The evaluation of force exertions and muscle activities when operating a manual guided vehicle Chih-Long Lin, Ming-Shan Chen, Yu-Li Wei, Mao-Jiun J. Wang * Department of Industrial Engineering and Engineering Management, National Tsing Hua University,101 Sec. 2 Kuang Fu Rd., Hsinchu, Taiwan 30013, ROC article info Article history: Received 17 September 2008 Accepted 20 August 2009 Keywords: Manual guided vehicle (MGV) Pushing and pulling force exertion EMG abstract A manual guided vehicle (MGV) is used to handle heavy materials in thin film transistor-liquid crystal display (TFT-LCD) manufacturing clean rooms. This study focuses on evaluating the force exertions and muscle activities in MGV operations. The independent variables include gender, force direction, handle height, load handled and wheel diameter of the MGV. The results show the force direction, handle height and load handling effects are significant in most measures except for F ending (the peak force required to stop the MGV) and the EMG of the anterior deltoid. The wheel diameter had a significant effect on F initial (the peak force required to move the MGV) and F ending responses. Gender did not significantly effect any measures. Moreover, the pushing and pulling force is less at 115 cm handle height than at 101.5 cm and 88 cm handle heights. Using 15.3 cm (6 inch) diameter wheels requires less force than 20.3 cm (8 inch) diameter wheels because the two front wheels are fixed and the two rear wheels are rotatable. The design implications are discussed. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction A manual guided vehicle (MGV) is commonly used to transport flat panel glasses in the TFT-LCD manufacturing workplace. The MGV is made of stainless steel to provide high tensile strength and dustless conditions. It also has a shock absorbing mechanism for reducing shock and vibration to avoid damaging the glass panels during transport. Operators in TFT-LCD production clean rooms push and pull a MGV from one station to another to load and unload flat panel glass during operations. Musculoskeletal problems, particularly in the lower back, shoulder, and forearm areas, develop due to pushing and pulling (Chaffin, 1987; Macfarlane et al., 2000). The risk of musculoskeletal disorders increases as the pushing and pulling force exertion increases (Hoozemans et al., 1998). The force exertion evaluation can be divided into initial force (to start the cart), sustained force (to keep the cart moving at a constant velocity), and ending force (to stop the cart). The initial force is usually greater than the sustained force and ending force (Van der Beek et al., 2000), and has been used to evaluate pushing and pulling tasks (Al-Eisawi et al., 1999b; Resnick and Chaffin, 1996). The maximum voluntary pushing or pulling strength of males is greater than that of females (Kumar et al., 1995; Shih and Wang, 2002). The initial pushing force exerted by females is similar to males during normal dynamic cart pushing and pulling tasks (Al-Eisawi et al., 1999b; Resnick and Chaffin, 1995). However, in a maximum voluntary pushing task, the initial pushing force of females is only about 60 percent of that of males (Resnick and Chaffin, 1995). For maximum acceptable initial and sustained pushing forces, both forces exerted by females while pushing a cart are about 70 percent of that of males (Ciriello, 2004; Ciriello et al., 2007). When pushing a cart on a low coefficient of friction floor, the acceptable initial pushing force for males and females are about 65 percent and 72 percent of that of a high coefficient of friction floor, respectively (Ciriello, 2005; Maikala et al., 2009). Beside the strength capability difference between genders, other factors such as motivation and working skill may also affect force exertion. For the effect of force direction, Resnick and Chaffin (1996) reported that the initial pushing force is smaller than the pulling force while moving a 68 kg pushing simulator. Al-Eisawi et al. (1999b) showed that the initial pushing force for the horizontal direction is 93.5 percent of the pulling force and the difference was significant when moving a 181 kg four-wheel cart. Knapik and Marras (2009) recently used an electromyography-assisted biomechanical model to evaluate the force direction effect on the spine load and found that pulling produced greater spine compressive loads than pushing. Other factors such as the handle height and amount of load handled also affected the initial pushing and pulling force. The vertical height of the cart handle is an important factor affecting force * Corresponding author. Tel.: þ886 3 5742655; fax: þ886 3 5734461. E-mail address: mjwang@ie.nthu.edu.tw (M.-J.J. Wang). Contents lists available at ScienceDirect Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo 0003-6870/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2009.08.005 Applied Ergonomics 41 (2010) 313–318