On-body sensors for ergonomic analysis: influence of supports for order picking

1

Keywords:

• Physical ergonomics
• electrogoniometers
• musculoskeletal disorders
• order picker

1. Introduction

The success of drive-in supermarkets necessitated the development of new jobs dedicated to picking orders in storage area. These workers are continuously performing repetitive movements, awkward sustained postures, manual handling tasks followed by pushing and pulling tasks. It has to be added that workers may use two different manual supports to prepare orders: a cart or wheeled-based containers. All these manual tasks represent potential risk factors to develop work-related musculoskeletal disorders (WMSDs). Indeed, drive-in employees expressed physical pains at lower-limbs, lumbar region, shoulder joint, elbow joint, wrist joint and hand levels (Brion et al., 2018). In this study, upper-limb physical constraints related to elbow and wrist joints will be investigated.

To prevent the development of WMSDs, joint comfort zones have been defined in the literature (e.g., Kee & Karkowski, 2001). Concerning the elbow joint, a comfort area between 60 and 100° of flexion can be deduced from the Rapid Upper Limb Assessment (RULA) methodology which aims at providing a score of exposure to WMSDs for a whole-body posture (McAtamney & Corlett, 1993). At wrist level, a flexion/extension of the wrist between 0 and 15° may be considered at ease (HSE, 1990). The same values can be defined for radial deviation. For ulnar deviation, the range of comfort motion would be between 0 and 5° (Hsiao & Keyserling, 1991).

Thus the aim of the current study is to analyse joint comfort zones during order picking in order to assess the influence of manual support, i.e., cart or wheeled-based containers.

2. Methods

2.1. Participants

17 workers took part in this study (7 women and 10 men). Mean age, years of experience, height and weight were 27.7 ± 7.5 years, 3.2 ± 2.5 years, 1.71 ± 8.7 cm, 68.4 ± 12.6 kg, respectively.

2.2. Materials

Four wireless electrogoniometers (Biometrics Ltd, UK) were used to measure right and left joint angles at elbow (flexion/extension) and wrist (flexion/extension and radioulnar deviation) levels during the task. Electrogoniometers were sampled at 1000 Hz and they were initialized in anatomical neutral position. A video camcorder was also used to film workers during order picking (30 Hz).

2.3. Procedure

After installing electrogoniometers, each participant was asked to pick order as usual as possible. Data were collected during orders performed with a cart (two repetitions), and orders performed with wheeled-based containers (two repetitions). As data collection duration differed according to each order (between 10 and 30 minutes), results are expressed in percentage of total time spent at each risky angle.

2.4. Statistical analysis

Independent variable corresponded to the type of manual support used during order picking, i.e., cart or wheeled-based containers. Dependent variables were the percentage of time spent at a risky angle for flexion/extension of the elbow, flexion/extension of the wrist, and radioulnar deviation of the wrist for both sides. Normal distribution of data was assessed through the Shapiro-Wilk test. If the distribution was normal, a paired sample student t-test was performed to evaluate significance. If not, the Wilcoxon test was applied. Statistical significance was obtained for p < 0.05.

3. Results and discussion

3.1. Wrist joint angles

According to joint angle thresholds from the literature, it can be noticed that wrist extension was maintained at risk during almost half of the task (right side, containers: 48.7 ± 16.5%; cart: 49.3 ± 18.9%; left side, containers: 42.3 ± 11.9%; cart: 41 ± 12.4%). A risky wrist radial deviation was also sustained (right side, containers: 40.2 ± 17.9%; cart: 38 ± 17.8%; left side, containers: 35.9 ± 14.2%; cart: 31.4 ± 14.2%). However, there was no statistical difference of the percentage of time spent at a risky level for the wrist between orders performed with a cart and with containers. This statement concerned risky angles of flexion, extension, radial and ulnar deviation. Despite the fact that the wheeled-based containers induced supplemental manual handling tasks compared to the cart, the percentage of time spent at a risky level while picking orders was not significantly different in that condition (see Figure 1). It has to be noted that the fact that one hand was holding a scanner during the task has been investigated, but this parameter had no consistent influence on the time spent at a risky joint angles.

Figure 1. Percentage of time spent at a risky angle for right (A) and left (B) sides (*p < 0.05; ***p < 0.001).

3.2. Elbow joint angles

At elbow level, participants spent significantly more time at a joint angle over 100° when using the cart, although absolute values appear low (right side, containers: 8.3 ± 7%; cart: 11.1 ± 7.7%; left side, containers: 8.2 ± 11%; cart: 12.2 ± 17.2%). This posture was likely used by workers to push the cart more efficiently with increasing load while picking order (Resnick & Chaffin, 1995). Conversely, participants were under 60° of elbow joint angle significantly more often when using wheeled-based containers. Moreover, this risky angle was sustained by workers during almost half of order picking (right side, containers: 48.4 ± 16.7%; cart: 38.9 ± 15.9%; left side, containers: 64.5 ± 23.3%; cart: 56.1 ± 25.6%). Given the fact that an elbow joint angle between 0 and 60° also means having forearm in an extended posture, it might be more hazardous for the development of WMSDs, e.g., with a load in the hand (Rose et al., 2000). Thus it appears necessary to increase height of the containers, or to add a steering bar to wheeled-based containers. This bar would be fixed between 90 and 115 cm, as suggested by the ISO 11228-2 standard (2007).

4. Conclusions

This study aimed to investigate the influence of the type of support (wheeled-based containers vs. cart) on elbow and wrist joint angles while picking orders in a drive-in supermarket. Outcomes showed that an elbow joint angle under 60° was sustained for the longest time with both supports. Using the wheeled-based containers had a significant impact on this angle compared to employing the cart. Thus ergonomic adjustments need to be implemented to this support to ensure prevention of WMSDs for picking order workers.

Acknowledgements

The authors want to thank management team and workers from the E-Leclerc drive-in supermarket located in Cormelles-le-Royal, France, for the conduct of this study.