International Journal of Occupational Safety and Ergonomics (JOSE) 2007, Vol. 13, No. 3, 279–289              Biomechanica...
280 J.T. ALBERS & S.D. HUDOCKwere upper extremities (73%) including wrist/          1.2. Work Methodshands/fingers (48%) a...
REBAR TYING BIOMECHANICS             281                                                    The National Institute for Occ...
282 J.T. ALBERS & S.D. HUDOCKrequired, to become familiar with the use of the       The subjects were directed to tie the ...
REBAR TYING BIOMECHANICS             283number of ties each worker made. This consisted      2. guidelines for acceptable ...
284 J.T. ALBERS & S.D. HUDOCK2.3.3. 3DSSPPTM                                      3.1. Wrist KinematicsThe University of M...
REBAR TYING BIOMECHANICS                  285TABLE 3. Comparison of Measured Wrist Velocity (degrees/second) to High- and ...
286 J.T. ALBERS & S.D. HUDOCKTABLE 5. Time in Trunk Inclination During            TABLE 6. Estimated Forces on the L5/S1 D...
REBAR TYING BIOMECHANICS             287PTE for 4 hrs or more each day slightly exceeded      concrete slab) reduces worke...
288 J.T. ALBERS & S.D. HUDOCKobserved using their free hand to support their         The study was not conducted in a mann...
REBAR TYING BIOMECHANICS           289  Worker productivity, as measured by the            11. American Conference of Gove...
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Biomechanical assessment of three rebar tying techniques jos

  1. 1. International Journal of Occupational Safety and Ergonomics (JOSE) 2007, Vol. 13, No. 3, 279–289 Biomechanical Assessment of Three Rebar Tying Techniques James T. Albers Stephen D. Hudock U.S. Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, USAThe National Institute for Occupational Safety and Health (NIOSH) conducted a study of ironworkersto evaluate their risk for developing back and hand injuries from hand-tying reinforcing steel bar and toinvestigate whether power tying tools can be an effective intervention for the prevention of work-relatedmusculoskeletal disorders. A field investigation of biomechanical loading when using 3 techniques totie together rebar was conducted. Researchers measured employees’ wrist and forearm movement withgoniometers and videotaped and analyzed trunk postures. Manually tying rebar at ground level involvedsustained deep trunk bending and rapid, repetitive, and forceful hand–wrist and forearm movements. Using apower tier significantly reduced the hand–wrist and forearm movements and allowed the ironworkers to useone free hand to support their trunk posture while tying. Adding an extension handle to the power tier allowedthe ironworkers to tie rebar while standing erect, minimizing sustained trunk flexion. construction biomechanics reinforcing ironwork1. BACKGROUND 1.1. Workplace Injuries and Physical Risk FactorsIn the USA, construction ironwork can be divided According to the Bureau of Labor Statistics [1],into four primary specialties: (a) structural construction workers for the year 2004 sufferedironwork, (b) ornamental ironwork, (c) machinery work-related nonfatal injuries at a rate of 6.4moving/rigging ironwork, and (d) reinforcing per 100 full-time employees compared with 4.8ironwork (also known as rod, rebar, or concrete per 100 full-time employees for all industries.reinforcement work). This report focuses on the Construction workers specializing in highway,last of these specialties. In reinforcing ironwork, street, and bridge construction had an incidenceworkers place steel rods into concrete forms and rate of 6.4 nonfatal injuries per 100 full-timetie the rods together using wire prior to filling employees, one third higher than the nationalthe forms with concrete. The embedded steel average.rods provide additional support for the concrete In a survey of approximately 1 000 constructionstructures being built, including bridge decks and ironworkers including 250 reinforcing ironworkersvertical walls. (RIWs), respondents were asked if they had ever experienced a musculoskeletal disorder (MSD) over their entire ironworker career [2]. For RIWs, the prevalence of self-reported MSD symptoms The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institutefor Occupational Safety and Health. Mention of company names or products does not constitute endorsements by the National Institute for Occupational Safety and Health. Correspondence and requests for offprints should be sent to James T. Albers, NIOSH, MS C-24, 4676 Columbia Parkway, Cincinnati,OH 45226, USA. E-mail: <JAlbers@cdc.gov>.
