Resisted side-stepping: the effect of posture on hip abductor muscle activation

Abstract

Study Design

Controlled laboratory study, repeated-measures design.

Objectives

To compare hip abductor muscle activity and hip and knee joint kinematics in the moving limb to the stance limb during resisted side-stepping and also to determine if muscle activity was affected by the posture (upright standing versus squat) used to perform the exercise.

Background

Hip abductor weakness has been associated with a variety of lower extremity injuries. Resisted side-stepping is often used as an exercise to increase strength and endurance of the hip abductors. Exercise prescription would benefit from knowing the relative muscle activity level generated in each limb and for different postures during the side-stepping exercise.

Methods

Twenty-four healthy adults participated in this study. Kinematics and surface electromyographic (EMG) data from the gluteus maximus, gluteus medius, and tensor fascia lata (TFL) were collected as participants performed side-stepping with a resistive band around the ankle while maintaining each of 2 postures: 1) upright standing and 2) squat.

Results

Mean normalized EMG signal amplitude of the gluteus maximus, gluteus medius, and TFL was higher in the stance limb than the moving limb (P≤.001). Gluteal muscle activity was higher, while TFL muscle activity was lower, in the squat posture compared to the upright standing posture (P<.001). Hip abduction excursion was greater in the stance limb than in the moving limb (P<.001).

Conclusions

The 3 hip abductor muscles respond differently to the posture variations of side-stepping exercise in healthy individuals. When prescribing resisted side-stepping exercises, therapists should consider the differences in hip abductor activation across limbs and variations in trunk posture.

INTRODUCTION

Weakness of the hip abductors is present in individuals with a range of musculoskeletal conditions, including: femoroacetabular impingement (FAI),⁸ iliotibial band syndrome (ITBS),^{17, 20} patellofemoral pain (PFP),^{9, 33, 40, 43} and chronic ankle sprains.²² While the majority of studies measure overall hip abductor strength, reduced strength is typically interpreted as gluteus medius weakness.^{20, 21, 44} This interpretation is scientifically supported in that the gluteus medius has the largest volume and physiological cross sectional area of the hip abductors.^{18, 19} Furthermore, the lower extremity kinematics associated with hip abductor weakness, hip adduction and internal rotation and knee abduction during the weight acceptance portion of the stance phase,^{10, 28} are consistent with a reduced activation of the posterior portion of the gluteus medius.^{20, 21, 34}

Gluteus medius weakness is hypothesized to result in compensatory excessive use of the tensor fascia lata (TFL).^{5, 41} Increased TFL recruitment may subsequently lead to further gluteus medius atrophy.²⁰ Because the TFL also internally rotates the hip,³⁴ it is theorized that excessive TFL activity may further exacerbate the abnormal lower extremity movement patterns related to gluteus medius weakness.^{5, 41}

Therapeutic exercises are commonly used by clinicians to increase gluteus medius strength and enhance functional muscle recruitment patterns. These exercises often include a variation of resisted hip abduction, which activates all of the hip abductors, including the TFL.³⁴ For some patient populations, it is important for clinicians to be mindful of excessive use of the TFL during therapeutic exercises, and select exercises which preferentially increase gluteus medius activation.

One popular hip abductor strengthening exercise is side-stepping with an elastic resistance band secured around the lower extremities. Several studies have compared hip abductor muscle activity during resisted side-stepping to other strengthening exercises. Jacobs et al²⁷ found no difference in gluteus medius activity between resisted side-stepping and side-lying abduction, or between weight-bearing and non-weight-bearing standing hip abduction in patients following total hip arthroplasty. Similarly, DiStefano et al¹³ found no significant difference in gluteus medius activity during resisted side-stepping (61% maximum voluntary isometric contraction [MVIC]) and sidelying hip abduction (81% MVIC). The authors also found gluteus maximus activity to be lower during resisted side-stepping (27% MVIC) than side-lying hip abduction (39% MVIC) and all of the other exercises studied.

To our knowledge, only 2 previous studies^{6, 42} have measured both gluteus medius and TFL muscle activation during resisted side-stepping. Selkowitz et al,⁴² using fine-wire EMG, found lower TFL (13.1% MVIC) compared to gluteus medius (32.2% MVIC) activation while Cambridge et al⁶ reported TFL activity of approximately 21% MVIC compared to 29% MVIC for the gluteus medius.

