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ISSN : 1598-7248 (Print)
ISSN : 2234-6473 (Online)
Industrial Engineering & Management Systems Vol.18 No.4 pp.591-599

Exploratory Study on the Impacts of Handle Vibration on the Hand and Forearm

Josefa Angelie Revilla*, Ping Yeap Loh, Satoshi Muraki
Department of Human Science, Graduate School of Design, Kyushu University, Fukuoka, Japan Department of Industrial Engineering, University of the Philippines Los Baños, Laguna
Department of Human Science, Faculty of Design, Kyushu University, Fukuoka, Japan
Corresponding Author, E-mail:
May 7, 2019 September 17, 2019 October 18, 2019


This study investigated the immediate impacts of short-term handle vibration on hand functions, upper limb discomfort, and forearm muscle responses during hand grip test and task performance of seven healthy young adults. The task was to grip the handlebar for 5 minutes with 50% perceived strength under two conditions: with handle vibration (HV) and without handle vibration (NHV). Activities of forearm muscles namely flexor carpi radialis (FCR), flexor of the fingers (FF), flexor carpi ulnaris (FCU), and extensor digitorum (ED) were recorded using surface electromyography (EMG), while post-task hand tests for finger dexterity, strength, and sensibility were also measured. These finger functions as well as muscle responses did not differ significantly between HV and NHV. The lacking effects might be associated to the inconsistencies on grip force during task, perhaps participants let go of the handle during the latter part. Meanwhile, perceived discomfort on the shoulder was significantly higher after HV than NHV and activity of FCR, FF, and ED during maximal grip test were significantly different (p < 0.05) as well. Specifically, muscle activities were lower by 12-15% after HV than NHV, indicating that HV might have influenced the ability to grip hardly. In conclusion, maximal grip test and discomfort rating can be a predictive parameter to determine the instantaneous effects of handle vibration.



    Physical exposure to vibration has been proven to have desirable and undesirable effects to humans (Fuermaier et al., 2014;Schuhfried et al., 2005;Egan et al., 1996;Carlsoo, 1982). For instance, moderately employed whole-body vibration (WBV) can stimulate and improve muscle activities which is widely applied in various sports training and medical field (Fuermaier et al., 2014;Schuhfried et al., 2005;Cardinale and Wakeling, 2005;Torvinen et al., 2002). Although, when too much vibration is transmitted to the human body, vibration-related injuries can manifest. The hand-arm area is one of the most common part that is being exposed to such vibration level, specifically with the integration of industrial and other modernized machineries in the workplace. The international labor organization set a threshold (of 1 m/s2), action (of 2.5 m/s2), and exposure (of 5 m/s2) limit value of handle vibration that can be referred to when using vibration machineries. The exposure limit value is the maximum daily exposure that an unprotected operator may be exposed to (Stellman, 1998), and any amount greater than this gives higher health and safety risks.

    Individuals who have been exposed to jackhammer, chainsaw, driller, and even some dental tools are those who are prone to hand-arm vibration syndrome (HAVS). In Europe alone, 25 million workers are exposed to vibration at work and are at risk to the said injuries. While in the US, 2 million workers are exposed to hand-arm vibration and 50% of them are likely to develop HAVS (Trotto, 2015). Furthermore, 72,000 to 144,000 cases of HAVS have been reported in Canada as of 2017 (Shen and House, 2017). The Health and Safety Executive (2017) stated that there are approximately 3,000 cases of vibration white finger being reported per year. The numbers show that many people have high chances of acquiring vibration-related injuries from their line of work.

    Agriculture is one work sector that is known to have significant risk factors when it comes to exposure from vibration equipment such as tractors, lawn mowers, harvesters, chainsaws, and rice-planting equipment (Vallone et al., 2016). Hence, in an agricultural country like the Philippines, where 26% of the 30-million-hectare land area is devoted for agriculture, a significant number of people are exposed to such risks. For instance, research showed that hand tractor operators are constantly exposed to vibration greater than the exposure limit value (Revilla et al., 2015) and this may lead to injuries commonly known as HAVS (Youakim, 2012). HAVS is defined as injuries in the fingers, hands, and arms resulting to numbness, impaired sensibility, limited dexterity, impaired grip force, and reduced mobility. These are significant hand functions that are necessary to perform daily life activities and work-related tasks, impairing any of them can lead to low quality life.

