See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/41002100 The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during... Article in Ergonomics · January 2010 DOI: 10.1080/00140130903323232 · Source: PubMed CITATIONS READS 15 32 3 authors: Jaclyn N Hurley (Chopp) Steven L Fischer 21 PUBLICATIONS 114 CITATIONS 36 PUBLICATIONS 153 CITATIONS University of Waterloo SEE PROFILE University of Waterloo SEE PROFILE Clark R Dickerson University of Waterloo 88 PUBLICATIONS 640 CITATIONS SEE PROFILE All content following this page was uploaded by Steven L Fischer on 27 April 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. This article was downloaded by: [Canadian Research Knowledge Network] On: 18 January 2010 Access details: Access Details: [subscription number 918588849] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 3741 Mortimer Street, London W1T 3JH, UK Ergonomics Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713701117 The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work Jaclyn N. Chopp a; Steven L. Fischer a; Clark R. Dickerson a a Department of Kinesiology, University of Waterloo, 200 University Avenue W, Waterloo, Canada Online publication date: 12 January 2010 To cite this Article Chopp, Jaclyn N., Fischer, Steven L. and Dickerson, Clark R.(2010) 'The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work', Ergonomics, 53: 1, 83 — 91 To link to this Article: DOI: 10.1080/00140130903323232 URL: http://dx.doi.org/10.1080/00140130903323232 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Ergonomics Vol. 53, No. 1, January 2010, 83–91 The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work Jaclyn N. Chopp, Steven L. Fischer and Clark R. Dickerson* Department of Kinesiology, University of Waterloo, 200 University Avenue W, Waterloo, ON N2L 3G1, Canada Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 (Received 28 July 2008; final version received 5 September 2009) Overhead work has established links to upper extremity discomfort and disorders. As many jobs incorporate working overhead, this study aimed to identify working conditions requiring relatively lower muscular shoulder load. Eleven upper extremity muscles were monitored with electromyography during laboratory simulations of overhead work tasks. Tasks were defined with three criteria: work configuration (fixed, stature-specific); target angle (7158, 08, 158, 308 from vertical); direction of applied hand force (pulling backwards, pushing forwards, downwards, sideways, upwards). Normalised electromyographic activity was greater for fixed configurations, particularly when pulling in a backward direction (total activity ¼ 108.3% maximum voluntary exertion (MVE)) compared to pushing down or forward (total activity ranging from 10.5 to 17.3%MVE). Further, pulling backwards at angles of –158 and 08 showed the highest muscular demand (p 5 0.05). These results suggest that, if possible, positioning overhead work in front of the body with exertions directed forwards will result in the lowest upper extremity muscle demand. Statement of Relevance: Overhead work pervades occupational settings and is associated with risk of upper extremity musculoskeletal disorders. The muscular intensity associated with performing overhead work was assessed in several combinations of work placement and hand force direction. These findings should have utility for designing overhead work tasks that reduce muscular exposure. Keywords: electromyography; overhead work; work design; muscle demand 1. Introduction Upper extremity discomfort resulting from working overhead is a common concern in the modern workplace (Bernard 1997). As it may not be possible to modify or design all jobs to eliminate overhead working components, it may be beneficial to identify preferable work designs that minimise shoulder muscular demands. This may in turn decrease shoulder discomfort and risk of pathology. Over the past 10 years, an annual average of 5700 shoulder-related injury claims were reported to the Workplace Safety and Insurance Board of Ontario (2006). Furthermore, Silverstein et al. (1998) examined the incidence of work-related upper extremity disorders in the US state of Washington from 1987 to 1995 and found that shoulder disorders constituted 43.2% of lost time claims per year, with a mean of 6146 claims per year. This highlights the need for more shoulder-focused workplace design. Overhead work is strongly associated with the development of upper extremity discomfort and disorders (Bernard 1997, Punnett et al. 2000, Grieve and Dickerson 2008). Specifically, past research indicates *Corresponding author. Email: cdickers@uwaterloo.ca ISSN 0014-0139 print/ISSN 1366-5847 online Ó 2010 Taylor & Francis DOI: 10.1080/00140130903323232 http://www.informaworld.com that awkward postures, that is, deviation from a neutral posture in extension, flexion and abduction greater than 458, require a high level of muscle activity (Herberts et al. 1984). Furthermore, Punnett et al. (2000) identified an association between shoulder flexion and abduction greater than 908 (for 410% of the work cycle) and shoulder disorder development. Sporrong et al. (1998) earlier