Raina, Quadflieg and Stolyarov2016; Xiong et al., Reference Xiong, Chen and Lee2021). The twentieth-century need for functional textiles sped up the development of synthetic fibers like polyester, polypropylene, and polyurethane, as well as innovative production methods like thermal drawing, microfluidic spinning, and printing (Xiong and Lee, Reference Xiong and Lee2019; Dong et al., Reference Dong, Peng and Wang2020; Zhang et al., Reference Zhang, Zhang, Zhang and Chen2020). Through creative approaches, including materials development, component integration, structural design, and fabrication processes, elastic textiles have been made possible by advances in materials and technologies. Elasticity is one of the major material properties that help materials recover from mechanical bending, stretching, twisting, and shearing (Liu et al., 2017; M. Park et al., 2012). Material or structure allows elastic textiles to deform and recover from external stimuli. They also accommodate and maintain deformations for specific configurations or circumstances (Hu et al., Reference Hu, Lu and Zhu2008; Levitt et al., Reference Levitt, Zhang, Dion, Gogotsi and Razal2020).

Elastic fibers can be made from synthetic and natural materials. Natural elastic fibers, such as rubber, utilize the malleable microstructures found in plants and animals (Wang et al., Reference Wang, Zhou, Liu, Peng and Zhou2022). Conversely, synthetic elastic textiles generally depend on a deformable matrix and are prone to dynamic molecular displacement and reassembly. Elastic polymers are composed of a dynamic mixture of long, flexible, and rigid short chains (Hu et al., Reference Hu, Lu and Zhu2008). The extension of the flexible long chains happens by the axial displacement of the amorphous region (Brzinski and Daniels, Reference Brzinski and Daniels2015) (Figure 4a). The flexible amorphous chain region can relax once the external stressors are eliminated, allowing the material to recover. A long, flexible polyethylene glycol chain is reported to stretch up to 500 times its initial size (Hu et al., Reference Hu, Kumar and Lu2020; Zhang et al., Reference Zhang, Yin, Nian and Suo2021; Guo et al., Reference Guo, Pang, Xie, Xu and Yuan2022). One of the market’s most widely utilized elastic polymers is polyurethane (PU). PU is frequently utilized as an elastic matrix when creating functional elastic textiles, which are processed using conventional spinning and production methods such as wet spinning, thermal drawing, extrusion, casting, coating, templating, and printing (O. Akindoyo et al., Reference Akindoyo, Beg, Ghazali, Islam, Jeyaratnam and Yuvaraj2016). These technologies improve elastic textiles’ utility and applicability by adding functional components. Popular elastic textiles such as latex and rubber elastic bands are used in fitness and rehabilitation workouts to offer resistance for strength training and muscle toning (Martins et al., Reference Martins, De Oliveira, Carvalho, De Oliveira Damasceno, Da Silva and Silva2013). These elastic bands come in various shapes, sizes, and resistance levels to assist users in achieving their fitness goals or rehabilitation (Lindberg, Reference Lindberg2024).

Figure 4.

Elongation of fibers and textiles. (a) Elastomer under strain (Brzinski and Daniels, Reference Brzinski and Daniels2015). (b) Knitted textiles under strain (“The Why and How of Stretch Fabrics”, n.d.).

Textile elasticity can be accomplished through the yarn structure forming the textile specified by the fabrication method. Textiles can be classified as woven, knitted, or nonwoven forms based on their manufacturing type. Knitted textiles, for example, can be elongated significantly due to the interlaced loop structure of the yarns (Sundstrup et al., Reference Sundstrup, Jakobsen, Andersen and Andersen2014; Poincloux et al., Reference Poincloux, Adda-Bedia and Lechenault2018). The interconnected loops can withstand tension and contract when the elongating strain is released (Figure 4b). On the other hand, the warp-weft structure of woven fabrics tightly interlaced at the right angle does not allow much room to stretch compared to the knitted textiles. The nonwoven textiles, a web of fibers bonded mechanically, thermally, or chemically, sometimes allow for a limited range of elongation according to the fiber arrangements but often do not recover when the tension is removed (Yilmaz et al., Reference Yilmaz, Sabuncuoglu, Yildirim and Silberschmidt2020). Overall, incorporating elastic yarns into knitted and woven textiles is comparable, with careful consideration of yarn and fabric structure critical to defining the fabric’s elasticity and mechanical qualities (Huang et al., Reference Huang, Wang, Hu and Zheng2020; Zhang et al., Reference Zhang, Zhou, Zhang, Zhang, Yong and Xiong2022).

