Physical Therapy Korea

Hip abductors provide the lumbo-pelvic-hip complex stability during unilateral weight-bearing activities. The gluteus medius (Gmed), gluteus minimus, and tensor fasciae latae are attached to the pelvis, and are considered the pelvic stabilizer muscles and hip abductors [1]. The Gmed plays an important role in the frontal plane stability of the pelvis in functional activities, including one-leg lifting [2,3]. The attachment of the Gmed from the ilium to the greater trochanter of the femur allows for eccentric control of the adduction-medial rotation of the femur and contralateral pelvic drop during single-leg weight-bearing activities [4,5]. In several studies, weakness of the Gmed has been reported in patients with lower extremity injuries such as hip osteoarthritis [6,7] and greater trochanteric pain syndrome [8], and increased activation of the Gmed has been shown to compensate for the lack of eccentric control. In clinical practice, various measurements are performed to evaluate the abnormal movements caused by weakness of Gmed.

Among the static measurements, the conventional Trendelenburg test has been widely used to measure the Gmed function [9-11]. Many researchers have investigated uncontrolled pelvic motion, such as the pelvic drop angle and lateral sway, during the Trendelenburg test [11-13]. However, the Trendelenburg test has a ceiling effect on the pelvic drop angle [11,14]. Even if the test result for the pelvic drop angle is negative, it has been reported that people with nonspecific low back pain have Gmed weakness [15]. The pelvic drop angle may not sufficiently reflect the functional decline of the Gmed. Therefore, both superior-inferior motion (pelvic drop angle) and medio-lateral motion (lateral pelvic shift) are considered for the evaluation of the Gmed. A pelvic shift occurs spontaneously in any individual, regardless of the Gmed weakness, as the center of gravity of the body moves to the contralateral side during one-leg lifting. According to Sahrmann [16], excessive pelvic shift with ipsilateral hip medial rotation during one-leg lifting can be influenced by habit of lying on one side. They reported that if people with hip adduction syndrome participated in sports such as running or cycling, the imbalance between the hip flexor-medial rotator muscle and extensor-lateral rotator muscle would increase. In the pelvic shift test, when the pelvic shift distance is > 10 cm or the asymmetry of both pelvic shift distances is > 2 cm, it is determined as excessive hip adduction of the standing leg and failed load transfer [16-18]. The amount of uncontrolled pelvic shift is associated with isometric and eccentric control of the Gmed [17]. Therefore, the distance of pelvic shift can be useful in evaluating people with and without Gmed weakness. However, laboratory-based measurements for pelvic motion analysis, such as 3D motion capture system [13,19] or two-dimensional video analysis [20], are inconvenient for clinical use in that measurement equipment is required.

Side-lying hip abduction (SHA) has been used as a dynamic test because static test do not sufficiently reflect the function of Gmed and are inconvenient to analyze. Distefano et al. [21] showed that SHA produced approximately 16% more activation of the Gmed in electromyography than other exercises, such as single-limb squat, band walk, and sideway hop. To investigate the relationship between strength of the Gmed and uncontrolled pelvic motion, the correlation between SHA strength and pelvic drop angle was investigated, but no significant results were found [22]. The reason for these controversial results may be that the abduction force of the lower leg against the floor was not controlled during SHA. Contralateral hip adduction during SHA can be proposed. The abduction force of the Gmed and the adduction force of the contralateral adductor magnus bring the pelvis to a neutral position through co-contraction [23,24]. Simultaneously, the base of support is reduced by lifting the lower leg in SHA. It was expected to decrease the angle of lateral pelvic tilting and increase the activity of the Gmed muscle [25]. Therefore, hip abduction performance with contralateral adduction in the side-lying position (HAPCA) can be used to measure Gmed performance.

Although some studies have investigated physiological characteristics such as muscle activation or muscle thickness in the HAPCA position [25,26], no studies have determined whether the number of HAPCA correlates with the lateral pelvic shift distance during one-leg lifting. Measuring the number of performances has the advantage of being able to provide test results with quantitative data. Determining the relationship between the number of HAPCA and lateral pelvic shift distance during one-leg lifting may be a meaningful contribution for evaluating and managing pelvic stability. Therefore, this study aimed to determine the effect of the number of HAPCA on the lateral pelvic shift distance during one-leg lifting. It was hypothesized that there would be a negative correlation between the number of HAPCA and lateral pelvic shift distance and that the number of HAPCA may influence the lateral pelvic shift distance.