    The tendency to fully develop vibration injuries takes several years. For instance, loggers may develop it after 17 years of exposure while mechanics may develop it after 24 years of exposure (Youakim, 2012). The ISO 5349-1:2001 described it as: chances are, 10% of workers with 8-hour of daily exposure to 2.5 m/s2 of vibration magnitude for 12 years are going to acquire vibration white finger. Aside from the mechanical source of vibration and exposure duration, the amount of vibration transmitted to the body is also influenced by biomechanical factor such as grip strength and possibly other hand-arm posture (Carlsoo, 1982;Burstrom, 1994). In the study of Burstrom and Lundstrom (1994), they concluded that tight grip and high vibration frequency (> 60 Hz) elicited high vibration absorption.

    Although it may take a few years for HAVS to completely manifest, its effects are irreversible once acquired. Thus, it is extremely important to detect its immediate effects in the early stages of exposure. As an initial step, this study explores the effects of vibration on the hand-arm area when a specified grip force is employed. Specifically, it aims to investigate the impacts of a 5-minute handle vibration on hand functions, upper limb discomfort, and forearm muscle responses during hand grip strength test and exposure to task with and without vibration.

    2. METHOD

    2.1 Participant

    Our study recruited seven young male adults (23.6 ± 2.1 years old) without any long-term exposure to vibration and have not had any recent injuries in the hand-arm area. Mean height and weight are 175.3 ± 5.5 cm and 71.9 ± 7.4 kg, respectively, while hand measurement is presented in Table 1. All participants were right hand dominant based on Edinburgh handedness inventory (EHI) test (Oldfield, 1971).

    2.2 Vibration Device Used for the Tasks

    A custom-made vibration machine (VP-15D-110V-60HZ) installed with a bicycle handlebar was used as vibration source. The vibration table has a vibration frequency of 60 Hz and can have a maximum load of 10 kg while the bicycle handlebar is made up of iron, has a dimension of 550 x 270 x 100 mm (width, length, and height), and has a mass of 580 g with pipe diameter of 22.2 mm. The bicycle handlebar was attached to the vibration table through the handlebar attachment, which is composed of aluminum as frames, c-clamps as fixtures, and device mounting stand with round brace clamp as handlebar stand (see Figure 1).

    2.3 Experiment Procedure

    The experiment was conducted in an indoor laboratory set-up. First, the participant was introduced to the objectives of the study, test procedures, equipment to be used, experiment tasks, and experiment duration. After changing to experiment clothing and measuring the height, weight, hand anthropometry, and EHI, electrodes of surface electromyography (EMG) were placed on the flexor carpi radialis (FCR), flexor of the fingers (FF), flexor carpi ulnaris (FCU), and extensor digitorum (ED) of the dominant side as shown in Figure 2. Then, pre-task hand tests mainly: (1) forearm muscle responses during maximal grip strength test, (2) 9-hole peg test for finger dexterity, (3) pinch gauge test for pinch strength, and (4) two point discrimination test for finger sensibility were performed to assess baseline hand functions.

    After the pre-task tests, the participant was instructed to rest for 5 minutes in a sitting position to allow the hands and forearm muscles to relax before the actual task. Then, he was led to stand in front of the machine for 1 minute while being instructed on how to grip the handlebar and how to position the arms during the task. The task was to grip the handlebar in a specified arm posture using 50% strength for 5 minutes under two task conditions: (1) with handle vibration (HV) and (2) without handle vibration (NHV). There was no specific elbow or wrist angle during task performance and instead, the participant was just asked to position his arm in the manner shown in Figure 2 where in the upper arm was closed to the trunk, the elbow was approximately angled at 145°, the forearm was in neutral position, the wrist was slightly in ulnar deviation and in flexion, and the hand held the handlebar at 50% perceived grip force. The participant was not encouraged to pull up, push down, or apply any force to the handlebar during task performance instead he was only instructed to hold it in static position. This hand-arm posture was employed since it is closely identical as to how two-wheel tractor operators hold the equipment during farming operation.

    During both task conditions, the responses of FCR, FF, FCU, and ED were transmitted and recorded in PowerLab and LabChart software. After the 5-minute HV task, a 1-minute adjustment period was given to the participant as he remained standing in front of the vibration machine while he was asked to rate discomfort felt on the hand, forearm, upper arm, and shoulder. He was also instructed not to rub or wipe his hands because this might affect the results of the succeeding hand tests. Forearm muscle responses during maximal and 50% grip strength test were measured and recorded again, and the other hand tests were performed immediately after. Afterwards, a 5-minute complete rest was given before performing the exact same sequence for NHV task. The schematized experimental procedure is presented in Figure 3.

