Different hip rotations influence hip abductor muscles activity during isometric side-lying hip abduction in subjects with gluteus medius weakness

Journal of Electromyography and Kinesiology 24 (2014) 318–324

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Different hip rotations influence hip abductor muscles activity during isometric side-lying hip abduction in subjects with gluteus medius weakness Ji-hyun Lee a,1,2, Heon-Seock Cynn a,⇑, Oh-Yun Kwon b,1,3, Chung-Hwi Yi b,1,4, Tae-Lim Yoon a,1,2, Woo-Jeong Choi a,1,2, Sil-Ah Choi a,1,2 a b

Applied Kinesiology and Ergonomic Technology Laboratory, Department of Physical Therapy, The Graduate School, Yonsei University, South Korea Department of Physical Therapy, The Graduate School, Yonsei University, South Korea

a r t i c l e

i n f o

Article history: Received 31 August 2013 Received in revised form 20 December 2013 Accepted 25 January 2014

Keywords: Gluteus maximus Gluteus medius Hip rotation Side-lying hip abduction Tensor fasciae latae

a b s t r a c t The purpose of this study was to establish the effects of different hip rotations during isometric side-lying hip abduction (SHA) in subjects with gluteus medius (Gmed) weakness by investigating the electromyographic (EMG) amplitude of the Gmed, tensor fasciae latae (TFL) activity, and gluteus maximus (Gmax), and the activity ratio of the Gmed/TFL, Gmax/TFL, and Gmed/Gmax. Nineteen subjects with Gmed weakness were recruited for this study. Subjects performed three isometric hip abductions: frontal SHA with neutral hips (SHA-N), frontal SHA with hip medial rotation (SHA-MR), and frontal SHA with hip lateral rotation (SHA-LR). Surface EMG amplitude was measured to collect the EMG data from the Gmed, TFL, and Gmax. A one-way repeated-measures analysis of variance was used to determine the statistical significance of the Gmed, TFL, and Gmax EMG activity and the Gmed/TFL, Gmax/TFL, and Gmed/Gmax EMG activity ratios. Gmed EMG activity was significantly greater in SHA-MR than in SHA-N. TFL EMG activity was significantly greater in SHA-LR than in SHA-N. The Gmed/TFL and Gmed/Gmax EMG activity ratios were also significantly greater in SHA-MR than in SHA-N or SHA-LR. The results of this study suggest that SHA-MR can be used as an effective method to increase Gmed activation and to decrease TFL activity during SHA exercises. Published by Elsevier Ltd.

1. Introduction The gluteus medius (Gmed) is the primary muscle that acts as a hip abductor (Standring et al., 2005), a pelvis stabilizer in a unilateral stance against gravity (Al-Hayani, 2009; Gottschalk et al., 1989), and a controller hip adduction and internal rotation eccentrically (Moore and Dalley, 1999). Accordingly, Gmed weakness can lead to lateral hip pain (Strauss et al., 2010),

⇑ Corresponding author. Address: Baekwoon-kwan, Yonsei University, 1 Yonseidae-gil, Wonju, Kangwon-do, South Korea. Tel.: +82 33 760 2427; fax: +82 33 760 2496. E-mail addresses: [email protected] (J.-h. Lee), [email protected] (H.-S. Cynn), [email protected] (O.-Y. Kwon), [email protected] (C.-H. Yi), [email protected] (T.-L. Yoon), [email protected] (W.-J. Choi), [email protected] (S.-A. Choi). 1 Address: Baekwoon-kwan, Yonsei University, 1 Yonseidae-gil, Wonju, Kangwondo, South Korea. 2 Tel.: +82 33 760 2497; fax: +82 33 760 2496. 3 Tel.: +82 33 760 2721; fax: +82 33 760 2496. 4 Tel.: +82 33 760 2429; fax: +82 33 760 2496. http://dx.doi.org/10.1016/j.jelekin.2014.01.008 1050-6411/Published by Elsevier Ltd.

