Workload and Timing of the Shoulder Muscles in the Horizontal Repetitive Work Seung-Min Mo 1, Kyung-Sun Lee 2, Myung-Chul Jung 3 1 Department of Industrial & Chemical Engineering, Suncheon Jeil College, Suncheon-si, 57997 2 Department of Industrial Safety Management, Suncheon Jeil College, Suncheon-si, 57997 3 Department of Industrial Engineering, Ajou University, Suwon-si, 16499 ABSTRACT Objective: The purpose of this study is to analyze the workload and activated duration of shoulder muscles during repetitive one-handed work, based on surface electromyography. Background: Most workers have suffered from chronic shoulder pain. The shoulder joint has widely range of motion and complicated system. Hence, it required to quantitative study analyzed to the workload and activity timing of shoulder muscles according to the movement direction. Method: Five neuromuscular intact male subjects in their 20s were recruited. This study considered leftward, rightward, forward and backward directions in the horizontal plane. Also, the arm and shoulder muscles were considered such as biceps brachii, anterior, upper trapezius. The workload and timing of muscles analyzed to linear envelope detection. The onset and offset time of muscle activity was defined that exceeds from the mean value and twice standard deviation of the resting interval of the electromyography signal. Results: The mean muscle activity of leftward direction was analyzed significantly lower than other directions. It may consider that internal rotation of glenohumeral joint was greater strength than external rotation due to the mass of anterior musculature. Leftward and backward directions analyzed to low activated duration because concentric ballistic motion. Conclusion: This study shows that the shoulder motion efficiency of the latitudinal direction was lower than longitudinal direction. Application: This study can be applied to the ergonomic design of workstation to minimize shoulder workload of workers in the horizontal plane. Keywords: Repetitive work, Manual material handling, Working direction, Shoulder, Electromyography 1. Introduction 인력운반작업 (MMH; Manual material handling) 은작업자힘에의한중량물들기, 밀기, 운반, 지지하는등의작업으로써산업현장에서빈번하게발생하고있다. 최근로봇, 머신비전, PLC (Programmable logic controller), FA (Factory automation) 등의산업자동화시스템이보편화되면서무인자동화를구축하고있지만아직까지작업자의물리적인근력을이용한단순반복적인인력운반작업은잔존하고있다. 반복스트레스는근조직손상, 근골격계질환등을유발시키는주원인이다. 