1. Participants

Thirty healthy participants (19 males, 11 females; age = 26.9 ± 2.59 years; height = 167.77 ± 14.06 cm; body mass = 72.40 ± 16.56 kg) were recruited. The sample size was calculated using the G-power 3.1 program (Franz Faul, Kiel University) based on pilot study data including six participants. As a result of using a priori sample-size power analysis with a power of 0.8, α = 0.05, and an effect size of 0.339, at least 26 participants were required. For a sufficient sample size, 30 participants were recruited. Participants were excluded if they could not perform hip abduction because of hip joint pain, low back pain, inability to maintain a one-leg standing position, or any ankle joint disability. The dominant leg was selected to kick the ball. Ethical approval was obtained from the Institutional Review Board of Yonsei University Mirae campus (IRB no. 1041849-202103-BM-041-01). A signed informed consent was obtained from all participants after informing them of the procedures and purpose of the study.

2. Procedures

An one-leg lifting test was performed to measure the lateral pelvic shift distance (Figure 1). A step was placed in front of the wall, and the camera was fixed 150 cm vertically from the end of the step. The endpoint of the camera tripod was marked such that the position of the camera did not change during the experiments. The measurement process was recorded using a smartphone (iPhone 6s; Apple Inc.). Two reflective markers were attached to the anterior superior iliac spine (ASIS) of each participant. Participants stood on a step with their feet 10 cm apart [10]. The feet were placed 1 cm from the wall to prevent pelvic rotation during leg lifting. The participants performed hip flexion of the non-dominant side leg to the target bar. It was held for 5 seconds [22]. Because the pelvic posterior tilt may be induced by excessive hip flexion [9,27], the height of the target bar was set at the hip flexion of 30°. Another target bar was set at the ASIS level, and the participants were not allowed to raise their pelvis above the target bar. The one-leg lifting test was repeated five times. All trials were video filmed, and two images were captured in each trial. One was the phase of standing on both legs, and the other was the phase of maintaining one-leg lifting. The lateral pelvic shift distance was analyzed using the image J software (National Institutes of Health) [28] and the average of five trials was used for the analysis.

Figure 1. One-leg lifting test and measurement of the pelvic lateral shift distance.

The participants were placed in a side-lying position with the dominant leg on top. A towel was placed under the waist to prevent pelvic tilting and lumbar side-bending. The target bars were placed at an abduction angle of the upper leg of 35° and an adduction angle of the lower leg of 10° [25,29]. The angle of the leg was determined using the angle measurement application. Unlike SHA (Figure 2A), the participant repeated hip abduction between the two target bars while keeping the contralateral leg in contact with the lower target bar (Figure 2B). The tempo of hip abduction was set using an application “Metronome: Tempo Lite” (Frozen Ape Pte. Ltd.), and the metronome beat was determined at 50 bpm (1 beat per 1.2 seconds) through a pilot experiment. The examiner fixed the pelvis of the participant and checked whether a lateral pelvic tilt occurred. The researcher counted the number of successful movements in which the subject touched both target bars at the beat of the metronome. If the participant could no longer raise the leg to a given height with the metronome, the test was stopped.

Figure 2. (A) Side-lying hip abduction. (B) Measurement of the hip abduction performance with contralateral adduction in side-lying position.

3. Statistical Analysis

Data were analyzed using IBM SPSS software (ver. 25; IBM Co.). For normally distributed data, the Shapiro–Wilk test was used. A simple linear regression analysis was performed to investigate the relation between the number of HAPCA on the lateral pelvic shift distance during one-leg lifting. The Pearson correlation coefficient was used to determine the relationship between the number of HAPCA and lateral pelvic shift distance during one-leg lifting. The r-value was interpreted by Mukaka [30] as follows: 0.0–0.3 negligible, 0.3–0.5 low, 0.5–0.7 moderate, 0.7–0.9 high, and 0.9–1.0 very high. The level of significance was set at α = 0.05.

A summary of the results is provided in Table 1. All variables were normally distributed. There was a negative correlation between the number of HAPCA and lateral pelvic shift distance during one-leg lifting (r = –0.630, p < 0.05; Figure 3). The relation between the number of HAPCA and the lateral pelvic shift distance was determined as shown in Table 2. The number of HAPCA accounted for 39.7% of the variance in the lateral pelvic shift distance during one-leg lifting, and the regression equation was statistically significant (F = 18.454, p < 0.001). The regression equation is (Pelvic shift distance) = 156.024 + [–1.793 × (the number of HAPCA)].

Table 1 . Summary of the results.