iliotibial-band friction syndrome (Fredericson et al., 2002; Lee et al., 2012), patellofemoral pain syndrome (Cichanowski et al., 2007; Robinson and Nee, 2007; Magalhães et al., 2010; Nakagawa et al., 2012), and osteoarthritis of the knee (Hinman et al., 2010). Therefore, many therapeutic exercise protocols have focused on Gmed activity for prevention and rehabilitation in clinical and athletic training settings. Many previous studies have examined the effectiveness of various exercises to increase Gmed activity including a single-leg stance, hip clams, side steps, bridging, sideways hop exercises, and side-lying hip abduction (SHA) (Bolgla and Uhl, 2005; Distefano et al., 2009; McBeth et al., 2012; Selkowitz et al., 2013). Of these various exercise, SHA exercise is frequently used in rehabilitation sessions because it can be performed early in a rehabilitation program to generate proper neuromuscular control and strength since it is a less demanding exercise as an open kinematic chain exercise. Additionally, SHA is effective in targeting Gmed muscle activity. Previous study reported that Gmed activity was greater than almost 16% of maximal voluntary isometric contraction (MVIC) than single-limb squat, band walk, single-limb

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deadlift, and side-way hop exercise (Distefano et al., 2009). Bolgla and Uhl (2005) also reported that SHA induced greater electromyographic (EMG) amplitude of Gmed than the non-weight bearing standing hip abduction exercises used in their study. Thus, SHA exercise has been recommended for inducing Gmed activation. When prescribing exercise for Gmed activation, one should consider the relative activation of all synergist muscles. Synergist muscles act together and affect each other during movement (ChanceLarsen et al., 2010; Page et al., 2009). Accordingly, previous investigators have examined the Gmed, tensor fasciae latae (TFL), and gluteus maximus (Gmax) as hip abductors during hip rehabilitation (Cambridge et al., 2012; Homan et al., 2013; Distefano et al., 2009; McBeth et al., 2012; Selkowitz et al., 2013). TFL is an abductor and medial rotator at the hip (Gottschalk et al., 1989; Neumann, 2010). However, when a person’s gait exhibits a movement pattern with excessive medial rotation, the TFL may be dominant or over-active as a hip abductor compared to the Gmed (Sahrmann, 2003). An over-active TFL can also put lateral force on the patella through connections to the iliotibial band (Kwak et al., 2000; Merican and Amis, 2008, 2009). This movement pattern has been associated with patellofemoral pain (Powers, 2003, 2010; Souza and Powers, 2009). The upper portion of the Gmax also acts as hip abductor and lateral rotator during gait (Blandine, 1993; Lyons et al., 1983; Neumann, 2010). Thus, the Gmed and Gmax control excessive hip medial rotation and adduction in ambulation (Delp et al., 1999; Lyons et al., 1983). However, the TFL is an abductor and medial rotator of the hip. Additionally, less activation of the Gmax muscle compared to the TFL can be observed in degenerative hip joint pathology (Grimaldi et al., 2009). Abnormal hip kinematics and impaired hip muscle performance have been associated with these various musculoskeletal disorders. For this reason, this study decided to investigate Gmed, TFL, and Gmax activity during SHA exercises. In a previous study, McBeth et al. (2012) reported the TFL was more active than the Gmed and Gmax during SHA when subjects’ hips were laterally rotated. The theoretical rationale was that hip lateral rotation (LR) activated the Gmed as a hip lateral rotator. However, a hip lateral rotator cannot act against gravity during SHA-LR compared SHA-medial rotation (MR). It is possible that SHA-MR can facilitate the Gmed and Gmax as hip lateral rotators with regard to gravity. Thus, SHA-MR can be used to increase Gmed and Gmax activity relative to TFL activity. However, a limitation of previous studies was that they evaluated Gmed, TFL, and Gmax muscle activity during SHA in healthy subjects; they did not investigate these muscle activities during SHA with different hip rotations in subjects with Gmed weakness. Therefore, an examination of Gmed, TFL, and Gmax activation with different hip rotations during SHA in subjects with Gmed weakness will provide new, valuable information. The purpose of this research was to establish the effects of SHA with different hip rotations in the frontal plane (frontal SHA with neutral hip, SHA-N; SHA with hip MR, SHA-MR; and SHA with hip LR, SHA-LR) on Gmed, TFL, and Gmax EMG activity, and the EMG activity ratios of Gmed/TFL, Gmax/TFL, and Gmed/Gmax during isometric SHA in subjects with Gmed weakness. The hypothesis was that Gmed and Gmax EMG activity and the EMG activity ratios of the Gmed/TFL, Gmax/TFL, and Gmed/Gmax would increase and that TFL activity would decrease in SHA-MR, compared with the other SHA exercises in subjects with Gmed weakness. 2. Methods 2.1. Subjects G-power software provided power analyses. The necessary sample size of seven subjects was calculated from data obtained