  2. 2. 280 J.T. ALBERS & S.D. HUDOCKwere upper extremities (73%) including wrist/ 1.2. Work Methodshands/fingers (48%) and shoulders (34%), On this particular project, the RIWs werelower back (52%), and lower extremities responsible for placing and tying together the(53%) including knees (37%) and ankles/feet rebar used to reinforce the concrete deck and(27%). Only 12% of the RIWs reported having walls of a highway bridge. The rebar was usedexperienced no MSD during their career. The to reduce the tensile stresses (i.e., bending andrespondents were also asked if they had ever stretching) acting on the concrete deck. The rebarreceived a doctor’s diagnosis for a MSD during was either No. 10 metric size (10-mm diameter)their career. For RIWs, the prevalence of doctor- at 0.56 kg/m, or No. 16 metric size (16-mmdiagnosed MSDs were tendonitis (19%), carpal diameter) at 1.56 kg/m, and 6.1 m in length.tunnel syndrome (16%), ruptured back disk Two rebar mats were placed inside the concrete(14%), and shoulder bursitis (11%). Almost half form over the full length of the bridge. Each mat(49%) of the RIWs reported no doctor-diagnosed consisted of rebar placed perpendicular to eachMSD during their career. other and spaced about 18 cm apart. The mats Ergonomic exposure assessment protocols were supported above the metal decking usingbased on work sampling strategies with wire “chairs” and the chairs were used to separateobservations over extended periods of time the top and bottom mats. The specifications forwere found suitable to accurately estimate the job, common for highway bridge decks in theexposure to workplace hazards associated with USA, required tying 50% of the intersecting rebarconstruction work [3]. Forde and Buchholz on the bottom mat and 100% of the intersectionsconducted quantitative exposure assessments on the top mat. The bridge was approximatelyfor seven construction ironwork tasks, including 1 600 m long by 18.3 m wide (approximatelytying rebar in a highway base layer [4]. Nearly 30 000 m2). The contractor estimated 2.2 million14 000 observations were made overall. Twelve wire ties were made on the bridge to secure theRIWs were observed over 6 days and over rebar. Weather permitting, the ironworkers4 500 separate observations were recorded. These typically worked a 5-day, 40-hr work week. Theworkers, in general, had the worst trunk, arm and amount of rebar placed and tied on any givenleg postures of the seven groups of construction day depended on several factors, including theworkers. For trunk postures, the RIWs were in number of workers on the job, the environmentalneutral posture only 52% of the time, in slight conditions, and the pace of the work precedingflexion (>20°) 13%, and in severe flexion (>45°) rebar installation.28% of the workday. For arm postures, the RIWs The ironworkers typically employed twohad one elbow at or above the shoulder 14% of techniques to tie the intersecting rebar together:the time and two elbows at or above the shoulder (a) pliers and a spool of wire and (b) a battery-6%. The RIWs stood on unstable or uneven operated power tier (Model RB392 rebar tier,ground 70% of the time, primarily standing on MAX USA Corp., USA). Pliers were used tothe rebar mats. It was observed that these workers wrap and twist the wire around the rebar whencarried heavy weight (>23 kg) 19% of the more secure ties were necessary while framingworkday. For a typical 8-hr shift, this amounted the side walls of the bridge deck or making theto 1.5 hrs, primarily positioning the rebar into first ties for each mat. The power tier (PT) wasthe mat structure. Another 6% of the shift, about used most frequently to make the remaining ties.0.5 hrs, the RIWs carried moderate weights of Traditional tying (Figure 1) required the use of4.5–23 kg. The RIWs were observed positioning two hands—one to use the pliers to pull, wrap,rebar 36% of the time, approximately 3 hrs each twist, and cut the wire and the other to pull andshift. The workers spent 29% of the time (2.3 hrs) push the wire. Only one hand was necessarytying together the rebar mats, typically in a severe to operate the PT (Figure 2). Both techniquestrunk flexion posture. required frequent and sustained deep trunk bending (>60° trunk flexion) to tie the rebar.JOSE 2007, Vol. 13, No. 3