noted this association for overhead precision tasks. Additionally, humeral elevation above 608 is known to decrease the subacromial space, potentially leading to development of supraspinatus and biceps tendon impingement (Flatow et al. 1994, McFarland et al. 1999, Bey et al. 2007). Several risk factors may contribute to upper extremity discomfort, including task repetition, high hand force, awkward postures, direct pressure, vibration and prolonged constrained postures (Rempel et al. 1992). Although these risk factors may exist in the absence of elevated arm posture, their presence, in addition to working overhead, may place the upper extremity at elevated risk. Specifically, prolonged activity in overhead working postures creates strain and fatigue on shoulder muscles (Herberts and Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 84 J.N. Chopp et al. Kadefors 1976). A strong relationship also exists between muscle fatigue and increased ratings of discomfort (Oberg et al. 1994). Furthermore, postural discomfort is known to occur when the arms are required to work overhead (Wiker et al. 1989). It follows that workers performing jobs with overhead components are at an increased risk of developing shoulder disorders (Svendsen et al. 2004, Miranda et al. 2005). Despite the strong association between overhead work and musculoskeletal disorders, it is sometimes difficult to avoid these postures in practice. However, in industrial tasks that require arm elevation above shoulder height, flexibility in the location of the task with regard to the body and the required hand force direction may exist. Increased muscular demand as documented through electromyography has been linked to shoulder pain and discomfort (Wiker et al. 1989). Thus, if muscle activity required to produce the same target force is decreased at a specific work configuration or with a specific direction of force application, then that condition lowers muscular demand and can be considered more optimal in terms of lowering injury risk. Working configurations, target angles and hand force directions have all been examined individually in the literature; however, limited research examines how these factors interact. Extensive research exists to relate specific arm angles, working heights and upper extremity discomfort (Flatow et al. 1994, Sporrong et al. 1998, McFarland et al. 1999, Palmerud et al. 2000, Punnett et al. 2000, Anton et al. 2001, Garg et al. 2002, Svendsen et al. 2004, Okunribido and Haslegrave 2008). Further, existing research generally suggests eliminating certain types of overhead work by recommending specific arm angles and postures to avoid; for example, arm flexion or abduction over 908 (Bernard 1997, Punnett et al. 2000). However, few studies have evaluated the implications of different hand force directions (a notable exception is Haslegrave et al. 1997) in conjunction with work height. During sustained work, postures or force application directions that require low-level muscle activation may be less likely to trigger the development of a musculoskeletal disorder than those that require higher activations. Higher muscle activation may also accelerate fatigue development, which is associated with an increased probability of postural discomfort (Wiker et al. 1990). The aim of this research is to quantify shoulder muscle activity for several overhead working configurations, target angles and hand force directions to determine how differences in overhead work conditions influence specific muscle activation. The results from this research will help in establishing which working postures minimise muscle activity, encourage safer job design and assist with injury prevention in jobs that require overhead work. The goal is to determine which of the set of overhead working conditions minimises shoulder muscular load. 2. Method 2.1. Participants In total, 14 right-hand dominant, university-aged male students (aged 22 + 2 years) participated in the study. Male participants were selected for ease of comparison with earlier published results and subject availability. Participants had a mean height and mass of 1.85 + 0.08 m and 85.6 + 7.5 kg respectively. Participants were excluded from participation if they self-reported any upper extremity disorders within the past year. The research protocol was approved by the university ethics review board. 2.2. Equipment Exerted hand force and muscle activation were measured in this study. Force was quantified using a multiaxis loadcell (Figure 1) (MC3A; AMTI, Watertown MA, USA). Visual force feedback was provided through a custom designed program written with Labview software (National Instruments, Austin Texas, USA), which provided visual feedback to the participant on the level of force applied to the force transducer. Feedback was displayed on three bar graphs corresponding to the direction of force application; forward/backward (X), upward/ downward (Y), medial (Z). Each graph had a 30 N threshold line. Subjects were instructed to exert the specified force level on the transducer until the Figure 1. Diagram showing the five hand force directions: pulling backwards and pushing upwards, downwards, forwards and sideways (A–E). The exertion is an open-handed push or pull with the palm of the hand centred on the opposing face of the force transducer. Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 Ergonomics moving bar on the screen met the threshold line. Offaxis force levels were monitored and kept below 5 N or the trial was repeated. They were given ample practice before commencing trials and collected force values indicated that participants were capable of staying within +5 N for each trial. Surface electromyographic (EMG) signals were collected using the Noraxon T2000 telemetered system (Noraxon, Scottsdale, Arizona, USA). Force and electromyography were measured at 1500 Hz, synchronised through the Vicon Nexus 1.2 software (Oxford, UK). Target angles were confirmed using a goniometer. EMG signals were collected from 11 different sites, overlying muscles on the right side of the upper extremity using bi-polar Ag-AgCl Noraxon dual surface electrodes with a fixed 2 cm inter-electrode spacing (Noraxon). Specifically, electrodes were placed on the biceps brachii, triceps brachii, anterior deltoid, middle deltoid, posterior deltoid, upper trapezius, lower trapezius, latissimus dorsi, infraspinatus and pectoralis major (both clavicular and sternal insertions) using published placements (Cram and Kasman 1998). Prior to electrode placement, the skin overlaying the muscle was shaved and cleansed with alcohol to minimise impedance. 2.3. Experimental procedures 2.3.1. Experimental protocol Muscle specific maximal voluntary exertions (MVEs) were performed for the 11 muscles monitored using the protocol recommended by Cram and Kasman (1998). Each exertion was 5 s in duration, during which participants ramped up to their maximum (1 s), sustained their maximum (3 s) and then ramped down (1 s). Participants performed three repetitions of each MVE in order to increase the reliability of the results (Fischer et al. 2009). In addition, there was a minimum of 2 min rest between each exertion to minimise the likelihood of fatigue and promote recovery (Mathiassen et al. 1995). Following instrumentation set-up and the completion of the MVE protocol, participants were seated under the force transducer. Three overhead work parameters were manipulated: working configuration; target angle; hand force direction. The work configuration variable was defined by either ‘fixed’ or ‘stature scaled’. In the fixed condition, the transducer was placed at 120 cm from the stool; independent of the other two variables, the height of the force transducer remained the same for all trials. This height was chosen as it reflects the maximum reach distance of a 5th percentile male and thus the study findings would be germane to the entire working population (Chaffin 85 et al. 2006). It was also low enough to constitute an overhead work location for the range of tested statures. In the stature scaled condition, the transducer spatial location was anthropometrically scaled to the participant and was placed at a distance of 70% of the participant’s maximum overhead reach, measured vertically from the greater trochanter to the centre of the palm. This distance was maintained along all target angles (Figure 2). Within each of the two work configurations, four target angles were examined: 158; 08; 158; 308. These angles were defined as the angle (with a positive angle being a clockwise rotation) between vertically upright and a line connecting the centre of the participant’s pelvis (the 3-D centre of the greater trochanters) to the centre of the force transducer, while the participant was seated (Figure 2). Thus, a 08 target angle resulted in the transducer being located directly overhead and positive angles creating an overhead reach in front of the body and negative angles behind the body. Within each target angle participants exerted 30 N of force onto the force transducer, in each of five globally defined hand force directions: pushing forward, sideways (medial), downward, upward, and pulling backward (Figure 1). Each combination (work configuration-target angle-hand force direction) lasted 5 s and was repeated three times; all combinations were randomised. The completion of the protocol entailed 120 sub-maximal exertions: two work configurations 6 four target angles 6 five hand force directions 6 three repetitions. Participants were given a minimum of 1 min rest between trials, with the option of more rest at their discretion. 