from a pilot study of seven subjects to achieve a power of 0.80 and an effect size of 0.40 (calculated by the partial g2 of 0.14 from the pilot study), with an a level of 0.05. We recruited 27 participants in the beginning of the study. Nineteen subjects (eight males, 11 females) with weak Gmed participated in current study through the manual muscle testing (Table 1). Subjects were between 18 and 30 years of age. Inclusion criteria included being free from past or current inflammatory arthritis and lowerextremity or back dysfunction, and being able to maintain five seconds of isometric hip abduction in the side-lying position (Kim et al., 2011). All exercise was performed on the dominant leg for each subject, defined as the leg preferred for kicking a soccer ball (Bolgla and Uhl, 2005; McBeth et al., 2012). Then, Gmed weakness was confirmed by performing manual muscle testing. To confirm Gmed weakness, subjects assumed a side-lying position on the treatment table. Each subject’s bottom leg was flexed for comfort and stability, and the test leg was aligned with the rest of the trunk. The hip of the test limb was abducted to 50% of the hip abduction total ROM, and the investigator’s hand was placed 10 cm proximal to the lateral femoral epicondyle (Khayambashi et al., 2012). An isometric hold was performed for five seconds against resistance. The principal investigator (JHL) provided verbal encouragement to facilitate maximal performance and gave instructions to avoid any medial rotation or flexion of the hip through recruitment of the TFL or any hip hiking through use of the quadratus lumborum (Fredericson et al., 2002). Subjects took a three-minute rest between the two trials (Friel et al., 2006). Muscle grading was based on the method described by Kendall et al. (2005). Strength was graded as 0, 1, 2, 3, 4, or 5/5, then grouped as either ‘weak’ (3/5 or less) or ‘strong’ (4 or 5/5) (Bewyer et al., 2009). The reliability for individual muscle groups ranged from 0.63 to 0.93 (Kendall et al., 1993, 2005). Cohen’s Kappa was 0.89 for the two investigators in this study. The study excluded subjects with past or present musculoskeletal, neurological, or cardiopulmonary diseases that could interfere with SHA. Additionally, using Craig’s test, those with excessive femoral anteversion above 42° were excluded because the mean peak Gmed was decreased in subjects with femoral anteversion above 42° (Nyland et al., 2004). Overweight or obese subjects were also excluded, as fatty tissue, acting as a low-pass filter, could interfere with EMG signals (Wong, 1999). Subjects were identified as ‘overweight’ and ‘obese’ if they had a body mass index (BMI) > 25 (Flegal et al., 1998). BMI is defined as a subject’s weight divided by the square of his or her height, in units of kg/m2. Prior to the study, subjects signed a written consent form to participate. The University Institutional Review Board approved the protocol for this study, and each volunteer gave informed consent prior to participation.

Table 1 Characteristic of participants (mean ± sd). Characteristics

Participants (n = 19)

Age (years) Height (cm) Weight (kg) BMI (kg/m2) Modified Ober test (°) Hip abduction total ROM (°) Hip medial rotation total ROM (°) Hip lateral rotation total ROM (°)

21.00 ± 1.73 166.00 ± 0.07 59.79 ± 9.61 21.54 ± 2.56 0.20 ± 5.97 46.11 ± 15.17 49.66 ± 7.21 49.31 ± 11.11

Abbreviation: BMI: body mass index, ROM: range of motion. Hip adduction (below horizontal plane) was recorded as a negative number and abduction was recorded as a positive number.