Variable	Total (N = 30)	Male (n = 19)	Female (n = 11)
HAPCA (n)	29.03 ± 9.19	28.47 ± 10.17	30.0 ± 7.58
Pelvic shift distance (mm)	103.96 ± 26.16	110.30 ± 29.13	93.0 ± 15.78

Values are presented as mean ± standard deviation. HAPCA, hip abduction performance with contralateral adduction in the side-lying position..

Table 2 . Results of the simple linear regression analysis.

Variable	B	SE	β	t	p	F	R2
Constant	156.024	12.695		12.291	< 0.001	18.454	0.397
HAPCA	–1.793	0.417	–0.630	–4.296	< 0.001

HAPCA, hip abduction performance with contralateral adduction in the side-lying position..

Figure 3. Negative correlation between the number of the HAPCA and the pelvic lateral shift distance during one-leg lifting. HAPCA, hip abduction performance with contralateral adduction in the side-lying position.

This study identified the relationship between the number of hip abductions with contralateral adduction in the side-lying position and lateral pelvic shift distance during one-leg lifting. The hypothesis was that there is a relationship between the number of HAPCA and lateral pelvic shift distance. The number of HAPCA significantly affected the lateral pelvic shift distance during one-leg lifting, and there was a negative correlation between the two variables, which was consistent with our hypothesis.

In previous studies, there have been various attempts to determine the relationship between uncontrolled pelvic motion and hip abductor strength [4,12,22,31,32]. Kendall et al. [22] compared the pelvic drop angle of the posterior superior iliac spines during the Trendelenburg test and the SHA strength measured with a hand-held dynamometer (HHD) between low back pain and control groups. There was no difference in the pelvic drop angle between the two groups, and there was no correlation between the SHA strength and pelvic drop angle. In addition, after 3 weeks of abductor strengthening exercises in the low back pain group, a 12% increase in SHA strength was found, but there was no significant difference in the pelvic drop angle. Kendall et al. [22] noted that, although SHA strength may contribute to pelvic stability, the Trendelenburg test is limited in measuring hip abductor performance. In healthy participants, the Trendelenburg test may not be sufficient to detect changes in lumbopelvic and hip biomechanics [14]. For individuals with hip abductor strength greater than 30% of their body weight, pelvic kinematics cannot be assessed using the Trendelenburg test [33,34]. Therefore, functional movements that are more sensitive to changes should be used to assess hip abductor performance. Lateral pelvic shift is a motion that occurs during one-leg lifting, and with greater hip adduction on the standing side (contributing to lateral pelvic shift), optimal hip abduction force generation is provided according to the length-tension curve [12]. In addition, the lateral pelvic shift is closely related to the function of the Gmed because it reduces the moment arm of the body weight [35]. As the lateral pelvic shift increases, the center of gravity moves toward the standing limb, and the lever arm between the center of rotation of the hip and the line of body weight shortens. It leads to a decrease of the counteracting abduction force. Possibly, people with insufficient Gmed function may compensate for the lack of abduction force with a more lateral pelvic shift. However, previous studies did not consider the lateral pelvic shift [4,22,31], and using only the pelvic drop angle may not have reflected the function of the Gmed. A recent study reported a significant relationship between uncontrolled pelvic motion and hip abductor strength. Allison et al. [12] compared the hip abduction strength and uncontrolled pelvic motion during a single-leg stance between gluteal tendinopathy and control groups. The measured uncontrolled pelvic motion variables were the hip adduction angle (angle between the inter-ASIS line and femur), pelvic obliquity (angle between the inter-ASIS line and horizontal line), and lateral pelvic shift distance (the vertical line of the calcaneus-ASIS midpoint [distance between the vertical line of the calcaneus and inter-ASIS midpoint]) at different phases of leg lifting. Compared with the control group, the gluteal tendinopathy group showed greater hip adduction and less contralateral pelvic rise during the single-leg stance phase, and a 12% greater lateral pelvic shift occurred during the toe-off phase. Based on statistical estimates, they reported that the differences between the two groups could be attributed to hip abductor strength. This suggested that hip abductor strength affects pelvic kinematics, and it was different from previous studies that reported there was no correlation between uncontrolled pelvic motion and hip abductor strength.

