The Effect of Varying Work Order Sequences on Physiological Responses in Combined Manual Material Handling

Abstract

Despite technological advances in the workplace that reduces the physical requirements of the worker, musculoskeletal disorders (MSDs) continue to be a concern. This is particularly the case in the manual material handling (MMH) industry. Although a considerable amount of effort has been made to address injury occurrences in MMH, such as the development of guidelines for a vast range of physiological, psychophysical, and biomechanical responses, these efforts fall short of understanding and encompassing all the complex characteristics of real world MMH jobs. One such characteristic is the variation in work intensity while performing combined manual material handling (CMMH) activities. This is often seen in occupations such as warehouse distribution order picking where loads of drastically different weights must be handled in a stochastic pattern. Research has shown that fatigue, both whole-body and localized muscle fatigue, is an important factor that should be considered in the effort to minimize the occurrence of MSDs. However, little is known about the impact workload variation has on the physiological responses that have been used as basis of designing work to control fatigue and injuries; responses such as heart rate, heart rate kinetics, RPE, and energy expenditure. It is also unclear how this variation affects localized trunk muscle fatigue development and activity; a relatively new but increasing research area for lower intensity repetitive MMH work for its noted relationship to the potential development of back MSDs. Therefore, more understanding of these responses in this type of MMH work was warranted. The purpose of this body of research contained herein was to initiate the discussion on and begin investigations of these responses that account for this type of operation. Furthermore, in an effort to mimic real world situations as much as possible, the research was designed to further encompass combined manual material handling (CMMH); another significant aspect of MMH that is limited in terms of research focus. Thus, this work expanded on that research area as well. The first study sought to determine if initiating and altering the intensity of a previous (baseline) workload in a simulated order picking task had an effect on the various physiological responses of a subsequent workload. It also sought to determine if these responses were affected if the order of the workload sequence was re-arranged. The results of the study suggested that a previous workload of very low to moderate intensity would have no significant impact on heart rate related responses of a subsequent load. The results also suggested that reordering the sequence order of very low to moderate intensity workloads does not significantly impact the overall heart rate related responses of the task. Because it was speculated that the insignificant differences found in the first study was due to the relatively low level of work intensities evaluated, a second study was conducted to evaluate the impact workload sequence order had on the physiological responses in a higher intensity simulated order picking scenario that included lifting and lowering from low levels and higher weights. An additional objective was to determine the applicability of traditional energy prediction models in MMH work involving varying workloads. The results of this study suggested that in higher intensity work, sequence order does affect the heart rate related responses of the work sequences as well as the whole-body perceived exertion of the individual workloads within the respective sequences. The conflicting magnitude of the results, however, suggested that the response could be dependent upon specific dose-response characteristics. The results of the study furthermore suggested that sequence order need not be considered when energy expenditure prediction models are utilized. The final study was conducted to examine the effects varying the workload sequence order had on the static and dynamic fatigue development and muscle activity in localized trunk muscles. The surface EMG and rated perceived exertion results of the study suggested that muscle activity of select trunk muscles reacted significantly different in the various sequence orders but muscle fatigue, either static or dynamic, was not found to be affected. This suggested that muscle activity may be a more important factor when evaluating work ordering. The overall conclusion of this body of research was that work variation in the form of workload sequence order in MMH work does have an impact on the various physiological responses. Research on additional MMH work variation characteristics (e.g., varied workload picking durations or pick locations) should be considered to further enhance the understanding of this type of work to possibly aid in developing work ordering optimization techniques and design guidelines to minimize worker fatigue and thus the potential occurrence of MSDs in the industry.