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2.2. Surface EMG recording and data processing EMG data were collected from a Tele-Myo DTS EMG instrument with a wireless telemetry system (Noraxon, Inc., Scottsdale, AZ, USA). The sampling rate was 1000 Hz. A digital band-pass filter (Lancosh FIR), which filtered the raw signals, was between 20 and 450 Hz and a common mode rejection ratio of 92 dB at 60 Hz. Root-mean-square values were calculated with a moving window of 50 ms, and Myo-Research Master Edition 1.06 XP software analyzed the EMG data. Data were collected from the Gmed, TFL, and Gmax on subjects’ dominate side. An investigator prepared the electrode sites by shaving the subjects’ hair from the immediate vicinity of the muscle belly and cleaning the skin, using isopropyl alcohol with a sterile gauze pad to diminish impedance to the EMG signal; then, disposable Ag/AgCl surface electrodes were fixed on the proper sites (Cram et al., 1998; Hermens et al., 2000). Electrodes were positioned over the midsection of subjects’ muscle bellies, as in previous studies determining the sites of gluteal muscles (Ayotte et al., 2007; Bolgla and Uhl, 2005) and detailed by Rainoldi et al. (2004). The electrode pairs were placed parallel to the target muscle fibers. Two electrodes were placed approximately 20 mm apart in the direction of the muscle fibers For the Gmed muscle, electrodes were placed directly superior to the greater trochanter of the femur, one-third of the distance between the iliac crest and the greater trochanter of the femur. For the TFL muscle, electrodes were placed 2 cm inferior, and slightly lateral, to the anterosuperior iliac spine (Cram et al., 1998). For the Gmax, two active electrodes were placed half the distance between the second sacral spinous process and the greater trochanter in the middle belly on an oblique angle at the level of the trochanter or slightly above it. Proper placement of the electrodes was confirmed by viewing the subjects while they completed five repetitions of SHA. Electrode contacts were checked before all contractions (Cram et al., 1998). MVIC in the standard manual muscle-test position was used to normalize the Gmed, TFL, and Gmax (Kendall et al., 2005). To obtain the MVIC values for the Gmed, subjects assumed a side-lying position on the treatment table with the test leg up and the bottom hip flexed at 45° and knee flexed at 90° for stability. The test leg was abducted to approximately 50% of hip abduction, and the hip was placed in extension and slightly laterally rotated. An investigator applied downward force to the ankle while maintaining the hip with the other hand. To obtain the MVIC value for the TFL, the subjects assumed a supine position on the treatment table with the test hip flexed and slightly medially rotated with the knee extended. The investigator applied downward force to the ankle in the direction of the hip extension. To obtain the MVIC value for the Gmax, the investigator tested each subject’s resistance to hip extension by having him/her lie fully prone, with the knee flexed to 90°, and applying a downward force to the posterior thigh. Subjects performed the MVIC twice for the Gmed, TFL, and Gmax muscles. The mean value from the two trials was used for data analysis. Subjects performed this for five seconds with a three-minute rest between contractions. Subjects had a three-minute rest between muscles tested (Soderberg and Knutson, 2000). The EMG amplitudes collected during each exercise were expressed as a percentage of the average MVIC (%MVIC). To calculate the muscle activity ratios of the Gmed/TFL, Gmax/TFL, and Gmed/Gmax, the normalized Gmed amplitude was divided by the normalized TFL amplitude, the normalized Gmax amplitude was divided by the normalized TFL amplitude, and normalized Gmed amplitude was divided by the normalized Gmax amplitude, respectively. Bolgla and Uhl (2007) reported an intraclass correlation coefficient (ICC) of 0.93 for the hip abductors when using an MVIC as a reference. ICC for MVIC of Gmed was 0.91 (95% Confidence Interval (CI): 0.78–0.96, Standard Error of Measurement (SEM): 31.8) in current study. ICCs for MVICs of TFL and Gmax were

