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Brazilian Journal of Physical Therapy

Print version ISSN 1413-3555

Braz. J. Phys. Ther. vol.17 no.3 São Carlos May/June 2013

http://dx.doi.org/10.1590/S1413-35552012005000095 

Original Articles

30º inclination in handles of plastic boxes can reduce postural and muscular workload during handling

Luciana C. C. B.  Silva1 

Ana B.  Oliveira1 

Danilo C.  Silva2 

Luis C.  Paschoarelli2 

Helenice J. C. G.  Coury1 

1Department of Physical Therapy, Universidade Federal de São Carlos (UFSCar), São Carlos, State of São Paulo (SP), Brazil

2Department of Industrial Design, School of Architecture, Art and Communication, Universidade Estadual Paulista (UNESP), Bauru, SP, Brazil

ABSTRACT

BACKGROUND:

The handling of materials, which occurs in the industrial sector, is associated with lesions on the lumbar spine and in the upper limbs. Inserting handles in industrial boxes is a way to reduce work-related risks. Although the position and angle of the handles are significant factors in comfort and safety during handling, these factors have rarely been studied objectively.

OBJECTIVE:

To compare the handling of a commercial box and prototypes with handles and to evaluate the effects on upper limb posture, muscle electrical activity, and perceived acceptability using different grips while handling materials from different heights.

METHOD:

Thirty-seven healthy volunteers evaluated the handles of prototypes that allowed for changes in position (top and bottom) and angle (0°, 15°, and 30°). Wrist, elbow, and shoulder movements were evaluated using electrogoniometry and inclinometry. The muscle electrical activity in the wrist extensors, biceps brachii, and the upper portion of the trapezius was measured using a portable electromyographer. The recorded data on muscle movements and electrical activity were synchronized. Subjective evaluations of acceptability were evaluated using a visual analog scale.

RESULTS AND CONCLUSIONS:

The prototypes with handles at a 30° angle produced the highest acceptability ratings, more neutral wrist positions, lower levels of electromyographic activity for the upper trapezius, and lower elevation angles for the arms. The different measurement methods were complementary in evaluating the upper limbs during handling.

Key words: ergonomic design; usability; electrogoniometry; electromyography; grip perception

Introduction

Handling materials occurs in industrial sectors and during activities of daily living. This activity is considered to cause most of the health-related costs and problems in work environments1.

Preventative studies have evaluated the lumbar spine during handling activities2 at the expense of the upper limbs, even though the upper limbs are the second-most commonly affected region of the body when musculoskeletal disorders occur while performing handling activities3 4 5.

Inserting handles on boxes is one way to reduce occupational risks and increase safety during handling6. Handles provide greater comfort and efficiency7 8 and increase the maximum acceptable weight during handling9. However, the positioning of handles, which is a fundamental factor that has a great influence on the variables mentioned7, has not been thoroughly studied in an objective manner.

Drury et al.10 evaluated the use of handles during actual handling activities and found that inserting lateral handles close to the top and bottom surfaces of the boxes resulted in lower levels of discomfort, especially for heavy loads (13 kg). Chung and Wang11 evaluated two types of boxes with handles and found that handles with a 0° horizontal incline caused a large ulnar deviation of the wrist, whereas the box with 90° handles caused a large radial deviation. The authors suggested that box handles should be positioned at an angle between 30° and 45° so that the wrists could remain in a more neutral position during handling. Similarly, Wang et al.12 evaluated the effects of handle angles in relation to the maximum acceptable weight for handling and wrist deviation. These authors also recommended an angle between 30° and 45° to provide a better grip.

To evaluate the benefits of an ergonomic intervention, objective quantitative measures obtained simultaneously are required to evaluate the workload imposed when performing occupational activities13. Although studies have evaluated the position and angle of handles on industrial boxes7 11, only wrist movements were directly evaluated, indicating that there is also a need to evaluate other upper limb joints such as the elbows and shoulders. It is equally important that muscle activity, a significant biomechanical parameter, be evaluated, preferably in records obtained simultaneously.

