Comparison of grip and pinch strength in young women with and without hyperkyphosis: A cross-sectional study

1. Introduction

From a biomechanical perspective, hyperkyphosis is one of the most common deformities of the spinal column that is not age-dependent [1]. Hyperkyphosis is characterized by a kyphotic angle equal to or greater than 40 degrees [2, 3]. Several conditions including congenital anomalies, trauma, neuromuscular and inflammatory disorders can lead to hyperkyphosis. Moreover, unsuitable postural habits predispose people to develop this deformity [3]. The prevalence of hyperkyphosis in a group of 11-year-old children was reported as 15.3%, and the prevalence among people aged 20 to 50 years old has been reported as 38% [4].

Hyperkyphosis is a common malalignment syndrome in the upper quadrant of the body accompanied by serious complications that may lead to death. It increases the risk of degenerative processes, pain or dysfunction in the thoracic spine [4]. Upper limb function depends completely upon proximal stability and distal functional movement. The prescapular muscles provide proximal stability during distal movement [5].

The hand is the functional executer of activities of daily living, particularly those requiring fine control of movement. Grip strength is defined as the ability of the fingers to hold objects and is an important factor when evaluating hand function [6]. Pinch power is defined as the ability to hold small objects between the thumb and index finger together with overall movement of the hand [7]. Grip and pinch indices are not simple actions executed solely by the fingers and wrist, but require appropriate functioning of the forearm, prescapular and shoulder muscles [8]. Park et al. noted that hand function is affected by scapular stability [8]. Appropriate scapular position is known to increase the efficiency of upper limb function [9]. Scapular protraction, which results from hyperkyphotic posture, leads to scapulohumeral rhythm dissociation [4]. This in turn can compromise the stability of the proximal cuff and lead to decreased upper limb function. Excessive thoracic kyphosis limits motion of the scapula on thorax and this in turn can result in decreased shoulder range of motion [10]. Increased thoracic kyphosis was considered as a risk factor for limited shoulder range of motion [11]. Hyperkyphosis is an unsuitable postural habit inducing muscle strength imbalance [12]. Considering the upper extremity as a chain, we hypothesized that grip and pinch strength would be attenuated in women with hyperkyphosis secondary to limited shoulder range of motion and induced muscle imbalances.

The effect of hyperkyphosis on grip and pinch strength has yet to be determined, so the aim of this study was to compare grip and pinch strength in women with and without hyperkyphosis.

2. Materials and methods

This cross-sectional study was conducted between August and October 2016. The inclusion criteria were age between 18 and 40 years and kyphotic angle equal to or greater than 40 degrees. Women diagnosed with scoliosis, a history of spinal surgery, neuromuscular disorders, uncorrected visual impairment, or vestibular dysfunction was excluded. The study was carried out at the biomechanical laboratory of the School of Rehabilitation Sciences, Shiraz, Iran. Thirty women with hyperkyphosis were included in this study by convenience sampling, and 30 healthy women matched for age and body mass index (BMI) took part in the study as a control group. The sample size was calculated based on earlier studies [5] considering an alpha level of 0.05 and a power of 0.80. All participants signed an informed consent form before participating in the study, which was approved by the ethics committee of Shiraz University of Medical Sciences in accordance with the standards of the Helsinki Declaration. Kyphosis angle was carefully measured with a 50 cm flexible ruler (Today Sport Researches Comp., Tehran. Iran).

To measure the kyphosis angle we asked participants to stand barefoot with their feet shoulder-width apart. The flexible ruler was placed over the spinous processes of the 7th cervical (C7) and 12th thoracic vertebrae. The exact point of the 12th thoracic vertebrae was determined with reference to the highest point of the iliac crests. The line between the crests was aligned with the level of the fourth lumbar vertebra, and the exact location of T12 was determined by counting up to the spinous process. To locate C7, the flexion- extension method was used. In this method, the two most prominent cervical spinous processes were palpated with the examiner’s middle and index fingers of the examiner while the patient’s cervical spine was in flexion. Then, the cervical spine was moved into extension through assisted movement. If palpated upper cervical spine moved anteriorly while the lower one remained stationary, the lower cervical spine was labeled C7. If both palpated spinous processes remained stationary, the upper one was known as C7. The palpation process was repeated by moving one level cephalad at a time to confirm the exact level of C7 [13]. The accuracy of this method was shown to be greater than the conventional method [13].

Participants were asked to look at a point located on the wall 2 m in front of them. They held this position for about 3 min to attain their routine alignment [2]. The flexible ruler was placed on the spinous process of the C7 and 12th thoracic vertebrae in order to record the original shape of the thoracic spinal curve. Once the thoracic curve was identified, the ruler was transferred to white paper without changing its curve and the spinous process of the C7 and 12th thoracic vertebrate were marked on the paper. These two points were connected with a straight line (L), and the bisector of the line was drawn (H). After that, the kyphosis angle was calculated with the formula: θ = 4 arctan(2H/L). If the calculated angle was equal to or greater than 40 degrees, the participant was assigned to the hyperkyphosis group [2]. The flexicurve method has been shown to be an appropriate quantitative clinical method for measuring the curvature of thoracic kyphosis [14].