  3. 3. REBAR TYING BIOMECHANICS 281 The National Institute for Occupational Safety and Health (NIOSH) introduced a commercially available extension handle developed for use with the PT. This was the third rebar tying technique used during the investigation (Figure 3).Figure 1. Traditional rebar tying with pliers andspool of wire. Figure 3. Power tier with extension (PTE). 2. METHODS NIOSH evaluated ironworkers’ exposures to risk factors for developing low-back and hand disorders during hand-tying of reinforcing steel bars (rebar) on concrete bridge decks, and investigated whether the use of rebar power tying tools could be an effective intervention for the prevention of work- related musculoskeletal disorders (WMSDs) of the upper extremities and back. The investigation was designed as a repeated measures-randomized ordered study. All RIWs working on the study site were enrolled as participants and all were familiar with using the pliers and the PT. The extension handle was brought to the site by NIOSH investigators and none of the employees had previously used theFigure 2. Tying rebar with power tier (PT). device. Participants were encouraged, but not JOSE 2007, Vol. 13, No. 3
  4. 4. 282 J.T. ALBERS & S.D. HUDOCKrequired, to become familiar with the use of the The subjects were directed to tie the rebarextension before measurements were made. placed for the bottom mat using three differentThe order that the three tying techniques were tying techniques: (a) pliers and wire spool, (b)used was randomly assigned to each employee. the PT, and (c) the PT with an extension handleIndependent variables were the three different (PTE).rebar tying methods. Dependent variables were The contractor had purchased the PTs(a) trunk position, (b) hand–wrist position, approximately 2 years prior to the study in part(c) hand–wrist movement (e.g., velocity and to reduce employees’ exposures to biomechanicalacceleration), (d) low-back position, and (e) the loading related to tying rebar using pliers. The PTnumber of ties per minute. still required deep forward bending when tying at deck level, but workers used one hand to hold the2.1. Demographics tier tool and the other to support their upper body while tying.The construction reinforcing and structural steelcontractor that collaborated in the study was a 2.2. Data Collectionminority-owned business providing structural andreinforcing steel services for new construction Each subject tied rebar for about 30 min usingprojects in a major metropolitan area. At the (a) pliers, (b) a PT, and (c) a PTE. A twin-axistime of the request, the contractor employed goniometer (Biometrics SG Series; Biometrics,100 ironworkers. Eight male RIWs participated UK) and a torsiometer (Biometrics Q110) werein the study. Participant mean age was 37 years used to measure the dominant wrist motion andold (±6) and mean height, weight and body position in the flexion/extension, ulnar/radial,mass index (BMI) were, respectively, 174 cm and pronation/supination planes during tying [5].(±6), 83.5 kg (±9.0), and 27.6 BMI (±1.9). The goniometers and torsiometer were calibratedAll study participants were members of the on-site using a special fixture to control crosstalkInternational Association of Bridge, Ornamental, (Figure 4) [6]. The calibration results for theand Reinforcing Iron Workers. Subjects reported electrogoniometers demonstrated that the wristtying rebar using pliers 8.1 hrs/week (±6.2) and position measurements were accurate andusing a battery-powered PT 6.1 hrs/week (±6.9). consistent.One worker reported an injury occurring during Observational methods were used to recordthe previous 12 months that affected his work. the position of the trunk during tying and the Figure 4. Calibration jig.JOSE 2007, Vol. 13, No. 3
  5. 5. REBAR TYING BIOMECHANICS 283number of ties each worker made. This consisted 2. guidelines for acceptable hand–wrist motionof videotaping each worker from three different (e.g., velocity and acceleration) to preventangles so that the trunk could be later viewed wrist cumulative trauma disorders (CTD) [12,from both the side and front. 13], Using the Borg Modified CR-10 scale, each 3. 3DSSPPTM to estimate the compressive andworker was asked to describe the physical effort shear forces on the L5/S1 spinal disc [9], andthey used with their hand–wrist and low back on 4. International Organization for Standardizationa scale from 0 (nothing at all) to 10 (extremely (ISO) Standard No. 11226:2000 [14].strong) when using the three tying methods [7].Personal information and work history wereobtained using self-administered questionnaires. 