Application of the predicted biomechanics approach in assessing work-related musculoskeletal discomfort among female brickfield workers in West Bengal,India

Abstract

BACKGROUND:

Manual brick manufacturing units in West Bengal, India are mostly unorganized sectors employing many migrant female workers in manual material handling tasks as brick moulders and brick carriers.

OBJECTIVE:

The study estimated the biomechanical postural stress developed due to manual load handling, carriage and brick transportation.

METHODS:

Body Part Discomfort Scale was used to assess the subjective pain felt by 18 adult female brick moulders and 12 brick carriers during work. Video recordings were taken during the actual field work and the images were analyzed for two-dimensional prediction kinetics and kinematics.

RESULTS:

Results show that the horizontal distance of the load from the body was 59.95±3.74 cm during the initial lifting of bricks by the brick carriers and so their biomechanical trunk angle inclination was significantly detrimental. The bending moment at the lumbosacral disc was much higher during the squatting posture while initial brick lifting. Moreover, the trunk and the neck pain of the brick carriers were caused by carrying 18% of body weight load.

CONCLUSIONS:

The brick carriers were the more vulnerable group in terms of work-related musculoskeletal disorders. Therefore, ergonomic interventions such as relay brick carrying and pole supported balanced brick carrying, may be implemented to reduce the prevalence of pain. This study is the first of its kind that validates the extent or the prevalence of work-related musculoskeletal disorders among the female brickfield workers through approaches of predicted biomechanics.

Keywords

Brick manufacturing units predicted biomechanics torque compressive force shearing force

1 Introduction

In this world of automation, manpower is mostly being replaced in large scale industries. However, small scale industries and unorganized sectors are a boon to the lower-income group with inherited skills, local technologies, cheap muscular power, limited economic investments, lack of infrastructure and the cognitive response to handle any kind of situation [1]. Brick making units are one of such manufacturing enterprises in industrially developing countries where heavy manual material handling (MMH) work still predominates.

Brick manufacturing units (BMUs) are unorganized sectors employing mostly migrant workers. It recruits a substantial quantum of female workforce viz, brick moulders (BM) and brick carriers (BC). Evaluation of workload [2], respiratory symptoms [3], health [4, 5] and nutritional status [6], environmental stress on work productivity [7] have already been studied on the brickfield workers. These female workers were at risk of developing musculoskeletal disorders (MSDs) and showed deteriorated health status with the burden of the task and poor working environment. A study done in Pakistan showed that among 105 men and 45 women from 15 brick kilns, digging and carrying jobs caused significant pain in body parts such as the neck, upper back, shoulders, lower back and hips [8]. Moreover, there is a relationship between MMH, enormous physical labour and unexplained mental fatigue [9]. The female brickfield workers reported higher discomfort in their hands, lower back and knees, than their male counterparts, probably due to the additional domestic responsibilities [10]. Again, body posture analyses by the Rapid Entire Body Assessment (REBA) method showed that 45% and 40% of the female brickfield workers reported low back pain (LBP) due to MMH and awkward lifting of heavy objects, respectively [11].

Questionnaire analyses or work posture assessments (without using software) are the indirect means of evaluating musculoskeletal disorders. Nevertheless, direct methods of evaluation, use certain electronic devices and sensors that are not always economically viable and are difficult in using in the real-time workplace [12].

However, present study is unique in the sense that musculoskeletal discomfort is analyzed using two-dimensional prediction kinetics. This is a semi-direct method using computer software program where precision is much more than the indirect methods of evaluation. Ergomaster software, as applied in the current study, uses two-dimensional images of participants during various activities to evaluate the biomechanics components like spinal compression and shear. Very few past studies used such software in evaluating low back pain or work-related musculoskeletal discomfort [13, 14]. So far no reported data is available in a field study using such a sophisticated method in the unorganized sectors of the Indian context.

Previous studies indicate that load carriage induced maximum stress on the lower lumbar spine. Therefore, the responses of L5/S1 disc (lumbosacral joint) have been used to represent the spinal stresses [15 –17]. Thus, the purpose of this study was to investigate the effect of biomechanical postural stress of the female brickfield workers on manual load handling, carriage and brick transportation.

