Physical Therapy Korea

The gluteus medius (Gmed) and gluteus maximus (Gmax) function as critical roles in standing, walking and several functional activities [1]. The primary function of the Gmed involves eccentric control of hip medial rotation and adduction. The Gmed serves as a hip abductor and contributes to pelvic stabilization during unilateral stance [2,3]. The Gmax primarily operates as the extensor and lateral rotator of the hip joint. The upper portion of the Gmax acts as a hip abductor due to the inferior and lateral orientations of its muscle fibers [4]. Weakness in these muscles can lead to musculoskeletal disorders, including chronic lower back pain, patellofemoral pain syndrome, iliotibial band syndrome and lateral ankle sprain [1,5-7].

Various exercises aimed at strengthening the Gmed and Gmax, including step-up, bridging, lunge, squat, clam, and side-lying hip abduction (SHA), are highly recommended. Among these, SHA is particularly helpful in activating the Gmed and enhancing strength and neuromuscular control during rehabilitation [8,9]. Earlier researchers have investigated the roles of the Gmed, Gmax, and tensor fasciae latae (TFL) as hip abductors in rehabilitation protocols [10,11]. Specifically, the TFL functions as synergistic muscle and interacts with other muscles, particularly in exercises aimed at activating the Gmed. The dominance or excessive activity of the TFL over the Gmed as a hip abductor is correlated with various pathologies affecting the hip, knee and ankle joints [12,13].

Prior studies have examined the possibility of reducing TFL activity during SHA. According to Lee et al. [10] the TFL demonstrated less activity during SHA when participants’ hip was internally rotated compared with the Gmed and Gmax. McBeth et al. [11] observed higher TFL activity than the Gmed and Gmax during SHA with externally rotated hips. Additionally, Baik et al. [14] found that the activity of the Gmed and Gmax was the greatest whereas activity of the TFL was the lowest in SHA with anterior rolling. However, prior studies addressed the reduced activity of the TFL through rotational movement in the transverse plane, neglecting considerations of the sagittal plane during SHA exercise.

Choi et al. [15] reported that applying Thera-band activates the Gmax prior to initiating a bridging exercise resulting in enhanced Gmax activity. In one of the few studies that used Thera-band during various hip joint exercises, Bishop et al. [16] demonstrated its utility as an alternative method for modifying direction and amount of resistance. The utilization of Thera-band resistance during SHA exercise aims to provide additional resistance to the tested leg, potentially influencing hip abductor muscle activities. Although SHA with Thera-band is common in clinical settings, no previous studies have proven an increase in the activities of the Gmed and Gmax while decreasing TFL activity with isometric hip extension (IHE) using Thera-band during SHA exercise.

The aim of this study was to determine the influences of IHE using Thera-band during SHA exercise on the activities of the Gmed, Gmax, and TFL, and the ratio of Gmed/TFL in participants with Gmed weakness. Our hypothesis was that the activities of the Gmed, Gmax, and TFL, and the activity ratio of Gmed/TFL would be different with IHE using Thera-band during SHA exercise.

1. Design

Participants attended a 45-minute testing session at the Applied Kinesiology and Ergonomic Technology Laboratory at Yonsei University. SHA exercises in the frontal plane (traditional [T], IHE, and isometric hip flexion [IHF]) served as independent variables. Surface activity and the muscle ratio (Gmed, Gmax, and TFL) using electromyography (EMG) were dependent variables.