2.3.1. ACGIH™ HAL TLV™ The ACGIH™ HAL TLV™ uses HAL and2.3. Data Analysis Methods peak hand forces to evaluate the risk factors for developing a hand, wrist, and forearm MSDThe goniometric data were converted to [11]. The HAL is based on the frequency ofreadable files using proprietary Biometrics hand exertions used and the job duty cycle (i.e.,Ltd. data management and analysis software. distribution of work and recovery periods). PeakStatistical analysis, including summary and force can be measured using a strain gauge orinferential statistics, was conducted using SAS® other instrumentation (and normalized on a scalesoftware version 8, including the MIXED model of 0 to 10) or estimated by a trained observerprocedure. using subjective exertion scales (i.e., Borg Videotape was analyzed using the Multimedia Modified CR-10 perceived effort scale) [7]. TheVideo Task Analysis (MVTATM) software total exposure is characterized in terms of averageprogram on a computer [8]. This software allows HAL and peak hand force. Scores between 0.56for frame-by-frame analysis of the video of the and 0.77 are at the Action Level and scores abovetying tasks identifying and cataloguing exposure 0.77 exceed the TLV™.to multiple risk factors, such as trunk andneck postures and wrist deviations. An analystrecorded the frequency and duration of the trunk 2.3.2. Hand–wrist motion guidelinesposture angles and the time required to tie rebar Marras and Schoenmarklin studied thefor each of the three tying techniques. relationship between wrist movement, including Analysts used the computer-based 3D Static the angle, repetition, velocity, and accelerationStrength Prediction ProgramTM (3DSSPP) to levels, and the risk of developing a CTD [12].estimate the pressure (compressive forces) on The study found that high wrist and forearmthe spinal disc between the fifth lumbar vertebrae motion (i.e., angular velocity and angularand the first sacral vertebrae (L5/S1 spinal disc) acceleration) during an 8-hr period wereduring rebar tying in stooped postures [9]. Spinal significantly associated with risk of developingdisc pressure is known to vary depending on an upper extremity CTD. Mean wrist accelerationthe amount and type (e.g., forward, sideward, levels associated with high and low CTD risk intwisting) of bending and external loads [10]. the radial/ulnar, flexion/extension, and pronation/ The following methods were used to evaluate supination planes were, respectively, 494 andthe biomechanical loading of the subjects’ low 2 2 301 angular degrees/second (d/s ), 824 andback and upper limbs for each rebar tying task of 2 2 494 d/s , and 1 824 and 1 222 d/s . In follow-upthe study (detailed descriptions follow): analysis, Marras, Schoenmarklin, and Leurgans [13] reported that wrist acceleration in the1. the American Conference of Governmental flexion/extension plane was the best predictor of Industrial Hygienists (ACGIH™) Hand a hand/wrist CTD. Activity Level (HAL) Threshold Limit Value (TLVTM) [11], JOSE 2007, Vol. 13, No. 3
  6. 6. 284 J.T. ALBERS & S.D. HUDOCK2.3.3. 3DSSPPTM 3.1. Wrist KinematicsThe University of Michigan 3DSSPP™ is a Wrist mean velocity and wrist mean accelerationcomputerized model which can be used to were significantly higher during pliers tyingevaluate the physical demands of a prescribed than PT or PTE tying (Tables 1–2). Mean wristjob [9]. Inputs to the model are the magnitude velocities measured in the flexion/extension andand direction of forces at the hands, angles of ulnar/radial planes during pliers tying exceededbody segments, and worker characteristics. velocities associated with high and low risksThe model calculates forces (i.e., moments) of developing a CTD [12] (Table 3). The lowon the joints of the body and estimates the CTD risk level was exceeded in the ulnar/radialpercentage of the workforce able to sustain the plane using each of the three tying techniques.inputted loads. Compressive and shear forces Operating the PT resulted in a mean wristfor the L5/S1 disc are calculated. The program velocity in the ulnar/radial plane exceeding theallows the analyst to estimate the compressive level associated with a high CTD risk.forces acting on the spine relative to the revised TABLE 1. Comparison of Mean Wrist VelocityNIOSH lifting equation [15]. In order to prevent Postures by Rebar Tying Method (Chi-Squarelow-back disorders, NIOSH recommends Difference of Least Squares Means)L5/S1 disc compression force should not exceed Pliers Pliers PT3 400 Newtons (N) during any single job activity Plane vs. PT vs. PTE vs. PTE[16] (1 N = 0.225 pound-force). Flexion/extension <.0001 <.0001 ns Ulnar/radial ns <.0500 ns2.3.4. ISO evaluation of static work postures Supination/pronation <.0001 .0001 ns Notes. PT—MAX RB 392 power tier (MAX USAThe ISO developed the consensus standard No. Corp., USA); PTE—MAX RB 392 power tier plusISO 11226:2000 [14]. It describes acceptable adjustable extension.trunk postures and maximum acceptable holdingtimes for potentially harmful postures. Postures TABLE 