2.3.2. Data analysis Raw EMG signals were band pass filtered from 10–500 Hz and differentially amplified (common-mode rejection ratio 4100 dB at 60 Hz, input impedance 100 MO) to generate maximum signal (in the range of the A/D board). EMG signals were A/D converted using a 16 bit A/D card with a +3.5 V range. Following collection, EMG signals were full wave rectified and filtered at 6 Hz (Dickerson et al. 2008) using a fourth order low pass Butterworth filter to produce a linear enveloped EMG response, then normalised to MVEs outlined in the experimental protocol. Within-trial means in normalised electromyography were calculated for each muscle. Then, the sum of all mean muscle activities during each combination was calculated to give an estimate of total shoulder effort for each trial. The rationale for this calculation is the assumption that lower total muscle activity may prevent potential injury and, Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 86 J.N. Chopp et al. Figure 2. Diagram showing both the fixed (top) and stature scaled (bottom) work configurations with target angles of 308, 158, 08, –15 (left to right). In the fixed configuration condition, the height of the force transducer remains consistent at 120 cm from the stool. In the stature-specific condition, the cube is moved vertically and horizontally to maintain the hip–hand distance at each specified angle (note the varying vertical location of the transducer relative to the participant). thus, total activity gives a holistic representation of muscular effort. A 2 6 4 6 5 repeated measures ANOVA was used to determine the effects of work configuration (fixed and stature-scaled), target angle (7158, 08, 158, 308) and force application direction (forward, backward, upward, downward, sideways) on total normalised EMG activity. A p-value of 0.05 was used to determine significance. Pairwise comparisons were examined post hoc using a Tukey with Bonferroni adjustments to ensure a strict p value, and reduce the risk of type 1 error. Effect size was calculated using the method outlined by Cohen (1988). All statistical analysis was done using JMP software (SAS Institute, Cary, NC, USA). 3. Results Significant influences of the tested factors, both main effects and interactions, were found that included all three manipulated condition levels (work configuration, target angle and hand force direction) for both total and individual muscle activity. 3.1. Total muscle effects Work configuration, target angle and hand force direction all independently and in conjunction influenced total muscle activity. Main effects of each of the individual factors were present in work configuration [F (1, 506) ¼ 28.5], target angle [F (3, 506) ¼ 11.4] and direction [F (4, 506) ¼ 289.1]. Also, two-way interaction effects were found between work configuration and direction [F (4, 506) ¼ 19.4] and target angle and direction [F (12, 506) ¼ 8.7]. The twoway interaction between work configuration and target angle and the three-way interaction between work configuration, target angle and direction were not found to be overall significant (Table 1). Thus, hereafter only interaction effects between work configuration and direction, and target angle and direction will be discussed. 3.1.1. Work configuration * direction interaction A statistically significant (p 5 0.05) interaction existed between the two work configurations (fixed, stature scaled) and five hand force directions (backwards, forwards, downwards, sideways, upwards). The combination with the highest total normalised electromyography was pulling backwards at a fixed configuration (total muscle activity ¼ 108.3%MVE) (Figure 3). A distinct pattern existed for work configuration–direction combinations. The level of activity from highest to lowest was first dictated by direction (backward, sideways, upwards, downwards, forwards), but within each direction (for backward, sideways and upwards) the fixed condition had significantly higher activity than the stature scaled. In contrast, the directions ‘downward’ and ‘forward’ 87 F(12,506) ¼ 1.08 F(4,506) ¼ 1.02 F(3,506) ¼ 0.89 F(4,506) ¼ 1.62 F(3,506) ¼ 0.56 F(1,506) ¼ 1.09 Figure 3. Comparison of total muscle activity (11 muscles) required to maintain 30 N of force between different work configurations * directions of hand force interactions; significantly different conditions (p 5 0.05) are indicated by different letters. MVE ¼ maximum voluntary exertion. showed no significant difference in activity between each other for either work configuration (total muscle activity ranged from 10.5 to 17.3%MVE for fixed-forward to stature-downward respectively). 3.1.2. Note: F ratios shown in bold indicate the main effects and interactions that were significant. ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ 7.42 5.56 1.81 1.95 7.29 3.07 5.11 4.27 6.65 1.08 ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ F(4,506) ¼ 5.59 F(4,506) ¼ 22.35 F(4,506) ¼ 2.19 F(4,506) ¼ 3.43 F(4,506) ¼ 3.86 F(4,506) ¼ 1.83 F(4,506) ¼ 21.63 F(4,506) ¼ 2.40 F(4,506) ¼ 11.58 F(4,506) ¼ 1.75 F (3,506) ¼ 4.31 F(3,506) ¼ 4.72 F(3,506) ¼ 2.58 F(3,506) ¼ 0.41 F(3,506) ¼ 1.25 F(3,506) ¼ 3.49 F(3,506) ¼ 9.27 F(3,506) ¼ 18.46 F(3,506) ¼ 2.48 F(3,506) ¼ 1.34 Anterior Deltoid Middle Deltoid Posterior Deltoid Biceps Triceps Latissimus Dorsi Upper Trapezius Lower Trapezius Infraspinatus Pectoralis Major (C) Pectoralis Major (S) F(1,506) ¼ 4.07 F(1,506) ¼ 69.47 F(1,506) ¼ 2.18 F(1,506) ¼ 4.65 F(1,506) ¼ 2.67 F(1,506) ¼ 0.42 F(1,506) ¼ 53.87 F(1,506) ¼ 0.30 F(1,506) ¼ 12.68 F(1,506) ¼ 1.23 F(4,506) ¼ 227.93 F(4,506) ¼ 104.54 F(4,506) ¼ 3.00 F(4,506) ¼ 90.10 F(4,506) ¼ 31.31 F(4,506) ¼ 53.56 F(4,506) ¼ 156.11 F(4,506) ¼ 68.11 F(4,506) ¼ 232.61 F(4,506) ¼ 17.59 F(3,506) ¼ 0.27 F(3,506) ¼ 3.45 F(3, 506) ¼ 0.86 F(3,506) ¼ 0.73) F(3,506) ¼ 2.87 F(3,506) ¼ 0.26 F(3,506) ¼ 3.70 F(3,506) ¼ 0.50 F(3,506) ¼ 0.63 F(3,506) ¼ 0.16 F(12,506) ¼ 1.18 F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) F(12,506) 0.59 0.86 1.35 0.29 1.12 0.30 0.63 0.31 0.54 0.60 Configuration * Angle * Direction Angle * Direction Configuration * Direction Configuration * Angle Direction Angle Configuration Muscles ANOVA results summary of all 11 muscles demonstrating main effects and interactions. Table 1. Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 Ergonomics Target angle * direction interaction A statistically significant (p 5 0.05) interaction existed between the four angles (7158, 08, 158, 308) and five hand force directions (backwards, forwards, downwards, sideways, upwards). Pulling backwards at angles of –158 and 08 showed the highest muscular demand and were not significantly different from one another (Figure 4). Further, higher muscle demand was present when pulling backwards at the two remaining angles than for any other hand force direction. Within each angle, backward pulling and sideways pushing produced higher muscle activity than other directions. An exception to this was at a 308 target angle. At this angle, pushing upwards had significantly higher activity (43.4%MVE) than sideways (29.8%MVE). 3.2. Individual muscle contributions Specific muscles contributed more toward achieving 30 N of hand force than others, with some having a combination of significant main effects and interactions (Table 1). Further, the activity elicited from each muscle during work configuration–hand force direction interactions and arm angle–hand force direction interactions varied considerably. Specifically, the work configuration–hand force direction combination that resulted in the largest level of total muscle activity was the ‘fixed configuration–backward hand force direction’ combination. In this 88 J.N. Chopp et al. Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 combination, certain muscles were far more activated than in the combination that resulted in the lowest level of total muscle activity (‘fixed configuration– forward hand force direction’) (Figure 5). Anterior deltoid, middle deltoid, biceps, lower trapezius and infraspinatus had 90% higher activation during the more ‘difficult’ work configuration–force combination than the ‘easier’ combination; ‘difficult’ was defined as the combination requiring the highest level of muscle activity and ‘easy’ as the combination requiring the lowest level of muscle activity. The remaining muscles showed relatively minimal change in activity level during different combinations. The target angle–hand force direction combination found to require the highest activation was ‘–158 target angle–backward hand force direction’ and lowest activation was ‘308 target angle–forwards hand force direction’. Similar to the work configuration–hand force interaction, anterior deltoid, middle deltoid, biceps, lower trapezius and infraspinatus showed the most variety between combinations, requiring the highest activation compared to lowest activation, with highest activation having 95% higher activity (Figure 6). 4. Discussion Figure 4. Comparison of total muscle activity (sum of % maximum voluntary exertion (MVE) for 11 muscles) required to maintain 30 N of force between different target angle * directions of hand force interactions; significant differences between conditions (p 5 0.05) are indicated by a difference in letters. This research quantified shoulder muscle activity for a variety of overhead working conditions to determine how differences in overhead work configurations, target angles and hand force directions influence muscular demands. The magnitude of normalised EMG activity was greater for the fixed height configurations (anthropometrics not considered) and when the target angle was –158 and when pulling backwards (effect size ¼ 4.1–6.3). These results highlight the utility of job modification to reduce shoulder muscular demands during overhead work, Figure 5. Comparison of individual muscle activity between ‘work configuration * hand force direction’ combination requiring highest level of total muscle activity (fixed configuration–backward hand force direction) and combination requiring lowest level of total muscle activity (fixed configuration–forward hand force direction) to maintain 30 N of force. MVE ¼ maximum voluntary exertion. Figure 6. Comparison of individual muscle activity between ‘target angle * hand force direction’ combination requiring highest level of total muscle activity (7158 target angle–backward hand force direction) and combination requiring lowest level of total muscle activity (308 target angle–forward hand force direction) to maintain 30 N of force. MVE ¼ maximum voluntary exertion. 89 Ergonomics Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 particularly in cases when overhead work is not entirely removable. 