2. Participants

The sample size was calculated using G-Power Software (ver. 3.1, Heinrich-Heine-Universität Düsseldorf). Based on data from a pilot study involving five participants, a necessary sample size of eight participants was calculated to achieve a power of 0.80 and an effect size of 0.525 (calculated by partial η2 of 0.216 from the pilot study), with an α level of 0.05. The study ultimately included 21 participants (Table 1). All exercises were performed each participant’s weaker leg. The leg that exhibited the lowest performance level during manual muscle testing was determined to be weaker [17]. Participants were between the ages of 20 and 30 years, exhibited Gmed weakness (performing manual muscle testing and grade 3 or lower), had no past or present lower extremity injuries including fractures or sprains, and demonstrated the ability to maintain SHA positions for 5 seconds [18]. Participants with past or present musculoskeletal, cardiopulmonary, neurological disease, inflammatory arthritis, low back pain and dysfunction of lower extremity were excluded. Additionally, those with shortened iliotibial bands were excluded based on the modified Ober test [17]. Due to the shortness of the iliotibial band, the tested leg did not drop beyond 10° toward the table in the modified Ober test. Because of the interference caused by fatty tissue, acting as a low-pass filter for electrical signals, participants classified as overweight or obese (body mass index ≥ 25 kg/m²) were excluded from the study [19]. The present study protocol received approval from the Institutional Review Board of Yonsei University Mirae campus (IRB no. 1041849-202112-BM-220-02). Prior to the study, participants were required to read and sign a written consent form.

Table 1 . Subject characteristics (N = 21).

Variable	Value
Sex (male/female)	19/2
Age (y)	23.5 ± 2.4
Height (cm)	170.0 ± 5.5
Weight (kg)	72.7 ± 12.4
Body mass index (kg/m2)	23.9 ± 3.4

Values are presented as number or mean ± standard deviation..

3. Instrumentation

The EMG signals from the Gmed, Gmax and TFL muscles were detected using DTS EMG 542 sensors and the Tele-Myo DTS Belt Receiver system (Noraxon, Inc.). The sensory features included a sampling rate of 1,500 Hz, an overall gain of 500, a common mode rejection ratio > 100 dB, and an input impedance of > 100 Mohm. The MyoStudy Master Edition (ver. 3.16, Noraxon Inc.) was used for the EMG signal processing and recording. A band pass filter was used (10–450 Hz) to filter the raw signals, alongside a 60-Hz notch filter. The root mean square was enveloped with a window size of 200 ms before recording.

4. Procedures

Prior to the test, participants engaged in a 5-minute jog around the gymnasium at a submaximal speed as a warmup to minimize potential pain or inconvenience during SHA exercises [20]. Subsequently, the participants familiarized themselves with the exercises until they could perform them accurately. Each SHA variation was performed in three trials with 3-minute interval between trials to avoid the impact of fatigue and was performed at a comfortable pace. To prevent learning effects, the order of the exercises was randomized. Data of EMG were obtained for 3 seconds during the isometric phase. These data were computed from the mean of the middle 3-second for each exercise to minimize potential starting or ending effects or skin-electrode interface issues. Statistical analysis was conducted using the mean values [10].

SHA-T (Figure 1) was performed in a side-lying position on the clinical bed to ensure alignment of the pelvis and upper trunk in an even line. To provide stability and comfort, the leg on the bottom side was placed with the hip and knee joints flexed. A target stick was positioned at the midpoint of the maximum range of motion for hip abduction. The tested leg was abducted to the midpoint of the maximum range of motion with the knee extended until the lateral side of the distal one-third of the fibula contacted the target stick. This position was maintained for 5 seconds, and then gradually returned to the initial position. The participants were instructed to touch the calcaneal region on a vertical pole and abduct the tested hip such that the calcaneal region could slide up the vertical pole to ensure SHA in the frontal plane.

Figure 1. Side-lying hip abduction traditional style. (A) Initial position and (B) end position.

SHA-IHE (Figure 2) was performed in the same manner as SHA-T with the use of Thera-band. One end of Thera-band was wrapped around the ankle of the tested leg. The other end was tied to a front vertical pole at the same height as the target stick. Before the SHA, the participants were asked to maintain the contact of the calcaneal region on a posterior vertical pole. The Thera-band’s elasticity while performing the SHA would provide relative isometric resistance in the sagittal plane for the hip extensor muscle and follow the same procedures as SHA-T. The tension of Thera-band was determined based on the participant’s ability to complete more than 10 repetitions of hip extension exercises using Thera-band in a side-lying position.

Figure 2. Side-lying hip abduction with isometric hip extension. (A) Initial position and (B) end position.

