The relationship of hip rotation range,hip rotator strength and balance in healthy individuals

1. Introduction

Hip internal rotation (IR) and external rotation (ER) play important roles in maintaining good pelvic and leg alignment while walking and during activities of daily living [1]. Hip IR and ER range of motion (ROM) are both typically about 45 ∘ , and an imbalance in hip rotation ROM could lead to a need for hip arthroplasty, such as a total hip replacement [2, 3, 4]. The factors that affect hip IR and ER include muscle length imbalance and skeletal malformation [5]. Skeletal problems of the hip joint include femoral neck anteversion or retroversion, and muscular problems are mainly caused by differences in muscle length or in the strength of the hip rotators due to a repetitive and habitual abnormal sitting, standing, or lying posture [5]. As a result, many interventions are applied to improve the agonist and antagonist ratios of hip strength and physical function [6, 7]. Overall, increased hip strength and greater balance performance have been reported to be associated with reduced musculoskeletal injury risk [8].

Balance is defined as the ability to maintain the center of mass (COM) over the base of support (BOS) under static and dynamic circumstances [9]. Among strategies for maintaining balance, the hip strategy is actively used to move the COM on the BOS during various activities and contributes to stabilizing posture during movement of the neck and trunk in the three body planes [10]. Therefore, hip strength plays important roles in stabilizing posture and balance and in anatomically and biomechanically connecting the upper and lower extremities. Although previous studies have examined the relationship between hip strength and ROM in the horizontal plane using simple clinical measurements in different positions [2, 11, 12], it remains unclear whether hip IR and ER strength and ROM actually influence static and dynamic standing balance. Therefore, in this study, we investigated the relationships of hip IR and ER ROM, measured using a three-dimensional (3D) motion capture system, and hip rotator strength with single-leg standing balance.

2. Methods

3. Results

The mean hip IR/ER ROM ratio was 1.15, and the mean hip IR/ER strength ratio was 0.88 (Table 1). The mean values for hip rotation ROM were IR = 43.3 ∘ and ER = 42.4 ∘ , and the mean values for hip rotator strength were IR = 8.7 kgf and ER = 10.3 kgf (Table 1). There was a significant difference between IR and ER strength ( p = 0.001) but not between IR and ER range ( $p>$ 0.05) (Table 1). We found a significant correlation between hip ER strength and each variable measured on the hip ROM test (IR range: r = - 0.44, 0.01; ER range: r = 0.35, 0.05; IR/ER ratio: r = - 0.46, 0.01) (Table 2). Furthermore, significant positive correlations were found between the hip IR/ER strength ratio and the hip IR/ER ROM ratio ( r = 0.72, 0.01) (Table 2).

During the static balance ability assessment, the surface area of the ellipse, moving distance, and mean velocity from the single-leg standing test showed no significant difference between the subgroups divided by the IR/ER ratio of hip ROM. However, during the dynamic balance ability assessment, the subgroup with a hip IR/ER ROM ratio of around 1 showed significantly better balancing ability than the subgroup with a hip rotator muscle imbalance ( 0.05) (Table 3). Similarly, in hip rotator muscle strength tests, the group with an IR/ER ratio close to 1 showed significantly higher dynamic balance ability ( 0.05) (Table 4).

The results of this study show that the hip IR/ER ROM ratio was significantly correlated with the hip IR/ER strength ratio ( r = 0.72, 0.01), indicating a possible link between hip rotator strength and hip rotation ROM. Few studies have reported the relationship between hip rotation range and hip rotator muscle strength. Previous studies have reported that differences in hip IR/ER muscle strength were dependent on the type of hip rotation symmetry or asymmetry [11, 14]. Cibulka et al. [11] reported that greater hip IR ROM relative to ER ROM might be correlated with hip ER muscle weakness, whereas greater hip ER range was often associated with weakness of the hip IR muscles. Although the results of our study are similar to those of previous studies, the range of joint motion in our study was measured using a biomechanical method with more reliable 3D motion analysis and a modified shank model. Previous studies have indicated that the reproducibility of goniometric measurements of hip motion was generally poor [17, 18, 19]. Chevillotte et al. [20] reported a lack of inter- and intra-observer reliability in measurements of hip ROM by physical examination. Although the present study found no significant differences in hip IR or ER ROM according to posture, Lin’s concordance correlation coefficient for hip ER measurement between two examiners showed poor agreement [2]. Therefore, 3D motion analysis based on eight infrared cameras overcomes the limitations of conventional measurement by plastic goniometer and allows a more objective and quantitative measurement of hip rotation ROM.