Grip strength was measured with a dynamometer (SAEHAN Hydraulic Hand Dynamometer, Sh 5001, SAEHAN Comp., Changwon, Korea). The validity and reliability of the SAEHAN dynamometer have been confirmed previously [15] and the values obtained from this dynamometer are comparable to the reference values recorded with a Jamar dynamometer [15]. Based on the Wasia method, half of the space between the tip of the index finger and the metacarpo-phalangeal joint of the thumb (flexion crease) was specified as standard grip size [16]. The participant stood with her shoulders in adduction and in neutral rotation, and with her elbow in full extension [17] and wrist and forearm in the neutral position. The dynamometer was held close to but not in direct contact with the body. The measurements were repeated twice and the average was recorded for final analysis [16].

Pinch strength was measured with a pinch meter (SAEHAN Hydraulic Pinch Gauge Sh 5005, SAEHAN Comp., Masan, Korea). The pinch gauge was calibrated according to the manufacturer’s manual. The participant sat on a chair with her arm beside her body, her elbow in 90 degree flexion and her wrist in the neutral position. The pinch meter was impinged between the pads of the thumb and index finger as much as possible. The average of three trials was recorded for final analysis [18].

Normal distribution of the data was confirmed with the Kolmogorov-Smirnov test. To compare grip and pinch strength between the groups, we used independent sample t -tests. Pearson’s correlation coefficient was used to determine the correlation between kyphosis angle and grip and pinch strength. Statistical significance was set a priori at 0.05.

The demographic data for the two groups are summarized in Table 1. No significant differences were observed between the groups in the demographic data. The kyphosis angle was approximately 22 degrees greater in the hyperkyphosis group than the healthy control group (52.14 ± 8.07 vs. 30.78 ± 5.35 degrees). Grip strength was significantly lower ( P = 0.03) in hyperkyphotic group (mean ± SD: 23.38 ± 5.41 kg) in comparison to healthy group (mean ± SD: 26.78 ± 6.22 kg). Pinch strength were significantly lower ( p = 0.04) in the group with hyperkyphosis (mean ± SD: 6.10 ± 2.14 kg) than in the healthy control group (mean ± SD: 7.24 ± 2.20 kg). Kyphosis angle showed a weak correlation with grip ( r = 0.26) and pinch strength ( r = 0.23).

4. Discussion

The results of this study showed that grip and pinch strengths were lower in women with kyphosis compared to age-matched healthy women. Increased kyphotic posture has been associated with inappropriate scapular positioning [19]. Dabholkar et al. examined grip strength in different ranges of the scapular plane (180, 120 and 90 degrees), and found that grip strength improved considerably in these planes compared to the standard position. They suggested that improved grip strength was due to optimal position of the scapula (abduction and upward rotation) [20]. In contrast, optimal scapular position was compromised secondary to hyperkyphosis in our sample of young women. In hyperkyphotic postures, the scapula is abducted and rotated internally, which may lead to reduced grip and pinch strength. Horsley and colleagues found a strong positive correlation between grip strength and strength of the shoulder lateral rotator muscles [21]. In many cases, forward head posture is accompanied by hyperkyphosis. One of the associated muscle imbalances of forward head posture is humeral internal rotation. This might occur secondary to scapular protraction and changes in the resting position of the humeral head in the glenoid fossa [22]. Hence, the reduced grip strength in the women with hyperkyphosis studies here may be attributable to reduced shoulder external rotator muscles strength secondary to the internally rotated posture of the shoulder region.

Yang et al. evaluated the effects of active scapular protraction on upper limb performance in a group of young women. They concluded that active protraction of the scapula improved performance and muscle activity in the upper extremity [5]. Considering the protracted position of the scapula in the hyperkyphotic posture, the contradictory results was obtained might be explained by two factors. Firstly, hyperkyphosis leads to chronic passive scapular protraction, which may be completely different from intentional active protraction of the scapula. Secondly, in active protraction, the scapula is in abduction and upward rotation, whereas in hyperkyphosis the scapula is abducted and internally rotated.