4.1. Work configuration in space Although work configuration did not influence activity to the same degree as hand force direction, the data suggested a stature scaled (anthropometric) workstation can reduce muscle activity for overhead work compared to a fixed working configuration (effect size ¼ 0.79). This finding is particularly pronounced with backward or sideways hand force directions (Figure 3). The anthropometrically scaled condition eliminated some excessive reaches and brought the task to an equal distance from the worker (Figure 2). These results coincide with Sood et al. (2007), who found that an increase in muscle activity is required (for anterior and middle deltoid) when the working height is moved from low to high. Further research showed that moving the task closer to the worker (moving up a step on a ladder) reduced muscle activity (Anton et al. 2001). Other research has shown that by reducing workstation dimensions, that is, by having a half reach instead of a full reach and lower height (elbow compared to shoulder), muscle activity was 3.5–3 times lower, respectively (Habes and Grant 1997). The present results support this previous research, which encourage the use of a scaled work design or a reduced reach distance in order to decrease muscle demands. 4.2. Angle of work from directly overhead As a main effect, target angle was found to be significant. However, results show that target angle interacts with hand force direction, in that certain target angles yield higher muscle activity dependent on hand force direction. Overhead arm angle has been previously examined by Anton et al. (2001), Haslegrave et al. (1997) and Garg et al. (2002). The present findings are similar to Anton et al. (2001), when the current data are limited to only the upward push force direction at close (08) and far (308) target angles (Figure 4). They only examined the effects of overhead arm angle, using an upward drilling task and were therefore not able to show the interactions discovered in the current study when using alternate force application directions at each of the angles. These interactions are important for job design to provide additional options for modification when the task is in a specified overhead location, but the worker is not constrained to a single posture. Analysis of muscle activity while reaching to a variety of overhead target angles provided insight into the demand on the shoulder. This quantification provided contrasting results with those examining other biomechanical factors. Garg et al. (2002) examined endurance times while holding various weights statically at varied overhead arm angles. Their research supports job design directed at minimising the external shoulder moment, allowing for greater endurance times. This is accomplished by reducing the applied force at the hand or reducing the perpendicular distance between the shoulder and the applied hand load. In their case, the applied hand load acted vertically downwards due to gravity; therefore, the lowest perpendicular distance and resulting lowest moments occurred when the arm was either placed parallel to the torso at the side or directly overhead (08). Their argument has limited application, however, as it lacks information about specific muscles. Although the moment created by external weights is greater when the arm is farther from an overhead position (in the case presented here at 308), this study found less total muscle activation was required in these forward reaches compared to when the target was placed directly overhead (08). The results from this study showed the target angle of –158 (158 backwards) required the most muscle activity (effect size ¼ 0.49–1.08), particularly when the hand force direction was also pulling backwards (Figure 4). Haslegrave et al. (1997) also examined the location of overhead tasks: 158 forward, 158 backwards, 158 medially and 158 laterally with regard to the vertical. Although forward and either side were not significantly different from one another, overall, participants had the lowest force capability in the backward direction. The results from this study compare with those of Haslegrave et al. (1997), as it is likely that the capacity to produce a greater force in the rearward angle position is less due to these findings that there is less muscle activation in reserve to increase force output. 4.3. Applied hand force direction Hand force direction was the most influential factor affecting muscle activity during the overhead working task examined. Most individual muscles had larger mean EMG recordings for the backward direction than for the forward and downward directions (Figures 5, 6). Further, pulling backwards had a higher level of total activity than all other hand force directions (effect size ¼ 4.1–6.3). This may be due to some gravitational assistance in pushing both forward and downward. Simply resting or leaning the hand on the force transducer in these directions may have contributed to producing the 30 N of required hand force (thus eliciting minimal shoulder muscle demands). Conversely, in the backward pull, the Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 90 J.N. Chopp et al. muscles would need to not only contribute to both the 30 N force, but also act to support the arm against gravity. Haslegrave et al. (1997) examined isometric strength capability for six different directions of force exertion (push, pull, medial, lateral, up, down) and found participants were most capable in the vertical direction (up or down). The results from this study showed that a downward push required the least muscle activity. Haslegrave et al. (1997) found that force capability decreased as pushes moved from vertical to horizontal (sagittal plane) and then to lateral (frontal plane). A similar pattern was found in this study, where activity was lower for those directions that corresponded to the greatest strength capability, with the exception of the backwards (horizontal) direction, which required the most activity overall. 