2. Comparison of Mean Wristwith trunk flexion greater than 60° [6] are not Acceleration Postures by Rebar Tying Method (Chi-Square Difference of Least Squares Means)recommended. Pliers Pliers PT Plane vs. PT vs. PTE vs. PTE3. RESULTS Flexion/extension <.0010 <.0010 ns Ulnar/radial <.0500 <.0009 nsManually tying rebar at ground level using Supination/pronation <.0001 .0001 nspliers and wire involved sustained deep trunk Notes. PT—MAX RB 392 power tier (MAX USAbending and rapid and repetitive hand and wrist Corp., USA); PTE—MAX RB 392 power tier plus adjustable extension.movements. Using a PT greatly reduced therapid and repetitive hand–wrist and forearm The workers’ mean wrist acceleration levelsmovements characteristic of tying with the pliers, measured during pliers tying exceeded levelsand freed one hand to support the weight of the found to be associated with (a) high CTD risk intrunk during tying. Adding an extension handle the flexion/extension (i.e., 824 d/s2) and ulnar/to the PT (PTE) allowed workers to tie rebar radial (i.e., 494 d/s2) planes and (b) low CTD riskstanding erect. The results show that manually in the pronation/supination plane (i.e., 1222 d/s2)tying rebar using pliers involves higher exposure (Table 4). Use of the PT exceeded accelerationto risk factors for developing a low-back WMSD levels related to low CTD risk level in the ulnar/than tying with the PTE. The study indicates that radial plane (i.e., 301 d/s2) [12].tying with the PT or PTE may lower exposureto risk factors for developing an upper limbWMSD.JOSE 2007, Vol. 13, No. 3
  7. 7. REBAR TYING BIOMECHANICS 285TABLE 3. Comparison of Measured Wrist Velocity (degrees/second) to High- and Low-Risk [13]Velocity Rates RiskPlane Statistic Pliers PT PTE High Low a M 85 20 21 42 29Flexion/extension SD 28 6 10 12 8 M 56a 39 22b 26 17Ulnar/radial SD 17 23 14 7 7 M 114a 43 40 91 68Pronation/supination SD 41 12 9 23 19Notes. PT—MAX RB 392 power tier (MAX USA Corp., USA); PTE—MAX RB 392 power tier plus adjustableextension; a—exceeds high cumulative trauma disorder (CTD) risk level, b—exceeds low CTD risk level. 2TABLE 4. Comparison of Measured Wrist Acceleration (degrees/second ) to High- and Low-Risk [13]Acceleration Rates RiskPlane Statistic Pliers PT PTE High Low a M 961 216 219 824 494Flexion/extension SD 260 63 98 266 156 a b M 782 454 242 494 301Ulnar/radial SD 205 270 154 142 125 b M 1335 462 452 1824 1222Pronation/supination SD 377 130 118 533 384Notes. PT—MAX RB 392 power tier (MAX USA Corp., USA); PTE—MAX RB 392 power tier plus adjustableextension; a—exceeds high cumulative trauma disorder (CTD) risk level, b—exceeds low CTD risk level.3.2. Self-Reported Data Analyses when pliers were used (2.5) and slightly exceeded when a PTE (HAL = 0.83) was used for 4 hrs orFive workers completed the perceived effort more each day. Use of the PT alone would notquestionnaire. Participants reported the lowest exceed the HAL TLV™.low-back effort when using the PTE (1.2 on the Workers were observed tying rebar with10 point Borg Modified CR-10 scale), followed extreme trunk flexion (≥90°) 94% of the timeby 2.8 for the PT and 5.8 for the pliers. The when using the pliers and 93% of the time whenlowest hand–wrist effort was reported for the PT using the PT (Table 5). With the PTE workers(2.8) and the PTE and pliers were, respectively, tied rebar using neutral trunk positions (<15°5.0 and 5.2. flexion) 83% of the time and moderate forward flexion (16°–30°) 16% of the tying time.3.3. Observational Analyses When using the PT, all workers used theirACGIH™ HAL TLV™ scores were calculated free hand to support the weight of their torso,using the workers’ perceived effort scores e.g., resting the hand/forearm on the knee/thigh,[7] (i.e., mean, low, and high) for each tying 92.4% ±4.0 of the time when they tied rebar attechnique. Using the mean scores, the HAL ground level1.TLV™ (HAL = 0.78) would be greatly exceeded 1 Workers use two hands to tie using the pliers, while only one hand is necessary using the PT or PTE. JOSE 2007, Vol. 13, No. 3