이에따라기업은인간공학적작업설계를통한작업환경개선과안전보건활동을수행하고있다 (Ciriello et al., 1999). 생산제조공정의작업자는손과팔의힘을이용한단순반복적인인력운반작업을수행하며이는어깨부위에스트레스를누적시켜근골격계질환, 경견완증후근, 회전근개증후근등만성적인어깨질환을유발시키는주요원인이다. 해외산업현장에서는어깨통증으로인한보건진료가빈번하게발생하고어깨부위근골격계질환에따른연간보건손실비용이약 70 억달러에육박하고있다 (Bongers, 2001; Dinnes et al., 2003, Urwin et al., 1998). 어깨의근골격시스템은볼 - 소켓형태로불안전성의특징이있기때문에삼각근, 승모근, 회전근개등다양한근육의상호작용이발생한다 (Wickham et al., 2010). 따라서어깨부위근골격계질환은만성적으로발병하고치료및수술을해도재발률이높기때문에사회, 경제적부담이높은질환으로조기예
방이중요하다 (Croft et al., 1996; Sommerich and Hughes, 2006; Falla et al., 2003). Michener et al. (2003) 은어깨부위통증, 근골격계질환의원인중약 55% 는견봉하충돌증후근 (Subacromial impingement syndrome) 에의해발병된다고보고하였다. 이는어깨가들려진채밀기, 뻗기, 당기기등의인력운반작업으로어깨의견봉과상완골대결절부위가충돌하고조직이손상되어발생한다 (Chopp et al., 2010; Michener et al., 2003). 제조및생산공정등의작업자들은입식작업대위수평면을기준으로정상작업영역에서밀기, 당기기동작이빈번하게발생하고어깨부위피로가누적되어만성적인어깨통증을호소한다. 어깨의반복적인동작은방향별로근활성패턴에차이가있기때문에 (Karst and Hasan, 1991) 근육부하및활성패턴의생체역학적데이터를기반으로인간공학적작업설계및개선이중요하다. 본연구의목적은입식작업대위반복적으로중량물을이동시키는인력운반작업을고려하여팔및어깨부위근활성패턴을정량적으로분석하는것이다. 이에따라표면근전도를활용하여수평면을기준으로중량물이동방향과어깨부위근육의부하및활성타이밍을종합적으로분석하였다. 2.1 Subjects 2. Method 본연구는 20 대남성 5 명이실험에참가하였으며모두오른손잡이였다. 팔및어깨부위에근골격계질환병력이없는피실험자를섭외하였으며본실험에참가하기전실험의전반적인프로토콜을숙지시켜친숙한환경에서실험을진행하였다. 피실험자의평균연령, 신장, 체중, 팔꿈치높이, 팔길이는 Table 1 과같다. Age (year) 26.80 (±1.06) Table 1. Anthropometric characteristics of subjects Height 175.54 (±4.58) Body weight (kg) 74.68 (±7.06) Elbow height 108.24 (±4.12) Arm length 61.90 (±2.20) 2.2 Equipment 반복적인인력운반작업에따른팔및어깨부위의근활성도를분석하기위하여 Telemyo 2400 DTS telemetry (NORAXON, USA Inc, Scottsdale, Arizona) 표면근전도측정장비를사용하였다. 16 비트아날로그출력리시버를사용하여근전도신호를디지털로변환후 1500 sampling rate 으로저장하였다. 높이조절이가능한입식작업대를사용하여중작업을기준으로피실험자팔꿈치높이의 -15cm 로설정하였다 (Sanders and McCormick, 1993). 피실험자는 7kg 의중량물을이용하여인력운반작업을수행하였으며 10cyc/m 의작업빈도를유지하기위해전자메트로놈을사용하였다. 2.3 Experimental design 본연구의독립변수는팔및어깨의근육과방향 2 가지로설정하였다. 실험에서측정한근육은상완이두근, 상완삼두근장두, 전면삼각근, 후면삼각근, 상부승모근총 5 부위였으며수평면을기준으로전, 후, 좌, 우 4 방향을고려하였다. Within-subject design 실험계획에따라피실험자 1 인당총 6 회의실험조건을수행하였으며각실험조건당 10 회반복수행하였다. 이에따른종속변수는각방향에따른근활성도와활성시간으로설정하였다. 2.4 Protocol 본실험에임하기전피실험자의팔및어깨부위근육에표면전극을부착하였다. 전극부착절차는 SENIAM (Surface Electromygraphy for Non-Invasive Assessment of Muscle) 권고사항을준수하였다 (Hermens et al., 1999). 표면전극센서는 Ag/AgCl 양극성전극으로전극의지름은 10mm, 전극중심간거리는 20mm 로설정하였다. 근전도장비셋업은다음과같다. CMRR (Common Mode Rejectrion Ratio) > 100dB, Noise < 1μV, Gain 1000, input impedance >10 Ω, first order high-pass filter 10 Hz+/-10%. 피실험자는실험조건별방향에따라시점의중량물을들고종점까지이동하여내려놓는인력운반작업을 10 회반복하였다 (Figure 1). 피로를최소화하기위하여실험조건이변경될때마다충분한휴식시간을제공하였다.