SHA-IHF (Figure 3) was performed in the same manner as SHA-T with the use of Thera-band. One end of Thera-band was wrapped around the ankle of the tested leg. The other end was tied to a vertical pole behind the participant at the same height as the target stick. Before the SHA, the participants were instructed to maintain the contact of the anterior ankle on the anterior vertical pole. The Thera-band’s elasticity while performing the SHA provided relative isometric resistance in the sagittal plane for the hip flexor muscle and followed the same procedures as SHA-T. The tension of Thera-band was determined based on the participant’s ability to complete more than 10 repetitions of hip flexion exercises using Thera-band in a side-lying position.

Figure 3. Side-lying hip abduction with isometric hip flexion. (A) Initial position and (B) end position.

5. EMG Data Collection

To prepare the electrode sites, the skin surrounding the muscle belly was removed of hair, and cleaned using a sterile gauze pad soaked in isopropyl alcohol. This process aimed to minimize the impedance of the EMG signal and ensure proper electrode adhesion. In line with prior study concerning the hip abductor muscles, electrodes were positioned at the midpoint of the muscle bellies, as outlined by Criswell et al [21]. For the Gmed muscle, electrodes were positioned proximally, at one-third of the distance between the iliac crest and the greater trochanter. For the Gmax muscle, electrodes were positioned halfway between the second sacral spinous process and the greater trochanter. For the TFL muscle, electrodes were positioned approximately 2 cm below the anterior superior iliac spine. The precision of electrode placement was validated by observing the participants perform five repetitions of the SHA. Verification of electrode contact was conducted before all contractions.

Normalization of the Gmed, Gmax and TFL was achieved using the maximal voluntary isometric contraction (MVIC) in the standard position of the manual muscle test [17]. MVIC was executed for 5 seconds with a 10-second break between contractions and a 3-minute break between testing each muscle. To acquire the Gmed MVIC, participants were placed in a side-lying position with the tested leg up and the bottom hip and knee flexed for stabilization. The participants then abducted the tested leg by approximately 50% hip abduction, with slight extension and lateral rotation of the hip joint. The investigator stabilized the hip with one hand and applied a downward force to the ankle with the other hand. To acquire the Gmax MVIC, participants were positioned prone with their tested legs flexed at 90° angles in the knee joint. The investigator applied a downward force at the posterior thigh, while the participants extended their hips with resistance. To acquire the TFL MVIC, participants were positioned supine with the tested leg flexed and internally rotated in the hip joint maximally with the knee extended. The investigator applied force at the ankle in the direction of hip extension. The investigator measured the MVIC for the Gmed, Gmax and TFL muscles twice and used the mean value from the two trials for data analysis. The EMG amplitudes obtained for the Gmed, Gmax and TFL muscles during each exercise were expressed as a percentage of the mean MVIC (%MVIC).

6. Statistical Analysis

IBM Statistics 24.0 software (IBM Co.) was used to perform all statistical analyses. Kolmogorov–Smirnov Z-tests were performed to assess the normality of distribution. One-way repeated measures analysis of variance was used to assess the statistical significance of Gmed, Gmax and TFL muscle activities and the activity ratio of Gmed/TFL during SHA exercises (SHA-T, SHA-IHE, and SHA-IHF). Statistical significance was set at 0.05. If a significant difference was found, a Bonferroni adjustment was used to avoid a type I error (with α = 0.05/3 = 0.017). The effect size (ES) was calculated to determine meaningful changes among the interventions.

Significant differences were found among the three exercises during SHA in the activities of the Gmed (F = 11.705, p < 0.001), Gmax (F = 18.927, p < 0.001), TFL (F = 16.911, p < 0.001), and Gmed/TFL ratio (F = 8.733, p = 0.002). SHA-IHE showed a significantly higher Gmed, and Gmax activities, and Gmed/TFL ratio than SHA-T (p < 0.001, ES = 1.374; p < 0.001, ES = 1.334; p = 0.007, ES = 0.759, respectively) and SHA-IHF (p < 0.001, ES = 1.048; p < 0.001, ES = 1.329; p = 0.001, ES = 0.929, respectively). SHA-T had a significantly higher Gmed/TFL ratio than SHA-IHF (p = 0.003, ES = 0.847). SHA-IHF showed a significantly higher TFL activity than SHA-T (p < 0.001, ES = 1.295) and SHA-IHE (p < 0.001, ES = 1.044) (Table 2, Figures 4, 5).