Given the lack of studies that address the workload for the upper limbs during handling and the need for new box and handle designs that provide a better grip, the goal of this study was to compare the handling experience using a commercial box and prototypes. Both boxes were made from plastic material and developed especially for this study. The present study also aimed to evaluate the effects on upper limb posture, muscle electrical activity, and the perceived acceptability of different grip angles during simulated handling activities at different heights. The prototypes included handles that changed in position (top and bottom) and angle (0°, 15°, and 30°) with respect to the horizontal plane.

Method

Subjects

A sample calculation was made using the ENE software (V.2.0) to determine the number of subjects needed. To perform the calculation, the parameters for shoulder abduction were used with statistical power of 80%, a standard deviation of 14.50°, and between-group difference of 10°. Because the calculation showed that 34 subjects were needed for a sufficient sample, 40 male students were initially recruited.

The following inclusion criteria were used for the subjects: right-handedness, height between 1.65 and 1.75 m, and maximum body mass index of 29.9 kg/m2. The goal of subject recruitment was to obtain a homogeneous sample of individuals who were inexperienced with handling activities so that the study would not be influenced by prior training. Three students were excluded from the sample because they displayed one or more of the exclusion criteria: musculoskeletal symptoms, intolerance of palpation, skin lesions, general illness, or balance problems. The level of physical activity was evaluated, and the subjects who were considered athletes were also excluded.

The participants had an average age of 23.85±3.97 years, an average height of 1.71±0.03 m, and an average weight of 73.95±10.35 kg. This age group was chosen because it is included in the largest economically active group (20 to 29 years of age) in Brazil as of 200914.

The subjects were informed about the collection procedures and signed a free and informed consent form, which was approved by the Ethics Committee of the Universidade Federal de São Carlos - UFSCar, São Carlos, state of São Paulo (SP), Brazil under protocol number CAAE: 0054.0.135.000-07.

Plastic boxes and evaluated handles

A plastic box (55.5 × 36 × 31 cm) was designed especially for this study (Figure 1 - B.1 and C.1). The prototype had handles (13 × 4.5 cm) that could change position (top and bottom) on each side of the box and allowed for several different angles from the horizontal plane (0°, 15° and 30°), for a total of seven different box conditions: 1) A: regular commercial box; 2) B.1: prototype with handles at the top at a 0° angle; 3) B.2: prototype with handles at the top at a 15° angle; 4) B.3: prototype with handles at the top at a 30° angle; 5) C.1: prototype with handles at the bottom at a 0° angle; 6) C.2: prototype with handles at the bottom at a 15° angle; 7) C.3: prototype with handles at the bottom at a 30° angle. The handle inclinations were performed towards the radial deviation of the wrist.

Figure 1 The boxes used in the data collection: A = regular commercial plastic box; B.1 = prototype with handles in the high position at a 0° angle from the horizontal plane; C.1 = prototype with handles in the low position at a 0° angle from the horizontal plane. 

All of the boxes had a total mass of 15 kg, determined by previous studies15. The handles/grips were developed from polyester resin composites with calcium carbonate.

Handling activity

The boxes were moved between three surfaces: a fixed surface (FS) 102.5 cm above the ground, the ground (G), and an adjustable surface (AS). The adjustable surface was positioned in two ways: at the height of the greater trochanter of the right femur (GT) and at the height of the right acromion (A) of each of the subjects with respect to the ground. The fixed and adjustable surfaces were positioned perpendicular to each other. The complete task involved moving the box between the following positions: FS-G, FS-GT, and FS-A. The order in which the boxes were used for each of the surface heights was randomized. Each box was moved between each surface only once.

During the procedure, synchronized records were obtained by performing electromyography, inclinometry, and electrogoniometry of the upper limbs. After moving each box, the subject responded to a subjective scale to evaluate its acceptability16.

This study was conducted under the same experimental conditions as another study that evaluated cardboard boxes used by a local business17. Thus, although the goals and results of the two studies are completely different, certain aspects of the design and procedure were similar.

Record of muscle electrical activity of upper limbs

Active electrodes for surface electromyography (Model #DE-2.3, DelSys ®, Boston, USA) were used to capture the bilateral electromyographic activity (EMG) of the extensor muscles in the wrist, the biceps brachii, and the descending portion of the trapezius. The electrodes were attached to the skin with double-sided tape (DelSys ®).