    2.4 Measurement

    Data were recorded and categorized into three: pretask, post HV task, and post NHV task; while three major tests were analyzed: hand tests, forearm muscle responses during grip strength test, and forearm muscle activities during task performance. However, the baseline data were not used for comparison. Instead, only the post task data were compared since these data can give a clearer indication on the impact of handle vibration.

    The dependent parameters were: finger dexterity from the 9-hole peg test, lateral pinch strength, finger sensibility from the two-point disk test, subjective discomfort rating, and forearm muscle activities during grip strength test and task performance. On the other hand, the presence and absence of handle vibration was the independent variable.

    2.4.1 Hand Functionality Test

    A cardboard 9-hole peg panel from The Agency of Design was used to assess finger dexterity of both hands. The participant was instructed to sequentially put the pegs in the designated hole which were numbered from 1 to 9 then remove each of them starting from 9 to 1, as fast as he could. Three actual trials were made for the right hand first, followed by three trials for the left hand and each trial was timed using a stopwatch. If a peg fell on the floor, the trial was stopped and repeated but when it only fell on the table (where the peg board was placed), the time and trial was continued.

    Lateral pinch strength was measured using a B&L Engineering pinch gauge. Initially, the participant was instructed to have a proper arm posture where in the elbow was angled at 90° and placed beside the trunk while the forearm was in neutral position, hanging, and not anchored on a table. A lateral-pinch using 100% strength was performed three times for the right hand first, and another three trials for the left hand. The researcher supported the opposite side of the gauge as the participant pinched. A rest period of 10 seconds was given in between trials.

    The last hand test was the two-point discrimination test for finger sensibility. Firstly, the participant was asked to close his eyes and lend his right hand to the researcher. Then, the researcher randomly selected between a single point or any two-point (with 2 to 4 mm distance) from the Touch-Test two-point disk. A finger (starting from the thumb up to the pinky finger, one at a time) was poked with these points randomly for 7 times and the participant was asked if he felt one or two points. If at least 4 out of 7 were determined correctly, the next finger was tested. Else, gradually increase two-point distance until the participant correctly determined the number of points being pinned on his finger. The minimum two-point distance that was consistently determined correctly was recorded for that specific finger. Ideally, the greater the two-point distance the lesser tactile sensitivity that finger has. This was repeated for all the fingers on the left hand.

    2.4.2 Forearm Muscle Activity during Hand Grip Test, HV Task, and NHV Task

    Forearm muscle activities were measured during pre-task-, post HV task-, and post NHV task-hand grip strength test and during both HV and NHV task performance. After the placement areas namely: FCR, FF, FCU, and ED of the dominant hand side were located and cleaned, surface EMG electrodes were carefully placed on these forearm muscles. Then, the maximum voluntary contraction (MVC) was measured to normalize the muscle activity data across all participants.

    For the grip strength test, the participant was initially instructed to have a proper arm posture where in the elbow was angled at 90° and placed beside the trunk while the forearm was in neutral position, hanging, and not anchored on a table. It was ensured that the base of the Takei hand grip dynamometer was rested on the heel of the palm and the handle was rested on the middle of the four fingers before the participant squeezed with 100% strength for 10 seconds. This was done once for the dominant hand then a rest period of 1 minute was given. He was again asked to squeeze the dynamometer for 10 seconds, but this time with 50% strength which was done to practice and familiarize him on how he should hold the handlebar during task performance. During both grip tasks, the participant was instructed to maintain the same force for the 10-second span by looking at a real-time digital value of his grip force. Forearm muscle responses were recorded during the test.

    2.4.3 Subjective Discomfort Rating after HV and NHV Task

    After performing each task, a subjective discomfort rating of the hand, forearm, upper arm, and shoulder was asked. Using the Wong-Baker FACES Foundation (2018) pain rating scale, discomfort was ranked from 0 (no discomfort) to 10 (worst possible discomfort). The participant’s dominant side was always the main reference during the assessment.

    2.5 Statistical Analysis

    Paired t-test was used to compare post HV task and post NHV task for finger dexterity, pinch strength, subjective discomfort rating, and forearm muscle responses during grip strength test. It was also used for comparing forearm muscle activity during HV and NHV task. Meanwhile, post HV task and post NHV task for the two-point disk test was compared using Wilcoxon-signed rank test. A non-parametric analysis was used for this hand test since it failed normality (via Shapiro-Wilk test). Lastly, all hand tests for the right and left hand were compared separately between post task periods and no statistical comparison was made between the two since a natural difference in strength and efficiency was expected between the dominant and non-dominant hand which can cause a misleading contribution to the effects of handle vibration. The level of significance used in the study was 0.05.