4.2. Mechanical properties in comparison to human musculoskeletal tissue

Material performance measurement is essential for ensuring safety, cost-effectiveness, and marketability. Textile elasticity is assessed through Young’s modulus, tensile strength, and elongation at break. Young’s modulus measures a textile’s stretchability and recovery under force; higher values indicate rigidity, while lower values suggest flexibility (Omnexus, 2024). Elastic textiles generally have lower Young’s modulus than other materials such as metals, ceramics, and glasses. For example, nylon has a Young’s modulus of 2.7 GPa, while glass fibers have 72 GPa (Omnexus, 2024). The tensile strength of elastic textiles defines the material’s ability to endure stretching forces without breaking (Hamburger, Reference Hamburger1948). Textiles often show a linear-elastic behavior under tensile stress, and the deformation happens until it reaches its ultimate tensile strength (Rempel et al., Reference Rempel, Ricker and Feiri2021). Understanding this behavior is vital for designing textiles with optimal stretchability and durability. Finally, the elongation at break of elastic textiles means the amount of deformation under tension before failing, assisting in assessing stretchability and durability (Hamburger, Reference Hamburger1948). Product developers incorporate elastane in textiles to increase the elongation capability. For example, the elongation at break was higher in textiles with a higher percentage of elastane fibers (Jovanović et al., Reference Jovanović, Penava and Vrljičak2022) (Ballard, Reference Ballard1995).

When developing a wearable assistive system using elastic textiles, understanding the properties of both human body tissues and elastic materials is necessary to assign proper materials and guarantee the system’s performance (Lamers and Zelik, Reference Lamers and Zelik2021). Figure 5 and Table 1 compare the mechanical properties of the commonly used materials for elastic textiles – natural/synthetic rubbers and biomedical-grade polyurethane – and various human tissues in the musculoskeletal system – skeletal muscle, tendon, ligament, bone, and skin – highlighting the difference in stretchability and strength.

Figure 5.

Young’s modulus of human muscle elements and elastic textile materials. The plot shows the average of the min–max value of the range described in Table 1.

Table 1.

Mechanical properties of elastic textile materials and human muscle elements (Butler et al., Reference Butler, Grood, Noyes, Zernicke and Brackett1984, Reference Butler, Kay and Stouffer1986; Collinsworth et al., Reference Collinsworth, Zhang, Kraus and Truskey2002; Devkota and Weinhold, Reference Devkota and Weinhold2003; Francois Francois, Reference Francois2008; Garrett et al., Reference Garrett, Nikolaou, Ribbeck, Glisson and Seaber1988; Johnson et al., Reference Johnson, Tramaglini, Levine, Ohno, Choi and L-Y. Woo1994a; Kuthe and Uddanwadiker, Reference Kuthe and Uddanwadiker2016; Mark, Reference Mark2006; Martin et al., Reference Martin, Burr, Sharkey and Fyhrie2015; Ní Annaidh et al., Reference Ní Annaidh, Bruyère, Destrade, Gilchrist and Otténio2012; Noyes and Grood, Reference Noyes and Grood1976; Takaza et al., Reference Takaza, Moerman, Gindre, Lyons and Simms2013; Vermette, Reference Vermette2001; Vincentelli and Grigoroy, Reference Vincentelli and Grigoroy1985; Vogel, Reference Vogel1987; Wood, Reference Wood1977; Xiao et al., Reference Xiao, Wang, Sun and Zheng2020)