The electrodes had the following characteristics: two parallel silver bars (1 mm2 x 1 cm) with an inter-electrode distance of 1 cm; Common Mode Rejection Ratio (CMRR) >92 dB; input impedance >1015Ω in parallel with 0.2 pF; voltage gain of 10 V/V ±1%; and noise of 1.5 µV (root mean square, RMS).

A self-adhesive reference electrode (5 x 5 cm) was also attached to the flexor surface of the right distal wrist. The rest signal was obtained while the subject stood with relaxed upper limbs for 30 seconds.

The signal conditioner (Myomonitor IV Wireless EMG System, DelSys ®), with a bandwidth of 20-450 Hz and noise <1.2 µV (RMS), improved the signals by 1000 V/V. The data sampling rate was 1000 Hz.

Before attaching the active electrodes, the skin under the electrodes was shaved and cleaned with alcohol. For the wrist extensors, the subject was asked to contract the forearm in the pronated position, enabling the palpation and identification of the muscle18. For the biceps brachii, the electrodes were attached at a third of the distance between the cubital fossa and the acromion19. For the descending portion of the trapezius muscle, the midpoint between the spinous process of the seventh cervical vertebra and the acromion was located, and the sensor was attached 2 cm laterally from this point20.

The signals were normalized using the maximum electrical activity obtained in two maximal voluntary isometric contractions (MVICs) of 5 seconds each. The MVICs of the extensor muscles, the biceps brachii, and the trapezius were registered using the procedure described by Akesson et al.21 and Freriks and Hermens19. The examiner provided verbal feedback while recording the MVICs.

Recording arm-lifting movements

To record arm-lifting motions, digital inclinometry (INC) sensors were used (Logger Teknologi HB, Akarp, Sweden). The accuracy and reliability of the sensors were 1.3° and 0.2°, respectively22. The acquisition frequency was 20 Hz.

The inclinometers were fixed below the deltoid muscle insertion using plastic plates because of the bulging in the region. The subject was asked to hold a weight of 2 kg vertically above the ground to record the reference position of the upper limbs23. The arms were elevated in the scapular plane to record the direction of the movement23.

Recording elbow and wrist movements

Electrogoniometers, models SG65 and SG110 (Biometrics ® , Gwent, UK), were used to record wrist movements (flexion(+)/extension(-) and ulnar(+)/radial(-) deviation) and elbow movements (flexion(+)), respectively. The acquisition unit DataLog (Biometrics ® , Gwent, UK) was used for recording and the storing data from the electrogoniometer (EGM).

The sensors were attached to the wrist and elbow joints according to the instructions provided by the manufacturer (Biometrics ® , Gwent, UK). The maximum error measured by the sensors in a test conducted prior to the data collection aligned with the manufacturer's recommendations24. The acquisition frequency was 20 Hz.

To analyze the recorded wrist movements, the subject maintained a neutral wrist position for one minute. The subject was asked to remain standing with elbows flexed at 90°, pronated forearms resting on a flat surface and hands in a neutral position. The neutral position of the elbows was recorded while the subject kept the elbows fully extended with palms facing the body for one minute.

Evaluation of acceptability

After handling each box at each height, the subject responded to a subjective acceptability scale. Using this scale, which consisted of a 100-mm horizontal line, the subject evaluated the contact between his hand and the new handle configurations. The anchors "lack of acceptability" (associated with a value of zero) and "maximum acceptability" (associated with the value of 10) were located at the left and right ends of the horizontal line, respectively16.

Data processing and analysis

The data from the EMG, INC, and EGM were processed using routines developed in MatLab ® (version 7.0.1, MathWorks Inc., Natick, USA). The Butterworth filter with zero phase delay was used to filter all of the signals, and the design and cutoff frequency were defined for each signal.

The EMG data were filtered by a 20-450 Hz fourth-order band-pass filter, and the RMS was obtained by windowing (with a 25-millisecond duration and 50% overlap). The maximum RMS value was identified in the central portion (discarding the first and last second of collection) of the MVIC for each muscle to normalize the signal. The 30-second average RMS was calculated and subtracted from the baseline recording to correct for possible noise.

