Grip Strength Survey Based on Hand Tool Usage

Hand grip strength is broadly used for performing tasks involving equipment in production and processing activities. Most professionals in this field rely on grip strength to perform their tasks. There were three main aims of this study: i) determining various hand grip strength measurements for the group of hand tool users, ii) investigating the effects of height, weight, age, hand dominance, body mass index, previous Cumulative Trauma Disorder (CTD) diagnosis, and hand tool usage experience on hand grip strength, and iii) comparing the obtained results with existing data for other populations. The study groups comprised 71 healthy male facility workers. The values of subjects’ ages was observed between 26 and 74 years. The data were statistically analyzed to assess the normality of data and the percentile values of grip strength. The results of this study demonstrate that there were no significance differences noted between dominant and non-dominant hands. However, there were highly significant differences between the CTD group and the other group. Hand grip strength for the dominant hand was positively correlated to height, weight, and body mass index, and negatively correlated to age and tool usage experience. Hand dominance, height, weight, body mass index, age and tool usage experience should be considered when establishing normal values for grip strength.


Introduction
Hand grip strength is defined as a measurement of the strength of different muscles in the hand and the forearm [1].It is measured in either kilograms or Newtons by squeezing a hand grip strength dynamometer with one's maximum strength [2].Because gripping is important for many daily functions, grip strength is frequently used in clinical practice as a representation of long-term body strength and health [3,4,5].Hand grip strength is broadly used for performing tasks involving equipment in production and processing activities [6].Most professionals in this field rely on grip strength to perform their tasks.Facility workers, especially working in small companies, may be more likely to use equipment and hand tools that are old and cheap, most of them not ergonomically designed [7].The use of poor tools might be a potential factor that increases the risk of Cumulative Trauma Disorder (CTD).
In the USA, hand and finger injuries constitute 35% of the total work-related injuries [8].In order to get information about the capacity of workers, the assessment of grip strength is very important.The obtained information can be considered to design tools, equipment, and workstations [9].
There are various factors which affect hand grip strength such as gender, weight, height, age, and hand dominance [10].There is a non-linear relationship between grip strength and age [11].According to gender differences, male participants have greater grip strength than female participants [12].The dominant hand is almost 10% stronger than the nondominant hand [13].In addition, there is a positive relationship between grip strength, weight and height [14].
Several authors have evaluated the grip strength of various populations.For instance, Fernandez and Uppugonduri [15] have compiled data on hand grip strength from South Indian male workers (n=128) in the electronic industry.Chau et al. [16] have compiled grip strength data from French males (n=55) and females (n=45).Kamarul et al. [17] have compiled grip strength data from Malaysian males (n=212).Mandahawi et al. [18] have compiled grip strength data from Jordanian males (n=115) and females (n=120); the subjects were mainly carpenters, vehicle drivers, electrical technician, and others.Recently, Cakit et al. [19] have compiled grip strength data from Turkish male (n=92) and female (n=73) students studying at dentistry faculty.There were three main aims of this study: i) determining various hand grip strength measurements for the group of hand tool users, ii) investigating the effects of height, weight, age, hand dominance, CTD, and hand tool usage on hand grip strength, and iii) comparing the obtained results with existing data for other populations.

Sample size
The minimum required sample size was predicted applying the equation used in "General requirements for establishing anthropometric databases" [20]: (n: sample size, CV: coefficient of variation, : the percentage value of relative accuracy, ̅ : the mean value of the population and s: the standard deviation fort he corresponding population).
Based on the inital pilot study of 15 participants, A relative accuracy was assumed as 5%, and using the mean (46,4 kg) and standard deviation (6,45kg) values were calculated and a relative accuracy was assumed as 5%.The minimum required sample size was calculated as 70 from the equation above.Thus, seventy-one US-based healthy male facility workers were randomly selected for this study.

Participants
Seventy-one US-based healthy male facility workers participated in the study.Of the 71 participants, only one was left-handed, the remaining 70 were right-handed.The demographic data of participants are summarized in Table 1.Some of the tools that participants have used mostly are screw drivers, pliers, drill, electric testing, etc. (Figure 1).The hand tool experience was changed between 6 and 55 years and some participants had CTD before.

Apparatus and measurements
The Jamar dynamometer has been used to measure grip strength of both the right and left hands; it is a hydraulic instrument that measures in pounds (lbs) and kilograms (kg), up to 200 lbs or 90 kg, respectively (Figure 2).Maximal voluntary contraction (MVC) is recorded when the manometer needle stops at each particpant's optimal grip point, which remains in place until it is manually reset to zero.