Table 2 . Comparison of muscle activity in the Gmed, Gmax, and TFL among SHA-T, SHA-IHE, and SHA-IHF.

Muscle	SHA style	Muscle activity (%MVIC)
Gmed	SHA-T	36.6 ± 10.6
	SHA-IHE	51.2 ± 15.2
	SHA-IHF	31.6 ± 14.3
Gmax	SHA-T	24.6 ± 12.8
	SHA-IHE	52.2 ± 27.8
	SHA-IHF	17.2 ± 12.9
TFL	SHA-T	31.0 ± 18.7
	SHA-IHE	29.8 ± 22.1
	SHA-IHF	52.4 ± 25.7

Values are presented as mean ± standard deviation. %MVIC, percentage of maximum voluntary isometric contractions; Gmed, gluteus medius; Gmax, gluteus maximus; TFL, tensor fasciae latae; SHA, side-lying hip abduction; SHA-T, SHA traditional style; SHA-IHE, SHA with isometric hip extension; SHA-IHF, SHA with isometric hip flexion..

Figure 4. Comparison of muscle activity in the (A) Gmed, (B) Gmax, and (C) TFL among different SHA styles. Gmed, gluteus medius; Gmax, gluteus maximus; TFL, tensor fasciae latae; SHA, side-lying hip abduction; SHA-T, SHA traditional style; SHA-IHE, SHA with isometric hip extension; SHA-IHF, SHA with isometric hip flexion; %MVIC, percentage of maximum voluntary isometric contractions. *Significant difference by Bonferroni adjustment (p < 0.017).

Figure 5. Comparison of muscle activity ratio in the Gmed and the TFL among different SHA styles. Gmed, gluteus medius; TFL, tensor fasciae latae; SHA, side-lying hip abduction; SHA-T, SHA traditional style; SHA-IHE, SHA with isometric hip extension; SHA-IHF, SHA with isometric hip flexion. *Significant difference by Bonferroni adjustment (p < 0.017).

The primary aim of this study was to determine whether the activities of the Gmed, Gmax, and TFL, as well as the activity ratio of Gmed/TFL, would differ among the three SHA exercises (SHA-T, SHA-IHE, and SHA-IHF). The results of this study support the hypotheses. As far as we know, this study is the first to demonstrate that incorporating IHE or IHF during SHA using Thera-band affected the activities of the Gmed, Gmax, and TFL, and Gmed/TFL ratio.

The activity of the Gmed in SHA-IHE was significantly higher than that in SHA-T and SHA-IHF. A possible mechanism for the significantly increased Gmed activity during SHA-IHE was the application of IHE with Thera-band facilitated the Gmed activity during SHA. The Gmed, Gmax and TFL exhibited synergistic relationships and affected each other during SHA. The inhibition of TFL activity was caused by facilitated Gmax activity against the line of force from Thera-band during IHE in the sagittal plane, which is explained by the mechanism of reciprocal inhibition [14]. Related to Gmed and TFL during SHA, when the activity of one muscle decreases, a compensatory increase occurs in the activity of another muscle to achieve the same range of motion [10]. Decreased TFL activity result in increased Gmed activity during SHA-IHE, which is referred to as synergistic dominance [10,14].

These findings suggest that the EMG activity of the Gmax in SHA-IHE was significantly higher than that in SHA-T and SHA-IHF. Previous studies have reported that SHA does not effectively target the Gmed and Gmax simultaneously [10]. However, this outcome could be explained by SHA-IHE facilitating the Gmax as a hip extensor against Thera-band. The Gmax was preceded by an extension movement in the opposite direction against Thera-band, thus activating the Gmax, which functions not only as the primary hip extensor, but also as the hip abductor during SHA-IHE [22]. Our current findings are consistent with those of prior research, indicating the impact of muscle pre-activation on muscle firing. French et al. [23] confirmed that the increased amplitude of EMG following the 3-second contraction likely indicates that preconditioning contractions lead to a higher number of activated motor units. Choi et al. [15] reported that performing isometric hip abduction before a bridging exercise through muscle pre-activation results in increased EMG signals recorded from the Gmax. Our investigation suggests that pre-activation of the Gmax against the line of force from Thera-band activates an increased number of motor unit recruitments during exercise.