One variation of this resisted side-stepping exercise is related to the desired amount of hip and knee flexion.²⁶ Patients can either maintain an upright standing posture while side-stepping or assume a squat posture. Cambridge et al,⁶ Selkowitz et al,⁴² and DiStefano et al¹³ used resisted side-stepping performed in a squat posture while Jacobs et al²⁷ had participants maintain an upright posture. Because no studies tested both postures, it is not possible to compare results across studies. Determining changes in muscle activation of the hip abductors based on posture variations is potentially useful to optimize exercise prescription and strengthening programs.

Additionally, it may be clinically important to know the relative muscle activation level of the stance versus the moving limb during resisted side-stepping. Youdas et al,⁴⁷ using resisted side-stepping in a squat posture, found higher levels of muscle activation for the gluteus medius (49.9% MVIC) and gluteus maximus (18.1% MVIC) in the stance limb compared to the moving limb (32.8% MVIC and 12.1% MVIC, respectively). 40% MVIC is assumed to be the minimum activation level necessary during an exercise to produce strengthening in untrained individuals.¹ Using this cutoff, adequate strength stimulus would be occurring only for the gluteus medius of the stance limb.

As exemplified by individuals with FAI who often have a combination of limited hip abduction range of motion^{29, 31, 38} and decreased hip abduction strength,⁸ there are potential benefits gained from a better understanding of the hip and knee joint motion that take place during side-stepping. To date, to our knowledge, the hip and knee joint kinematics of the side-stepping exercise have not been well investigated.

The purpose of this study was to analyze hip abductor (gluteus maximus, gluteus medius, and TFL) muscle activity and selected hip and knee joint kinematics during resisted side-stepping to determine the relative level of activation between the stance and moving limb and the effect of posture (upright standing versus squat). We hypothesized that, for all muscles, activation would be greater for the stance versus the moving limb. We also hypothesized that for the gluteal muscles, activation would be greater in the squat posture than in the upright standing posture, and conversely for the TFL.

METHODS

Participants

A convenience sample of 24 healthy college aged adults (12 males, 12 females; mean ± standard deviation age 22.9 ± 2.9 years; height 171.1 ± 10.5 cm; mass 68.6 ± 12.9 kg) participated in this study. Written informed consent, as approved by the Boston University Institutional Review Board, was obtained from each participant prior to testing. To be included in the study, participants had to be between 18 and 50 years old and report being healthy. Exclusion criteria included back, hip, knee, or ankle pain lasting greater than 2 weeks within the previous year.

Instrumentation

A surface electromyography (EMG) system (Bagnoli™, Delsys Inc, Natick, MA, USA: frequency response of the system = 20–450 Hz; CMRR > 100 dB; input impedance > 10¹⁵ Ohms // 0.2pF) was used to collect data at a sampling rate of 1000 Hz. A transmitter belt unit was worn by participants during data collection and transmitted the EMG signal to the receiver unit via a shielded cable. EMG data were collected using single differential surface EMG sensors (DE-2.1, Delsys Inc, Natick, MA, USA). These sensors have 2 parallel bars which are 1 cm long and 1 mm wide each and have an interbar distance of 1 cm. The skin was prepared by scrubbing the area with a cotton ball soaked with rubbing alcohol. Electrodes were placed over the muscle bellies of the gluteus maximus, posterior portion of the gluteus medius, and TFL bilaterally according to manufacturer's guidelines for surface electrode placements.³⁰ A disposable ground electrode was placed on the posterior elbow. EMG signal amplitude for each muscle was visually inspected to ensure proper electrode placement.

Three-dimensional trunk and lower extremity kinematic data were collected using a 10-camera motion capture system (Vicon Motion Systems Ltd, Centennial, CO, USA) at a sampling rate of 100 Hz and synchronized with the EMG data in Vicon Nexus (Version 1.8.5). Retroreflective markers were placed bilaterally on the participant's trunk, pelvis, and lower extremities, and secured with tape. Specifically, markers were placed over the first and fifth metatarsal heads, the calcanei, the medial and lateral malleoli, the medial and lateral femoral epicondyles, the greater trochanters, the anterior superior iliac spines, the sacrum between the posterior superior iliac spines, the iliac crests, the spinous process of the seventh cervical vertebra, the xiphoid process, and the acromion processes. Plastic shells with 4 non-collinear markers each were placed laterally over the shanks and thighs.⁷

Procedures

After securing the surface EMG electrodes over their desired locations, we collected EMG data during maximal voluntary isometric contraction (MVIC) trials. For the MVICs, manual resistance was applied to each muscle group using standard manual muscle testing techniques.²³ Following instruction and a practice trial, participants performed a single repetition and held the contraction for at least 3 seconds with verbal encouragement.