Natural rubber and synthetic rubber display a high flexibility with Young’s modulus ranging 1–6 MPa and 0.3–20 MPa, respectively, while stretching significantly up to about 900% (Francois Francois, Reference Francois2008; Mark, Reference Mark2006; Wood, Reference Wood1977). Considering these properties, rubbers can be ideal for products requiring both high resilience and elasticity. Biomedical-grade PU, meanwhile, shows a higher Young’s modulus (25–62 MPa) and textile strength (29–49 MPa), and comparable substantial stretchability up to 800% (Francois Francois, Reference Francois2008; Vermette, Reference Vermette2001). Compared to the elastomers, the human tissues exhibit distinct differences. Skeletal muscle displays a much softer and more flexible nature with its much lower Young’s modulus (4.9–13.5 kPa) and tensile strength (234–440 kPa) (Collinsworth et al., Reference Collinsworth, Zhang, Kraus and Truskey2002; Kuthe and Uddanwadiker, Reference Kuthe and Uddanwadiker2016; Xiao et al., Reference Xiao, Wang, Sun and Zheng2020). It elongates up to 225%, which may be optimal and not necessarily extreme for body movements (Garrett et al., Reference Garrett, Nikolaou, Ribbeck, Glisson and Seaber1988; Takaza et al., Reference Takaza, Moerman, Gindre, Lyons and Simms2013). In contrast, the tendon, connecting a skeletal muscle to a bone, and ligament, binding bone to bone, are much stronger, with Young’s modulus values of 612–660 MPa and 65–111 MPa, respectively (Noyes and Grood, Reference Noyes and Grood1976; Butler et al., Reference Butler, Grood, Noyes, Zernicke and Brackett1984, Reference Butler, Kay and Stouffer1986; Johnson et al., Reference Johnson, Tramaglini, Levine, Ohno, Choi and L-Y. Woo1994a; Devkota and Weinhold, Reference Devkota and Weinhold2003). Their stretchability is also limited, up to 26% in the case of tendons and 60% for ligaments. The tendon and ligament’s properties reflect their role in joint stability while allowing some flexibility. Bone is the strongest and the most rigid among human tissues, displaying 10–17 GPa in Young’s modulus and a very low elongation capability (2–3%), showing its solid focus on the structural support (Vincentelli and Grigoroy, Reference Vincentelli and Grigoroy1985; Martin et al., Reference Martin, Burr, Sharkey and Fyhrie2015). The skin exhibits a relatively wide range of Young’s modulus of 15–150 MPa as well as tensile strength (5–32 MPa) and elongates up to 115% (Vogel, Reference Vogel1987; Ní Annaidh et al., Reference Ní Annaidh, Bruyère, Destrade, Gilchrist and Otténio2012). The skin covers the whole body, and its need to accommodate various needs, such as protection, movement, and growth, mirror the variability in the elastic properties.

The differences between elastic materials and human tissue shed light on the need for design considerations in wearable systems that mimic or complement the mechanical nature of the human body to provide necessary support without restricting biomechanical movement. Soft and wearable applications made of elastic textiles should enhance body performance while ensuring protection and comfort during demanding exercises, heavy load-bearing tasks, and repetitive bending. Balancing the rigidity to provide mechanical support and the flexibility not to hinder the joint movement is the key to design exoskeletons and resistance exercise outfits, which requires more research specialized in the target body area and motions on a deeper scale.

5.1. Upper limb

The passive exosuit has shown vast potential in relieving upper limb movements in diverse conditions. These assistive devices can help with tasks in daily activities as well as in industrial and clinical settings. The upper limb exosuits could reduce muscle effort and support arm movements, benefiting individuals with motor impairments and musculoskeletal disorders. The arm muscles are one of the weakest muscles in the body and can become fatigued more frequently than the other larger muscle groups, such as the back and lower limbs (Chen, Reference Chen2000). Localized fatigue in the upper arm muscles can cause lower back pain because the compensatory lifting posture may rely more on the back muscles, thereby increasing stress on the lower back. Therefore, assisting the arm muscles is recommended to prevent lower back injuries (Wan et al., Reference Wan, Ishioka, Kanda, Osawa, Kodama and Tanaka2022b). Reflecting the rapidly increasing needs for the upper limb assistance in various industrial sectors, such as manufacturing, construction, and shipping, a considerable amount of passive exoskeletons has been introduced both in the academia and the market (Moeller et al., Reference Moeller, Krell-Roesch, Woll and Stein2022; Ashta et al., Reference Ashta, Finco, Battini and Persona2023). However, most of them utilize rigid structures made of metal or plastic to employ a solid exoskeletal lever and/or torque system (Balser et al., Reference Balser, Desai, Ekizoglou and Bai2022; van Sluijs et al., Reference van Sluijs, Scholtysik, Brunner and Lambercy2024). Still, these rigid exoskeletons provided clues and opportunities to replace metal components, like springs and supports, with high elasticity and strength textiles.

