| ObjectivesTo investigate the occurrence of musculoskeletal disorders and muscle fatigue among car assembly workers and explore the related inducing factors.To explore the sEMG characteristics of local active muscles during car assembly tasks, and preliminarily discuss the sensitive objective indexes in evaluating local muscle fatigue by analysizing the consistency between the subjective index and objective index.MethodThis study includes two parts. The first part is occupational epidemiological survey and the second part is field individual sEMG test and analysis.A cross-sectional and longitudinal study of498male car assembly workers in assembly plant were carried out using a modified version of the Nordic questionnaire and Hazard Zone Checklist. The prevalence of musculoskeletal disorders, local muscle fatigue, and related awkward ergonomics factors were collected in the questionnaire. The questionnaires were made out by informant themselves. According to the survey data, analysed the annual incidence of musculoskeletal disorders; Univariate analysis and multivariate analysis were performed to explore the related inducing factors; Chi-square test was used to study the correlation between local muscle fatigue and musculoskeletal symptoms, and also the correlation coefficients were calculated.Based on the results of epidemiologic survey, the workers on assembling water tank post and on fixing damping cushion post were selected as the research objects. Collect their surface electromyograpghy signals of forearm flexor, biceps, deltoid and trapezius in actual field assembling tasks, and meanwhile inquire their feelings once every ten minutes by the Borg’s RPE; and record the sEMG signal of Maximal Voluntary Contraction testing before and after work to remove the influence of individual difference and anlaysis the changes before and after work. By above-mentioned methods, taking assembling cycle and reference activities (Weight Lifted and overhead work) as units, analyse the dynamical changes of the sEMG indexes of RMS and MF, and explore the correlation between subjective and objective indexes.ResultsOccupational epidemiological survey results:In this study,455effective questionnaires (the valid response rate is91.4%) have been collected and analyzed statistically.(1)Eighty-four percent of the respondents had been troubled with musculoskeletal symptoms in the last year. The commonest musculoskeletal symptoms were from the shoulder (50.6%), neck (49.7%), lower back (43.4%), wrist and leg (both38.4%).(2)The occurence body sites of occurring MSDs are different according to the different awkward ergonomics factors in different sections. The main existing factors in interior ornament and classis is "Working with the neck bent more than45°", followed by "High repetitive and forceful exertions with the hands","Working with the back bent forward more than30°" and "Working with the back bent forward more than45°", and the predominant body sites of MSDs are mainly shoulder, neck, low back, wrist and leg. While the main existing factors following in the order in split charging are "Working with the neck bent more than45°","Working with the back bent forward more than30°","pinching with a force of4.5kg or more and Highly repetitive motion" and "Working with the back bent forward more than45°", and the most common body sites of MSDs is leg, followed by neck/lowback/wrist, shoulder and thigh. And the other sections also present similar difference.(3)Musculoskeletal disorders of different body sites are related to awkward ergonomics factors and other influencing factors. Univariate and multivariate analysis indicated that the factors affecting neck musculoskeletal symptoms include "Working with the neck bent more than45°","pinching with a force of4.5kg or more with poor wrist posture" and drink; the factors that affecting shoulder MSDs include "Kneeling","daily manual lifting task more than45kg" and "Working with the back bent forward more than30°"; The factors that affecting low back MSDs include "Kneeling","manual lifting task more than4.5kg and more than twice per minute","Working with the back bent forward more than45°" and BMI; The factors that affecting low back MSDs include "pinching with a force of4.5kg or more with poor wrist posture".(4)The results of correlation analysis between local muscle fatigue(include neck, shoulder, low back, wrist and leg) and MSDs showe that there is a remarkable statistical association between muscle fatigue and MSDs.Field individual sEMG test and analysis(1)The change characteristics of RPERPE was increased with the assembly cycles and the time(assemly cycles) of perceived fatigue was quite different in different post or different body sites. On assembling water tank post, neck and shoulder were the first body sites occurring fatigue (both at the15th cycle), followed by wrist (at the18th cycle) and upper arm (at the20th cycle). On fixing damping cushion post, shoulder was the first body sites occurring fatigue (at the22th cycle), next was upper arm (at the23th cycle), then wrist and neck (both at the23th cycle).(2)The change characteristics of sEMG①The changes of RMSmvc during MVC testing before and after workOn assembling water tank post, the RMSMVC of deltoid and left/right trapezius after work was significantly lower than that before work and there was no obvious change of RMSMVC of forearm flexor and biceps. On fixing damping cushion post, the RMSMVC of deltoid and left trapezius after work reduced obviously compared with before work, while the RMSMvc of forearm flexor, biceps and right trapezius didn’t change significantly.②The change characteristics of MVE%and MFOn assembling water tank post, the MVE%of biceps, deltoid and right trapezius with development of assembly work were obviously changed and presented a dropped tendency; the MVE%of left trapezius, the MVE%and MF of forearm flexor didn’t chang significantly. On fixing damping cushion post, the change of the MVE%of forearm flexor and the MF of right trapezius were in a decrease trend, and the MF of forearm flexor was in an increase trend; there was no obvious change of the MVE%and MF of biceps, deltoid and left trapezius and the MVE%of right trapezius.(3)The correlation between objective index and subjective indexOn assembling water tank post, the MVE%and MF of biceps, deltoid and trapezius, the MF of forearm flexor showed obvious downtrend when the perceived fatigue was occurring and the decrease of sEMG indexes were slowed down or no obvious change during the development of fatigue. On fixing damping cushion post, the MF of forearm flexor and biceps, the MVE%of deltoid and left/right trapezius were increased significantly in perceived fatigue; the MF of deltoid and left/right trapezius were decreased significantly.ConclusionThe occurrence of musculoskeletal symptoms is high in car assembly workers. It is probably related to its existing awkward ergonomics factors, and occurrence body sites are different according to the existing awkward ergonomics factors. Drink, BMI and education level may be important fators which affect occurrence of musculoskeletal symptoms. There is a remarkable statistical association between local muscle fatigue and musculoskeletal symptoms.Whether static or dynamic activities in car assembly tasks, a significant decrease of the MF in most tested body sites when perceived fatigue occurs was observed, while few body cite showed a increase trend; and the changes of the MVE%in static activities showed a descending trend, while the changes of the MVE%showed inconsistent results for dynamic activities. It is suggested that the MF could be used to evaluate muscle fatigue in field work. |
PDF Full Text Request