Figure 2. Jamar dynamometer used in experiment
The American Society of Hand Therapists (ASHT) recommends standardised positioning: subject seated, shoulders adducted and neutrally rotated, elbow flexed at 90 degrees, forearm in neutral, and wrist between 0 and 30 degrees of extension [21].
Several studies have reported no significant difference in grip strength with subjects either sitting or standing [22,23,24].For this purpose, in our study, standardised positionining described by ASHT was considered as stated above (Figure 3-a and 3-b).The popular approach for assessing maximum grip strength is to calculate the average value of three trials [29].Participants performed three maximum attempts with each hand, while the average value was recorded in kilograms.In addition, participants' body mass index (BMI) was determined as well, according to weight and height factors.

Statistical analysis
To perform the normality of data and the important percentile values of grip strength, all data were statistically analyzed using IBM SPSS® version 20.
The mean, standard deviation (SD), and percentile values were obtained in order to summarize the descriptive statistics.T-tests were used to compare dominant hand versus non-dominant hand, CTD group versus other group, and American versus other populations for existing hand grip strength measurements in previous research.The levels of statistical significance were declared at p value as 5 % or 1%.

Results and Discussion
Important percentile values (first, 25th, 50th, 75th, and 99th) of grip strength data for the dominant and opposite hands were calculated to represent the extreme values (Table 2).Hand tool designers might be interested in the extreme values of grip strength (first and 99th percentiles) since they influence fit and comfort.The mean and standard deviation for each hand in CTD situations is shown in Table 3.The percentage difference between dominant and non-dominant hands and the significance test results for the differences are represented in Table 4.The results of this study demonstrate that there were no significance differences noted between dominant and non-dominant hands.However, there were highly significant differences between the CTD group and the other group.The results suggest that individuals with CTD lost some grip ability on hand tools.The last objective of this study was to compare US-based grip strength data with some of the international data.
Several studies have been carried out on hand grip strength in different countries.Grip strength was used to compare with South Indian (n=128 male) [15], French (n=55 male) [16], Malaysian (n=212 male) [17], Jordanian (n=115 male) [18], Turkish (n=92 male) [19] (Table 5).Based on the results of the T-tests for the significance between American and other populations, there were significant differences between U.S. males and males from other countries, excluding the nations of Jordan, Malaysia, and Turkey (Table 6).Based on the comparison of the results, grip strength is significantly different in distinctive populations.According to the results obtained, the grip strengths of American males were significantly higher than South Indian males and significantly lower than French males.Table 7 depicts the correlations among the variables.This study supported the results obtained from the study of Amosun et al. [30] that a positive relationship between grip strength, weight and height was found.As shown in Table 7, hand grip strength for the dominant hand was positively correlated to height, weight and body mass index (p<0.05for height); negatively correlated to age and tool usage experience (p<0.05 for age).

Discussion and Conclusion
In conclusion, there were no significance differences found between dominant and non-dominant hands.However, there were highly significant differences between the CTD group and non-CTD group, and between U.S. males and males from other countries, excluding the nations of Jordan, Malaysia, and Turkey.The hand grip strengths of U.S. male workers appeared to be stronger than those of other populations aside from French males; the dominant hand had greater grip strength than the nondominant hand.Such differences have practical implications for the design of hand tools.However, more data are needed to reliably establish these differences.Hand dominance, height, weight, body mass index, age and tool usage experience (for facility workers) should be considered when establishing normal values for grip strength.On the other hand, tools should be selected more carefully to minimize possible problems such as excessive vibration and force that might lead to a CTD diagnosis.Furthermore, tools should be balanced properly and designed based on hand anthropometry such as palm width, length, etc.Workers should be trained to use tools properly and how to report problems with tools.

Figure 1 .
Figure 1.Some hand tools used frequently by participants Hand grip strength measurement was taken for both hands.At the beginning of the study, the subjects were informed of the objectives of the study, and those who agreed to voluntarily participate in the study signed a Consent to Participate form.This study was approved by the Institutional Review Board for Research with Human Subjects at University of Central Florida.A hand injury or any related disability was not observed at the time of the study.

Figure 3 .
Figure 3.Typical subject positioning (left), Front view (right)As shown in Figure3.a,there is no support for the arm, since the dynamometer is positioned vertically[25].The span is adjustable with five different grip distances (2.5, 3.8, 5.1, 6.4 and 7.6 cm apart) according to a comfort scale.It has been found that maximal grip strength is usually calculated when the scale is in the second or third position[26,27,28].In the current study, maximum grip strength values were obtained in the second position in centimeters (3.8cm).
The values of subjects' ages was observed between 26 and 74 years.The average values and standard deviations of age, height and weight of the subjects were 47.44 16.34 years, 170.73 10.29 cm, and 80.04 16.58 kg, respectively.

Table 1 .
Subject demographics

Table 3 .
The grip strength(kg) mean and standard deviation for both hands and CTD diagnose.

Table 4 .
Comparison of grip strength between dominant and opposite hands, CTD-Other group

Table 5 .
Summary data of grip strength of American male facility workers and other populations (n=71)

Table 6 .
Comparison of grip strength between American male facility workers and other nationalities

Table 7 .
Correlation between dominant hand grip strength and other parameters * Statistically significant (p<0.05)