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УДК: 612.744.22: 616-056.17
БИЛАТЕРАЛЬНАЯ РАЗНИЦА В МАКСИМАЛЬНОЙ СИЛЕ МЫШЦ КИСТИ: ЭФФЕКТ РУКОСТИ И СИЛЬНЕЙШЕЙ РУКИ
Ю. Зверев - к.м.н., доцент
Нижегородский филиал Сочинского государственного университета Медицинский колледж университета Малави, Блантир, Малави, Южная Африка
BILATERAL DIFFERENCE IN MAXIMAL GRIP STRENGTH: EFFECT OF HAND PREFERENCE AND STRONGER HAND
Y. Zverev - MD, PhD, Professor College of Medicine, University of Malawi Blantyre, Malawi, Southern Africa N. Novgorod branch, Sochinskiy State University N. Novgorod
e-mail: yzverev@yahoo. com
Ключевые слова: билатеральная асимметрия, максимальная сила мышц кисти, динамометрия, рукость.
Аннотация. В настоящем исследовании сравнивается сила мышц кисти доминантной и недоминантной рук у правшей, левшей и амбидекстров. Результаты подчёркивают, что для более точного прогнозирования билатеральной асимметрии максимальной силы мышц кисти необходимо разграничение рукости, определённой на основе предпочтения руки для выполнения точных движений, и рукой, имеющей преимущества в силе.
Key words: bilateral asymmetry, maximal grip strength, dynamometry, handedness
Summary. This study compares dominant and non-dominant hand strength in right-, left-, and mixed handed participants. The results suggest that distinction between stronger hand and hand preferred for skilled activities should be made and considered when predicting the bilateral asymmetry in the MGS and assessing limitations in hand strength.
INTRODUCTION
Grip strength is critical in various daily life, social and professional activities as well as in manual actions in physical culture and sport. Maximal grip strength measurements are commonly used as an objective index of the functional integrity of the upper extremity and indicator of overall health, nutritional status and physical fitness of people [5, 12]. The estimation of hand grip strength is of immense importance in assessment of disability ratings and in determining the efiency of different treatment strategies of hand and also in evaluation of, patients rehabilitation effectiveness with damaged upper extremity, neurological and neuromuscular disorders, stroke, cerebral palsy, and etc.
The grip strength is affected from many conditions and muscle strength is one of these factors. The synergistic action of flexor and extensor muscles and the interplay of muscle groups is an important factor in the strength of resulting grip [3]. In gripping activities, the flexor muscles in the
hand and forearm create the grip strength, while the extensor muscles of the forearm stabilize the wrist [3].
The goal of patients with limitation of hand functions is physical rehabilitation to restore upper extremity strength to the pre-injury or pre-illness level. However, prediction of this initial level of grip strength may present some problems due to several reasons.
First, comparison of hand strength of a person with reference values may not be very useful due to high variability of this parameter within reference populations. Therefore, this approach gives only approximate degree of hand weakening due to pathological conditions. Second, in unilateral limitation of hand functions the strength of the two hands can be compared. This is useful when initial muscle strength is equal in both hands. Problems arise when this is not the case. Therefore, some guidance is required for comparison of the grip strength of the damaged and intact hands. The classic "ten percent rule" servers this purpose. The rule suggests that the dominant hand is 10% stronger than the non-dominant hand [1, 10].
Bilateral asymmetry in grip strength may be attributed to the central mechanism of the origin of handedness and lateralization of functions in the brain [2]. Some researchers believe that maximal voluntary muscle force (especially grip force) only has a mild level of lateralization [14]. Environmental factors also affect the differences between hands. The world we live in is designed for right handedness as most tools and daily appliance are designed for the right hand. Therefore, the right hand of both right and left handed people is exercised more often on a daily basis and develops more strength than the left one [5]. Some authors have suggested that peripheral factors such as differences in biomechanical properties of the two hands also contribute to the bilateral asymmetry in maximal grip strength [14].
The utilization of the "ten percent rule" was questioned by several studies conducted in different populations. First, it is not clear whether the rule can be applied to all three categories of handedness, namely right-, left-, and mixed-handedness. For example, it has been demonstrated that the dominant hand is stronger in right handers only while in left handers the non-dominant hand strength is equal to or higher than the dominant hand [1, 4, 9, 10]. Second, even within the same handedness category the rule cannot be equally applied to people with strong and weak degree of hand preference [2, 14]. Third, the degree of bilateral asymmetry in grip strength in different studies varies from 0% to more than 30 % [2, 4, 5, 7-10].