Joshi et al. (Reference Joshi, Beck, Seth and Santina2022) developed a soft passive exosuit to assist the shoulder that primarily adopts elastic resistance on the back and an inextensible 2-mm-diameter cable reaching the elbow and crossing the shoulder (Figure 6a). The elastic resistance band, serving as a single exotendon in this suit, provides the elastic force, thereby reducing the shoulder load when the wearer raises their arm. In their experiments in shoulder flexion, the mainly contributing muscles, such as the deltoid and trapezius, showed significantly decreased muscle activity with the exosuit (Joshi et al., Reference Joshi, Beck, Seth and Santina2022). Liao et al. (Reference Liao, Ishioka and Mishima2020) placed the elastic band directly in place of the biceps to assist the load-bearing tasks (Figure 6b). The full-body suit, including the shirt and anchors under the knee and at the sole, is called e.z.UP^® evenly distributes the heavy loads across the entire body, reducing the strain on the arm and back muscles (Liao et al., Reference Liao, Ishioka and Mishima2020). This was an updated version of their previous assistive upper body suit, which could support the arm muscles but not the back (Liao et al., Reference Liao, Kodama, Ishioka, Lee and Tanaka2018). Abs-Suit was designed for parcel delivery workers to reduce the increasing musculoskeletal disorders in the arms and the lower back (Yoo et al., Reference Yoo, Lee and Cho2024). It has a strong elastic band on the lower back that is connected to the load-bearing strap hanging around the front chest (Figure 6c). While the strap through which the delivery worker can place the heavy load is inextensible, the connected elastic band on the lower back provides distribution of the load and is configurable for the overall performance (Yoo et al., Reference Yoo, Lee and Cho2024).

Figure 6.

Upper limb passive exosuits using elastic materials. (a) Elastic band located on the back assisting shoulder movements (Joshi et al., Reference Joshi, Beck, Seth and Santina2022). (b) Full body assisting suit based on the combinations of deformable/non-deformable textiles (Liao et al., Reference Liao, Ishioka and Mishima2020). (c) Vest with a loading strap (Yoo et al., Reference Yoo, Lee and Cho2024). (d) Assisting elbow extension for poststroke patients (Phan et al., Reference Phan, Nguyen, Vermillion and Lee2019). (e) Elastic band across the whole arm for arm movement assistance (Liao et al., Reference Liao, Ishioka and Mishima2020).

An assistive device named PEMA (portable elbow movement assistant) was developed for stroke survivors, as some of them experience excessive muscle tone causing resistance to extend the elbow (Phan et al., Reference Phan, Nguyen, Vermillion and Lee2019). The PEMA has elastic cords anchored on each side of the elbow to promote the elbow extension, as the elastic bands create recovering moments when the arm flexes (Figure 6d). The whole system mimics the musculoskeletal structure: while the elastic cords support the elbow extensor muscles, and the 3D-printed base frames, like a tendon, bind the cord to the arm. The PEMA significantly decreased the muscle activation of the triceps brachii, the main elbow extensor muscle, while making no changes in movement time, velocity, or biceps activation. In their another previous assistive exosuit, Liao et al. (Reference Liao, Ishioka and Mishima2020) also reported a suit design where the elastic bands covers the whole arm joints, including the shoulder, elbow, and wrist (Figure 6e). They only referred to this as Prototype-3 without further details in terms of the arm support, but it showed the possibility of creating an undershirt soft exosuit that fully provides supports for all the muscles in the arm (Liao et al., Reference Liao, Ishioka and Mishima2020).

5.2. Torso

The torso, the core of the human body, is one of the most actively explored for opportunities of assistance in musculoskeletal load bearing (Ashta et al., Reference Ashta, Finco, Battini and Persona2023). This trend originated from the fact that 41% of workers report backache complaints (De Kok et al., Reference De Kok, Vroonhof, Snijders and Isusi2019), and more than 370,000 incidents of back injuries resulted in missing workdays, according to the Bureau of Labor Statistics (Statistics, Reference Statistics2016). As a few occupations, including operators, fabricators, and laborers, account for more than half of back injury cases (NIOSH, 2004), the assistive solutions have been focused on the protection and support of the lumbar region of those performing heavy-lifting, load-carrying, and repetitive bending/twisting tasks.

