Abstract
BACKGROUND:
Grip strength is frequently used in research and clinical work and is commonly compared to normative data in clinical settings. To enable accurate interpretation of grip strength values, normative reference values should be close to the demographic, occupational, cultural and geographic characteristics of the individual tested.
OBJECTIVE:
Investigate the effects of gender, age, work-group and hand dominance on grip strength for the Israeli population, and to compare the grip strength to two normative studies from the United States.
METHODS:
A cross sectional study. The grip strength of 637 healthy adults was measured using a Jamar dynamometer. The effects for age, gender, hand dominance, and work strain were investigated. Israeli sample results were compared to US norms.
RESULTS:
Within the Israeli sample, a significant age effect was found for both men and women. This effect was most apparent among the 70+ age groups, in which grip strength was weaker than all the other groups. Males were significantly stronger than females, in both hands, and the dominant hand was significantly stronger, regardless of gender. Results also demonstrated a medium to large effect for type of work on grip strength portraying high manual strain workers to have stronger grip strength. Overall, the grip strength in the Israeli sample was weaker than both US samples.
CONCLUSION:
Clinicians should be cautious when comparing grip strength to published norms from a different culture/geographical region. The amount of manual strain invested in various occupational roles should be considered in the assessment and intervention process.
Introduction
Hand function is a broad term based on several components, including strength, range of motion, and dexterity. Normal function of the hand is an important factor in a person’s ability to independently engage in daily activities and occupations [1]. Damage to one or more of these components can lead to dysfunction of the hand and limit participation in everyday life [2].
Hand Grip Strength is the measurable ability to exert maximum voluntary force with the combined contraction of the extrinsic and intrinsic flexion muscles of hand and fingers [3]. Grip strength is commonly used in research and clinical work as an objective measure for hand function. It contributes to setting treatment goals, evaluating performance and assessing treatment effectiveness [4, 5]. Studies associated different hand conditions, such as trigger finger, carpal tunnel syndrome, distal radius fractures and osteoarthritis with lower grip strength [1, 6–9]. Furthermore, moderate correlations were demonstrated in these studies between grip strength and upper extremity related function.
In the past two decades, research and clinical practice expanded the use of grip strength evaluation beyond upper extremity related conditions. There is a growing body of evidence that supports the use of grip strength evaluation for different health conditions [10–12]. It was found to have prognostic value for mortality, frailty and cardio-vascular diseases [5, 13].
Grip strength is commonly compared to normative data in clinical settings [14]. Therefore, validated, up-to-date, normative data are required as they serve as the gold standard for comparison. The normative data established by Mathiowetz et al. [15] in a United States (US) sample are still widely used as a gold standard reference value in many clinics all over the world [16]. In 2006, Bohannon et al. [17] produced a meta-analysis of consolidated grip strength norms from 12 studies, based mainly on data from developed countries. They hypothesized that normal grip strength may be more representative of the worldwide population if derived from larger cohorts in multinational settings.
Other studies [18, 19], based on very large cohorts, show contradicting evidence, claiming that grip strength must be interpreted using regional/ethnic-specific reference values. The relationship of anthropometric factors, such as height, weight, body mass index and palm length to grip strength has also been studied [3, 20]. Werle et al. [16] and Kamarul et al. [21] found that grip strength significantly correlated with height, weight and occupation. Schlüssel et al. [3] showed significant association to body mass index, and Wu et al. [22] found palm length to be an effective predictor of grip strength.
The normative grip strength data used as reference value should be as close as possible to the demographic, cultural and geographic characteristics of the individual to avoid the possibility of an erroneous conclusion of impairment. For example, in comparing both the Malaysian and the Taiwan Chinese populations to Caucasians, grip strength reference values were significantly higher among Caucasians and therefore cannot be applied as a reference value [21, 22]. Many studies [3, 21–28] were conducted in various geographical areas in an attempt to establish culturally relevant normative grip strength data sets.
Based on clinical observation, Israeli grip strength often falls below the age-gender adjusted mean when compared to the classical gold standard of Mathiowetz [15]. This discrepancy could be attributed to social cultural differences between Israeli and US populations. However, it could also be explained by the social and technological changes together with the shift from physical labor to more sedentary work during the past 30 years that occurred in the western world. The frequent use of technology in everyday activities instead of manual hand force may have influenced grip strength [29]. A recent study comparing the grip strength of healthy young US citizens born after 1980 (20–34 years old) to Mathiowetz’s normative data found significant lower grip strength amongst the study group [30].