After reflective markers were properly attached to the participant, we collected a static standing trial with the participant in a neutral posture. This trial was used to create a model which included joint centers for the hips and knees. The medial knee and ankle markers were removed after the static trial so that they did not impede movement.

The participant then stood with each foot aligned with the sides of a 12 inch (approximately 30 cm) square floor tile. A resistive band (TheraBand™, The Hygenic Corporation, Akron, OH) was wrapped around the participant's ankles just proximal to the malleoli and tied so that it was gently stretched (approximately 110% of full unstretched length). The majority of our participants used a red (medium) band, with 2 of our stronger participants using a blue (heavy) band. The resistive band position and tension was not altered between trials, making it likely that the resistance level was the same for each trial. The participant was then instructed to side-step a distance of 1 floor tile (12 inches), resulting in the feet being 24 inches apart. The participant then moved the other foot so that the feet were once again 12 inches apart and aligned with the edges of the floor tile. The participant repeated this sequence of movement until reaching the other side of the laboratory, approximately 8 side-steps. The participant, facing in the same direction, then side-stepped in the opposite direction to return to the starting location. The stepping distance of 1 tile for all participants, instead of a height adjusted distance, was selected for ease of clinical application and was deemed acceptable given the single-group repeated measure design of the study.

Postures

Participants performed resisted side-stepping while maintaining 1 of 2 postures: 1) upright standing and 2) squat (FIGURE 1). For upright standing, the participant was instructed to stand up straight, and maintain that posture while side-stepping. For the squat posture, the participant was instructed to squat and to maintain the squat while side-stepping. The participant was allowed to “self-select” the squat posture. That is, the participant was not given feedback with regard to depth of the squat or trunk position during the squat. The order of testing of the postures was randomized.

FIGURE 1.

Participant side-stepping to the right in the upright standing posture and the squat posture.

Data Processing

Raw EMG signals were band-pass filtered between 20 and 390 Hz using a 4^th order Butterworth filter with zero phase lag.³² Filtered EMG signals were processed using root-mean squared (RMS) smoothing with a moving window of 100 msec. RMS data were normalized to peak mean amplitude calculated over a 10 msec period measured during MVIC testing.

Marker trajectories were low-pass filtered at 6 Hz using a 4^th order Butterworth filter. Commercially available software (Visual3D, C-Motion, Inc, Germantown, MD) was used to calculate joint kinematics based on the marker data. Knee and hip joint angles were defined as the angle between the distal segment and the proximal segment. Joint angles were determined using a Visual3D 8 segment hybrid model with a Cardan X-Y-Z (mediolateral, anteroposterior, vertical) rotation sequence.¹² The pelvis was defined using the CODA pelvis model.² Trunk segment angles were determined with respect to the global coordinate system.

Data Analysis

Muscle Activity

We calculated the average of the smoothed normalized EMG of each muscle for both the stance and the moving limb from when the moving foot left the ground (foot off) until the same foot contacted the ground again (foot on). Foot off and foot on were determined from the lateral velocity of the calcaneal marker of the moving limb, and were verified visually within Visual3D. The stance limb and moving limb were determined by the direction of side-stepping, with the stance limb being opposite the direction of stepping. For example, when stepping to the left, data for both the left and right muscles were calculated from left off to left on with the left limb being the moving limb. Approximately 8 steps were used to calculate the average muscle activity.

Kinematics

We were interested in the knee, hip, and trunk positions maintained as well as the amount of hip abduction occurring at each hip during the side-stepping exercise. Therefore, we calculated the average knee, hip, and trunk flexion angles throughout the side-stepping cycle as defined by foot off to subsequent ipsilateral foot off. Cycle was defined for both the leading limb (in the direction of side-stepping) and trailing limb (opposite the direction of side-stepping). We also calculated the hip abduction excursion (maximum abduction angle minus minimum abduction angle) of each limb for each step and averaged for each posture.