0.96 (95% CI: 0.90–0.98, SEM: 30.6) and 0.99 (95% CI: 0.98–0.99, SEM: 11.2), respectively. 2.3. Three-dimensional ultrasonic motion analysis system The three-dimensional ultrasonic motion analysis system (CHSHS, Zebris Medizintechnik GmbH, Isny im Allgau, Germany) calculated the hip rotation range of motion (ROM) of each subject and monitored compensatory pelvic movement (pelvic tilting in the sagittal plane, pelvic rotation in the horizontal plane, pelvic obliquity in the frontal plane) during SHA exercises. One triple marker was placed above the lateral femoral epicondyle to measure the amounts of hip rotation (Kiss and Illyés, 2012). The other was located on the midline of the pelvis by fastening a strap around the pelvis at the level of both posterior superior iliac spines to monitor pelvic movement (Oh et al., 2007; Park et al., 2011). The measurement sensor, consisting of three microphones, was positioned to the back of the subject so that it faced the markers. The side-lying position with neutral hip rotation was calibrated to zero as a reference position. The sampling rate was 20 Hz. SHA-MR and SHA-LR were calculated using 50% of the maximal ROM for each MR and LR. If the angle of hip rotation and pelvic movement exceeded 5°, the data were regarded as deviations and discarded. 2.4. Experimental procedures Before testing, subjects jogged for five minutes at a sub-maximal speed to warm up and to decrease possible discomfort or pain while performing SHA exercises (Hunter et al., 2003). The muscle activities of the Gmed, TFL, and Gmax were collected during the SHA exercises in the frontal plane (SHA-N, SHA-MR, and SHA-LR) in randomized order; subjects drew lots to avoid learning effects or fatigue. To ensure that each subject performed exercises at a standard speed, a metronome was set at one beat per second (Nyland et al., 2004). EMG data were collected for five seconds during the isometric phase and calculated from the middle three seconds of each exercise to avoid any connecting element of skin-electrode and possible starting or ending effects (Ayotte et al., 2007; Soderberg and Knutson, 2000). Subjects performed three trials under each SHA condition with a three-minute rest between exercises (Sykes and Wong, 2003). The mean value was used for data analysis (De Luca, 1997). A modified Ober test measured the shortness of the iliotibial band. A normal iliotibial band would stretch when the hip adducted beyond 10°. If the iliotibial band was short, it did not allow the test leg to drop in adduction toward the table beyond 10° (Kendall et al., 2005). The modified Ober test recorded hip adduction below the horizontal plane as a negative number and abduction above the horizontal plane as a positive number. Plastic guides were aligned near the mid-thigh and heel to maintain the frontal plane during the SHA. If the anterior midthigh or the posterior heel touched the vertical plastic guides, the position was regarded as a deviation from frontal SHA, and the data were discarded (Fig. 1). The unit presented visual feedback to avoid unwanted changes in body position during SHA exercises. Using a pressure biofeedback unit can decrease compensation from the quadratus lumborum and prevent excessive lateral tilting of the lumbopelvic region in the frontal plane (Cynn et al., 2006). The pressure biofeedback unit was placed between each subject’s lumbar spine and the treatment table with the subject in the side-lying position. A familiarization period was necessary to use the pressure biofeedback unit effectively during SHA exercises. The principal investigator (JHL) explained how to use the pressure biofeedback unit with verbal cues. Before SHA exercise, the peak pressure of the pressure biofeedback

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Fig. 1. Location of vertical plastic guides.

unit was measured during SHA exercise. Then, the subjects had to maintain the pressure biofeedback unit at 80% of their maximal pressure during all SHA exercises, and the principal investigator (JHL) monitored pressure changes to ensure the maintenance of pressure. Each subject was pain-free and comfortable with use of the pressure biofeedback unit after the familiarization period. 2.4.1. Side-lying hip abduction with neutral hip Subjects lay on their sides with the upper trunk and pelvis aligned in a straight line on the treatment table. The bottom side hip joint could be flexed at 45°, and knee joints were flexed at 90° for stabilization (Selkowitz et al., 2013). A wooden target bar was placed at 50% of the hip abduction ROM, accommodating the ROM of each subject. Thus, the test leg was abducted to 50% of the maximal ROM with knee extension until the lateral aspect of the distal one-third of the fibula touched the target bar, maintained in the position for five seconds, and then slowly returned to the starting position. The pressure biofeedback unit monitored the quadratus lumborum and the three-dimensional ultrasonic motion analysis system monitored compensatory pelvic movement during the five-second period (Fig. 2A). 2.4.2. Side-lying hip abduction with hip medial rotation Subjects performed this in the same way as in the SHA-N, excluding the hip medial rotation. The hip was rotated medially 50% of the total ROM. Subjects performed SHA-MR until the posterolateral aspect of the distal one-third of the fibula touched the target bar. The principal investigator monitored the ROM of MR during a five-second period through the three-dimensional ultrasonic motion analysis system (Fig. 2B). 2.4.3. Side-lying hip abduction with hip lateral rotation Subjects performed this in the same way as in the SHA-N, except for the hip lateral rotation. The hip was rotated laterally 50% of the total ROM. Each subject was asked to perform the SHA-LR until the anterolateral aspect of the distal one-third of the fibula touched the target bar. The principal investigator monitored the ROM of LR during a five-second period through the three-dimensional ultrasonic motion analysis system (Fig. 2C). 2.5. Statistical analysis The PASW Statistics 18 software (SPSS, Chicago, IL, USA) was used to perform all statistical analyses. A one-way, repeated-measures analysis of variance (ANOVA) assessed the statistical significance of the Gmed, TFL, and Gmax EMG activity and the Gmed/TFL,

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Fig. 2. Hip abduction exercise in side-lying (A) side-lying hip abduction with neutral hip (B) side-lying hip abduction with hip medial rotation (C) side-lying hip abduction with hip lateral rotation.