There are several possible reasons for these different results regarding the relationship between hip abductor strength and uncontrolled pelvic motion. First, the position of the contralateral leg during measurement may have affected the hip abductor strength. To measure the hip abductor strength, the HHD is typically applied in the side-lying position, whereas Allison et al. [12] applied it in the supine position. The position of the non-measuring leg can influence the SHA strength of the testing side. This is because the pressure that occurs between the non-measured leg and floor can reinforce the hip abduction strength of the test leg. Therefore, when measuring the SHA strength, it is necessary to adjust the position of the non-measuring leg. Similarly, the SHA strength can be affected by the base of the support, which can vary according to the position of the non-measuring leg. In previous studies, the hip and knee joints of the non-measuring leg were placed at 90° flexion [22,31] or full extension [25,32]. The base of support varies depending on the degree of flexion of the lower leg, and a decrease in the base of support can increase Gmed activation during SHA [25]. The HAPCA was designed to eliminate the effect of the non-measuring leg and reduce compensatory motion such as pelvic tilting, allowing more accurate hip abduction performance [25]. Linear regression analysis showed that the number of HAPCA explained 39.7% of the lateral pelvic shift distances in this study. The pelvic shift distance, which reflects only one-leg performance, may be closely related to HAPCA than to SHA, which is influenced by the opposite leg. Second, lateral pelvic shifts with and without other pelvic motions may have different outcomes for Gmed performance. Uncontrolled pelvic motion is a complex of pelvic shift, pelvic rotation, and pelvic tilt and can occur in various patterns during one-leg lifting. To lift the leg, the participants used a “self-selected” strategy for balance [27]. For example, participants may experience excessive pelvic shift instead of lateral pelvic tilting or pelvic rotation. In the pelvic shift test by Comerford and Mottram [17], it was said that if the lateral pelvic shift occurred by more than 10 cm, a lack of Gmed control could be suspected; however, the test did not limit pelvic rotation and pelvic tilt. In this study, the ASIS level was aligned with the target bar to minimize the pelvic hiking of the lifting side, and the wall was used as feedback to limit pelvic rotation to measure the lateral pelvic shift distance during the one-leg lifting test. The participants in this study had lateral pelvic shift distances ranging from 5.3 cm to 18.1 cm. Among them, participants with a lateral pelvic shift distance > 10 cm showed an average number of HAPCAs of 25.7, and they were healthy participants without Gmed pathology. A lateral pelvic shift > 10 cm in healthy people may account for the possibility that a greater pelvic shift was required instead of pelvic rotation or pelvic tilt. The lateral pelvic shift during the one-leg lifting test and HAPCA are both highly related in that they limit compensatory motions such as pelvic tilt. Perhaps these are the reasons for the correlation between lateral pelvic shift distance and the number of HAPCA.

As previously mentioned in Kim et al. [25], HAPCA can be a useful exercise because it limits compensatory motion, such as pelvic tilting, and increases Gmed activation. In this study, we used the number of HAPCA as a variable indicating the hip abduction performance. In general, an HHD is used to measure force [12,22,31,32]. However, the HHD has a subjective effect on the measurement results depending on the user, and it is difficult to quantify the force in the absence of the HHD. If hip abductor performance can be expressed by the number of HAPCA, it is clinically useful because it is quantifiable. In addition, it has the advantage of being intuitive for the participant to recognize the test results and does not require other specialized tools for measurement.

Our study has several limitations. First, because this study involved only healthy participants, it is unknown whether the HAPCA is applicable to individuals with Gmed weakness. HAPCA requires a higher level of Gmed activation than the SHA, and if the Gmed is pathologically weak, there is a possibility that HAPCA cannot be performed. If patients with lumbar spine disorder or pathological weakness of Gmed are attempting HAPCA, it is suggested to do it against a wall. When HAPCA is performed against the wall, it is expected to reduce the difficulty of the motion while maintaining the minimization of the compensatory pelvic tilt thorough the counteraction of contralateral adductor. Second, our findings did not tell us about the pelvic multiplanar motions. In this study, only the frontal plane motion of the pelvis was measured; motions in the other planes were excluded. Although we measured only one plane due to the clinical advantage of two-dimensional analysis, a comprehensive movement analysis is needed to better understand the relationship between pelvic motion and HAPCA. In addition, the joints of the lower extremities were not considered during the one-leg lifting test. For example, femoral and tibial rotations, ankle inversion, and ankle eversion may occur. In this study, participants with knee or ankle problems did not participate; however, compensation that may affect the lateral pelvic shift distance could not be completely ruled out. Finally, HAPCA cannot be utilized if the contralateral hip adductors are weak.

The number of HAPCA correlated with the lateral pelvic shift distance during one-leg lifting. This result indicates that the number of HAPCA can be used as a clinical test to estimate hip abductor muscle performance related to lateral pelvic shift.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: DL, GG, YW, OK. Data curation: DL, YW. Formal analysis: DL, JK, GG. Investigation: DL. Methodology: DL, JK, GG, YW. Project administration: DL. Supervision: JK, OK. Validation: DL. Writing - original draft: DL. Writing - review & editing: JK, OK.