Any injury of the soft tissues (muscles, tendons, ligaments, joints, and cartilage) and nervous system could be classified as a musculoskeletal disorder or MSD (OSHA, 2000). These injuries, which often damage upper limbs/extremities (arms, from fingers to shoulder, and neck), lower limbs/extremities (legs from hips to toes) and back (http://www.hse.gov.uk/msd/index.htm), could cause numbness, stiff joints, or muscle loss. More specifically, MSDs include “cases in which the nature of the injury or illness is pinched nerve; herniated disc; meniscus tear; sprains, strains, tears; hernia (traumatic and nontraumatic); pain, swelling, and numbness; carpal or tarsal tunnel syndrome; Raynaud's syndrome or phenomenon; musculoskeletal system and connective tissue diseases and disorders, and when the event or exposure leading to the injury or illness is overexertion and bodily reaction, unspecified; overexertion involving outside sources; repetitive motion involving microtasks; other and multiple exertions or bodily reactions; and rubbed, abraded, or jarred by vibration” (https://www.bls.gov/news.release/osh2.t06.htm).

The Bureau of Labor Statistics has reported that in 2015 MSDs accounted for 31 percent of all cases of nonfatal occupational injuries and illnesses requiring days away from work (https://www.bls.gov/news.release/osh2.nr0.htm). The incidence rate of MSDs that year in the construction industry was 34.6 per every 10,000 full-time workers (https://www.bls.gov/news.release/osh2.t01.htm). Data from National Safety Council (NSC) also show that MSDs are as twice likely as a combination of amputations, fractures, bruises, contusions, cuts, lacerations, burns, and chemical burns to occur in workers’ compensation claims. The National Institute for Occupational Safety and Health (NIOSH) has named construction tradespeople and construction industry among the highest-risk workers and industries for MSDs.

Ignoring ergonomics principles could expose workers to physical stressors, including vibration, awkward postures, repetitive motions, etc., which could lead to serious MSDs such as carpal tunnel syndrome (CTS). While various risk factors could contribute to MSDs, this study has focused on one specific condition: awkward postures in overhead tasks. Among all body parts, the upper extremities were involved in the largest number of non-fatal occupational incidents (32.4 per 10,000 full-time workers) in 2015 (https://www.bls.gov/news.release/osh2.t06.htm). Bernard et al. (1997), in a report for the Department of Health and Human Services, identified a strong association between MSDs and shoulder postures (with greater than 60 degrees of flexion or abduction), especially when the postures are combined with other physical factors such as “holding a tool while working overhead” (Bernard et al. 1997, page 3-1). Many construction workers work under these conditions, as several construction tasks such as welding, installing lights, etc., require employees to work with their hands stretched out at chest height or overhead. These awkward postures could put significant strain on the arms and shoulders, resulting in the need for more breaks from work (reduced productivity) and more injuries. Furthermore, awkward postures combined with holding construction tools in a stable position would localize the stresses to arms and shoulders. For instance, Herberts and Kadefors (1976) clinically examined ten welders between the ages of 50 to 65 who were experiencing pain around their shoulders. The authors concluded that heavy static loading of overhead postures on the supraspinatus muscle, along with the constant traction in tendons, could contribute to the degeneration of the cuff and shoulder injuries.

Ergonomic tools are designed to fit a worker’s body to reduce physical stress and eliminate serious MSDs. To address the risks of MSDs during overhead tasks, Engelhoven et al. (2018) introduced three main design criteria for upper limb exoskeletons:
•  During elevated postures, the exoskeleton should support shoulders. However, during neutral postures this support should be removed;
•  The devices should not limit a worker’s shoulder/spine movements (two degrees of freedom at the spine and three degrees of freedom at the shoulder);
•  The device should not interfere with a worker’s performance.

The first criterion is also known as optimized support: when the arms are elevated at more than 20 degrees, most of the forces will be transferred to the hips; during resting postures, removal of support will allow workers to move freely (Engelhoven et al. 2018). Moreover, to reduce fatigue, the arms on some models lock when workers are completing overhead tasks (http://worksaversystems.com/2016/11/09/exoskeleton-technology/).

Three recent studies have quantified the effects of upper limb exoskeletons on different shoulder muscles. First, Sylla et al. (2014) examined the performance of an upper limb exoskeleton in a screwing task in the automotive industry. The results revealed that exoskeletons could significantly reduce the mechanical energy (i.e., joint torques) on the wearer’s shoulders. However, the joint angle trajectories were not significantly different with or without the device, and the task took more time with the exoskeleton. Next, Gillette and Stephenson (2017) assessed the safety performance of exoskeletons by collecting muscle contractions using EMG signals. The results showed that the most benefit was provided to two areas: anterior deltoid and biceps brachii. Lastly, Engelhoven et al. (2018) acknowledged that the use of exoskeletons reduces muscle activity, suggesting that these devices can change how different industries deal with overhead work.

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