Author(s): Fermín Basso Della Vedova; Ignacio Bermejo Bosch; Juan López Pascual; Jesús Sellés Vizcaya; Alicia Piedrabuena Cuesta; Juanma Belda Lois; Mario Lamas Rodríguez; Mercedes Sanchis Almenara.

Exoskeletons provide an ergonomic solution that can reduce the physical strain that specific segments of the body are exposed to while performing certain tasks, especially in industrial environments. Before incorporating an exoskeleton into a workstation, it is important to conduct a biomechanical analysis to assess both the improvements and benefits of its introduction into the workstation, as well as any associated risks. Any such biomechanical analysis should record the muscular activity, movements and reaction forces of the body segments that are mainly involved in the performance of the work activity. The procedure followed by the Instituto de Biomecánica (IBV) includes the study of movements in the workstation, the definition of biomechanical variables, the performance of tests and the analysis of the results. To carry out the biomechanical analysis of an exoskeleton, the IBV uses Kinemov/IBV, a tool that allows the synchronized recording of the different variables required for this analysis. In conclusion, a biomechanical analysis is essential to assess the desirability of incorporating an exoskeleton into a work environment and Kinemov/IBV is a useful tool to perform this analysis.

INTRODUCTION

The efforts required to perform specific work tasks can lead to discomfort, aches and pains, or even occupational diseases.

In recent years, passive exoskeletons have been introduced, particularly in the industrial sector, in an effort to improve comfort and reduce musculoskeletal disorders. Their goal is to reduce the physical strain on the body segments involved in performing certain tasks that require manual handling of loads or forced postures.

However, there are a number of considerations that need to be taken into account before introducing an exoskeleton into the workstation. On the one hand, it is important to know which parts of the body it will protect, which body parts it will exclude, and which other body parts will be affected. This requires a biomechanical analysis of the worker’s muscular activity and of the postures and forces he or she assumes and applies with and without the aid of the exoskeleton. On the other hand, it is also essential to know how the user feels about the exoskeleton: whether he or she feels comfortable wearing it, whether they would incorporate it into their workstation, etc. This will help to achieve a full integration between the workstation, the worker and the exoskeleton.

This article focuses on explaining the role of biomechanical analysis in the incorporation of exoskeletons into the workstation.

THE CHALLENGE

Today, it is necessary to carry out a biomechanical analysis to quantify the ergonomic impact of using an exoskeleton in the work context.

However, due to the need to develop different recording technologies, synchronizing the recordings, defining the appropriate biomechanical models and programming how the variables of interest should be calculated, etcetera, make these analyses, however, complex and time-consuming.

This article presents the methodology, procedures and metrics for biomechanical analysis.

Biomechanical analysis is made possible by recording and analyzing three sources of data:

- Electromyography (EMG), which records the physiological signal from the electrical activity of the muscles and the nerves that control them. Electrodes placed on the surface of a person’s skin record the electrical activity of the muscles at rest and during muscle contraction.

- Motion analysis, in order to record the rotations and displacements of the various body segments involved in performing a task. This provides flexion and joint ranges of motion, which can be used to assess body postures during the performance of the task in question. These can be measured using inertial sensors or photogrammetry.

- Reaction forces, recorded using force platforms or templates. Reaction forces make it possible to calculate models for estimating muscle fatigue and musculoskeletal effort.

The IBV uses the Kinemov/IBV system as a biomechanical recording and analysis platform that allows the integration of the above technologies. In the context of the biomechanical analysis of exoskeletons, Kinemov/IBV is able to record all the metrics required to carry out this type of analysis in a synchronized and accurate manner. The tool can record motion analysis using both photogrammetry and inertial sensors, EMG signals, and ground reaction forces, by means of force platforms. The application can be used both in a laboratory environment (photogrammetry version) and directly in the workstation (inertial sensor version).

THE PROCEDURE INVOLVED IN A BIOMECHANICAL ANALYSIS

Before performing a biomechanical analysis of the use of an exoskeleton in a workstation, it is necessary to define a procedure that makes it possible to identify the risk factors that can lead to musculoskeletal injuries and disorders. The following defines the methodology used by the IBV once the exoskeleton and the workstation to be analyzed have been selected.

PHASE 1: STUDY OF THE MOVEMENTS PERFORMED IN THE WORKSTATION.

The first step is to analyze the workstation. This involves visualizing the workstation in which the exoskeleton is to be incorporated and studying the movements required to perform the tasks (Figure 1).

Once these movements have been observed and studied, we identify the biomechanical variables that merit study in order to evaluate the ergonomics of the workstation. The parameters selected are usually related to the most affected body parts and those at risk of musculoskeletal disorders.

For example, in a biomechanical analysis, one of the biomechanical variables to be studied is often the percentile of muscle activation. This variable can be used to observe the degree of activation of the muscle and the duration of these activations. Another example of a biomechanical variable that is often studied is the maximum flexion of the body region on which the exoskeleton acts. In this way, the differences between using an exoskeleton and not using an exoskeleton can be determined on a postural level, or whether the use of an exoskeleton leads to less freedom of movement for the workers who use it.

Figure 1: Example of a workstation where the worker is working in a forced posture.