  8. 8. 286 J.T. ALBERS & S.D. HUDOCKTABLE 5. Time in Trunk Inclination During TABLE 6. Estimated Forces on the L5/S1 DuringRebar Tying Manual Tying Using the 3DSSPP™ [10] Inclination Time (s) Angle Total Compression Total ShearTreatment (°) M SD Range Trunk (°) (N) (N)PTE Neutral 124** 35 55–150 Flexion –90 16–30 24* 36 0–95 Rotation 0 2427 306 31–45 1 3 0–8 Lateral 0 46–60 <1 <1 0–1 Flexion –90 61–75 <1 <1 0–<1 Rotation 0 2857 389 76–90 0 0 0–0 Lateral ±25 >90 0 0 0–0 Flexion –105PT Neutral <1 1 0–2 Rotation 0 1864 419 16–30 <1 <1 0–<1 Lateral 0 31–45 <1 <1 0–<1 46–60 <1 <1 0–<1 Flexion –105 61–75 <1 <1 0–1 Rotation 0 2322 480 76–90 5* 7 0–18 Lateral ±25 >90 140** 13 110–150 Flexion –115Pliers Neutral 0 0 0–0 Rotation 0 1511 401 16–30 0 0 0–0 Lateral 0 31–45 0 0 0–0 Flexion –115 46–60 <1 <1 <1–1 Rotation 0 1930 501 61–75 <1 1 0–2 Lateral ±25 76–90 6* 7 1–23 >90 141** 9 126–150Notes. Total sample time = 150 s; PT—MAX RB 392 3.5. Productivity Analysespower tier (MAX USA Corp., USA); PTE—MAX RB392 power tier plus adjustable extension; *p < .05(comparing differences between PT + PTE and pliers Tying with the PT or PTE is faster than+ PTE), **p < .0001 (comparing differences between using the pliers. The mean number of ties perPT + PTE and pliers + PTE). 2.5 min completed using the pliers, PT, and PTE, respectively, was 42 (±6.8), 84 (±10.3), and 52 (±9.0). Tying times were significantly3.4. Other Analyses different for the three techniques.Using 3DSSPPTM [9], compressive and shearforces acting on the L5/S1 disc were estimated 4. DISCUSSIONto be between 1 511 and 2 857 N during two-hand rebar tying using the pliers without lateralbending or trunk rotation (Table 6). Lateral 4.1. Risks Associated with Pliers Tyingbending (25° left or right) increased both totaland shear compression forces when the trunk The ACGIH™ HAL TLV™ and the CTDinclination did not change. Highest estimated risk estimates developed by Marras andshear forces were 501 N, at the deepest trunk Schoenmarklin [12] assume exposure occurs,inclination (–15°) and 25° lateral bending. respectively, for ≥4 hrs and 8 hrs each day. The RIWs reported tying rebar for about 16 hrs per week using the pliers or PT. Tying rebar using the pliers for 4 or more hours each day exceeded the ACGIH™ HAL TLV™ threefold. Using theJOSE 2007, Vol. 13, No. 3
  9. 9. REBAR TYING BIOMECHANICS 287PTE for 4 hrs or more each day slightly exceeded concrete slab) reduces workers’ exposures tothe HAL TLV™. low-back disorder risk factors. During pliers tying, wrist motion (i.e., meanvelocity and acceleration) in the flexion/extension 4.2. Tying Using the PT and PTEand ulnar/radial planes was about twice as fast as Use of the PT resulted in mean hand-activitythe motion associated with either low and high level scores below the ACGIHTM HAL TLV™.CTD risk reported by Marras and Schoenmarklin The PTE resulted in the lowest angular velocity[12]. The upper limb CTD risk assessment they and acceleration rate measured, but also in aconducted was for an 8-hr work day. RIWs who mean hand-activity level slightly higher than thedo not continuously tie rebar with pliers for 8 hrs TLV™. This difference can be explained by themay not exceed these parameters. higher perceived effort scores RIWs gave the PTE Tying using pliers requires rapid wrist on the Borg Modified CR-10 questionnaire [7].movement in all three planes of motion in Unlike the PT, workers did not have experiencecombination with hand forces necessary to pull, using the extension prior to the study. Use of thetwist, and cut the wire. Marras and Schoenmarklin extension increases the distance of the hand to the[12] did not describe the combined effect of tying location, and possibly reduces control of therapid motion in multiple planes and, therefore, tool during placement. Perhaps more importantly,the CTD risk levels reported may underestimate positioning and holding the PTE away from theactual risk when rapid motions are required in body results in higher forces on the hand, arm,all three planes. The results of the study show and shoulder due to the associated larger moment.that manually tying rebar using pliers exposes These factors likely increased the perceived effortworkers to risk factors for developing WMSDs of scores during PTE use and could be addressedlow back and the upper limbs. Use of the power with training. There was no significant differencerebar tier significantly reduced workers’ exposure between the wrist movements (i.e., velocity andto the risk factors for upper limb CTDs. acceleration) when comparing the use of the PT Observed trunk postures workers used during and PTE.pliers and PT rebar tying involved 90° of trunk In a study to evaluate the potential reduction inflexion over 90% of the time which exceeded the the risk of WMSDs to RIWs when using the PTEISO standard [14] recommendations. The ISO [18], Vi found that RIWs reported subjectivestandard recommends that working trunk postures scores in the very good to good range when usingnot exceed 60° of forward bending at any time. the PTE for the categories of level of comfort, Estimated low-back (L5/S1) compressive hand force, hand/wrist discomfort, shoulderforces did not exceed the NIOSH recommended discomfort, ease of use, and