The results of our study show that ER strength was significantly greater than IR strength ( 0.01). Our study confirmed that hip ER strength measures were higher in healthy individuals, which is consistent with the findings of previous studies [6, 21]. Kemp et al. [6] reported that hip ER to IR strength tested at a neutral hip position were significantly different, with a ratio of hip ER to IR strength of 1.32. However, our results differ from those of Kemp et al. [6] and Leetun et al. [21] in that we found no difference between men and women in hip rotation strength. In addition, these studies used different testing tools and procedures when measuring hip rotator strength. Although all participants in this study were healthy, the differences between IR and ER hip rotator strength resulted in an overall significant relationship between hip rotation range and hip rotator strength ( 0.05) (Table 2). This finding is consistent with the length-tension relationship in the muscular system, which indicates that the greatest muscle force develops when the position of the muscle is at about mid-length [22]. A muscle increases in length by adding sarcomeres, which shifts the length-force curve to the right, whereas a muscle decreases in length by losing sarcomeres, which shifts the length-force curve to the left [23].

Previous studies have reported that an abnormal hip IR/ER ROM ratio creates various musculoskeletal problems of the lower extremity, including excessive femur anteversion or retroversion, genu valgum or genu varum, pes planus, and metatarsus varus [24, 25, 26]. It has also been suggested that movement impairment syndrome of the lower extremities occasionally results from increased hip IR, increased knee valgus, and excessive foot pronation [3, 5, 14]. However, increased hip ER and a knee varus deformity have been associated with medial knee osteoarthritis [27]. In addition, excessive hip IR in the horizontal plane results in misalignment of the lumbar spine and pelvis. Particularly during repetitive movements such as walking, the muscle activity of specific muscles is increased and may cause pain around the hip joints [28].

There were significant differences between of the subgroups defined by our two hip IR/ER ratio categories in the dynamic one-leg standing balance ability (Tables 3 and 4). The single-leg standing test for static balance measures balance ability based on the level of static postural sway. In contrast, the pendular test of dynamic balance reflects the maximum motion capability of COM in eight radial directions [9]. Three well-known strategies have been proposed for balance control during different tasks: hip, ankle and stepping strategies [29]. The ankle strategy contributes most to maintaining static balance on a stable support surface, whereas the goal of the hip strategy is to maintain balance when COM moves on an unstable BOS [29].

Thus, a lack of a significant overall difference in the static single-leg standing test between two subgroups was likely a result of the fact that the subjects were generally healthy individuals who did not have any significant difficulty in performing daily activities. Because the static single-leg standing test used in this study can be adequately performed using an ankle strategy to maintain static standing posture, rotational torque of the hip joint was not required. However, the dynamic pendular test reflected the overall movement area of the COM during postural sway, which was more sensitive for measuring balance ability with engaged hips. Therefore, in this study, the relationship between hip strength and balancing strategies was found to greatly contribute to dynamic balance.

The present study has several limitations. This study was conducted on healthy individuals with no hip joint problems, such as pain or ROM limitations, who were mostly younger than those suffering from musculoskeletal dysfunction. Therefore, additional studies are needed to investigate the relationship between hip rotation ROM and hip rotator strength with various balance abilities in individuals with hip abnormalities, such as hip anteversion or retroversion.