PHASE 2. DESIGN OF A MEASUREMENT PROTOCOL

Before starting the recordings, a measurement protocol must be defined. The following aspects should be considered in this phase:

- Tasks to be reproduced. The measurement protocol consists mainly of reproducing the critical tasks in a laboratory or in the actual workstation itself (Figure 2).

- Analysis conditions. The normal procedure is to analyze each subject under the same conditions with the exoskeleton and without the aid of the device. This helps to reduce bias.

- Biomechanical metrics and biomechanical model: An iterative process is used to select the biomechanical metrics and the biomechanical model. The most appropriate biomechanical metrics should be selected according to the expected effect of the exoskeleton. On the other hand, each biomechanical model will yield specific metrics. Therefore, an iteration is necessary in order to obtain a viable biomechanical model that provides the most appropriate variables. To make it easier to choose, Kinemov/IBV describes which biomechanical models are available, how to instrument the subject and what kind of variables can be calculated.

Figure 2: Reproduction of a task involving the handling of 3 types of loads in a laboratory.

PHASE 3. CARRYING OUT THE STUDY

Once we have selected all the details of the study, the next step is to perform the measurements. The procedure is as follows:

1.- Create a new data set in the Kinemov/IBV software in order to manage all the measurements that have been performed on the subject and to control all the relevant data of the same.

2.- Select the biomechanical model.

3.- Instrument the subject according to the instructions provided by the software.

4.- Calibrate the instrumented subject (Figure 3)

5.- Record each of the selected tasks in the defined conditions (with and without the exoskeleton).

Figure 3: Example of a calibration using Kinemov/IBV

PHASE 4. ANALYSIS OF RESULTS

When performing the first measurements or pilot tests, the user will usually follow an iterative process in which different types of tables, graphs and variables are displayed based on the findings. This process is carried out on the Kinemov/IBV analysis screen, as it allows the user to select the variables of interest, the display format and various settings that facilitate automatic, real-time analysis (Figure 4). In this way, the user can easily select the graphs and parameters to be used for all the measurements in his or her study. The ease of use of the program means that the user does not necessarily have to be an expert in biomechanical modeling. The program is designed so that healthcare professionals and occupational risk prevention technicians can also use the tool.

After each measurement, it is recommended that the user should review the biomechanical metrics defined with Kinemov/IBV and qualitatively check the results. This ensures that there are no errors of any kind. This is usually done quickly since the graphs, tables and formats to be displayed with the tool have already been selected in the pilot phase.

Figure 4: Analysis window of the Kinemov/IBV program.

Figure 4 shows an example of the different results provided by the Kinemov/IBV tool. First, the movement recorded by the photogrammetry markers can be observed together with the actual movement recorded by the documentation cameras. In the upper right part, you can see the result of the EMG signal from different muscle groups recorded during the movement, all of them perfectly synchronized. In the lower part of the image, you can see the joint angles calculated from the markers (left), and what the force platforms have recorded (right).

Once the preliminary analysis is complete, Kinemov/IBV can automatically generate a report with the selected data and graphs. It is standard practice to generate report templates using Kinemov/IBV. By doing so, when we analyze a new study, we only have to load the template to see the selected data and graphs.

Figure 5: Schematic of the phases using Kinemov

In addition to these results, Kinemov/IBV allows you to export the data obtained from the different recording systems in case you want to perform a more detailed analysis of these data. This is useful for the type of analysis we normally perform at the IBV, where we usually perform more complex analyses using scripts programmed in Excel, SPSS, R, Python, OpenSIM or Anybody, among others. For example, the IBV has developed a biomechanical model by means of which, by introducing the movement data and anthropometric measurements of the participants in the study, it is possible to carry out simulations from which to extract information that cannot be measured directly during the course of the study, such as joint loads, for example.

PHASE 5. DISCUSSION OF THE RESULTS AND CONCLUSIONS

Once the field study has been completed, the results obtained from the objective measurements and the subjective information are analyzed together.

The objective tests typically show a number of parameters that are improved by the use of the exoskeleton, thus reducing the negative impact of the workstation. However, it is also common that some aspects are made worse by the use of the exoskeleton. For this reason, it is essential to quantify the benefits and drawbacks before making a decision on whether or not to incorporate an exoskeleton.

On the other hand, subjective evaluation is key to the success of implementing an exoskeleton in a company. If aspects such as comfort, learning, device adaptation or usability are not taken into account, the implementation project is likely to fail.

CONCLUSIONS

Nowadays, it is necessary to perform biomechanical analyses of the impact of an exoskeleton in the workplace prior to its implantation. These studies make it possible to take into account different aspects such as the design of the devices, the muscular fatigue suffered by the body segments involved, or even the quantification of excessive muscular fatigue.

In addition to biomechanical information, other data, such as subjective data, are necessary for a complete evaluation of the exoskeleton in a particular job.

By following a well-defined evaluation procedure, it is possible to objectify the impact of an exoskeleton on a workstation.

We recommend the use of analysis technologies such as Kinemov/IBV that make it possible to record and analyze the most relevant data in a simple and agile manner.

AUTHOR'S AFFILIATION

Instituto de Biomecánica de Valencia
Universitat Politècnica de València
Edificio 9C. Camino de Vera s/n
(46022) Valencia. Spain