A widely adopted style combines the vest or straps for the upper body and the shorts for the lower body to create anchors around the shoulders, the waist, and the thighs, respectively. This lightweight, low-profile design mostly includes a strong and thick elastane crossing the back to evenly distribute pressure across the body, providing substantial support for tasks such as leaning and lifting (Figure 7a). When the user leaned forward or lifted objects, these bands elongated, exerting tension forces that ran parallel to the lumbar extensor muscles and ligaments. This motion resulted in a moment of extension around the lumbar spine, which relieved the lower back muscles and decreased muscle activity during tasks (Lamers et al., Reference Lamers, Yang and Zelik2017, Reference Lamers, Soltys, Scherpereel, Yang and Zelik2020). Another version of the same idea aimed to provide biomechanical assistance to the solders’ back and hip, which alleviates tension on the muscles and intervertebral discs (Goršič et al., Reference Goršič, Song, Dai and Novak2021). This study showcased the effectiveness of an elastic, mode-switching back Exosuit in mitigating back pain and facilitating more straightforward lifting for soldiers during dynamic and realistic field operations (Slaughter et al., Reference Slaughter, Rodzak, Fine, Ice, Wolf and Zelik2023) (Figure 7b). The high-friction elastomer on the inside of the thigh sleeves, along with a durable fabric on the outside, provided a secure attachment point for the elastic bands in the garment. Variable-length elastic bands connecting the upper body section provide the lumbar extension torque (13–16 N/m at 30° of trunk flexion and 17–24 N/m at 60° of trunk flexion) to help reduce strain on the user’s back muscles during forward bending. The researchers concluded that the surgical procedures in the medical field that generate significant exposure to trunk flexion angles of 20° or greater would benefit from this Exosuit by reducing muscle fatigue and discomfort (Kang and Mirka, Reference Kang and Mirka2023).

Figure 7.

Exosuits assisting torso load management. (a) Low back load reducing suit (Lamers et al., Reference Lamers, Yang and Zelik2017). (b) Soldier Assistive Bionic Exosuit for Resupply (SABER) prototype for US Army (Slaughter et al., Reference Slaughter, Rodzak, Fine, Ice, Wolf and Zelik2023).

5.3. Lower limb

Lower limb capability is essential in human locomotion and is critically aligned with the individual’s quality of life. Those with Parkinson’s disease, developmental disorders, or those impacted by stroke often experience gait issues and need external help to move from one place to another. Anchoring strong elastic bands at the waist and the right above the knee is a popular tactic to assist hip flexion during the gait (Schmidt et al., Reference Schmidt, Duarte, Grimmer, Sancho-Puchades, Wei, Easthope and Riener2017; Panizzolo et al., Reference Panizzolo, Bolgiani, Di Liddo, Annese and Marcolin2019; Barazesh and Sharbafi, Reference Barazesh and Sharbafi2020; Yang et al., Reference Yang, Park, Kim, Park and Lee2021; Kowalczyk et al., Reference Kowalczyk, Mukherjee and Malcolm2023). The elastic bands perform like a flexor or extensor muscle depending on the location. For example, the hip joint extension will stretch the elastic back on the front thigh, creating an elastic potential force promoting the leg flex back (Figure 8a). The force generated by the elastic bands can be adjusted by modifying the preload, and the anchor straps around the waist and thigh remain stable without causing unnecessary discomfort (Panizzolo et al., Reference Panizzolo, Bolgiani, Di Liddo, Annese and Marcolin2019). These elastic passive exosuits have contributed to correcting asymmetrical gait patterns and reducing metabolic costs (Panizzolo et al., Reference Panizzolo, Bolgiani, Di Liddo, Annese and Marcolin2019; Yang et al., Reference Yang, Park, Kim, Park and Lee2021; Kowalczyk et al., Reference Kowalczyk, Mukherjee and Malcolm2023). A relatively unusual case would be a natural latex cord anchored at both sides of the shoes (Figure 8b). It creates moments whenever each leg flexes/extends, and the distance becomes longer than the cord’s relaxed lengths, eventually reducing energy consumption by 6.4% (Simpson et al., Reference Simpson, Welker, Uhlrich, Sketch, Jackson, Delp and Hawkes2019).

Figure 8.

Lower body passive exosuits. (a) Elastic band assisting hip flexion (Yang et al., Reference Yang, Park, Kim, Park and Lee2021). (b) Elastic cord connecting ankles to reduce energy cost (Simpson et al., Reference Simpson, Welker, Uhlrich, Sketch, Jackson, Delp and Hawkes2019). (c) Two elastic bands on a knee assist sit-to-stand (Lee et al., Reference Lee, Kim and Park2020). (d) Configurations of the elastic band on the lower body (Krishnan et al., Reference Krishnan, Adeeko, Washabaugh and Kalpakjian2024).