Statistical Analysis

Muscle Activity

To determine differences in muscle activity, we ran 3 linear regressions, one for each muscle, with 3 within-subjects factors: posture (upright standing versus squat), analyzed limb (stance versus moving), and side (left versus right side of the body). As there were repeated measures within each subject, a generalized estimating equation (GEE) correction was applied to the model. Separate models were run for each muscle.

Kinematics

We ran 3 linear regressions with GEE correction to compare the average knee, hip, and trunk flexion angles between the 2 postures. There were 3 within-subjects factors: posture (upright standing versus squat), analyzed limb (leading versus trailing), and side (left versus right side of the body). We also used linear regression with GEE to compare the hip abduction excursion of the leading limb to that of the trailing limb throughout the cycle in each posture and for each side.

All analyses were conducted in IBM SPSS Statistics version 20 (IBM Corporation, Armonk, NY) with an alpha level of .05. The Holm's sequentially rejective test was used to adjust reported P-values for the linear regressions to reduce type I error.²⁵ The Holm's sequential procedure is less conservative than the standard Bonferroni correction,¹⁴ which can significantly increase type II error.³⁷ All levels of the GEE that were significant were followed up with pairwise comparisons.

Effect sizes (ES) for paired comparisons were computed using Cohen's d and the pooled variance across conditions for each muscle. ES can be interpreted as small, medium, and large based on ES values of 0.2, 0.5, and 0.8 respectively.¹¹ The mean difference between conditions and 95% Wald confidence interval (CI) for the difference were also calculated.

RESULTS

Muscle Activity

For the gluteus maximus, gluteus medius, and TFL, the individual GEE models revealed main effects of posture (P< .001) and of analyzed limb (P< .001; TABLE 1). There was also an interaction effect between limb and posture (P< .001) for the gluteal muscles. There were no effects of side (P≥ .108).

TABLE 1.

Muscle activity level for each limb in each posture.^*

					Statistical Analysis†
	Upright Standing Posture		Squat Posture		Limb	Posture	Side	Limb × Posture
Muscle	Moving Limb	Stance Limb	Moving Limb	Stance Limb	P-value	P-value	P-value	P-value
Gluteus Maximus	8.9 ± 4.3	12.6 ± 6.7	12.1 ± 7.3	24.6 ± 12.8	<0.001	<0.001	0.756	<0.001
Gluteus Medius	18.7 ± 8.0	22.9 ± 9.5	23.3 ± 11.2	35.7 ± 13.8	<0.001	<0.001	0.610	<0.001
Tensor Fascia Lata	45.2 ± 20.3	56.2 ± 24.5	33.7 ± 16.5	38.6 ± 25.0	<0.001	<0.001	1.000	0.066

^*Values are mean ± standard deviation in % maximum voluntary isometric contraction.

^†Statistical analysis includes linear regression with generalized estimating equation (GEE) correction.

For the gluteus maximus and gluteus medius, average RMS EMG signal amplitudes were greater in the squat posture compared to the upright standing posture. Analysis of the interaction using pairwise comparisons revealed that average muscle activation of the gluteus maximus and gluteus medius was greater in the stance limb than in the moving limb for both the upright standing posture (P≤ .001, ES = 0.44 and 0.39, respectively) and the squat posture (P≤ .001, ES = 1.49 and 1.15 respectively) with the difference between limbs, as reflected by the larger ES values, being greater for the squat posture (TABLE 2). The average EMG signal amplitudes in the stance limb were largely greater in the squat posture than in the upright standing posture (P≤ .001, ES = 1.43 for the gluteus maximus and 1.19 for the gluteus medius). There was, however, no difference between the moving limb in the squat position and the stance limb in the upright position (P≥ .633).

TABLE 2.