Purpose of the study
To the best of our knowledge, grip strength norms of the Israeli population have not, as yet, been established. Therefore, the objectives of this study were to: (a) Describe the grip strength distribution in a large sample of normal healthy Israeli population stratified by gender, age and hand dominance; (b) Investigate the effects of gender, age-group, work-group and hand dominance on grip strength for the Israeli population; (c) Compare the grip strength data of the Israeli population with the gold standard data of Mathiowetz [15] and to up to date norms [30].
Methods
Design
A cross sectional research study was conducted over a three-year period. Participants were recruited using convenience sampling.
Ethical considerations
The Institutional Helsinki Committee approved the study protocol. Informed oral and written consent was obtained from all participants.
Participants
The sample size was calculated using an estimated population of six million adults living in Israel. The confidence level value was set at 95% with a margin of error of 5%. The minimal sample size required was 385. Six hundred and thirty-seven healthy adults, recruited through word of mouth and advertisement in social media, participated in the study. Prior to the grip strength testing the participants’ medical history was documented. Inclusion criteria comprised adults aged 18 and above, independent in daily living with absence of any impairment (sensory or motor) affecting upper extremity function according to self-report. Participants’ age, gender, handedness, and type of work –whether high or low manual strain- were recorded. If there was uncertainty regarding handedness we presumed dominance according to the hand that was used for writing.
Instruments
The Jamar Dynamometer is designed to measure gross power fist grip and is considered to be the most accurate test for this skill. The American Society of Hand Therapists (ASHT) recommends the use of this tool for the assessment of grip strength [31]. Psychometric testing established good inter-rater reliability and high test re-test reliability [15, 32]. In congruence with the ASHT recommendations, participants were asked to sit on a chair without armrests, with both feet on the ground. Their shoulders were in neutral position and elbows flexed to 90°, with arms by their sides and forearms in a neutral position. The Jamar Dynamometer was set to the second handle position. The dominant hand was measured first, and each hand was measured 3 times alternately. The mean score of the three trials was recorded and used for data analysis.
Procedure
Data was collected over a period of six months using a convenience sample of volunteers with fairly equal distribution across age groups and gender. All consenting volunteers received information regarding the objectives and value of the study. They were screened for any health conditions according to exclusion criteria prior to initiation of data collection. Participants that met the inclusion criteria were asked to complete a demographic questionnaire followed by grip strength measurement.
Statistical analysis
Results were analyzed using SPSS 25 (SPSS Inc., Chicago, IL, USA). The distribution of variables in the study met the criteria for normality based on the Shapiro-Wilk test of normality (p > 0.05). Descriptive and inferential statistics were used to analyze the data. Descriptive statistics included the assessment of frequencies, percentages, means and standard deviations for socio-demographic and normative data.
One-way analysis of variance (ANOVA) was used to compare grip strength between age groups. Post hoc Scheffe comparisons were conducted.
An independent sample t-test was used to compare grip strength between men and women. A paired sample t-test was used to compare grip strength between the dominant and non-dominant hand. Due to unequal group size a Welch’s t-test was used to compare grip strength between high manual strain workers and low manual strain workers. Effects sizes were calculated according to the statistical test, partial eta squared for ANOVA [33], and Cohen’s d for t-test [34]. A One sample t-test was used to compare the Israeli normative data stratified by age, gender and hand dominance to two sets of published norms: Mathiowetz [15] and Larson and Ye [30]. Both data sets were reported in pounds. Therefore, for the purpose of these comparisons, the Israeli data was transformed into pounds. Mathiowetz presented his norms divided into 12 age groups while Larson and Ye divided into 5 age groups. The Israeli sample was divided accordingly, i.e. into 12 groups when comparing to Mathiowetz and into five groups when comparing to Larson and Ye.
Results
Six hundred and thirty-seven participants took part in the study. Three hundred and thirty four participants were males (54%) and 293 females (46%); their mean age was 48 (SD = 18.4). Twenty percent of the participants worked in high manual strain jobs and 65% in low manual strain jobs. The rest were unemployed or retired (Table 1).