The inclinometry data were processed to identify the reference positions, the direction of movement,22 and the elevation angles for the shoulders. Next, both the INC and the EGM data were filtered by a second-order low-pass filter with cutoff frequencies of 3 and 5 Hz, respectively. A residual analysis was performed to determine the cut-off frequencies. The average values obtained from the recordings of the neutral wrist and elbow positions were subtracted from the data collected during the box handling task.

An amplitude probability distribution function (APDF) analysis was conducted on the EMG and the movement (INC and EGM) data. The APDF is widely used to describe occupational workloads25 based on EMG and movement records26. This type of analysis was considered appropriate because the average value obtained for these signals had low representativeness.

The normality and homoscedasticity of the data were analyzed using the Shapiro-Wilks and Levene's tests, respectively. Because the assumptions were not met, the EMG, INC, and EGM results and the scores on the subjective acceptability scale were compared for each of the boxes at each handling height using the Kruskal-Wallis test (P<0.05) and the post-hoc Mann-Whitney test with the Bonferroni correction (P<0.002). The statistical analyses were conducted using SPSS 11.5 (SPSS Inc., Chicago, IL, USA).

The postures and movements of the joints were analyzed according to the preventative guidelines and reference values described in the literature16.

Results

Records of muscle electrical activity for the upper limbs

The average values and standard deviations of surface muscle activity (EMG), as well as the significant differences, are shown in Table 1 by muscle group. The 90th percentiles are shown because they represent the greatest muscle workload.

Table 1 Means and standard deviations (±SD) for right (R) and left (L) EMG of the biceps brachii (90th biceps), the wrist extensors (90th wrist extensors), and the upper trapezius descendens (90th upper trapezius) for the various types of boxes (1 to 7) at each handling height (FS-G, FS-GT, FS-A). The superscript numbers indicate a statistically significant difference between the boxes (P<0.002). FS-G: handling from a fixed surface to the ground; FS-GT: handling from a fixed surface to an adjusted surface at the height of the right great trochanter; FS-A: handling from a fixed surface to an adjusted surface at the height of the right acromion. The data are presented as percentages of maximum voluntary contraction. 

The lowest percentage values for the EMG in the trapezius muscle tended to occur when the subjects moved the C3 prototypes both to/from the low surfaces (FS-G) and high surfaces (FS-A). The values were high between the intermediate surfaces (FS-GT) for all of the boxes, but they were slightly lower for the B3 prototype. Regarding the muscle activity of the biceps brachii and the wrist muscles, smaller values were associated with the commercial box.

Higher percentage values of electromyographic activity (i.e., a greater muscle workload) were recorded when the boxes were moved between FS-GT for all of the muscle groups evaluated.

The data for the muscle electrical activity of the left wrist extensors for one individual were lost. Therefore, all of the electromyographic activity variables for that muscle group in that subject were eliminated from the analysis.

Recorded arm-lifting movements

With regard to the arm-lifting range of motion, the lowest values were recorded for the C3 prototype in all of the conditions evaluated (Table 2). The statistically significant differences are shown in Table 2.

Table 2 Means and standard deviations (±SD) for the right (R) and left (L) upper arm elevation (90th upper arm elevation) for the various types of boxes (1 to 7) at each handling height (FS-G; FS-GT; FS-A). The superscript numbers indicate a statistically significant difference between the boxes (P<0.002). FS-G: handling from a fixed surface to the ground. FS-GT: handling from a fixed surface to an adjusted surface at the height of the right great trochanter. FS-A: handling from a fixed surface to an adjusted surface at the height of the right acromion. The motion data are presented in degrees (°). 

The largest ranges of arm-lifting motion (and therefore the greatest postural workload) were observed when the subjects lifted commercial box A.

Recorded wrist and elbow movements

The average values and standard deviations collected with the electrogoniometer are shown in Table 3. The 10th and 90th percentiles are shown because they represent, respectively, the highest levels of postural workload for wrist extension (10th percentile) and ulnar deviation and elbow flexion (90th percentile). The statistically significant differences are also shown in Table 3.

Table 3 Means and standard deviations (±SD) for right (R) and left (L) elbow flexion (90th elbow flexion, +), wrist extension (10th wrist extension, -) and wrist ulnar deviation (90th wrist ulnar deviation, +) for the various types of boxes (1 to 7) at each handling height (FS-G; FS-GT; FS-A). The superscript numbers indicate statistically significant differences between the boxes (P<0.002). FS-GT: handling from a fixed surface to an adjusted surface at the height of the right great trochanter. FS-A: handling from a fixed surface to an adjusted surface at the height of the right acromion. The motion data are presented in degrees (°). 

