Most occupational tasks involve some level of mental/cognitive processing in addition to physical work; however, the etiology of work-related musculoskeletal disorders (WMSDs) due to these demands remains unclear. The aim of this study was to quantify the interactive effects of physical and mental workload on muscle endurance, fatigue, and recovery during intermittent work. Twelve participants, balanced by gender, performed intermittent static shoulder abductions to exhaustion at 15, 35, and 55% of individual maximal voluntary contraction (MVC), in the absence (control) and presence (concurrent) of a mental arithmetic task. Changes in muscular capacity were determined using endurance time, strength decline, electromyographic (EMG) fatigue indicators, muscle oxygenation, and heart rate measures. Muscular recovery was quantified through changes in strength and physiological responses. Mental workload was associated with shorter endurance times, specifically at 35% MVC, and greater strength decline. EMG and oxygenation measures showed similar changes during fatigue manifestation during concurrent conditions compared to the control, despite shorter endurance times. Moreover, decreased heart rate variability during concurrent demand conditions indicated increased mental stress. Although strength recovery was not influenced by mental workload, a slower heart rate recovery was observed after concurrent demand conditions. The findings from this study provide fundamental evidence that physical capacity (fatigability and recovery) is adversely affected by mental workload. Thus, it is critical to determine or evaluate occupational demands based on modified muscular capacity (due to mental workload) to reduce risk of WMSD development.

Overhead work is an important risk factor for upper extremity (UE) musculoskeletal disorders. We examined the potential of a mechanical arm and an exoskeletal vest as a wearable assistive device (WADE) for overhead work. Twelve participants completed 10 minutes of simulated, intermittent overhead work, using each of three payloads (1.1, 3.4 and 8.1 kg) and with/without the WADE. Ratings of perceived discomfort (RPDs) and electromyography (EMG) were obtained for the upper arms, shoulders and low back. Using the WADE, UE RPDs decreased by ∼50% with the heavier payloads, whereas smaller (∼25%) and non-significant increases in low-back RPDs were found and were relatively independent of payload. Changes in RPDs with WADE use were consistent with physical demands indicated by EMG, though EMG-based differences in fatigue were less apparent. Participants generally preferred using the WADE, particularly with heavier payloads. These results supported the potential utility of a WADE as an intervention for overhead work.

Inertial motion sensors (IMSs) combine three sensors to produce a reportedly stable and accurate orientation estimate in three dimensions. Although accuracy has been reported within the range of 2 deg of error by manufacturers, the sensors are rarely tested in the challenging motion present in human motion. Their accuracy was tested in static, quasistatic, and dynamic situations against gold-standard Vicon camera data. It was found that static and quasistatic rms error was even less than manufacturers' technical specifications. Quasistatic rms error was minimal at 0.3 deg (+/-0.15 deg SD) on the roll axis, 0.29 deg (+/-0.20 deg SD) on the pitch axis, and 0.73 deg (+/-0.81 deg SD) on the yaw axis. The dynamic rms error was between 1.9 deg and 3.5 deg on the main axes of motion but it increased considerably on off-axis during planar pendulum motion. Complex arm motion in the forward reaching plane proved to be a greater challenge for the sensors to track but results are arguably better than previously reported studies considering the large range of motion used.

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The purpose of this study was to investigate the effectiveness and user acceptability of a Personal Lift-Assist Device (PLAD) at an automotive manufacturing facility, with operators who perform an on-line assembly process requiring forward bending and static holding. Surface EMG data were collected at six sites on the low back and abdomen, and an accelerometer was used to measure trunk inclination. Use of the PLAD significantly reduced the thoracic and lumbar erector spinae activity and EMG-predicted compression at the 10th, 50th, and 90th APDF percentile levels (p < or = 0.05), without significantly increasing rectus abdominus activity or trunk flexion. Similarly, ratings of perceived exertion were found to be significantly lower when wearing the PLAD (p = 0.006). Subjective opinions were positive, with 8/10 subjects indicating they would wear the device everyday. With slight changes, workers felt that the PLAD could be beneficial at reducing forces and discomfort in similar industrial or manual materials handling tasks that place excessive physical demands on the low back.