Demographic and clinical characteristics of study groups
Demographic and clinical characteristics of study groups
The average grip strength, stratified by gender, age and hand dominance is presented in Table 2. Overall, a significant effect for age was found in both hands for men (dominant hand F (5,343) = 22.795, df = 5, p = 0.000 η2p = 0.252; non-dominant hand F (5,343) = 18.897, p≤0.001 η2p = 0.218) and women (dominant hand F (5,292) = 12.190, p≤0.001, η2p = 0.175; non-dominant hand F (5,292) = 11.908, p≤0.001 η2p = 0.172). Post hoc comparisons revealed significant differences (p < 0.05) in grip strength between the 70+ age group to all other age groups for both hands in males and females. The 70+ group demonstrated weaker grip strength than all other groups. Among the men, significant differences (p < 0.05) were found between the 30–39 age group and the 60–69 group for both hands. In the dominant hand results showed significant differences (p < 0.05) between the 30–39 age group and 50–59 group.
Normative Israeli grip strength stratified by gender, age and hand dominance
Normative Israeli grip strength stratified by gender, age and hand dominance
Significant differences (p < 0.05) between men and women were found for both dominant and non-dominant hand, showing the grip strength of men to be stronger than the grip strength of women for both hands (Table 3). Comparisons between dominant and non-dominant hand revealed significant differences for both men and women, where the dominant hand was stronger than the non-dominant one in both groups. Among the women, the dominant hand was 9.6% stronger than the non-dominant hand and for men, the dominant hand was 6.8% stronger than the non-dominant hand (Table 4).
Mean grip strength differences between men and women
Mean grip strength differences between dominant and non-dominant hand
In order to compare grip strength between high manual strain workers (n = 129) and low manual strain workers (n = 411) we conducted a Welch’s t-test. The results demonstrated a medium to large effect for type of work on grip strength, more specifically, portraying high manual strain workers to have stronger grip strength (Table 5).
Mean grip strength differences between high and low manual strain workers
We compared Israeli normative grip strength data to the norms published by Mathiowetz et al. [15] in 1985 and to the recent data on grip strength, published by Larson and Ye [30] in 2017. The comparison between grip strength of males and females in the present study to that of the participants in Mathiowetz’ study revealed that the US cohort was significantly (p < 0.05) stronger than the Israeli cohort. A similar trend was found when comparing the current study to an updated US cohort in Larson’s study (Supplementary Table S1).
Comparing the current study to Mathiowetz’ norms depicted significant differences in more than half of all possible age group comparisons for both hands in males. In general, the grip strength of the Israeli male population was weaker than for men in the Mathiowetz study. Moreover, when comparing Israeli data to the Larson study, 50% of the comparisons were significant and the grip strength of the Israeli male population was found to be weaker in all the comparisons.
Among the females, 62% of all possible age group comparisons for both hands were significant compared to the Mathiowetz findings. The grip strength of the Israeli females was weaker in all but one comparison. Similarly, all comparisons, for both hands, between the current study and Larson’s study were significant, demonstrating weaker grip strength for the Israeli female population (Supplementary Table S1).
Discussion
This study presents Israeli normative reference values for grip strength based on data obtained from 637 healthy volunteers. Grip strength for the Israeli sample was compared to two data sets: The gold standard grip strength norms of Mathiowetz from the 1980’s and the more recent normative grip strength values in Larson’s study. The purpose of these comparisons was to examine the need for population specific norms.
Within the Israeli sample a significant age effect was found for both men and women. This effect was most apparent among the 70+ age groups, in which grip strength was weaker than all the other groups. Similar trends of grip strength differences between distant age groups, and no statistical significant differences with nearby groups were demonstrated in previous studies. In a recent study of a large US cohort (N = 1232), Wang et al. [35] reported that grip strength for participants aged 18 to 24 years was not statistically different from participants in nearby age groups (25–59 years), but was significantly greater than that of participants in age groups of 60 years and older. Similar findings were conveyed in a Turkish sample [23]. In a Slovenian cohort, age group 65–79 years obtained the lowest grip strength values, regardless of gender [36]. An Iranian study demonstrated that the weakest grip strength was found in participants who were 75 years of age or older [34]. In yet another study, pertaining to a large British sample, a small negative correlation was found between grip strength and age, demonstrating a decline in strength as age progresses [37]. These worldwide studies of grip strength accentuate that regardless of gender, hand dominance and culture there is an age effect for grip strength that is most evident between the age group of 60 + and the younger groups.