Gmax/TFL, and Gmed/Gmax EMG activity ratios during SHA exercises with three different hip rotations (SHA-N, SHA-MR, and SHA-LR). Level of significance was set at 0.05. If a significant difference was found, a Bonferroni adjustment was performed (with a = 0.05/3 = 0.017). 3. Results 3.1. Gmed, TFL, and Gmax EMG activity There were significant differences in Gmed muscle activity (F2,17 = 6.455, P = .009) and TFL muscle activity (F2,17 = 3.965, P = .039) among the three hip abduction exercises. SHA-MR showed significantly greater Gmed muscle activity than SHA-N (P = .003). SHA-LR produced significantly greater TFL muscle activity than SHA-N (P = .010). However, there was no significant difference in Gmax muscle activity among SHA-N, MR, and LR (F2,17 = 2.286, P = .132) (Fig. 3). 3.2. Gmed/TFL, Gmax/TFL, and Gmed/Gmax muscle activity ratios There were significant differences in the Gmed/TFL muscle activity ratio (F2,17 = 6.925, P = .006) and the Gmed/Gmax muscle activity ratio (F2,17 = 6.769, P = .007) among the three hip abduction exercises. SHA-MR produced a significantly greater Gmed/TFL muscle activity ratio than SHA-N (P = .001) and SHA-LR (P = .010). SHA-MR also resulted in a significantly greater Gmed/Gmax muscle activity ratio than SHA-N (P = .002). However, there was no significant difference in the Gmax/TFL muscle activity ratios among SHA-N, MR, and LR (F2,17 = 3.206, P = .066) (Fig. 4). 4. Discussion The purpose of the current study was to investigate whether Gmed, TFL, and Gmax EMG activity and Gmed/TFL, Gmax/TFL, and Gmed/Gmax EMG activity ratios would be different in various hip rotations during three isometric SHA exercises (SHA-N, SHAMR, and SHA-LR) in subjects with Gmed weakness. The findings indicate that Gmed EMG activity and the Gmed/ TFL and Gmed/Gmax EMG activity ratios were significantly greater in SHA-MR than in SHA-N or SHA-LR in subjects with Gmed weakness. This finding can be explained by the SHA-MR facilitating Gmed as hip lateral rotators with regard to gravity. To the authors’

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Fig. 3. Comparison of muscle activity in the gluteus medius, tensor fasciae latae, and gluteus maximus among different hip rotations during side-lying hip abduction exercises. SHA-N: side-lying hip abduction with neutral hip. SHA-MR: side-lying hip abduction with hip medial rotation. SHA-LR: side-lying hip abduction with hip lateral rotation. *Indicates a significant difference by Boferroni adjustment (p < .017).

Fig. 4. Comparison of muscle activity ratio in gluteus medius, tensor fasciae latae, and gluteus maximus among different hip rotations during side-lying hip abduction exercises. Gmed, gluteus medius. TFL, tensor fasciae latae, Gmax, gluteus maximus. SHA-N: side-lying hip abduction with neutral hip. SHA-MR: side-lying hip abduction with hip medial rotation. SHA-LR: side-lying hip abduction with hip lateral rotation. *Indicates a significant difference by Boferroni adjustment (p < .017).

knowledge, this study is the first to examine the effects of different hip rotations in SHA exercise on the hip abductors’ EMG activity in subjects with Gmed weakness. Gmed EMG activity was significantly greater in SHA-MR than in SHA-N (by 16.12%). Gmed EMG activity in SHA-MR was also greater, by 12.38%, when compared to SHA-LR; however, this difference was not statistically significant. These findings partially support our research hypothesis that Gmed EMG activity would increase more in SHA-MR than in other SHA exercises. Earl (2004) also showed that Gmed EMG activity was significantly greater in hip abduction with MR exercise than in hip abduction with LR exercise during single-leg stance. Additionally, the current findings indicate that Gmed muscles (61.34%MVIC) were more active than all the other muscles (Gmax: 23.67%MVIC, TFL: 38.51%MVIC) during SHA-MR. The results indicate that this exercise is very effective in targeting Gmed weakness and in preferentially activating the Gmed muscles. These findings may have clinical relevance. TFL EMG activity was significantly greater in SHA-LR than in SHA-N (8.94%). This finding is in agreement with data reported by McBeth et al. (2012), who reported that TFL activity (70.9 ± 17.2% MVIC) was significantly greater than Gmed EMG activity (53.0 ± 28.4% MVIC) during SHA-LR. The authors of this previous study explained that gravity on the lower extremity would pull the hip joint backward (hip extension); thus, the TFL would be challenged to maintain a neutral position during SHALR. The different position of the hip joint in SHA-MR may also explain the increased Gmed EMG activity in the present study when compared to other exercises. The possible mechanism for these findings may be that the Gmed muscles counteract the anterior roll of the pelvis in the transverse plane during SHA-MR. To overcome the greater external torque, the Gmed would be likely to generate greater force. Another possible mechanism for these findings is the

