**4. Measurement of hand stress**

#### **4.1. JSI-system**

program should offer a choice of three hand types with little, much as well as medium subcutaneous fat content. The work of [28] differ fleshy, tendinous and normal skin types. In addition, it should be possible to automatically scale the hand models by percentile and gender. A

A first approach for Comfortyping was developed with Recurdyn (see **Figure 1**). Here the grip of an iron bender was taken and defined the reduction of pressure peaks as well as a homogeneous pressure distribution as a target function. The MBS/FEM program Recurdyn has an Autodesign function and can iteratively optimize geometries. In the example, a shape change handle was constructed with six cylinders and pressed onto the palm of the hand with 200 N. In addition, the handle was gripped with 50 N gripping force. The geometry and material properties from the hand (skin type dependent) and the force and movement conditions from the iron bender (product-dependent) were selected from the database. For the target definition, the reduction and homogenization of the pressure distribution was selected manually (**Figure 4**).

The results show before the optimization on the hand regions O and Q high pressure loads. The radius of all steps is R14. The pressure loads were redistributed to the hand center P with the optimization. For this purpose, the program changed the radius of the steps of the handle until a desired pressure distribution is obtained. To do this, the radius in the hand center changes to R22. This information is output as a spline into the CAD model of the handle. A pressure evalu-

For comfort evaluation, the shape change handle was designed. The developed shape change handle consists of six spreader jaws, which can be moved by a threaded pin rotation. The threaded pins have right-hand and left-handed threads. The so-called entraining jaws spread the counter-jaw during an outward movement. To conceal the edges of the jaws, a rubber covering was applied. These six jaws have taken together the width of a male palm of the 50th percentile of about 95 cm. The whole mechanism was tested by FEM for strength and is made

ation with different subjects confirmed the optimized grip shape as comfortable.

of ABS. Various tools can be connected to the shape change handle (**Figure 5**).

snap function should allow the hand model to take the product automatically.

22 Anatomy, Posture, Prevalence, Pain, Treatment and Interventions of Musculoskeletal Disorders

**3.2. Results**

**Figure 4.** Comfortyping in RecurDyn.

The development of the JSI-System (Job-Strain-Index System) was made for a male person of the 50th percentile between 20 and 30 years (see **Figure 6**). As glove, a model (TouchGrip of UPIXX) in microfibre fabric was used. This remains firm in the fixed position of the sensors on the hand. For the sensors to measure the wrist angle ulnar and radial, a bending sensor (Interlink FSR 408) is mounted on the thumb side of the hand, at the level of the wrist. For the wrist angle, palmar and dorsal a potentiometer was used. To this on the back of the glove a sheet metal was sewn to attach the potentiometer. In the forearm a velcro strip was applied and on the velcro a sheet metal and a rod were attached. To measure the force distribution on the palm (thenar, hypothenar, palmar) and on the fingertips, were force sensors Interlink (FSR 402, FSR 400) adapted to the preparatory work of [21].

The wiring was led through a slot outwardly toward the back of the hand, so this could cause no hindrance on the palm. All signals of the sensors were lead to the microcontroller (Arduino MEGA 2560). The Arduino has been built with an LCD-Display in a plastic housing. Thus, it is possible to wear the evaluation system on a belt. For calibration, the raw data are transferred on the computer in Excel (PLX-DAQ: Parallax Data Acquisition Tool). Approximation

**Figure 6.** Prototype: JSI-system.

functions are derived from the raw data with the using of load cell and angle template. For the validation, several measurements by different forces and postures were accomplished. It was shown that the sensors measures are reproducible. For the evaluation of the JSI the measurements were compared with the limit values. The evaluation was performed in Excel according to the principle of the case distinction. Decisive for the strain variables are mainly maximum forces, angles and times [14].

of the hand posture on the strain magnitude should be examined. In the third variant, the handle of the torque wrench was grabbed in the middle to increase torque and also the force. Here should be examined the influence of an increased force on hand posture and the intensity

Analysis for Objective Evaluation the Stress of the Hand http://dx.doi.org/10.5772/intechopen.71474 25

For the purpose of a subjects study the experiments were filmed. One variant took about 3 min. The first variant was performed ten times to analyze the results of the JSI-System for measuring a similar task several times. The subjects were tested independently from each other and were assigned to perform the JSI evaluation using the videos from the three experi-

The evaluation of the JSI in the subject's study took about 10–15 min for each variant. The measurement took as long as the activity, therefore 3 min. The comparison between the subject's study (average of individual JSI ratings) and the measurement shows that the estimation is varying a bit from the measurement. The largest deviation arises in the third test variant. The highest hand stress emerges from the extreme hand posture in the second variant. Examiner 5, for example, tends to give higher estimations than examiner 3. For the repetition

ments. Apart from the videos the examiners got following information:

**1.** The speed of work were felt as normal.

**5.** The lever in the third variant was 15 cm.

**3.** The torque was about 20 Nm.

**2.** For the evaluation 8 h of work per day were assumed.

**4.** The lever in the first and second variant was 29 cm.

of the exertion [14].

