Upper body kinematic and low-back kinetic responses to precision placement challenges and cognitive distractions during repetitive lifting

Abstract

Exposure to cognitive distractions and precision placement challenges during manual lifting exertions may alter lumbar spine loading by influencing movement and/or motor patterns. In this study, the impact of such demands on upper body kinematics, trunk muscle activation, and lumbar spine loading during a repetitive lifting task was examined. Nine male volunteers performed four different sagittal plane lifting tasks in which a 15.9 kg mass was lifted repetitively at a fixed rate of 7.5 lifts/min for a total duration of 30 min (216 lifts/task). The physical dimensions of the experimental setup remained constant between tasks. Tasks differed based on the presence or absence of simultaneous cognitive distractions and/or precision placement challenges. Upper body kinematics and trunk muscle activation levels were monitored, and a two-dimensional dynamic biomechanical model was used to estimate L4/L5 joint loads. Reductions in elbow flexion (14.3%), elbow extension (16.6%), shoulder flexion (11.7%), and shoulder extension (10.2%) velocities were observed when precision placement challenges were imposed during lifting, and mean lift times increased by 27% as a result. This led to greater cumulative L4/L5 moments (23%), joint compression (15%), reaction shear (17%), and joint shear (10%) forces when precision placement constraints were imposed. Performing a cognitive distraction challenge while lifting had no impact on cumulative L4/L5 joint loads. The magnitude of cumulative L4/L5 loads was highly dependent on the preferred time taken to perform single-lift exertions, as there were no differences in peak L4/L5 loads between the tasks examined. This was further supported in that very few biomechanically significant changes were observed in trunk muscle activation levels or three-dimensional trunk kinematics as a function of the tasks performed. The results suggest that upper limb kinematic adaptations to precision placement constraints in repetitive lifting may alter the risk of reporting low back pain. Relevance to Industry The results of this study indicate that often overlooked task demands, particularly precision placement constraints, may modify the level of risk associated with the performance of repetitive manual lifting tasks. There was no evidence to suggest that exposure to simultaneous cognitive distractions while lifting would modify the level of risk under the conditions examined. Accordingly, these findings may be relevant in the design and biomechanical risk assessment of industrial lifting tasks.