positioning of the Gmed in the highest position in the transverse plane of the upper thigh while sustaining SHA-MR. Additionally, a change of the length-tension relationship in Gmed could be caused by SHA-MR. The length of Gmed could be increased, which can produce more muscle activity by sarcomere change of the muscle during SHA-MR. Thus, SHA-MR may become a preferable position to increase Gmed activation in the frontal plane. The Gmed/TFL EMG activity ratio significantly increased in SHAMR by 49.76% and 42.99% compared to SHA-N and SHA-LR, respectively. The greater Gmed/TFL EMG activity ratio suggests Gmed activation increased and TFL activation decreased. These Gmed/ TFL EMG activity ratio results support the research hypothesis. No previous studies have examined the Gmed/TFL activity ratio during SHA; thus, it is not possible to compare the results of other work. However, by following the EMG activity ratio of the Gmed to the TFL during SHA, one might assume that SHA-MR is the best exercise for increasing the Gmed/TFL EMG activity ratio and selective activity for Gmed activation relative to TFL activation among the three SHA exercises. Gmax EMG activity and the Gmax/TFL EMG activity ratio were not significantly different among SHA-N, SHA-MR, and SHA-LR. However, the Gmed/Gmax EMG activity ratio was significantly greater in SHA-MR than in SHA-N (by 24.94%). Additionally, the Gmax was not activated to levels consistent with Gmed and TFL, and the Gmax/TFL ratio was below 1 in this study, suggesting the EMG activity of the Gmax was less than that of the TFL. Previous studies (McBeth et al., 2012; Selkowitz et al., 2013) also reported that TFL muscles had significantly greater activity that Gmax muscles during SHA. Gmed and TFL muscles are primary hip abductors (Neumann, 2010). On the other hand, the Gmax is the primary hip lateral rotator (Neumann, 2010), and it may be a secondary hip abductor in SHA exercises unlike the Gmed and TFL (Selkowitz

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et al., 2013). Thus, SHA is not effective for targeting the Gmed and Gmax synchronistically. This study chose SHA with maximal 50% of MR and LR in the frontal plane. Compared with previous study (Lee et al., 2013) that investigated Gmed and TFL muscle activity in SHA with maximal of MR and LR, Gmed muscle activity was greater during SHA with maximal 50% of the hip MR (61.34%MVIC in current study) compared with SHA with maximal ROM of MR (45.30%MVIC in previous study). TFL muscle activity was also greater during SHA with maximal 50% of the hip MR (38.51%MVIC) compared with SHA with maximal ROM of MR (32.60%MVIC). The possible mechanism for these findings may be that length-tension curve. Muscles operate with greatest active force when close to an ideal length. Thus, maximal 50% of the hip abduction with 50% maximal MR and LR may become a preferable range to increase Gmed activation. This study has several limitations. First, it was cross-sectional; thus, the long-term effects of SHA-MR cannot be determined. Second, crosstalk might have occurred between the Gmed and TFL muscles, although the authors took all safety measures to ensure the reliability of the EMG signal. Future researchers should investigate the long-term effects of SHA-MR on Gmed performance. 5. Conclusions In this study, the authors investigated the effects of different hip rotations on Gmed, TFL, and Gmax EMG activity and on the Gmed/ TFL, Gmax/TFL, and Gmed/Gmax EMG activity ratios during SHA exercises in subjects with Gmed weakness. Gmed EMG activity was significantly greater in SHA-MR than in SHA-N. TFL EMG activity was significantly greater in SHA-LR than in SHA-N. The Gmed/ TFL activity ratios in SHA-MR were significantly greater than were those in SHA-N and SHA-LR. The Gmed/Gmax activity ratios in SHA-MR were significantly greater than those in SHA-N. Thus, SHA-MR is the most effective exercise for eliciting greater Gmed muscle activation and for obtaining a higher Gmed/TFL EMG activity ratio among the three SHA exercises. Gmax is a secondary hip abductor in SHA exercise. Conflict of Interest The authors confirm that there are no conflicts of interest associated with this article. Acknowledgement The authors would like to thank Hyo-jung Jeong for providing subjects and her assistance with data collection. References Al-Hayani A. The functional anatomy of hip abductors. Folia Morphol (Warsz) 2009;68:98–103. Ayotte NW, Stetts DM, Keenan G, Greenway EH. Electromyographical analysis of selected lower extremity muscles during 5 unilateral weight-bearing exercises. J Orthop Sports Phys Ther 2007;37:48–55. Bewyer KJ, Bewyer DC, Messenger D, Kennedy CM. Pilot data: association between gluteus medius weakness and low back pain during pregnancy. Iowa Orthop J 2009;29:97–9. Blandine CG. Anatomy of movement. Seattle: Eastland Press; 1993. Bolgla LA, Uhl TL. Electromyographic analysis of hip rehabilitation exercises in a group of healthy participants. J Orthop Sports Phys Ther 2005;35:487–94. Bolgla LA, Uhl TL. Reliability of electromyographic normalization methods for evaluating the hip musculature. J Electromyogr Kinesiol 2007;17:102–11. Cambridge ED, Sidorkewicz N, Ikeda DM, McGill SM. Progressive hip rehabilitation: the effects of resistance band placement on gluteal activation during two common exercises. Clin Biomech 2012;27:719–24. Chance-Larsen K, Littlewood C, Garth A. Prone hip extension with lower abdominal hollowing improves the relative timing of gluteus maximus activation in relation to biceps femoris. Man Ther 2010;15:61–5.