productivity. In Vi’sspinal compression force (3 400 N) [16] during study, where the RIWs had prior experience withpliers tying. These forces, however, are exerted using the PTE, they expressed a preference foron the low back for several hours each day using the PTE over the pliers method.thereby resulting in high cumulative force over In the current study, use of the PTE resulted inmany years of activity, which have been reported nearly eliminating forward bending during rebarto increase the risk of developing a low-back tying. If workers receive instruction before usingdisorder [10]. In addition, a recent experimental the extension handle, i.e., correct adjustmentstudy has shown that static lumbar flexion, of the handle height and the advantage of tyingwhich occurs during extreme trunk flexion, is a close to the body, the additional stress on therisk factor for developing a low-back disorder upper extremities will be reduced.[17]. RIWs tying rebar placed at ground level Although the PT trunk positions were notmay be at increased risk of developing low-back very different from the pliers trunk positions, itdisorders. Using an extension handle with the is reasonable to assume there is less loading onpower rebar tier whenever RIWs tie rebar placed the lumbar spine. The PT only requires the useat ground level, (e.g., bridge and freeway deck, of one hand to tie rebar and all workers were JOSE 2007, Vol. 13, No. 3
  10. 10. 288 J.T. ALBERS & S.D. HUDOCKobserved using their free hand to support their The study was not conducted in a manner thatupper body weight during the majority of the could determine the real productivity differencestime spent tying with the PT. The use of the hand among the three tying techniques. Using the PTor arm to support the upper body should reduce nearly doubled the number of ties completedthe compressive forces in the lumbar spine. during the analysis periods, while the PTE resultedParticipants appear to have confirmed this when in a slight increase. The observation time, however,they reported significantly less perceived effort was too short to consider possible nonproductive(Borg CR-10) for the low back using the PT than time related to using the PT, such as the potentialthe pliers (2.8 for the PT and 5.8 for the pliers), productivity loss due to changing batteries anddespite the similar posture. PT mechanical failure (e.g., wire jam). Despite this shortcoming, both RIWs and management4.3. Other Activities representatives expressed confidence that the PT increased workers’ productivity.Although RIWs perform additional job activities It would be beneficial if future research werethat require “maximum muscle force to lift, push, conducted to characterize the following aspects ofpull, or carry objects” [19], NIOSH analyzed tying reinforcing steel: (a) the types of reinforcedonly rebar tying during this study due to the concrete construction that would benefit from thenature of the request and time constraints. For use of PTs; (b) the optimal design of an extensionexample, in addition to lifting and carrying rebar, handle for power rebar tiers; and (c) RIWs’workers must also separate individual rebar from biomechanical loading, especially to the back andthe bundles transported to the immediate work shoulders, during rebar manual material handling.area. Rebar transported in bundles can becomeintertwined, which makes the separation ofindividual rebar lengths difficult. Workers were 5. CONCLUSIONSobserved separating rebar using sudden (i.e.,jerking) muscle forces—often in stooped and The preferred method for preventing andasymmetrical postures—to separate individual controlling WMSDs is to reduce or eliminaterebar from the bundles. exposure to the risk factors. The most effective way to do this is to design jobs, workstations, tools, and other equipment to match the4.4. Study Limitations physiological, anatomical, and psychologicalThere are several limitations of this study. characteristics and capabilities of the worker [21].The sample size was small consisting of only Under these conditions, exposures to task factors8 subjects. However, this was limited to the considered potentially hazardous will be reducedavailable workforce at the participating site. The or eliminated.participants’ time using the PTE was extremely Tying rebar using the PT significantly reducedshort, measured in minutes rather than hours. hand and wrist movements that can contribute toSubsequently, most, if not all, participants had upper limb WMSDs. Deep forward bending wasinsufficient time to become accustomed to using still necessary to tie the rebar, but the free armthe extension. The weight of the