Such discrepancy in the reported data on the utilization of the "ten percent rule" can be explained by differences in methodological approaches used in various studies in assessment of grip strength and handedness of participants, type of a criterion (strict or lax) used for categorization of handedness and different sample sizes. Many studies were focused on right and left handers categorized on the basis of hand preference for unilateral activities and hand skillfulness without considering stronger hand of participants.
Lack of a definitive picture of the difference between dominant and nondominant hand strengths prompted this study which was designed to evaluate the maximal grip strength differences between sides for right, left and mixed handed persons.
MATERIALS AND METHODS
The study was conducted at the College of Medicine (COM), University of Malawi and Queen Elisabeth Central Hospital, Blantyre, Malawi. In total 259 male volunteers were selected randomly from visitors to the patients admitted at the hospital and students of the college. The inclusion criteria to the study sample were as follows: 19-22 years age, lack of previous history of neuro-muscular and skeletal disorders of the upper extremity and trunk and lack of, hands functions limitation.
Maximal grip strength (MGS) of both hands was measured with the standard factory- calibrated adjustable-handle Jamar grip strength dynamometer and the standard procedure and arm positioning were followed [6]. For the tests, the subjects were seated with their shoulder adducted and
neutrally rotated, elbow flexed at 90°, forearm in neutral position, and wrist between 0° and 30° dorsiflexion and between 0° and 15° ulnar deviation. Such standardized position of the hand and arm facilitates maximal power grip strength [11]. The dynamometer was lightly held around the readout dial by the examiner to prevent inadvertent dropping. The method was demonstrated to each subject and volunteers were allowed to practice and familiarize themselves with the apparatus. Grip strength of each hand was measured three times and recorded to the nearest 0.1 kg. The highest reading was taken as MGS of the hand and the hand with higher value of the MGS was classified as the stronger hand. The percentage difference in the MGS of the two hands was calculated using the following equation:
Difference in MGS between the two hands (kg) x 100% MGS of the weaker hand (kg)
Handedness was assessed using questionnaire containing eight questions on hand preference for different types of unimanual activities (spontaneous activities not shaped by outside influences, activities influenced and marked by imitation and education and activities shaped by the design of technical equipment and tools) [13]. The instruction allowed a person to select the use of both hands for a specific activity. Subjects were classified as left or right handers if they preferred the left or right hand respectively for all activities and as mixed handers if hand preference was inconsistent.
A standard version of SPSS 11.0.0 was used for data analysis. Results are presented as means and standard errors of means. We used one-way chi-square analysis to compare proportions and the Students't test to compare means. Analysis of variance was used to test the equality of more than one population means. The level of statistical significance was fixed at p < 0.05.
RESULTS
The proportions of right, left, and mixed handers in the study sample were 88.0%, 5.4%, and 6.6%, respectively. The right hand exhibited higher MGS in 69.5% subjects (table 1). Significant association was found between handedness and stronger hand (x =8,62, n=2, p=0.01). For most of the left and right handers, the MGS of the preferred hand was highest while most of the mixed handed subjects exhibited higher MGS of the left hand.
Table 1
Relation between stronger hand and hand preference (values are numbers of subjects with percentages in brackets)
Stronger hand Hand preference Total
Right Left Mixed
Right 168 (73.7) 5 (35.7) 7 (41.2) 180 (69.5)
Left 60 (26.3) 9 (64.3) 10 (58.8) 79 (30.5)
Total 228 (100) 14 (100) 17 (100) 259 (100)
The MGS values had normal distribution for all groups of volunteers. Table 2 shows the bilateral asymmetry in MGS values according to the handedness category. Without stratification by the stronger hand a significant bilateral difference in the mean MGS values was found in the subgroups of right and left handers but not in mixed handers. The mean values of bilateral differences in the MGS of the right and left handers were similar (p > 0.05) and significantly higher than in the mixed handers (p < 0.05).
Table 2
Maximal grip strength (means±SEM) of right, left and mixed handers (*p < 0.05 comparing to cor-
responding value of right hand)
Handedness Maximal grip strength
Right hand (kg) Left hand (kg) Bilateral difference (%)
Right 44.4±0.6 42.6±0.6* 4.3±0.5
Left 41.3±1.6 43.6±1.9* 5.6±0.8
Mixed 40.5±2.6 40.1±3.6 1.0±2.2
The stratification of the left, right and mixed handers by a stronger hand (table 3) increased the levels of bilateral differences in the MGS in all subgroups of subjects.
