A Study On The Load And Occupational Exposure Limit Of Manual Lifting Task

Objective Manual lifting task(MLT) is frequent in industrial production, and it is the maincause of workers’ musculoskeletal disorders (MSDs). European and American developedcountries have established relevant occupational exposure limit, but few reported based uponChinese population. Therefore, studies on the load of manual lifting task and its occupationalexposure limit which is adaptive for Chinese people are necessary. In this study, maximumisometric strength of workers and university students were examined, influencing factors ofmanual lifting task were studied and the maximum acceptable weight of lifting (MAWL)between Chinese and occidental were compared. Modifications were given to the liftingthreshold limit value(TLV) of ACGIH, which aimed to make the modified TLV moreadvisable for Chinese population.Methods Back lifting strength, left hand lifting strength, right hand lifting strength, armlifting strength and shoulder lifting strength of938Chinese workers and college studentswere examined in the first section, the isometric strength of Chinese were compared withthose of occidental. In the second section,43male college students lifted different weights indifferent horizontal distances and vertical distances, surface electromyography (sEMG)signals of erector spinae muscles were recorded and analyzed. Rating of perceived exertion(RPE) was graded at the end of each lifting task. In the third section,10male collegestudents lifted and lowered a16kg weight from middle of shin height to Knuckle height for60times in10min, one technicist marked to distinguish lifting task and lowering task.Rating of perceived exertion (RPE) was graded after the whole experiment. MAWL ofChinese population was compared with that of occidental in section four to understand thegap of lifting capacity between the two groups.Results In present study, the mean isometric strength for back lifting, left hand lifting, righthand lifting, arm lifting and shoulder lifting is107.7kg、57.7kg、60.9kg、31.8kg and46.5kg for male, respectively. While that is59.7kg、30.7kg、34.1kg、18.2kg and25.2kg forfemale, respectively. The back lifting strength of male workers aged from20to29is greaterthan that of male college students in the same age group(P>0.05), while it is opposite infemale(P <0.05).The percentage of the maximum voluntary electrical activation (MVE%) of erector spinae muscles is36.15、45.63and52.35when the lifting weight is3.6kg、7.5kg and11.4kg,respectively. MVE%of LT10and RT10are greater when the horizontal distance is fromfloor to reach limit than that is from floor to shoulder (P <0.05). MVE%of LL3and RL3arealso greater when the horizontal distance is from floor to reach limit than that is from floor toshoulder (P <0.05, P>0.05). In addition, the result suggests that the MVE%of7kg/70cmcombination is greater than14kg/50cm combination, and the latter is greater than18kg/30cm combination (P <0.05). Borg rating has the similar result as MVE%.MVE%of trapezius are25.05and20.92in lifting and lowering, i.e. lifting is higher thanlowering (P <0.05). MVE%of erector spinae muscles are25.05and20.92in lifting andlowering, i.e. lifting is also higher than lowering (P <0.05). Borg grade of lowering is17%lower than lifting (P <0.05).Conclusion The isometric strength of Chinese male is15~23%lower than occidental, and15~18%for female. Muscle strength of female is lower than that of male, approximately halfof male. Different lifting strengths are in decreasing order as back lifting strength, right handlifting strength, left hand lifting strength, arm lifting strength and shoulder lifting strength.The muscle strength of male decreases with the growth of age, but the trend is not significantin female. Lifting weight, horizontal distance and vertical distance of lifting are factorswhich affect the muscle load, should be taken into account in formulating the lifting limit.MAWL, compare with maximum strength, is more acceptable to be the basis of regulationfor lifting TLV of ACGIH, finally, the lifting TLV tables were down-regulated21%fortable1,25%for table2and28%for table3. Although the muscle activity of lifting is higherthan lowering, up-regulation of lifting weight in limit should be more cautious. So the samelimit value is adopted in lifting and lowering.