PTE (3 kg) was used to support the weight of the trunk, mostwas significantly heavier than the pliers (0.5 kg) likely reducing forces on the L5/S1 disc.and so the wrist, elbow, and shoulder may be Tying rebar using the PTE eliminated thesubjected to larger biomechanical loads when sustained deep forward bending required whenthe tool is positioned and held away from the tying with the pliers and PT. Wrist movementbody [20]. Effects for other body parts possibly was significantly lower in all three planes ofaffected, including the neck, were not evaluated. motion using the PTE. Some participants,Inexperience using the extension may also however, reported hand–wrist effort similar toexplain some participants higher hand–wrist using the pliers, resulting in a mean HAL scoreperceived effort ratings. slightly higher than the ACGIH™ HAL TLV™.JOSE 2007, Vol. 13, No. 3
  11. 11. REBAR TYING BIOMECHANICS 289 Worker productivity, as measured by the 11. American Conference of Governmentalnumber of ties completed, increased when the PT Industrial Hygienists (ACGIH). Handor PTE were used during the study. activity level Threshold Limit Value™. In: ACGIH™ TLV/BEI documentation. Cincinnati, OH, USA: ACGIH; 2001.REFERENCES 12. Marras WS, Schoenmarklin RW. Wrist1. U.S. Department of Labour. Bureau of motions in industry. Ergonomics. 1993; Labor Statistics. Table 1. Incidence rates of 36(4):342–51. nonfatal occupational injuries and illnesses 13. Schoenmarklin RW, Marras WS, Leur- by industry and case types, 2004. Retrieved gans SE. Industrial wrist motions and risk December 6, 2005, from: http://www.bls of cumulative trauma disorders in industry. .gov/iif/oshwc/osh/os/ostb1487.pdf Ergonomics. 1994;37(9):1449–59.2. Forde MS, Punnett L, Wegman DH. Prev- 14. International Organization for Standard- alence of musculoskeletal disorders in ization. Ergonomics—evaluation of static union ironworkers. J Occup Environ Hyg. working postures (Standard No. ISO 2005;2:203–12. 11226:2000). Geneva, Switzerland: ISO;3. Buchholz B, Paquet V, Punnett L, Lee D, 2000. Moir S. PATH: a work sampling-based 15. Waters TR, Putz-Anderson V, Garg A. approach to ergonomic job analysis for Revised NIOSH equation for the design construction and other non-repetitive work. and evaluation of manual lifting tasks. Appl Ergon. 1996;27:177–87. Ergonomics. 1993;36(7):749–76.4. Forde M, Buchholz B. Task content and 16. National Institute for Occupational Safety physical ergonomic risk factors in con- and Health (NIOSH). Work practices struction ironwork. Int J Ind Ergon. 2004; guide for manual lifting (NIOSH Technical 34:319–33. Report No. 81-122). Cincinnati, OH, USA:5. Biometrics. Management and analysis, U.S. Department of Health and Human version 3.0. Cwmfelinfach, Gwent, UK: Services, NIOSH; 1981. Biometrics, Ltd.; 2003. 17. Solomonow M, Baratta RV, Banks A,6. Buchholz B, Wellman H. Practical operation Fruedenberger C, Zhou BH. Flexion– of a biaxial goniometer at the wrist joint. relaxation response to static lumbar flexion Hum Factors. 1997;39(1):119–29. in males and females. Clin Biomech7. Borg G. Borg’s perceived exertion and (Bristol, Avon). 2003;18:273–9. pain scales. Champaign, IL, USA: Human 18. Vi P. Promoting early return to pre-injury Kinetics; 1998. job using a rebar-tying machine. J Occup8. University of Wisconsin–Madison; Ergono- Environ Hyg. 2005;2:D34–7. mics analysis and design research consor- 19. Occupational Information Network. Re- tium. User’s manual for Multimedia Video inforcing iron and rebar workers (47- Task Analysis™ (MVTA™). Madison, WI, 2171.00). O*Net OnLine. Retrieved USA: University of Wisconsin–Madison; December 6, 2005, from: http://online 2001. .onetcenter.org/link/summary/47-2171.009. University of Michigan; Center for Ergo- 20. Marras WS. Biomechanics of the human nomics. 3-D Static Strength Prediction body. In: Salvendy G, editor. Handbook of Program™ Version 4.3. Ann Arbor, MI, human factors and ergonomics. New York, USA: University of Michigan; 2002. NY, USA: Wiley; 1997.10. Seidler A, Bolm-Audorff U, Heiskel H, 21. National Institute for Occupational Henkel N, Roth-Küver B, Kaiser U, et al. Safety and Health (NIOSH). Elements of The role of cumulative physical work load ergonomics programs: a primer based on in lumbar spine disease: risk factors for workplace evaluations of musculoskeletal lumbar osteochondrosis and spondylosis disorders (NIOSH Publication No. 97-117). associated with chronic complaints. Cincinatti, OH, USA: U.S. Department of Occup Environ Med. 2001;58:735–46. Health and Human Services, NIOSH; 1997. JOSE 2007, Vol. 13, No. 3

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