In the present study, the grip strength of Israeli males was significantly stronger than the women’s grip strength in both hands. For both genders the dominant hand was significantly stronger than the non-dominant hand. These findings are congruent with previous studies [18, 34–38] in which the dominant hand was stronger than the non-dominant hand for both men and women. However, the difference between the hands was larger among women (9.6%) than men (6.8%). Most previous studies reported the dominant hand to be stronger than the non-dominant hand regardless of gender [23, 35–37]. Similarly, to the findings in the current study, Ekşioğlu [23] reported differences in grip strength between dominant and non-dominant hand to be larger amongst women. In women the dominant hand was 5% stronger than the non-dominant hand, whereas amongst men the difference was only 3%. These findings are noteworthy, because in clinical settings it is common practice to compare the grip strength of the injured hand to the non-injured hand, with the assumption that the dominant hand is expected to be 10% stronger. The results of the present study, along with previous reports, question this practice [3, 39].
This study also addressed the effect of type of work on grip strength. Previous studies did not demonstrate a significant difference between high and low manual strain workers. Klum et al. [39] measured the grip strength of 750 working adults, showing that high manual strain workers had slightly stronger grip strength than the low manual strain workers. However, these differences were not significant. Similar trends were reported in several other studies [23, 40]. Contrary to that, we found that men and women who were employed in high manual strain work had significantly stronger grip strength than those employed in low manual strain work. Josty et al. [41] support our findings, documenting the strongest grip strength in 104 high manual strain male workers. A recent study [42] demonstrating positive association between upper extremity strength and physically demanding jobs among industrial employees lends further support to these findings. Furthermore, normative data collected among 1740 male workers demonstrated significantly higher grip strength among light manual strain workers in comparison to office workers [43].
The grip strength of the Israeli sample was compared to two other data sets in order to explore the need for Israeli norms. The gold standard normative data set that is utilized in both clinical and educational settings in Israel is that of Mathiowetz from 1985 [15]. From our clinical experience it seems that many clients receive scores that are considered to be below the normal range in their unaffected hand. This clinical observation, along with reports from the literature regarding differences in grip strength in different nations, motivated us to conduct the current study. In order to rule out the possibility that the differences between the Israeli population and Mathiowetz norms are the result of the three decades that passed since these norms were established, we also compared our results to a relatively small, but more recent, US sample reported by Larson and Ye in 2017 [30].
Comparing the Israeli data to both US data sets demonstrated significant differences in at least 50% of the comparisons. The vast majority of the comparisons indicated that the grip strength of the Israeli sample is weaker compared to both American samples. These finding are in congruence with previous studies from other countries. A cohort of 2678 Australians was compared to 7 different studies of grip strength from the US, United Kingdom, Sweden and Finland. Overall the grip strength of the Australian population was found to be weaker than the other cohorts [24]. Similarly, grip strength of Portuguese older adults was generally lower compared to data sets from 14 countries including US, Australia, South America, Europe and China [44]. Ekşioğlu [23] reported similarities in grip strength between Turkish, British, US and Nigerian populations, however differences were found in comparison to Asian populations. Similar findings were reported in other studies comparing grip strength in Asian populations to norms derived from mostly Caucasian populations [21, 38]. In all these studies the grip strength of the Asian samples were weaker than those in Caucasians. These differences were mostly attributed to racial anthropometric differences, mainly in the size of the hand.
The current study is limited due to the employment of convenience sampling, possibly restricting the sample’s ability to represent the general Israeli population. Another limitation may lie in the disproportion between low and high manual strain workers, for example the relatively small group of high manual strain workers.
Conclusions
This pioneering study presents adult normative data for grip strength in a large Israeli cohort. Differences between the current Israeli sample and two US normative data sets accentuate the importance of establishing culturally sensitive norms. Furthermore, the difference in grip strength between high and low manual strain workers found in the present study underlines the possible need for a more detailed set of norms that will take into consideration specific manual loads. Finally, our results challenge the preconception of ten percent difference between the dominant and non-dominant hand, especially for males.