**Figure 7.** Experiment.

#### **4.2. Results**

In the experiment, it shall be proven, if by using the prototype, the evaluation of the hand stress is faster and exacter than the classic JSI method. To show the applicability of the JSI-System, different strain cases are evaluated using the classic method and compared to the measured JSI. As an experiment angle bracket are assembled on Aluminum profiles in three different variations (see **Figure 7**).

Following work steps were performed:


The experiment was performed in three variations to show the function of the JSI-System and the influence of force and angle on the result. In the first variant, the experiment is performed as described above. In the second variant during the tightening of the bolts using the torque wrench, a dorsal flexion of the wrist of about 45 degrees was provoked. Thereby the influence

**Figure 7.** Experiment.

functions are derived from the raw data with the using of load cell and angle template. For the validation, several measurements by different forces and postures were accomplished. It was shown that the sensors measures are reproducible. For the evaluation of the JSI the measurements were compared with the limit values. The evaluation was performed in Excel according to the principle of the case distinction. Decisive for the strain variables are mainly

24 Anatomy, Posture, Prevalence, Pain, Treatment and Interventions of Musculoskeletal Disorders

In the experiment, it shall be proven, if by using the prototype, the evaluation of the hand stress is faster and exacter than the classic JSI method. To show the applicability of the JSI-System, different strain cases are evaluated using the classic method and compared to the measured JSI. As an experiment angle bracket are assembled on Aluminum profiles in three

The experiment was performed in three variations to show the function of the JSI-System and the influence of force and angle on the result. In the first variant, the experiment is performed as described above. In the second variant during the tightening of the bolts using the torque wrench, a dorsal flexion of the wrist of about 45 degrees was provoked. Thereby the influence

maximum forces, angles and times [14].

**Figure 6.** Prototype: JSI-system.

different variations (see **Figure 7**).

Following work steps were performed:

**1.** Two slot nuts were placed in the slot.

**3.** Washers were placed on two bolts.

**2.** One angle bracket was placed on the slot nuts.

**4.** Both bolts were screwed into the slot nuts using an Allen wrench.

**5.** Both bolts were tightened with 20 Nm using a torque wrench

**4.2. Results**

of the hand posture on the strain magnitude should be examined. In the third variant, the handle of the torque wrench was grabbed in the middle to increase torque and also the force. Here should be examined the influence of an increased force on hand posture and the intensity of the exertion [14].

For the purpose of a subjects study the experiments were filmed. One variant took about 3 min. The first variant was performed ten times to analyze the results of the JSI-System for measuring a similar task several times. The subjects were tested independently from each other and were assigned to perform the JSI evaluation using the videos from the three experiments. Apart from the videos the examiners got following information:


The evaluation of the JSI in the subject's study took about 10–15 min for each variant. The measurement took as long as the activity, therefore 3 min. The comparison between the subject's study (average of individual JSI ratings) and the measurement shows that the estimation is varying a bit from the measurement. The largest deviation arises in the third test variant. The highest hand stress emerges from the extreme hand posture in the second variant. Examiner 5, for example, tends to give higher estimations than examiner 3. For the repetition of the first variant a mean for the JSI of 1.055 with a standard deviation of 0.32 was measured. According to this, the spread lets us expect that the JSI measurements lay at 1.055 ± 0.32. The maximum spread is approximately ±0.5. These results indicate that during the experiments different influencing factors like the grip and strength of fingers, posture of hand and fingers etc. have influences on hand stress.

allows the comparison of hand-held products and processes together. A misjudgement of hand stress can be ruled out. The measurement of hand stress takes much shorter than with the classic JSI-rating. The validation has shown that all sensors provide reproducible measurements for the short-term application. Also, the state of the art shows the use of the piezo (-resistive) sensors. However, for the industrial capability and long-term use, the sensors should be examined more closely to know limits as power level and duration of sensors. For further development, it is also advisable to integrate a simple replacement of the sensors, because the sensors can be damaged by their sensitivity during continuous use. Other criticisms of the prototypes include obstructing the movement of the hand as well as the outsiders of wired sensors. With the JSI-System is shown that strain variables from monitoring methods may be measured. This also means that other methods such as LMM or RULA can be performed with the measurement method. The high training costs for more accurate results, which are recommended by [29], can be reduced with systems such as the JSI-System. The fact that compressive forces on the hand and finger surface can be measured by the JSI-System opens the possibility of a pressure reduction for comfortable design of hand-held products. The JSI-System, combined with the work of [21] can be used to assess the pressure