X-tights with two embedded elastic bands cross a front knee to assist sit-to-stand maneuver (Figure 8c) (Lee et al., Reference Lee, Kim and Park2020). They motion-captured the skin deformation during the transition between the sitting and standing posture and installed the elastic band where the skin stretches most to get the maximized elastic energy return. Meanwhile, the potential of the elastic component in place of the plantar-flexor to decrease the soleus activation has been reported (Farris et al., Reference Farris, Robertson and Sawicki2013), Krishnan et al. (Reference Krishnan, Adeeko, Washabaugh and Kalpakjian2024) examined different configurations to assist or resist selected muscle groups with the elastic bands, including ankle plantar flexion/dorsiflexion (Figure 8d Krishnan et al., Reference Krishnan, Adeeko, Washabaugh and Kalpakjian2024). In general, the elastic bands to assist hip extensors or ankle dorsiflexors decreased tibialis anterior activation, while the ones placed on the back showed varied results (Krishnan et al., Reference Krishnan, Adeeko, Washabaugh and Kalpakjian2024. It shows both the potential and limitation of the passive exosuit pants, as they decreased the overall muscle activation but could not accommodate all the opportunities to assist the metabolic cost at the individual phase of a gait.

Table 2 lists the applications introduced in this review based on the applications, materials, target regions, and effects. Decrease in muscle activation indicates that the muscle is exerting less effort to perform a particular activity while the exosuit is skillfully assisting the movement, thereby reducing the physical demands on the user, typically measured by EMG (electromyography).

Table 2.

List of elastic textile-based assistive musculoskeletal modulation applications

6.1. Upper limb

Resistance garments with embedded elastic bands can increase muscle use during exercise or daily activities. The elastic bands often flex the upper limbs so that the wearer activates the muscles to extend the arms or the other way (Figure 9a). A passive fitness wearable device with embedded elastic bands on the arms allows the wearer to simulate upper-body workouts, such as chest presses and frontal raises (Park et al., Reference Park, Kim, Kim and Lee2022). The elastic band connected to the glove extends up through the elbow and upper arm and is attached to a cable that adjusts the strength of the resistance force (Figure 9b). A similar and popular design to add elastic resistance is to connect the multiple resistance bands directly between the gloves and the vest with straps (Cranke, Reference Cranke2015; Felkel, Reference Felkel2015; Grove, Reference Grove2021) (Figure 9c). The resistance bands were also placed between a pair of armbands and the side of the waistband (Cornish, Reference Cornish2017; Deinlein, Reference Deinlein2018), wrist cuffs and shoes (Matthews, Reference Matthews2015), or arm-to-arm (Bergman et al., Reference Bergman, Mclntyre, Lavallée and Laberge2015; Alaniz, Reference Alaniz2021). To prevent upper limb muscle atrophy during spaceflight, stretchy sleeves were designed for a neural posture in microgravity and anchored to the hands added at least 3 N to achieve full arm extension (Yilmaz and Goncu-Berk, Reference Yilmaz and Goncu-Berk2022).

Figure 9.

Upper limb resistance clothing. (a) Resistance elbow sleeve (“AGOGIE – Resistance Training Pants and Resistance Band Leggings”, n.d.). (b) Elastic band to increase the resistance to triceps (Park et al., Reference Park, Kim, Kim and Lee2022). (c) Multiple elastic cords connecting the gloves and the vest (Cranke, Reference Cranke2015). (d) SaeboGlove promoting the muscle growth for poststroke patients (“Hand Therapy Rehabilitation Glove for Stroke | SaeboGlove”, n.d.).

Most designs are reported as patents, so a limited number of studies evaluate the feasibility and effectiveness of resistance garments for upper limbs. The wearable fitness system with elastic band and cable, although it may not generate loads as high as gym equipment, resulted in a 1.5 times higher perceived exertion and twice the muscle activation compared to the bare body condition (Park et al., Reference Park, Kim, Kim and Lee2022). Similarly, a wearable resistance band harness increased total muscle thickness by 11% when the wearer was fully extended in the chest press position (Panizzolo et al., Reference Panizzolo, Bolgiani, Di Liddo, Annese and Marcolin2019). A resistance suit with two resistance bands attached at the elbow and the other side of the waist induced higher heart rates during push-ups and considerable discomfort in terms of tightness, scoring 2.83 out of 5 (Hansen et al., Reference Hansen, Youngblood and Johnson2021). Hand orthosis with an embedded spring or tension cord to assist hand dexterity after stroke (Figure 9d) significantly enhanced grip strength (Barry et al., Reference Barry, Saabye, Baker, Fogarty and Beck2006), which virtual reality applications can facilitate further (Adams et al., Reference Adams, Ellington, Armstead, Sheffield, Patrie and Diamond2019). Despite the limited assessments of the wearable systems, general elastic band-based resistance training protocols consistently showed a positive effect on muscle strength and athletic performance at similar levels to conventional methods (Lopes et al., Reference Lopes, Machado, Micheletti, de Almeida, Cavina and Pastre2019; Bauer et al., Reference Bauer, Schwiertz and Muehlbauer2021). It is expected that the elastic bands incorporated into the garment system may display such an effect on the upper limbs.