2 Methodology

2.1 Participants

A total of 30 female volunteers of which eighteen (18) of them are BM (with mean±SD: age of 24.2±3.51 years, height of 150.39±5.73 cm and weight of 42.58±3.95 kg) and twelve (12) BC (with mean±SD: age of 21.7±2.13 years, height of 151.45±3.67 cm and weight of 41.67±5.21 kg) with at least 3 years of working experience in the BMUs as moulders and carriers respectively. The volunteers did not have any prior history of musculoskeletal injuries and/or other illnesses as per their acquaintances.

2.2 Instrumentation

Videos were recorded with Sony Cyber-Shot camera and still photographs were extracted and analysed using Ergonomics evaluation software (ErgoMaster, M/s NexGen Ergonomics Inc., version 4.0, Canada) to get two-dimensional prediction kinetics and kinematics.

2.3 Experimental protocol

The female BM and BC carried out their work at their self-selected speed and accuracy without any disturbances in their regular work. The study was carried out randomly in actual field situation in two different brick manufacturing units situated each in the districts of Hooghly, Nadia and 24-Parganas (North) of the state of West Bengal. Videos of different activities of each group of participants were recorded. Prior permission and written consent for videography were taken from the managers of each brick manufacturing unit as well as from the volunteers.

The intensity of pain or different types of discomfort was measured by utilizing the Body Part Discomfort (BPD) Scale [18] as the subjective method of discomfort. The scale consists of marks from 1 to 10 and ranges from just noticeable discomfort to intolerable discomfort. A ‘0’ in the scale means no discomfort at all. The study was approved by the Departmental Ethical Committee and the principles of the Declaration of Helsinki were followed.

BM Trials = [(3 districts of West Bengal X 2 brick manufacturing units per district) X 3 random postures X 3 volunteers] = 54

BC Trials = [(3 districts of West Bengal X 2 brick manufacturing units per district) X 3 random postures X 2 volunteers] = 36

2.4 Data analyses

Images extracted from videos for each trial were analysed to obtain prediction kinetics (i.e., total compressive forces, total shearing forces, the compressive force due to load, shearing force due to load exerted on L5/S1 disc of the spine, torque) from two-dimensional biomechanical predictions of Lift Analysis tool of ErgoMaster software (Fig. 1). A total of 90 photographs were processed. To get the result of shear and compression forces, subject specifications were likewise entered, and a scale factor was defined for the image. The analyst identified points at the ankle, knee, hip, shoulder, ear, elbow, and hand for final estimation.

Fig. 1

Window of the two-dimensional Biomechanical Predictions of ErgoMaster software.

2.5 Statistical analyses

The data were analysed using the Statistical Package for Social Sciences (SPSS) for Windows version 21.0 (M/s SPSS Inc., Chicago, IL, USA). All descriptive statistics were presented as mean values and standard deviation (SD). Pearson’s correlation was calculated between the biomechanical angles of the working postures and the values of the BPD Scale. Paired t-test was done for all parameters between BC and BM. A value of p≤0.05 was accepted as statistically significant.

3 Results

3.1 Task performed by the brick carriers and brick moulders

The female BM in a manual brick manufacturing unit in West Bengal, India, constantly mould bricks out of mud under the sun, in open fields, at their convenient postures like sitting, squatting, half-sit or stooping positions. On the other hand, the BC load bricks on their head in squatting posture and then carry the load on their head (10 sun-dried/baked bricks) to and from the brick kiln. Each baked and sun-dried bricks weigh ∼ 4.5 kg and 5.0 kg respectively. They are paid as per the number of bricks moulded/carried.

3.2 Data Analyses

Figure 2 graphically represents the horizontal distance of the load from the body. The activity of moulding is carried out by the BM while other activities are carried out by BC. It can be seen that the horizontal distance of load is significantly higher while lifting bricks on the head at the start of the work cycle of a BC.

Fig. 2

Horizontal distance of load from the body during different activities in the brickfields. N.B: Vertical bars: SEM; ^*p < 0.05.

The biomechanical angles of different body parts of the BM and BC are represented in Fig. 3. The peak bending moment of the trunk caused due to manual load handling at different postures adopted by the BC has been graphically presented in Fig. 4. The bending moment or torque is significantly higher (p < 0.05) in the squatting posture adopted by the BC while loading bricks on the head than that of the other postures adopted by the carriers while carrying a load. On the other hand, the bending moment of the BM at the squatting posture is 101.72±35.46 N-m, which is significantly much less (p < 0.001) than that of the squatting posture adopted by the BM.