400 Muscle workload (μv) 300 200 100 167.0 163.2 122.1 125.5 0 Forward Backward Leftward Rightward Direction Figure 1. Repetitive manual material handling in the horizontal plane Figure 2. Result of muscle workload according to the direction independent variable 2.5 Signal processing and statistics 10 회반복에따른근전도신호는중간 4~7 번째신호를추출하여 4th order Butterworth bandpass filter (10-500Hz) 로필터링하였다. 모든신호를 full wave rectified 한후, Lowpass filter (cutoff 5Hz) 를적용하여 Envelope detection 기법으로평균근활성도및활성시간을분석하였다 (Hermens et al., 1999). 근육발화시간및종료시간은피실험자별로초기편안한근전도신호의표준편차 2 배를기준으로정의하였다. 근전도신호처리및데이터분석은 MATLAB 7.0.4 (Mathworks, USA) 를활용하였으며 SAS 9.1 (SAS, Institute Inc.) 을사용하여분산분석을통한주효과및교호작용의유의성을분석하였다 (α =0.05). 3.1 Muscle activity 3. Results 근활성도에따른분산분석결과방향 (F 3,12 =15.07 p=0.0002), 근육 (F 4,16 =13.92 p<.0001) 에따른주효과는통계적으로유의하게나타났으며교호작용도유의한차이로분석되었다 (F 12,48 =12.69 p<.0001). 전후측방향의근활성도는좌우측방향과비교하여상대적으로높게분석되었다. 또한상완이두근의근활성도가가장높게분석되었으며삼두근장두의활성도는낮게분석되었다 (Figure 2 to 3). Muscle workload (μv) 400 300 200 100 0 292.9 Biceps brachii 190.6 Anterior Figure 3. Result of muscle workload according to the muscle independent variable 3.2 Activated duration 118.9 Upper trapezius Muscle 91.4 Posterior 28.5 Triceps long 근활성시간에따른분산분석결과방향 (F 3,12 =25.42 p<.0001), 근육 (F 4,16 =9.66 p=0.0004) 에따른주효과는통계적으로유의한차이가있는것으로나타났으며교호작용도유의하게분석되었다 (F 12,48 =2.29 p=0.0211). 몸의앞쪽으로중량물을당기는좌후측방향의근활성시간이상대적으로짧게분석되었다. 근활성도와비례하여상완이두근과전면삼각근의근활성시간은높게분석되었다 (Figure 4 to 5).
Forward Rightward Backward Leftward Activated duration (sec) 0 1 2 3 4 5 1.39 1.38 2.40 2.81 Figure 4. Result of activated duration according to the direction independent variable Biceps brachii Anterior Activated duration (sec) 0 1 2 3 4 5 2.40 2.10 학적작업장레이아웃설계시고려되어야할것이다. 회전근개근육은심부근육으로써본연구에서는고려하지못했지만향후침전극과표면전극을함께활용하여어깨부위근육의다각적인근활성도분석연구가필요할것이다. 근활성시간이낮게분석된좌후측방향은중량물을몸중심으로가져오는동작으로써질량이높은앞쪽어깨부위의근육이활성화되고구심성탄도동작에따라순간적으로높은토크를발생한것으로판단된다 (Bosco et al., 1983; Julienne et al., 2007). 반면우측방향의외측회전동작은상완와관절및견갑대부위근육, 인대등에통증을유발할수있기때문에가급적지양할것을권고한다 (Crétual et al., 2015; Ropars et al., 2010). 본연구는동적근전도신호를분석하여 %MVC, %RVC 등정규화된값이아닌절대적인신호값 (μv ) 으로근활성도를분석하여다소편차가높게파악되었다. 향후최대동적근수축을기준으로동적근전도신호의작업부하를신뢰도높게분석할수있는정규화연구가선행되어야할것이다. Upper trapezius Posterior 1.93 1.86 5. Conclusion Triceps long 1.68 Figure 5. Result of activated duration according to the muscle independent variable 4. Discussions 본연구결과좌우측방향이전후측방향보다상대적으로낮은근활성도가분석되어적은근력으로중량물이동이가능한것으로분석되었다. 본실험에서좌우측방향은주로어깨관절의내 외측회전이발생하여회전근개가활성화되어낮은근활성도가분석된것으로판단된다. Kothiyal et al. (2001), Strasser and Müller (1999) 는수평면을기준으로 90 ~150 외전각도의중량물이동은어깨부위근육에부하가높게분석되어작업설계에부적절한방향으로보고하였다. 이러한결과를기반으로좌우방향으로의중량물이동은어깨부위근육에부하가낮기때문에인간공 본연구는수평면작업대위반복적인인력운반작업에따른팔및어깨부위근활성도를효과적으로분석하였다. 본연구결과를기반으로다양한방향및근육을확장하여근육스트레스를최소화할수있는인간공학적작업설계에응용할수있으며, 향후팔및어깨근육의정량적부하및근활성패턴데이터베이스를구축하여근력보조기기설계및재활의공학분야에서유용하게활용할수있기를기대한다. References Bongers, P.M, The cost of shoulder pain at work. British Medical Journal, 322(7278), 64-65, 2001. Bosco, C., Mognoni, P. and Luhtanen, P., Relationship between isokinetic performance and ballistic movement. European Journal of Applied Physiology and Occupational Physiology, 51(3), 357-364, 1983. Chopp, J.N., O Neill, J.M., Hurley, K. and Dickerson, C.R, Superior humeral head migration occurs after a protocol designed to fatigue
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