Previous Research Framework for Manual Material Handling Task

2.3.1 The Model of Dempsey (1998)

In Figure 2.1, for MMH systems, Dempsey (1998) model illustrates a systematic method in defining the ratio of “task demand to worker capacity”. He identified two basic elements, namely “task demand” and “worker capacity” that interact closely within the MMH systems. The variables of task demand are the characteristics of organization, workplace, material, and environment. Worker capacity is described through the following aspects: personal characteristics and the capacities of biomechanical, physiological, and psychological. The ratio of “task demand to worker

capacity” influences the incidents of potential negative outcomes, for example injury, fatigue or lethargy, and uneasiness, as well as positive outcomes, for example productivity and outgoing quality. It is important to understand that these outcomes are somewhat interrelated, for instance fatigue is detrimental upon productivity.

The concept can be adopted to study human lifting capacity in MMH task. The model shows the two basic elements and influences factors in the MMH task design that have to consider in optimizing the system, but the model did not indicate the pathways and factors contributing to WMSDs.

Figure 2.1: Primary factors influencing the task demands to worker capacity ratio

2.3.2 The Model of Van Der Beek and Dresen, 1998

Physical ergonomics is a discipline of knowledge that deals with the physical load on the human body. The external and internal exposures are differentiated in the framework by Van der Beek & Frings-Dresen (1998) in Figure 2.2. Human, when they are at work, are externally exposed to certain job settings, risks, and working methods. These lead to the adoption of specific postures, external forces, and movements of the body. They function to elevate the internal forces to another level of “energy expenditure” and make the mechanical and physiological responses take place in the short term (acute responses). Acute responses that lead to musculoskeletal disorders are among the long term effects of this energy expenditure. As shown in Figure 1, the responses highly depend on work capacity of individuals which consist of body dimensions, conditions, and physical fitness.

The model shows that the worker’s capacity is the important parameter associated with the exposure and the short and long-term effects. The ratio of task demands to worker capacity has been shown to influence the occurrence of potential undesirable outcomes in short and long-term effects, such as fatigue, discomfort, and injury. However, this model is merely a conceptual model, which was developed following a review of the past research. In addition, this model is not specific to a particular occupational setting and industry.

Figure 2.2: The exposure to physical loads and its short term and long term effects

2.3.3 The model of Raghunathan et.al (2014)

Raghunathan et al. (2014) demonstrated MMH conceptual model that depicts the causal relationship between exposure to health effects and ergonomic risk factors (Figure 2.3). It adapts the studies of Westgaard & Winkel (1997) and Wells et al. (2004) that identified the assessment on exposure and the framework intervention of this model. External exposure is described as job demand that causes biochemical forces and independently quantifies the workers, while internal exposure is the forces of biomechanical occurred as a result of job demand and estimated by worker measurement (Westgaard & Winkel, 1997). Work exposures differ in accordance with numerous interactions between human beings and their workplace. The interaction factors namely forceful exertion, posture, vibration, and repetition largely assist in identifying task that is ergonomically stressful or demanding. Hence, suitable solutions intervening this matter are needed to mitigate the ergonomic stress, for example force and posture have been emphasized in the existing literature of biomechanics and/or

ergonomics (Coenen et al., 2013; Norman et al., 1998). In terms of exposure quantification, time dimension contains significant effects on the degree of stress related to many physical activities. Both “peak” and “cumulative” doses, which are the biomechanical exposures, play a key role in WMSDs causation (Callaghan et al., 2001; Coenen et al., 2013; Norman et al., 1998; Xu et al., 2012). Furthermore, (Dempsey & Mathiassen, 2006; Mathiassen et al., 2013; Takala et al., 2010; Waters et al., 2011) gave an insight into various strategies and observational methods utilized in evaluating the exposures of biomechanical because of physical exertions.

Work System

characteristics Response measure

Response measure Worker Characteristics Team Composition Gender Task Demand Material characteristics Task/workplace characteristics Environmental characteristics Organizational characteristics Shape or Interface Material handling equipment Intervertion Interaction Dimension Biomechanical responses Physiological responses Psychological responses · Fatigue · Discomfort · Strain · Mental Stress WMSD · Force · Posture · Time

Figure 2.3: A conceptual model of the WMSD development as a result of MMH

The model is comprehensive and clearly shows the interaction between work system characteristics, response measures and the long term effects of WMSDs. The model serves as a reference to determine the worker’s capacity than can be used to design the MMH task based on the response measure on biomechanical, physiological and psychological response so that it will not give increase the WMSDs risks.

2.3.4 Summary of Significant Factors from Reviewed Models

Most of the existing research frameworks were developed based on a comprehensive review of relevant literature by the respective researchers. All of the frameworks proposed the same significant factors related to MMH design. The models developed by Dempsey (1998), Van Der Beek & Dresen, 1998 and Raghunathan et al. (2014) clearly indicate that work-related musculoskeletal disorder is one of the potential health risks due to MMH task. Therefore, the factors which are significantly associated with health (WMSDs) are adopted in this study to develop a methodological framework to determine the maximum acceptable lifting frequency (worker capacity). The identified factors are task demand, physiological responses, and psychological responses.

It is evident that there are no methodological framework specifically establish which correlate task demand, physiological response and psychological response on determine the maximum acceptable lifting frequency (worker capacity). Thus, the significant factors related to worker’s capacity and WMSDs from the reviewed framework are adopted to derive a methodological framework for this study.