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Ji-Hyun Lee is a Ph.D. Student in the Department of Physical Therapy at the Graduate School of Yonsei University. She received B.S. degree in Physical Therapy from Hanseo University, M.S. degree in Physical Therapy from Yonsei University. She is a member of applied kinesiology and ergonomic technology laboratory, and her research interests are kinesiology and movement analysis.

Heon-Seock Cynn is a professor in the Department of Physical Therapy at the College of Health Science of Yonsei University. He received B.S. degree in Physical Therapy from Yonsei University, M.A. degree in Physical Therapy from New York University, and Ph.D. degree in Physical Therapy from Yonsei University. He was a full time lecturer of Seoul Health College and an associate professor of Hanseo University. He is a director of applied kinesiology and ergonomic technology laboratory, and his research interests are identification of etiologic factors, classification, and intervention approaches for movement disorders and musculoskeletal diseases.

Oh-Yun Kwon is a professor in the Department of Physical Therapy at the College of Health Science of Yonsei University. He received his B.S. degree in Physical therapy and M.P.H. degree from Yonsei University in 1986 and 1992 respectively, and Ph.D. degree from Keimyung University in 1998. He had research experience in Program in Physical Therapy at Washington University in St Louis as a Post Doctoral Fellow. He is a director in Lab of Kinetic Ergocise based on Movement Analysis (KEMA). He is interested in the mechanisms of movement impairment, movement analysis, and prevention and management of the work related musculoskeletal pain syndrome.

Chung-Hwi Yi received his Ph.D. degree in physical therapy from Yonsei University in 1990. He joined the Department of Rehabilitation Therapy of Yonsei University in 1993. He was a president of The Korean Academy of University Trained Physical Therapists. From 1993 onwards he has been employed as a professor in the Department of Physical Therapy at the College of Health Science of Yonsei University. His research interests include motion, posture analysis, and the development of outcome measures for evaluating disability.

Tae-Lim Yoon is a Ph.D. Candidate in the Department of Physical Therapy at the Graduate School of Yonsei University. He received B.S. degree in Physical Therapy from Yonsei University, M.A. degree in Physical Therapy from New York University. He is a member of applied kinesiology and ergonomic technology laboratory, and his research interests are movement analysis and prevention and management of musculoskeletal problems.

Woo-Jeong Choi is a M.S. Student in the Department of Physical Therapy at the Graduate School of Yonsei University. She received B.S. degree in Physical Therapy from Yonsei University. She is a member of applied kinesiology and ergonomic technology laboratory. Her research interests are musculoskeletal problems including the scapular winging.

Sil-Ah Choi received her B.S. degree in Physical Therapy from Yonsei University, and M.S. degree in Physical Therapy from Yonsei University. She is a member of applied kinesiology and ergonomic technology laboratory. Her research interests are the clinical biomechanics associated with musculoskeletal problems.