Meanwhile, compression garments or sleeves without intended increased joint loading but their own stretchy textile did not significantly affect the physical performance of the upper extremities in general (Phan et al., Reference Phan, Nguyen, Vermillion and Lee2019). However, when strategically designed, compression sleeves for the upper extremity demonstrated the ability to produce a torsional force on the forearm for hours. Supination, pronation, or no rotation was induced by the targeted design of the Lycra^® compression sleeves (Gracies et al., Reference Gracies, Fitzpatrick, Wilson, Burke and Gandevia1997). A study in which a compression splint immediately improved arm movements in supination and pronation in children with cerebral palsy also supports the idea of creating a load through elastic textiles (Elliott et al., Reference Elliott, Reid, Alderson and Elliott2011).

6.2. Torso

Although passive exosuits with back supports aim to reduce lumbar load to increase work efficiency and decrease fatigue, resistance clothing seeks to enhance the body by adding loads that activate muscles or realign the spine. Prolonged missions in space have often caused spinal elongation in astronauts because of the low-gravity environment, which even increased in height (Zannat et al., Reference Zannat, Uddin, Mahmud, Prithu and Mia2023). Gravity-loading countermeasure suit was developed to add the axial load to the human body as much as on Earth (Waldie and Newman, Reference Waldie and Newman2011) (Figure 10a). This full-body suit is constructed of stretch/non-stretch weave or bungee cords to simulate upright loading across the body (Attias et al., Reference Attias, Philip, Waldie, Russomano, Simon and David2017). It was effective at creating stress on the torso and spine, but it also caused significant discomfort at a level that the participants could not wear all day (Waldie and Newman, Reference Waldie and Newman2011; Breen et al., Reference Breen, Carvil, Green, Russomano and Breen2023).

Figure 10.

Elastic suits increasing the load on the spine. (a) Gravity-loading countermeasure suit for astronauts (Waldie and Newman, Reference Waldie and Newman2011). (b) Elastic head support for those with amyotrophic lateral sclerosis (Hansen et al., Reference Hansen, Bedore, Nickel and Goldish2014).

Designed to align joints and strengthen muscle groups, especially for children with cerebral palsy, dynamic orthotic garments were inspired by astronauts’ resistance suits, because the motor pathology displayed a link to the negative effects of anti-gravity (Semenova, Reference Semenova1997). Rigid orthoses may provide better correction of spinal deformity and stability under gravity, but they have clear disadvantages in terms of discomfort and appearance, resulting in low patient acceptance (Wong et al., Reference Wong, Cheng, Lam and Tam2008). As in the microgravity system, dynamic orthotic suits made of elastic textiles apply loads to weak musculature and have a positive effect not only on facilitating muscle movements but also on balance (Semenova, Reference Semenova1997; Almeida et al., Reference Almeida, Fonseca, Figueiredo, Aquino and Mancini2017). Suit therapy using dynamic orthosis like TheraTogs (TheraTogs Inc., Telluride, CO), in conjunction with proper exercise programs, reinforced the action of weak spinal muscles and stretched the tight tissues around the spine, thereby reducing kyphosis and improving range of motion in the thoracic area (El-Kafy and El-Shamy, Reference El-Kafy and El-Shamy2022). Meanwhile, for those with weak neck extensors and difficulty controlling head drop, an elastic band between the back of the cap and the waistband or chest band can provide tension to support the head (Fast and Thomas, Reference Fast and Thomas2008; Hansen et al., Reference Hansen, Bedore, Nickel and Goldish2014) (Figure 10b).