Table 3
Maximal grip strength (means±SEM) of right, left and mixed handers stratified by stronger hand
(*p < 0.05, **p < 0.001 comparing to corresponding value of the right hand)
Handedness Stronger hand Maximal grip strength
Right hand (kg) Left hand (kg) Bilateral difference (%)
Right Right 46.1±0.9 41.8±0.9** 10.3±0.7
Left 39.8±1.1 44.8±0.9** 12.6±1.4
Left Right 42.6±1.2 39.2±1.3* 8.7±1.2
Left 40.5±1.9 46.0±1.6* 13.6±1.3
Mixed Right 41.7±2.0 35.7±2.1* 16.8±2.1
Left 39.6±1.1 43.1±1.3* 8.8±1.7
DISCUSSION
The present study differs from the previous publications [1, 4-6, 8, 10] in the way that it considers the effects of the three categories of handedness as well as the stronger hand on the level of bilateral asymmetry in the maximal grip strength values. Such approach seems to have some advantages as it allows to analyze the relationship between handedness and stronger hand and to form more homogenous groups of subjects.
The main aim of the present study was to assess the bilateral asymmetry in the MGS and to test the utility of the "ten percent rule" which states that the dominant hand is 10% stronger than the non-dominant hand.
Our findings did not confirm the rule. First, the study revealed association between handed-ness determined on the basis of hand preference for skilled activities and stronger hand. However, this association seems to be weak as hand preference for habitual activities did not correspond with stronger hand in about 26% of right handers, and 36% of left handers. Similar proportion of left handers with stronger right hand (33.3%) was reported by Incel et al. [4]. However, they reported a smaller percentage of right handers (10.9%) with stronger non-dominant hand grip than in our study.
While interpreting the results the multidimensional nature and presence of different aspects of handedness should be considered: the relative preference for one hand in performance of unimanual tasks, the higher skillfulness of one hand in performance of these tasks and the greater strength of one hand. In addition, hand preference and performance vary from task to task. For example, Steenhuis and Bryden [13] suggested 4 types of manual actions (skilled, reaching, power and bimanual actions) and 4 corresponding types of handedness. However, they believed that only hand preference for skilled activities is a true indicator of sidedness as these activities are more lat-eralized than actions that involve strength. Therefore, the assessment of the impact of the effect of handedness on grip strength ratios requires clarification of handedness definition and evaluation method [2]. The results of the present study suggest that it is more accurate to consider bilateral asymmetry in the MGS between stronger and the other hand rather than between dominant and nondominant hand for skilled activities. Superior hand might be identified with questions on hand preferred for activities which require strength or with force-exertion and force-controlling tests.
Second, significant bilateral difference in the mean MGS values was found in right and left handers. In mixed handers this difference was small and statistically nonsignificant. These findings concur with recent data reported by Koley and Singh [5]. However, they did not consider a category
of mixed handers and the prevalence of left handed males (31.8%) in their study population was significantly higher than that reported for culturally restricted societies such as India or Malawi. Earlier study also demonstrated higher prevalence of subjects with higher grip values at their nondominant side in left handed group than in right handed [10].
Third, the level of mean bilateral asymmetry in the MGS depended on whether the stronger hand was considered. Calculation of the mean bilateral differences in the MGS values in the three categories of handedness regardless of the stronger hand of subjects gave values of 4.3%, 5.6% and 1% in right, left, and mixed-handers, respectively. When the stronger hand was considered and mean bilateral differences in the MGS values increased to 10.3%, 8.7%, and 16.8% in right, left and mixed-handers with stronger right hand, respectively and 12.6%, 13.6% and 8.8% in right, left and mixed-handers with stronger left hand, respectively.
CONCLUSION
In conclusion, the bilateral asymmetry in the MGS varied from 8.8% to 16.8% in different groups of handers. Distinction between stronger hand and hand preferred for skilled activities should be made and considered when predicting the bilateral asymmetry in the MGS. So we are in agreement with studies suggesting that the "ten percent rule" cannot be generalized to the whole population.
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Статья поступила в редакцию: 16.10.2012 г.