Analysis for Objective Evaluation the Stress of the Hand http://dx.doi.org/10.5772/intechopen.71474 27

[1] Liebers F, Caffier G. Berufsspezifische Arbeitsunfähigkeit durch Muskel-Skelett-

[2] Dihlmann W. Gelenke – Wirbelverbindungen. Stuttgart, New York: Georg Thieme

[3] Fischer J. Schmerztherapie mit Lokalanästhetika - Injektionstechniken - einfach und

[4] Ilbeygui R. Selbsthilfe durch Taping: Schritt-für-Schritt-Anleitungen – medizinische

[5] Müller BH. Ergonomie, Bestandteil der Sicherheitswissenschaft. Berlin, Wien, Zürich:

[6] Moore JS, Garg A. The strain index: A proposed method to analyze jobs for risk of distal upper extremity disorders. American Industrial Hygiene Association. 1995;**56**(5):443-458

Erkrankungen in Deutschland. Dortmund, Berlin, Dresden: BAUA; 2009

distribution and influence factors.

University Wuppertal, Germany

Aydin Ünlü\*, Peter Gust and Frank Mersch

\*Address all correspondence to: aydin.unlu@outlook.com

sicher. Stuttgart, New York: Georg Thieme; 2000

Hin-tergründe – Zusatzübungen. München: Elsevier; 2015

**Author details**

**References**

Verlag; 2002

Hanser; 1997

#### **5. Conclusion**

The literature review has shown that no digital hand models exist that are used for the simulation and analysis of influencing factors with respect to the pressure simulation. Either realistic skin deformation is simulated [13] or part models to study material investigation are used [9]. Here are no statements taken to the sensation of pressure. With the known hand models, a derating of the pressure distribution can be indeed proven; however, the inclusion of influences by [28] is ignored. With the help of the digital hand model, there is the ability to save subjects, expensive prototypes and pressure sensor mats in the development of handheld products. The hand model can develop proposals for the design in a short time, and reduces the pressure load objectively. The comparison between the pressure simulation and pressure measurement showed that correct pressure loads are determined with the linear approach of the hand model. The literature review shows, in this context, such as the work of [10] that a linear material behavior on skin tissue can be assumed. However, the linear material behavior is permitted only for a certain range of forces and demands, for example, for the fingers typing e-modules. This effort can be eliminated in a non-linear material behavior. Since in most cases, skin damage due to shearing arises, for further development the expansion of the model for simulation of shear stress is recommended. Other criticisms of the model include the fixing of the metacarpals in the room. These ensure that the palm is immobile. In addition, the palm consists of a plurality of muscles which alter the mobility of the metacarpal bone. The application displays a new approach to produce an impact analysis. The results of the impact analysis can be implemented directly in the CAD model and give a product which produces low and evenly distributed pressure distributions on the hand tissues for the simulated factors. The impact analysis gives findings about which factors have an influence on the pressure distribution.

In summary, regarding the literature on the measurement of hand stress can be said, that there is no work, which uses a sensor glove to measure strain variables from the JSI method [14]. Very often sensor gloves with force sensors are used to obtain an indication of the power level, without taking the hand posture into account. In some works, such as [22], for example, only the movement or the hand posture for motion capture is measured with the sensor glove. Then the measurement data scored with RULA, without taking forces into account. None of the sensor gloves aims solely the ergonomic quality of a hand-held product. The developed JSI-System give an effective tool for assessing the hand stress based on JSI method. In addition, the examiners do not always have the same expertise and experience regarding the hand ergonomics and therefore evaluate differently. Regarding the standardization of JSI, with the JSI-System arises a potential for application in the industry. The JSI-System allows the comparison of hand-held products and processes together. A misjudgement of hand stress can be ruled out. The measurement of hand stress takes much shorter than with the classic JSI-rating. The validation has shown that all sensors provide reproducible measurements for the short-term application. Also, the state of the art shows the use of the piezo (-resistive) sensors. However, for the industrial capability and long-term use, the sensors should be examined more closely to know limits as power level and duration of sensors. For further development, it is also advisable to integrate a simple replacement of the sensors, because the sensors can be damaged by their sensitivity during continuous use. Other criticisms of the prototypes include obstructing the movement of the hand as well as the outsiders of wired sensors. With the JSI-System is shown that strain variables from monitoring methods may be measured. This also means that other methods such as LMM or RULA can be performed with the measurement method. The high training costs for more accurate results, which are recommended by [29], can be reduced with systems such as the JSI-System. The fact that compressive forces on the hand and finger surface can be measured by the JSI-System opens the possibility of a pressure reduction for comfortable design of hand-held products. The JSI-System, combined with the work of [21] can be used to assess the pressure distribution and influence factors.