None of the prototypes produced lower workloads for all of the joints we evaluated, but there were significant differences between different prototypes in different conditions.

With regard to elbow flexion, the B3 prototype (high handle at a 30º angle) produced ranges of motion that were closest to the biomechanical ideal (80º to 120°) in all the conditions evaluated. With regard to wrist extension, relatively smaller ranges of motion were observed when the subjects handled the commercial box (A) and the C1 prototype (high handle with no angle), whereas greater values (and therefore greater postural workloads) were recorded when the subjects handled the C3 prototype.

The ulnar deviation in the wrist was closest to the neutral range of motion when the subjects moved the C3 prototype from FS-G and FS-GT and when the subjects moved the B1 prototype from FS-A. The ulnar deviation values displayed greater ranges for the B1 prototype in the FS-G condition and for the A prototype in the FS-G and FS-A conditions.

Evaluation of handling preferences

The average values and standard deviations of the preferences of the participants for the different handles, recorded using the acceptability scale, are shown in Table 4.

Table 4 Means and standard deviations (±SD) of the acceptability scale for the various types of boxes (1 to 7) at each handling height (FS-G; FS-GT and FS-A). The superscript numbers indicate statistically significant differences between the boxes (P<0.05). FS-G: handling from a fixed surface to the ground. FS-GT: handling from a fixed surface to an adjusted surface at the height of the right great trochanter. FS-A: handling from a fixed surface to an adjusted surface at the height of the right acromion. 

Higher average values on the acceptability scale were found for the boxes with high handles (C2, A, and C3) when the boxes were moved toward the floor or picked up from the floor. Moving the boxes to the high surface prompted the worst evaluations for all of the boxes. The low handles (B1, B2, and B3) received the best evaluations when the boxes were moved between intermediate surfaces.

Discussion

The results suggest that positive conditions for one joint can occur at the expense of another joint when the entire upper limb has to handle boxes using different grips and support surfaces.

The EGM results for the biceps brachii muscle showed that the commercial box resulted in the lowest EGM values for all of the heights we evaluated. However, the commercial box also produced the best results for the wrist extensor muscles when the box was moved between FS-G and FS-GT. Moving the B1 prototype between FS-A resulted in lower electrical activity values for the wrist extensor muscles. With respect to the descending portion of the trapezius muscle, the C3 prototype produced the best results when the box was moved between FS-G and FS-GT. The B3 prototype was the best box evaluated for movements between FS-A. Antony and Keir27 evaluated shoulder muscle activation during isometric and prehension movements. The authors found 2% and 6% increases in the electrical activity in the biceps and trapezius muscles, respectively, when prehension movements were performed. The authors stated that changes in muscle activation patterns affect the internal load of the muscles, which plays a key role in preventing and treating musculoskeletal disorders.

With regard to arm lifting, the C3 prototype resulted in the smallest range of motion for all of the handling heights tested. A possible explanation for this result is the fact that the lower position of the handles allows the lifter to hold the load closer to the body, thereby reducing the distance from the arms to the support surfaces.

The results for the arm-lifting movements corroborate the results for the electrical activity in the trapezius muscle, in the sense that muscle activity was lower when there was a smaller distance for the arms. Nielsen et al.28, in evaluating the electrical activity of the trapezius and the erector spinae muscles, found that there is a workload change in the spinal region for the shoulders when objects are moved from very low surfaces (the floor) to surfaces close to shoulder level. The authors recommended that boxes be handled between heights 72.5 to 126.8 cm above the ground. The greatest EMG activity for the trapezius when handling box C occurred when the joints assumed extreme postures, hindering torque generation (the ratio of muscle length/tension). In these extreme positions, additional motor units can be recruited and/or the firing frequency of already activated motor units can increase29, which explains the increase in the EMG activity of the muscles evaluated.

During the electrogoniometric recording of elbow flexion, the B3 prototype produced the ranges of motion with the greatest biomechanical advantage (80° to 120°)30 for all of the handling heights we evaluated. This result was expected because the higher handle position favors elbow flexion.