6.3. Lower limb

Improving lower body mobility has been a major interest in resistance apparel. Elastic bands or cords often cross the legs and add loads during lower body movements to improve exercise quality and prevent muscle loss. A straightforward and popular design is to install elastic bands from the foot or ankle to the waist or torso so that the lower body bears the additional returning force of the elastic when the leg is fully extended or flexed (Benocci et al., Reference Benocci, Pezzuolo, Piga and Vendali2015; Cranke, Reference Cranke2019; Allen et al., Reference Allen, Keith and Haskew2022) (Figure 11a). The Russian Pingvin (Penguin) suit included two bungee cords connecting the foot and waist/shin, respectively (Figure 11b). It stresses the lower body musculoskeletal system, up to 70% of body weight on the treadmill, thus preserving calf muscle and reducing lumbar bone loss (Artiles et al., Reference Artiles, Trigg, Jethani, Tritchler and Newman2016; Carvil et al., Reference Carvil, Attias, Evetts, Waldie and Green2017). Another gravity-loading countermeasure suit also showed improved exercise performance in terms of blood lactate levels, initial ventilatory response, and perceived workload compared to regular gym clothing. (Attias et al., Reference Attias, Philip, Waldie, Russomano, Simon and David2017). These effects of tension along the longitudinal axis of the lower body on exercise are confirmed in training sessions where lunges with an elastic tube looped from the foot to the shoulder induced high levels of hip, knee, and back muscle activity (Sundstrup et al., Reference Sundstrup, Jakobsen, Andersen and Andersen2014).

Figure 11.

Resistance clothing for lower limbs. (a) Elastic band-embedded leggings (“AGOGIE – Resistance Training Pants and Resistance Band Leggings” n.d.). (b) Russian Pingvin gravity suit increasing the muscle load for astronauts (Artiles et al., Reference Artiles, Trigg, Jethani, Tritchler and Newman2016). (c) Elastic bands connecting the legs (Hansen et al., Reference Hansen, Youngblood and Johnson2021). (d) foot-ankle orthoses further activating ankle muscles (Guillebastre et al., Reference Guillebastre, Calmels and Rougier2009).

A pair of elastic bands sometimes cross each other and connect one side of the thigh or pelvis to the other to increase loads during unilateral leg movements or to encourage postures that activate target muscles (Mial, Reference Mial2015; Cornish, Reference Cornish2017; Stricker, Reference Stricker2020; Hansen et al., Reference Hansen, Youngblood and Johnson2021) (Figure 11c). The effect of such designs on exercise performance has been reported in exercise studies using an elastic tube around the thigh, where using elastic bands improved the muscle activity of the hip external rotator during squats (Forman et al., Reference Forman, Alizadeh, Button and Holmes2023). Another study demonstrated that a 4-week training program using elastic tubing around the thigh not only significantly improved hip abduction force, but also the effect was also comparable to that of heavy resistance isotonic exercise (Folkins et al., Reference Folkins, Sahni, Ryan, Wooden, Bushby and Radzinski2021). Wearable tension across the legs also contributed to correcting squat posture. While the light resistance elastic band of 1.5 N/cm did decrease knee width index at various portions of countermovement jumps, reductions in knee width index occurred more frequently with the medium resistance band of 2.0 N/cm (Gooyers et al., Reference Gooyers, Beach, Frost and Callaghan2012). Comparable results have been observed in research using a single resistance-level elastic band. Theraband^®’s lightest band, 1.7 kg of resistance at 100% elongation, improved muscular activity in several lower limb muscles while maintaining an unaltered knee width index between conditions (Foley et al., Reference Foley, Bulbrook, Button and Holmes2017).

Elastic dynamic orthoses worn on the thigh, ankle, or foot have shown positive effects on the activation of relevant musculature in treating gait abnormalities and postural instability. TheraTogs, a dynamic orthosis, facilitated hip extensor and abductor muscle activity in an adult patient undergoing rehabilitation by applying pressure through elastic straps connecting the upper body and thigh (Maguire et al., Reference Maguire, Sieben, Scheidhauer, Romkes, Suica and de Bie2016). Meanwhile, ankle–foot orthoses with an elastic band tensioned between the shank and foot (Figure 11d) improved gait abnormalities by increasing muscle activity, gait speed, and stride length, and alleviating foot equinus (Boudarham et al., Reference Boudarham, Pradon, Roche, Bensmail and Zory2014). Another study showed that this soft and dynamic orthosis can control plantar flexion without negatively affecting gait performance, unlike rigid orthoses that disturb gait velocity and step length (Guillebastre et al., Reference Guillebastre, Calmels and Rougier2009). The pressure applied by elastic ankle–foot orthoses also improved postural control by equalizing the weight bearing of the paretic and nonparetic limbs by activating muscle groups around the ankle (Kim et al., Reference Kim, Sim and Won2015).

Table 3 lists the applications introduced in this review based on resistive devices, their materials, target regions, and effects. Muscle activation increases by the resistive devices indicate that the exosuits offer resistance, necessitating more exertion from the wearer.

Table 3.

List of elastic textile-based resistive musculoskeletal modulation applications