Effect of customer demand in a self-paced mass customization assembly line on worker posture and recovery time
Authors: Carrasquillo Vernnaliz , Armstrong Thomas J. , Hu S. Jack 
Published: September 2016

Aims: This study aimed to evaluate worker posture, hand activity level (HAL), and recovery time on workers of a hospital kitchen with an asynchronous indexing assembly line during periods of different customer demand. Background: Workers in hospital kitchens use their hands to prepare and transport food around the kitchen. Unsteady customer demand leads to times when workers are continuously busy with little recovery. These working conditions result in high joint loads and insufficient recovery, causes of work-related musculoskeletal disorders (WMSDs) (NCRC, 2001). Injuries caused by repetitive motion and overexertion are the greatest source of reported injuries (Bureau of Labor and Statistics, 2014). In spite of this, studies have not examined the effect of workplace design, production volume variations, and product complexity and their effect on workers’ exposure to these risk factors. A study at a hospital kitchen mixed-model continuous moving assembly line found that the frequency of reaching increased and the recovery time decreased as variations increased (Carrasquillo, Armstrong, and Hu, 2011). Methods: An observation study was conducted at a hospital kitchen with 2 principal areas: food preparation and tray assembly. High and low demand periods were identified from historical data. Workers were video recorded during each demand period. Videos were used to calculate frequency of reaching and recovery time, and to rate the hand activity level (HAL). Reaches were categorized as: (1) Lower arm, (2) Upper arm, (3) Torso twist/bent. The upper arm and torso twist/bent reaches were further categorized into necessary and unnecessary. Results: HAL – Overall, it increased by 1.2 (from 5 to 6.2) as the demand increased (p < 0.05). Reach - The proportions of unnecessary upper arm and torso reaches were not significantly affected by changes in demand (p > 0.05). Recovery time – Increase in customer demand resulted in a decrease in the proportion of recovery time (p < 0.05). Overall, recovery decreased from 40% to 22%. Discussion and conclusion: Results show that HAL increased as the customer demand increased except for the worker on tray station 2, who had no significant difference between the two periods. This worker was required to wait for meals to be cooked before her job could be completed. This resulted in a greater amount of recovery (37% - 53%). In contrast, the proportion of unnecessary reaches did not change significantly with demand. However, the upper arm and the torso to bend/twist reaches represented more than half of the reaches. While many of them were necessary, >20% were unnecessary. In this assembly line, workers were in control of the pace and recovery time was available more than 10% of the time, factors that reduce workers’ exposure to risk of WMSDs. Although previous studies have found self-paced work increased risk due to decreased recovery (Dempsey, Mathiassen, Jackson, and O’Brian, 2010), this work demonstrated that it might not be the sole factor affecting risk. When products and production time vary and workers must wait for process (cooking), self-pacing appears to provide workers with some control over the frequency of unnecessarily reaching and recovery time.