Three mechanisms can be proposed for the attenuation of grip and pinch strength in our group with kyperkyphosis. The first mechanism is based on Chaitow’s theory, according to which stability and optimum posture of the scapula are provided through the coordinated cross-action of prescapular muscles. One of the axes of this cross-action comprises the serratus anterior and rhomboid muscles, which are activated reciprocally to provide scapular stabilization. Chronic shortness of the serratus anterior, which is the pattern observed predominantly in hyperkyposis, leads to protraction of the scapula. The other axis of the putative cross-action involves the trapezius and pectoralis minor muscles. The latter muscle is often shortened in patients with hyperkyphosis, and causes anterior tilting of the scapula on the rib cage [23]. According to this theory, stability of the scapula is compromised due to muscle imbalance in the proximal part of the upper limb. In this case reduced grip and pinch strength can be attributed to length-tension dissociation of the prescapular muscles.

The second mechanism is based on the muscle imbalance theory of Janda. Proximal stability of the upper limbs is compromised inhyperkyphotic postures, so decreased grip strength can be attributed to dysregulation of the length-tension relationship in the arm muscles with subsequent effects on distal areas. According to Janda’s theory, the upper limb constitutes a motion chain that begins with the cervical and thoracic vertebrae and ends with the fingers. All components of this chain must be activated in a coordinated manner to result inappropriate performance [24]. It was previously suggested that any changes in time and power generation within the chain may be the result of poor performance or damage in other parts of the chain [16].

Hyperkyphosis leads to muscle imbalance in different kinetic chains of the upper extremities, including the pectoralis minor in the flexor chain, the rhomboid in the extensor chain, the pectoralis major in the anterior chain, and the rhomboids and serratus anterior in the spiral chain [25]. Thus, disequilibrium in these kinetic chains may account for reduced grip and pinch strength.

The third possible mechanism is based on the Louis theory, according to which there exists a Y-V pattern of equilibrium in front of and behind the body which comprises the muscles and fascia. The pectoralis major and its related fascia constitute the stems of the Y pattern, and the rectus abdominis and its fascia form the base of the Y in front of the body. The V pattern is formed by the latissimus dorsi and its related fascia behind the body [26]. Tightness of these muscles has been reported in patients with hyperkyphosis [27], and this could impair the Y-V pattern. Because the fascia is a continuous, flexible, fibrous connective tissue surrounding all groups of muscles, grip and pinch strength attenuation can be attributed to length-tension impairment in the involved fascia.

Our results are in line with earlier studies. Ugwu and Ene evaluated the effect of kyphosis on physical function in a group of retired older community dwellers. They found that greater degrees of kyphosis were associated with older age, and may have led to reduced physical function and decreased entrepreneurship ability [28].

Other previous studies also evaluated the association of kyphosis with grip strength in older community-dwelling men and women. Katzman et al. evaluated the relationship between age-related hypekyphosis and its associated impaired mobility in older community-dwelling women. Kyphosis angle showed a negative correlation with grip strength, and these authors concluded that hyperkyphosis might be a useful clinical marker signaling the need for evaluation of the risk of vertebral fracture and falling [29]. In a similar study, Katzman and colleagues investigated physical function in older men with hyperkyphosis. They demonstrated that kyphosis was not associated with grip strength, but instead was associated with impaired lower extremity function [30]. Kado et al. evaluated the association between hyperkyphotic posture and physical functional ability in older community-dwelling men and women. Greater kyphotic angle was associated with less grip strength [31]. In another study, Katzman et al. determined the factors associated with kyphosis progression in older women. They found that grip strength was not associated with kyphosis [32].

We did not find any correlation between kyphosis angle and either grip strength or pinch strength. Considering the wide range of kyphosis angles (40–75 degrees) in our group with hyperkyphosis, other factors in addition to kyphosis angle might have influenced grip and pinch strength.

To the best of our knowledge, this study is the first to investigate the effects of hyperkyphosis on grip and pinch strength in a group of young individuals. Because young people are considered the most productive group within a community, reduced grip and pinch strength may be related with irreparable economic loss and emotional impairment on a community-wide level. Previous studies focused mainly on the effects of exercise programs on pain and postural alignment in patients with hyperkyphosis or in healthy persons [19, 33]. As shown in the present study, hyperkyphosis might lead to decreased grip and pinch strength in the absence of pain. Hence, the clinical importance of this study is that the early prescription of postural correction exercises for persons with hyperkyphotic might prevent secondary disabling symptoms and help them cope with routine daily activities.

Our study has some limitations. First, we studied an only woman, which reduces the external validity of our findings. Because of the importance of gender in grip and pinch strength [34], a similar study in males is advisable. Second, we did not evaluate the strength of prescapular muscles, which might have provided valuable information and more generalizable results. We did not collect data on perceived a pain in our participants, and pain may have influenced pinch and grip strength in our group of women with kyphosis. Although the method of assessment we used was valid and reliable, further studies are warranted to evaluate kyphosis angle with more precise methods such as X-rays.

In conclusion, hyperkyphotic posture led to reduced grip and pinch strength in a group of young women compared to a healthy group of age- and BMI-matched women.