Mixed-Model Assembly Lines and Their Effect on Worker Posture and Recovery Time
Authors: Carrasquillo Vernnaliz , Armstrong Thomas J. , Hu S. Jack 
Published: September 2017

Aim: The aim of this study is to describe the relationship between cycle-to-cycle task variations in mixed-model assembly lines and workers’ exposure to poor posture and insufficient recovery time and provide guidelines for mixed-model assembly line (MMAL) design. Background: In the US, 17.6 out of 100,000 manufacturing workers suffer musculoskeletal injuries while conducting tasks in assembly lines which may be paced (e.g. continuously moving) or self-paced (e.g. asynchronous indexing). MMALs produce multiple products in the same line. Pacing must be carefully considered to achieve productivity targets while providing workers sufficient time to complete the tasks and recover. The effect of pacing on worker fatigue and exertions is not well understood. In MMALs, the complexity of completing multiple work methods may increase exposure to these risk factors. These concerns also affect hospital kitchens. While kitchen workers don’t have standardized work cycles, the work tasks (e.g. get, move, put) resemble the cyclic nature of manufacturing plants. Studies have been conducted to describe the effect of cycle-to-cycle task variations on worker posture and recovery time in MMALs (Carrasquillo, Armstrong, and Hu 2011, 2016, 2017). Methods: Three studies were conducted: (1) field observation of continuous moving assembly line - hospital kitchen, continuous moving conveyor; (2) conveyor type, product mix/sequence laboratory experiment - simulated assembly task conducted under 9 conveyor and product mix/sequence configurations; (3) field observation of on demand, self-paced assembly line with varying demand - hospital kitchen where patients placed orders at any time. Workers were video recorded and worker posture and recovery time were assessed. Posture tracking was used in laboratory study (2). Results: The self-paced assembly lines (asynchronous indexing, and manual), provided workers control over their work pace. They reached within the reach envelop more frequently than in the paced, continuous moving conveyor (43% vs. 86% –99%). The workers on the self-paced assembly line had the greatest average recovery time (31.8%), the worker on the continuous moving conveyor had 9.5% and the subjects on the asynchronous indexing conveyor had 0.7% recovery time. Discussion and conclusions: In study (1) increases in variations increased the frequency of workers reaching outside the reach envelope and reduced recovery time. In Study (2), the self-paced, asynchronous indexing mixed-model assembly line was the most effective configuration in reducing the frequency of reaching unnecessarily to complete the job. However, subjects didn’t reduce the pace to recover; instead, they worked continuously. The workers’ pace in the Study (3) assembly line was constrained by the process time. In it, workers, chose to reach outside the reach envelop 14% of the time; less frequently than in Study (1). The process time provided time for recovery. It exceeded 10%, more than in any of the other assembly line configurations. These findings demonstrate that as work variations increase, so does the frequency of reaching while the recovery time is. Self-pacing reduced but did not eliminate awkward postures. Participants reached ahead and completed the assigned tasks before the work objects were within the reach envelope. Ensuring that a minimum work time is maintained for each product type is essential to providing workers with sufficient time for recovery.