Regarding wrist extension, handling the commercial box (A) produced ranges closer to the neutral position when the box was moved from FS-G and FS-A. For movements between FS-GT, the C1 prototype produced the ranges of motion that were closest to the neutral position. Deviations of the wrist from the neutral position cause increased pressure on the carpal tunnel and decreased the power of the intrinsic wrist muscles in the lever arm31. Forceful movements and extreme ranges of motion of the wrist should be avoided because they are associated with increased workload and musculoskeletal disorders32.

Handling the C3 prototype produced ranges of motion closest to the neutral position when the box was moved from FS-G and FS-GT. For movements between FS-A, the B1 prototype produced the best results in terms of deviation movements. These results are in agreement with those obtained in previous studies11 12 that have recommended handles angled at 30º or 45° from the horizontal plane.

When direct quantitative data are used to evaluate movement, it is important to consider the extent to which the significant differences observed between the conditions (boxes) affect the range of motion from the clinical or preventative point of view. In this study, 10% of the average range of motion for each joint (maximum range - minimum range) was considered a relevant measure for evaluating the differences between the boxes we evaluated33. Given this standard, the statistically significant differences we found between the boxes with regard to arm lifting, elbow flexion, wrist extension, and ulnar deviation are clinically relevant. The following average ranges of motion were recorded for each joint: 50° for arm lifting (5° difference between the conditions), 56° for elbows (6° difference between the conditions), 38° for wrist extension (4° difference between the conditions), and 31° for ulnar deviation of the wrist (3° difference between the conditions).

The subjects' preferences for different handles varied depending on the surface height. The prototypes with high handles tended to obtain higher scores for handling between intermediate surfaces, and high handles also tended to receive better evaluations for handling tasks involving the ground. The acceptability scores are in agreement with the elbow flexion results: for both variables, the best prototype was the B3 prototype.

Interestingly, smaller ranges of elbow flexion (greater workload) occurred in situations with greater ranges of ulnar deviation (greater workload) for the C1 prototype, apparently because synergistic strategies were used to handle this prototype at different heights. Similar to the C1 prototype, the commercial box (A) also has a handle in a high and straight position, but the same result was not observed for A. This result may have occurred because the handle on the commercial box is held with a closed hand, and the hole in the box may have produced greater synergy between the agonist and the antagonist wrist muscles.

Another synergistic strategy seems to have occurred when the subjects handled the C3 prototype (high and straight handle): the better positioning of the shoulder promoted lower levels of muscle activity in the trapezius and more neutral ulnar deviation positions, but some of the wrist extension motion was lost.

In general, the B3 and C3 boxes had a greater ratio of positive aspects to negative aspects with respect to muscle electrical activity, postures adopted, and acceptability for the subjects performing the activity. The B3 prototype would be most suitable for handling tasks below the waist, and the C3 prototype would be best for handling tasks above the waist. In both prototypes, the handles are tilted 30°. Training programs for users focusing on how to perform the necessary adjustments to the box depending on the surface heights are highly recommended.

Future studies that include workers who are experienced in handling tasks in real environments are necessary to validate these results and to make continued improvements in new prototypes. Including inexperienced people in this study produced the required homogeneity of the sample, but this sampling strategy could limit the external validity of the results for workers. Future studies could also evaluate other joints associated with the upper limbs to evaluate the workload on the musculoskeletal system more comprehensively. One strength of this study was that direct measurements were collected simultaneously, which should be required in future studies. It would be equally interesting to analyze the electrical signals of the antagonist muscles when the upper limbs are moving in future studies to verify whether the joints are stable during handling. Furthermore, prospective studies are needed to verify whether adjusting the handle designs on boxes reduces musculoskeletal symptoms among real users.

Acknowledgements

This study received financial support from the National Council on Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq Proc. No. 142408/2009-3 and Proc. 301772/2010-0).

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Received: July 12, 2012; Revised: November 12, 2012; Accepted: November 18, 2012

Correspondence Helenice Jane Cote Gil Coury Universidade Federal de São Carlos Departamento de Fisioterapia Rod. Washington Luiz, Km 235 CEP 13565-905, São Carlos, SP, Brasil e-mail: helenice@ufscar.br

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