Fig. 3

The biomechanical angles of different body parts of the brick moulders and the brick carriers. N.B: Vertical bars: SEM; ^*p < 0.05.

Fig. 4

The bending moment of the trunk caused due to load handling at different postures adopted by the female brick carriers. N.B: Vertical bars: SEM; ^*p < 0.05.

The correlation between the BPD Scale and the biomechanical angles of different parts of the body of the two groups of female brickfield workers have been given in Table 1. There is a positive significant relationship between the perception in the BPD Scale and the biomechanical angles of the neck and forearm of the BM and that of the upper arm and thigh of the BC. The subjective evaluation as per BPD Scale of the neck and forearm of the BM is 6.81±1.21 and 7.61±1.35 respectively. On the other hand, the subjective evaluation as per BPD Scale of the upper arm and thigh of the BC is 6.91±1.27 and 5.21±1.33 respectively.

Table 1

Correlation between the values of BPD Scale and the biomechanical angles of the working postures of the brick moulders and the brick carriers

Parts of the body	APA Format (brick moulder)	Inference	APA Format (brick carrier)	Inference
Neck	r(16) = 0.86, p < 0.00001	+ve; S	r(10) = 0.29, p = 0.34	+ve; NS
Forearm	r(16) = 0.94, p < 0.00001	+ve; S	r(10) = 0.08, p = 0.78	–ve; NS
Upper arm	r(16) = 0.15,p = 0.55	+ve; NS	r(10) = 0.58, p = 0.48	+ve; S
Trunk	r(16) = 0.23, p = 0.35	–ve; NS	r(10) = 0.03, p = 0.91	+ve; NS
Thigh	r(16) = 0.60, p = 0.007	–ve; S	r(10) = 0.71, p = 0.009	+ve; S
Leg	r(16) = 0.30, p = .22	+ve; NS	r(10) = 0.16, p = 0.60	+ve; NS

NB: APA –(American Psychological Association); S –Significant; NS –Non significant, +ve/-ve: positive/ negative correlation.

The compressive forces due to load handling have been graphically presented in Fig. 5, which showed that there are no significant changes in the compressive forces between the two groups of workers. The shearing forces caused due to the load handling of the female brickfield workers have been graphically presented in Fig. 6. The shearing force of the BM is 509.6±176.81 N as they constantly bend their trunk to mould bricks continuously.

Fig. 5

The compressive forces caused due to load handling by the brick moulders and brick carriers. N.B: Vertical bars: SEM; p < 0.05.

Fig. 6

The shearing forces caused due to load handled by the brick carriers and brick moulders. N.B: Vertical bars: SEM; ^*p < 0.0001; ^**p = 0.0002.

4 Discussion

When the load being lifted or carried has to be kept beyond 40 cm distance from the body, a deeper flexion of the knees and ankles and an anterior translation of the upper body and arms occur [19]. This study (Fig. 2) also shows a similar trend as the biomechanical angles of the legs and upper arms of the BC is significantly less than (p < 0.05) that of the BM (Fig. 3). Reducing the distance of the load from the body reduces trunk inclination as well as the load moment [20]. A study on male volunteers showed that the increase in the horizontal distance during initial lift increases the peak moment acting on the lumbar spine [19]. As the horizontal distance of the load is beyond 40 cm during the initial lift of the bricks at squatting posture, so the biomechanical angle of the trunk is significantly higher for the BC as has been graphically represented in Fig. 3.

Human lumbar torque limits during lateral bending and extension are 17.8 Nm and 17.6 Nm, respectively [21]. During weight lifting/carrying, the bending moment at the L5/S1 disc can become quite large. To counteract this moment, the muscles of the low back region, especially the erector spinae group, must exert correspondingly high forces, since they operate as small moment arms [22].