    3. RESULTS

    3.1 Hand Tests

    Paired t-test revealed that finger dexterity and pinch strength did not vary significantly after performing HV task and NHV task. This was true for both the right and left hand, although a slightly faster performance in the 9- hole peg test was observed and expected on the dominant hand as compared to the non-dominant hand (see Table 2).

    On the other hand, Wilcoxon-signed rank test revealed that the mean perceived two-point distance of all fingers did not vary (p > 0.05) between post HV task and post NHV task. Although, the distance of each finger in both post task periods were within the normal range of less than 10 mm (Moberg, 1990), as presented in Table 3.

    3.2 Subjective Discomfort Rating

    There was a significant difference (p = 0.03) in discomfort felt on the shoulder between post HV task and post NHV task. Moreover, it was observed that the hand had the highest discomfort (moderate to severe) among all upper limb area, followed by the forearm (mild to moderate), upper arm (mild to moderate), and shoulder (none to mild), as presented in Figure 4.

    3.3 Responses of Forearm Muscles during Grip Strength Test, HV Task, and NHV Task

    Paired t-test showed that muscle activity of FCR (p = 0.022), FF (p = 0.038), and ED (p = 0.046) between post HV task and post NHV task were significantly different. Specifically, they were considerably lower on post HV task than post NHV task (see Figure 5).

    On the other hand, although muscle activity of FCR, FF, and ED were relatively higher while FCU activity was slightly lower during HV task than NHV task (see Figure 6), no significant differences were found between both tasks.


    4.1 Hand Performance Tests

    There were no significant differences found between post HV and post NHV task for all the hand tests. Specifically, an insignificant minimal improvement in the 9-hole peg test performance and an insignificant difference between pre and post HV task pinch strength and two-point disk assessment were observed.

    The minimal increase in the 9-hole peg performance might have been influenced by another factor such as the human capacity to adapt and learn when subjected to repeated task for a certain period of time. Since the same simple pattern was assigned for each hand and repeated three times, participants might have memorized the sequence well making it easier to place and remove the pegs, without putting much thought as to where and how to put and remove each of them. Generally, the concept of learning states that as a task is performed repeatedly over time, performance improved because of acquired familiarity and gained knowledge on how to do the task more efficiently. This is most especially applicable when the interval period between each trial is considerably small, similar to the 9-hole peg test performed in our study.

    Meanwhile, the insignificant difference in pinch strength after performing HV task and NHV task might indicate that a 5-minute exposure to handle vibration had no effect on finger strength. Post task pinch strength were within the normal values of 9.8 ± 0.3 kgf (Imrhan et al., 1991) and 9.6 ± 1.8 kgf (Fernandez et al., 1991) indicated by previous studies. The lack of effect might be due to the differences in muscle group used to perform lateral pinch and hand grip. Intrinsic muscles of the hand are commonly activated during precision-pinch grip, while forearm muscles are used during hand grip. Hence, the lack of engagement of these hand muscles during task performance led to unaltered finger strength.

    Lastly, finger tips sensibility between post HV task and post NHV task did not differ significantly indicating that the presence of handle vibration had no effect on tactile acuity. This was in contrast with most research specifically with that of Forouharmajd et al. (2017) where they concluded that after exposure to vibration, the twopoint distance felt by the index and middle finger increased by 1 to 2 mm. The difference in results might be correlated with the wrist posture during task performance and the level of grip force employed. In our research, the participants held the handlebar with their wrist in a slightly ulnar deviation and in flexed posture while trying to sustain 50% grip force. On the other hand, in the study of Forouharmajd et al. (2017) wrist was in neutral posture during grip while grip force was not completely specified. This also explains why the index and middle fingers were affected, since median nerves were distressed during neutral grip posture. As for our study, the ring and pinky fingers were expected to be affected (if there were any) since ulnar deviation of the wrist stresses the ulnar nerves running along the side of the hand and through these fingers. The wrist posture differences lied mainly on the different handlebar structures used in both studies.

    In general, the lacking effects of handle vibration on finger dexterity, pinch strength, and finger sensibility might be caused by the difficulty in actually sustaining 50% grip force for 5 minutes. Participants reported that such grip force was hard to sustain for a longer period of time, hence they were only able to maintain it for the first minutes of performance and gradually let go of their grip as the 5-minute mark approached. This might suggest that the inconsistencies in grip force during task performance influenced the outcomes of the hand tests.