Pairwise comparisons for the gluteal muscles for each limb in each posture.^*

	Muscle
Condition	Gluteus Maximus	Gluteus Medius
Straight, Moving vs. Straight, Stance	−3.7 (−5.3, −2.0)†	−4.2 (−5.8, −2.6)†
Straight, Moving vs. Squat, Moving	−3.2 (−4.8, −1.5)†	−4.6 (−7.1, −2.1)†
Straight, Moving vs. Squat, Stance	−15.7 (−20.1, −11.2)†	−17.1 (−21.0, −13.1)†
Straight, Stance vs. Squat, Moving	0.5 (−1.5, 2.5)	−0.4 (−3.1, 2.2)
Straight, Stance vs. Squat, Stance	−12.0 (−15.4, −8.6)†	−12.9 (−15.9, −9.8)†
Squat, Moving vs. Squat, Stance	−12.5 (−16.4, −8.6)†	−12.5 (−15.4, −9.5)†

^*Values are mean difference between conditions and 95% Wald confidence interval in % maximum voluntary isometric contraction.

^†Significant differences between conditions (P< .001).

For the TFL, evaluation of main effects revealed that the average RMS EMG signal amplitudes were smaller in the squat posture than in the upright standing posture (P< .001, ES = 0.70, TABLE 2). The activity in the stance limb was higher than in the moving limb (P= .001, ES = 0.40).

Kinematics

The GEE revealed an effect of posture on average knee, hip, and trunk flexion angle (P< .001; TABLE 3). For the knee and hip, there was also an effect of limb (P< .001) and an interaction between limb and posture (P≤ .016). There was no effect of limb nor interaction between limb and posture for trunk flexion (P≥ .595). The average trunk flexion angle was largely greater in the squat posture than in the upright standing posture (P< .001, ES = 2.14, mean difference = 15.3°, 95% CI = 13.2°, 17.5°). The average knee and hip flexion angles were also largely greater in the squat posture than in the upright standing posture in both the leading limb (P< .001, ES = 4.22 for the knee and 2.75 for the hip) and trailing limb (P< .001, ES = 4.23 for the knee and 2.72 for the hip; TABLE 4). The knee and hip of the leading limb were in slightly less flexion than the trailing limb in both the squat posture (P< .001, ES = 0.19 for the knee and 0.07 for the hip) and the upright standing posture (P< .001, ES = 0.49 for the knee and 0.20 for the hip).

TABLE 3.

Average knee, hip and trunk flexion angles and hip abduction excursion of the leading limb and trailing limb.^*

					Statistical Analysis†
	Upright Standing Posture		Squat Posture		Limb	Posture	Side	Limb × Posture
Joint angle	Leading Limb	Trailing Limb	Leading Limb	Trailing Limb	P-value	P-value	P-value	P-value
Knee flexion, average	7.1 ± 6.8	10.4 ± 6.5	41.1 ± 9.2	42.7 ± 8.6	< 0.001	< 0.001	1.000	< 0.001
Hip flexion, average	13.7 ± 7.6	15.2 ± 7.4	43.4 ± 13.3	44.4 ± 13.2	< 0.001	< 0.001	1.000	0.016
Trunk flexion, average	9.7 ± 5.5	9.7 ± 5.6	25.0 ± 8.5	25.0 ± 8.5	0.595	< 0.001	1.000	0.701
Hip abduction, excursion	11.0 ± 2.1	16.5 ± 2.5	9.9 ± 2.2	15.2 ± 2.8	< 0.001	< 0.001	0.696	0.719

^*Values are mean ± standard deviation in degrees.

^†Statistical analysis includes linear regression with generalized estimating equation (GEE) correction.

TABLE 4.

Pairwise comparisons for average hip and knee flexion angles for each limb and each posture.^*

	Joint Angle
Condition	Knee flexion	Hip flexion
Straight, Leading vs. Straight, Trailing	3.3 (2.4, 4.1)†	−1.5 (−2.1, −0.9)†
Straight, Leading vs. Squat, Leading	33.9 (30.4, 37.5)†	−29.8 (−33.1, −26.4)†
Straight, Leading vs. Squat, Trailing	35.6 (32.2, 39.0)†	−30.7 (−34.2, −27.2)†
Straight, Trailing vs. Squat, Leading	30.7 (26.9, 34.4)†	−28.2 (−31.6, −24.9)†
Straight, Trailing vs. Squat, Trailing	32.3 (28.8, 35.9)†	−29.2 (−32.6, −25.8)†
Squat, Leading vs. Squat, Trailing	1.7 (1.0, 2.3)†	−1.0 (−1.5, −0.5)†

^*Values are mean difference between conditions and 95% Wald confidence interval in degrees.

^†Significant differences between conditions (P< .001).