The obtained data suggest that the activity of the TFL in SHA-IHF was significantly higher than that in SHA-T and SHA-IHE. This outcome can be explained by pre-activation of the TFL as the primary hip flexor during SHA-IHF. We assessed the activity of the TFL to verify the isometric contraction of the hip muscle against the line of force from Thera-band. Dominance or excessive activity of the TFL can result in reduced activity of the Gmed as a hip abductor in accordance with the concept of synergistic dominance.

The activity ratio of Gmed/TFL was significantly higher in SHA-IHE, SHA-T, and SHA-IHF, in that order. The higher activity ratio of Gmed/TFL suggests that the activity of the Gmed, whereas that of the TFL decreased. The results of the activity ratio of Gmed/TFL supported the study hypothesis. By analyzing this activity ratio during SHA, it can be inferred that SHA-IHE was the most effective exercise for enhancing the activity ratio of Gmed/TFL and promoting selective activity for Gmed relative to TFL activity among the three SHA exercises. Reporting relative activity instead of absolute activity is beneficial since synergistic muscles work simultaneously and mutually affect each other throughout the movement [12]. Furthermore, Lee et al. [10] investigated that SHA with hip medial rotation is one of the best SHA exercises for inducing changes in the transverse plane. In addition, in clinical settings, a combination of IHE and medial hip rotation could be the best exercise for patients with Gmed weakness.

To the best of our knowledge, this is the first study to investigate the effect of IHE on the activities of the Gmed, Gmax, and TFL during SHA in participants with Gmed weakness. The Gmed and Gmax activities during SHA-IHE were 51.18 and 52.15 %MVIC, respectively. Prior studies [24,25] suggested that a neuromuscular activation threshold within 40%–60% of the MVIC effectively induces muscle strength gains. Our findings indicate that Gmed and Gmax activities during SHA-IHE surpassed this threshold. Performing SHA-IHE as an early, low intensity exercise is recommended to strengthen the Gmed and Gmax muscles to avoid several musculoskeletal problems, such as patellofemoral pain syndrome, medial osteoarthritis and lateral ankle sprain in participants with Gmed weakness.

This study had some limitations. First, the cross-sectional design precluded the assessment of long-term effects. Second, all participants were aged 18–35 years, potentially limiting the generalizability of our findings. Third, despite implementing all safety measures, the potential for crosstalk between the Gmed and TFL still exists. Fourth, force measurements were not performed in this study. Fifth, although many previous studies have included the trunk muscles during exercise, we did not consider them in our analysis. Differentiating between the three exercise protocols by analyzing the EMG/fore data in future studies would be interesting. Further studies are required to consider the trunk muscles and determine the long-term effects of SHA-IHE on the Gmed, Gmax and TFL in participants of all ages with Gmed weakness.

This investigation established the effects of SHA with IHE using Thera-band on the activities of Gmed, Gmax, and TFL, as well as the activity ratio of Gmed/TFL. The activities of the Gmed and Gmax significant increased during SHA-IHE compared with SHA-T and SHA-IHF, and TFL activity significantly increased during SHA-IHF compared with SHA-T and SHA-IHE. The activity ratio of Gmed/TFL in SHA-IHE was significantly higher than that in SHA-T and SHA-IHF. Therefore, SHA-IHE is the most advisable exercise, among the three SHA exercises, for higher activities of the Gmed, Gmax, and a higher activity ratio of Gmed/TFL. In clinical setting, performing SHA exercise with IHE using Thera-band may improve the efficiency of early rehabilitation and result in selectively strengthening Gmed.

None.

None to declare.

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

Conceptualization: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Data curation: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Formal analysis: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Investigation: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Methodology: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Project administration: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Resources: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Software: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Supervision: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Validation: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Visualization: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Writing - original draft: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC. Writing - review & editing: Sae-hwa Kim, HL, Seok-hyun Kim, SB, HC.