Abstract
Introduction
Physical work tasks are rarely completed independent of mental demands like attentional requirements or time pressures which can potentially affect worker performance and job tactics. Some studies have examined the effects of mental demands on material handling tasks similar to those found in occupational settings (Srinivasan et al., 2016; Davis et al., 2002). In this study, a mental arithmetic task was completed simultaneously with a lifting task to varying placement destination heights. The muscle activity was compared to the same lifting tasks without the cognitive load using a within-subject experimental design. The hypothesis was that the addition of a cognitive load to the lifting task would result in increased muscle activity.
Methods
Eleven volunteers (10 males, 1 female; age range 18-27 yrs.) were enrolled in the study. Participants were paid a base incentive for participation and a bonus based on dual-task condition performance. Participants were tasked with completing a simulated distribution center replenishment task by lifting a 5 kg box in front of them and rotating 90° counter-clockwise to place the box in a simulated flow rack at various placement heights (low, middle, high) specified by the researcher prior to the lift. In addition to this placement task, the cognitive load conditions had participants verbally answer an arithmetic task while performing the lifting task (e.g., Determine the time to get to 4:10 from 3:27) and had to answer between the initial lift of the box and before its placement on the rack. Participants were encouraged to complete trials as quickly and accurately as possible using an incentive system. Electromyographic (EMG) signals from ten trunk and shoulder muscles were compared across the conditions for each height. Muscle activity levels were normalized as percentage of maximum voluntary contractions (%MVC) with cumulative activity and 90th percentile values extracted and analyzed using within subject statistical procedures.
Results
Cognitive workload conditions resulted in longer average task durations (x̄=3.02 seconds SE=0.096), compared to the no cognitive load conditions (x̄ =2.00 seconds SE=0.063). Significant cognitive load effects were observed for the left and right anterior deltoids, erector spinae, latissimus dorsi, and external obliques but not for the rectus abdominus muscles. The no cognitive load condition resulted in significantly larger 90th percentile values compared to the cognitive load conditions. This effect was observed across all destination height conditions. Overall, a 2.7% to 7.6% decrease in 90th percentile %MVC values was observed in muscles that significantly differed due to the added cognitive load. A significant cognitive load effect was also observed for the cumulative activity of the left and right erector spinae and the right rectus abdominus muscles. With the addition of the cognitive load, cumulative muscle activity significantly increased between 29% to 40% for these muscles. In the right erector spinae, a significant effect was observed for the low and middle heights but not the high height. For the other muscles, the cognitive workload effect was significant for each height.
Discussion
Adding a cognitive task to a physical task was observed to significantly change the muscle activity for the measured muscles. The 90th percentile values of nearly all measured trunk and shoulder muscles decreased with the added complexity while the core trunk muscles (erector spinae and right abdominal muscle) showed increased cumulative activity. Participants were likely dividing and switching their attentional and processing resources between the physical and mental task which resulted in slower movements with reduced peak activity values and longer task duration. This increase in task duration explains the larger cumulative activity observed due to the longer time under mechanical load. Joseph et al.’s study (2014) observed similar effects of precision and cognitive load on upper extremity joint reaction forces, moments, and muscle forces observing up to 18% smaller maximal forces and moments and 43% larger cumulative forces and moments compared to the control conditions. They similarly identified that the longer lift times lead to larger cumulative forces and lower peak forces. Other studies have also identified reduced peak muscle activity due to the addition of a cognitive task similar to what was seen in the current study (MacDonell and Keir, 2005; Au and Keir, 2007).
This study investigated the effects of a cognitively demanding task during a manual material handling task. The results showed that as cognitive load increased, participants exhibited slower movement and lower peak muscle activity, but greater cumulative muscle activity in selected trunk muscles. Greater cumulative activity could contribute to increased muscular fatigue with repeated exposure. The study adds to the existing research on dual-tasking, particularly for occupationally-relevant work, and provides insight into the complex interplay between cognitive and physical demands in the workplace.