4.4. Relationship to fatigue Determining a combination of working conditions that lower muscular demand may, in turn, decrease the risk of musculoskeletal injury. In exceeding the recommended arm angles (Bernard 1997) or exerting force in unfavourable directions (i.e. backwards), one may increase required muscle activity and in turn increase the probability of fatigue development (Rohmert 1973). Sood et al. (2007) stated that localised muscle fatigue is a valid concern when determining injury risk and, therefore, attempts should be made to minimise muscle activity and resulting fatigue. The present findings dovetail well with Sood et al. (2007), as the stature scaled work placement required less muscle activity, which would likely be less fatiguing. Further, the current results agree with recommendations that, if overhead work is unavoidable, all attempts should be made to keep the work as close to shoulder height as possible to reduce the likelihood of sustaining shoulder fatigue or discomfort (Wiker et al. 1989). Further, lower apparent muscular requirements for a stature-specific working condition support the notion of population scalability in work design, a stance historically advocated by ergonomists (Nussbaum 2001). Positioning the task further in front of the body (up to 308 forward from vertical) required lower shoulder muscle activity than directly overhead or behind the worker in nearly all cases. Thus, working in a target angle range of 15–308 from directly overhead is preferred when a forward or downward force application is used. If an upward push is required, then working directly above the head as close to the body as possible minimises shoulder muscular activity, but may pose issues with line-of-sight and neck fatigue. 4.6. Despite attempts to minimise the likelihood of error with the study design, there were certain limitations that should be addressed. Total muscle activity was used as a surrogate for shoulder muscular load and thus combinations resulting in higher total muscle activity were considered ‘better’ in terms of lowering the risk of upper extremity injury. There were, however, trials in which total muscle activity was lower but an individual muscle or muscles showed a greater response. This discrepancy would make it difficult to distinguish between advantageous and potentially dangerous combinations. Additionally, it was decided to perform the maximal exertions according to specific recommendations (Cram and Kasman, 1998). This is somewhat limited as EMG measurements are influenced by local body posture. However, to perform maximal exertions in each posture tested was prohibitive for both fatigue generation and time considerations. Thus, all trial values were normalised by the defined maximal exertions with the understanding of this limitation. 5. 4.5. Did any overhead work combination minimise shoulder muscular demand? When designing an overhead work task to minimise muscular demands, many factors are important. In order of greatest to least influence in the present experiment, these were: 1) force direction; 2) positioning of the work interface; 3) angular work placement. This study identified a benefit in terms of muscular demand associated with generating forward or downward compared to backward applied hand forces (or horizontal and up). Consequently, adjusting the direction of applied hand force appears to yield the largest benefits in terms of reducing muscle activity. Limitations Conclusion Certain tasks or jobs intrinsically demand overhead work postures. Therefore, it is important to determine if an overhead working condition exists that lowers muscular demand. The present results indicated that muscle demands in low-intensity overhead tasks are lowest when: 1) the required hand force is directed either forwards or downwards; 2) the task is scaled to individual anthropometry; 3) the task is moved closer. These findings delineate the muscular impact of performing overhead work across a range of conditions and may therefore be useful as a part of job design decisions where shoulder muscle demand is a concern. Ergonomics Acknowledgements Support for Jaclyn Chopp and Steven Fischer during completion of this study came from a Hallman Research Fellowship and an NSERC doctoral award, respectively. Additional project support came from an NSERC discovery grant held by Dr Clark Dickerson. Downloaded By: [Canadian Research Knowledge Network] At: 20:26 18 January 2010 References Anton, D., et al., 2001. The effect of overhead drilling position on shoulder moment and electromyography. Ergonomics, 44 (5), 489–501. Bernard, B., 1997. 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