    4.2 Subjective Discomfort Rating

    Perceived discomfort on the shoulder had significant difference between post HV task and post NHV task. Specifically, the discomfort felt after HV task was higher than NHV task indicating that handle vibration affected the shoulder area of the participants. This might be due to the instructed upper arm posture during task performance, where in the shoulder was kept in adduction the entire 5-minutes. Nevertheless, no force was applied to the handle structure and the hand-arm area was in static.

    On the other hand, discomfort on other upper limb area did not vary between post task periods. This might be due to the same reason that participants let go of the required grip as the tasks approached their end. Nonetheless, a consistent discomfort rating was observed across the upper limb area after performing each task. The hand, being closest to the vibration source, had moderate to severe discomfort while the shoulder, being the farthest to handle vibration, felt the least discomfort. Previous research concluded that the amount of transmitted vibration during farming operations was highest on the metacarpal, followed by the elbow, then the shoulder area (Revilla et al., 2015). This suggested that higher vibration energy was absorbed by body parts closer to the vibration source, possibly resulting to more perceived pain or discomfort.

    4.3 Forearm Muscle Activity

    A significantly lower muscle response was observed for FCR (by 12%), FF (by 15%), and ED (by 13%) on post HV task than post NHV task which might indicate that handle vibration stimulated higher muscle activity, possibly bordering to development of fatigue. Rota et al. (2014)stated that fatigue may induce a decrease in activity level of triggered muscles which can result to performance decline. In our study, the higher reduction in muscle response during grip strength test after performing HV task than NHV task might indicate the manifestation of fatigue caused by the presence of vibration combined with trying to sustain a hard grip.

    The different forearm muscle responses during grip test were associated with the wrist posture and grip level throughout task performance. For instance, FCR is activated during wrist flexion. During task performance, the wrist was constantly in flexion due to the differences in width between the shoulder and the handlebar structure used in our experiment. On the other hand, FF is stimulated during finger flexion. During the task, it was constantly activated through gripping the handlebar at 50% perceived strength. FCU is activated during ulnar deviation of the wrist, and since the handlebar height (98 cm from the floor) was slightly lower than the waistline of the participants (mean height = 175 cm), the wrist was marginally in ulnar deviation to neutral position for the entire duration. Lastly, ED is triggered during extension of the medial four digits of the hand and contributes to grip relaxation and wrist extension. During the latter minutes of task performance, participants tend to relax their grip due to tiredness and discomfort hence stimulating ED activity. Although there was no significant difference in forearm muscle activity during HV and NHV task, the presence of handle vibration in HV task stimulated signs of muscle fatigue development seen in declined muscle response during grip strength test. As concluded in the study of Souza et al. (2017), grip strength was reduced when wrist extensor muscles got tired of constant activation through repeated exercise.

    With a relatively high grip force (of 50%) and integration of handle vibration, more energy was absorbed by the hand-arm system causing FCR, FF, and ED to activate more leading to earlier development of muscle fatigue. This further led to reduced muscle response during maximal hand grip test as presented in Figure 3. Meanwhile, FCU was not fully activated since its posture during task performance was relatively in neutral position hence no effect in its activity was observed.

    While this study only considered muscle activities, determination and quantification of muscle fatigue can also be a good measure to know how handle vibration affects humans in short run exposure. Moreover, the study was not able to monitor the required constant grip force for the 5-minute task duration hence possibly influencing the outcomes of the hand tests. Ensuring that grip was constant the whole time can provide results that are mainly associated by the presence of handle vibration. This issue will be addressed in succeeding investigations regarding handle vibration.


    This study aimed to investigate the immediate impacts of short-term handle vibration on hand functions, upper limb discomfort, forearm muscle activities during hand grip test, and forearm muscle responses during HV and NHV task. Finger dexterity, pinch strength, and finger tips sensibility were not affected by the presence of a 5- minute handle vibration. Meanwhile, discomfort rating on the shoulder was significantly higher on post HV task than post NHV task and the activity of FCR, FF, and ED during maximal grip test were lower after HV task than NHV task indicating the fatigue effect of short-term handle vibration which affected hand strength. Lastly, forearm muscle activities did not differ significantly during 5-minute of HV and NHV task. It can be concluded that forearm muscle responses during grip test and subjective discomfort rating can be a good predictive parameter when determining the immediate effects of short-term handle vibration.



    Handle vibration equipment.


    Hand-arm posture and forearm muscle locations.


    Experimental design.


    Discomfort rating comparison.


    Forearm muscle activity during grip strength test.


    Forearm muscle activity during the tasks.


    Hand anthropometric data of the participants

    Finger dexterity and pinch strength comparison

    Finger sensibility comparison


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