Load placement is an important factor in physiological response along with the terrain, walking speed, distance covered and also the duration of walk throughout the day [23, 24]. At self-selected speed, the brick carriers walk to the brick kilns which are at a distance of 0.5–0.7 km. After baking the bricks in the kiln, they again carry the bricks (10 at a time on their head) to the field which are stacked and are finally transported/marketed. Many groups of the population carry the load on their head at their workplaces, which in the Indian context includes e.g. porters [25] and construction workers [26]. Studies on Indian soldiers carrying a load of 6.27 kg for 8 hours of work have been recommended in sandy terrain [27]. The current study shows that the female BC carry about 45 kg (No. of bricks X Weight of each brick) of load on their head continuously from dawn until sunset with one hour of lunch break at noon. Literature shows that domestic water carrying on the head by women accounts for 69% spinal pain and 38% back pain respectively [24]. This indicates that the trunk and the neck pain of the BC are caused by carrying a load of 18% of body weight on head. Studies on college students suggest that carrying loads of 10% of body weight and above should be avoided [25]. A study on African women also showed that they have developed a strategy to limit the muscular work required to carry a head-supported load by their characteristics pendulum-like gait [26]. Similar gait adaptations are observed among these female workers (BC) while carrying bricks on their head.

Load carriage above shoulder level produces higher torque around L5/S1 disc compared to lower load carriage which is probably due to the difference in rotational inertia of the load [27]. Thus, the difference in the torque between the two groups of workers may be because the horizontal distance of load (mud moulded into bricks) being handled was 40.64±11.38 cm away from the body for the BM as seen in Fig. 2.

Constant brick moulding throughout the day makes their neck and forearm vulnerable to pain and injury (Table 1). Constant brick loading/unloading on/from the head in squatting posture and carrying the load on their head from one to the other end of the brick field makes their upper arm and thigh vulnerable to pain and injury. This result is similar to a study on female brickfield workers where their Rapid Upper Limb Assessment Scores (RULA) averages to 7.0 [2]. The RULA grand score has positive correlation coefficients with the maximum holding time and other criteria for postural loads, such as whole-body discomfort and compressive force at L5/S1 [28].

For each group, all different postures have been taken into consideration as a whole in Fig. 5. As per the American Conference of Governmental Industrial Hygienists’ (ACGIH) threshold limit values (TLVs), the allowable limits using biomechanical criteria represent injury risk threshold equal to 3400 N for compression force at L4/L5 [29]. Thus, as per the aforesaid norms, biomechanically, the female brickfield workers theoretically are not subjected to injury risk. But one has to keep in mind that they are migrant workers, undernourished, have dual work at home and at the field and at the same time are exposed to the unhygienic and polluted environment [2, 6].

The lumbar spine may experience significant shear forces during occupational tasks due to the force of gravity acting on the upper body when bending the trunk forward. The recommended shear force limits are 1000 N and 500 N for maximum permissible limit (MPL) and action limit (AL), respectively but a 700 N shear limit would appear to be acceptable for frequent shear loading (100–1000 per day) [30].

Compressive forces have higher magnitudes when compared to shear force magnitudes, however, spinal structures are much stronger in the axial direction than when loaded in shear [30]. So, from the study, it can be concluded that the brick carriers are at a higher risk of pain and discomfort at their neck and back due to carrying heavy loads.

5 Conclusion

Brick carriers use squatting posture while loading/unloading bricks on/from the head and the horizontal distance between the load (18% of their body weight) and the body is beyond 40 cm. Even, the bending moment or torque at the L5/S1 disc is significantly higher at this squatting posture. The biomechanical angles are also significantly higher in the neck and the trunk region of the brick carriers. The compressive forces are much higher but the total shearing forces acting on the spine is significantly lower among the female brick carriers. The constant head-load carriage led them to adopt a characteristic pendulum-like gait. Thus, from the study it can be concluded that the female brick carriers have a prevalence of neck and back pain and this group is at the risk of being afflicted with work-related musculoskeletal disorders or injuries. Ergonomic interventions such as load-carrying in head-strapped baskets or transporting of load in relay, may help them to reduce this risk of injury.

Footnotes

Acknowledgment

The authors are grateful to Dr. Bhuvnesh Kumar, Director of Defence Institute of Physiology and Allied Sciences (DIPAS), Delhi, who allowed them to interact with the scientists and were allowed to use the facilities of the ergonomics laboratory for the data analyses. The authors also acknowledge the participation of all female brickfield workers and brickfield managers to allow the authors to carry out the study. Furthermore, Dr. Subhashis Sahu gratefully acknowledges the financial support from the DST PURSE-II, University of Kalyani to conduct the study.

Conflict of interest

The authors declare no conflict of interest.

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