For hip abduction excursion, the GEE revealed a main effect of posture (P< .001) and of limb (P< .001; TABLE 3). Hip abduction excursion was approximately 1 degree more in the upright standing posture than in the squat posture (P< .001, ES = 0.71, mean difference = 1.2°, 95% CI = 0.8°, 1.6°) and approximately 5 degrees more in the trailing hip than in the leading hip (P< .001, ES = 2.42, mean difference = 5.4°, 95% CI = 4.2°, 6.7°). There was no effect of side (P≥ .696) for any of the kinematic variables.

DISCUSSION

The primary findings of this study were that during resisted side-stepping, 1) muscle activity was greater in the stance limb than in the moving limb; 2) muscle activity in the TFL was less, while activity in the gluteal muscles was more, in the squat posture than in the upright posture; and 3) hip abduction excursion was greater in the stance hip than in the moving hip.

Understanding the muscular requirements of both the stance and moving limb is important when treating patients with hip abductor weakness. Our results in healthy individuals indicated that resisted side-stepping required higher activation of the hip abductors of the trailing stance limb than the leading moving limb. Greater hip abductor muscle activity in the stance limb can be explained biomechanically. During resisted side-stepping, the hip abductors of the stance limb have to produce sufficient torque to stabilize the pelvis and superimposed segments against gravity,^{3, 34–36} and also to translate the pelvis in the direction of side-stepping. Additionally, the hip abductors of the stance limb stabilize the pelvis to provide a stable fixation for the contralateral hip abductors to move the hip into abduction. By contrast, the hip abductors of the moving limb only have to produce torque to move the limb against gravity and to overcome the torque created by the elastic band. Our findings are in agreement with those of Youdas et al,⁴⁷ who reported that gluteus maximus and gluteus medius activation was greater in the stance limb (18.1% and 49.9% MVIC, respectively) compared to the moving limb during resisted side-stepping (12.1% and 32.8%, respectively) using a squat posture. The current study expands on their findings and show similar results for both the upright standing posture and the squat posture as well as for the TFL.

Bolgla and Uhl³ have also previously investigated the magnitude of activation of the hip abductors during various exercises. In their study, they determined that abduction of the left hip while standing on the right limb, done without external resistance, required 42% MVIC of the right gluteus medius. In comparison, abduction of the right hip, when standing on the left limb, only required 33% MVIC of the right gluteus medius. Our study extends their finding to the gluteus maximus and TFL and tests a slightly more flexed hip and knee position as well as a more dynamic movement against external resistance.

The muscle activity of the TFL was less in the squat posture than in upright standing. Biomechanically, the TFL, while a hip abductor, is also a hip flexor.³⁴ In upright standing, the TFL is active to both abduct the hip and to balance the pelvis on top of the stance limb. In upright standing, activation of the gluteals would extend the hip (or posteriorly rotate the pelvis) if not for the counterbalancing hip flexion moment from the TFL. In a squat position, however, the center of mass of the trunk is anterior to the hip, creating a hip flexion moment due to gravity, and thus reducing the need for the hip flexion moment from the TFL. Therefore, increased activity from the TFL would be counterproductive. This biomechanical explanation for the decreased TFL activity in the squat posture is further supported by the findings of Willcox and Burden⁴⁵ who investigated the effect of pelvis position and hip angle on hip abductor muscle activity during a sidelying clam exercise. They found that activation of the TFL was not affected by pelvis position or hip angle in the non-weight bearing, sidelying position. This indicates that our finding likely is due to biomechanical influences in weight bearing and not simply the position of the hip and pelvis.

Because it is hypothesized that the TFL can be a primary hip abductor if there is gluteus medius weakness, which may lead to further underuse of the gluteal muscles,^{5, 41} it is important to understand how alterations in exercise posture could help preferentially activate the gluteal muscles while reducing activation of the TFL. In this study, when compared to the upright posture, side-stepping in a squat posture led to reduced activation of the TFL while concurrently increasing gluteus medius and maximus muscle activity. This variation of the side-stepping exercise may be clinical advantageous if targeting activation of the gluteal muscles is desired. But, it must be noted that the relative level of activation of the TFL based on normalized EMG signal amplitude was, on average, higher than that for the gluteal muscles for both postures.

While EMG normalization has numerous challenges that preclude the strict interpretation of muscle activation on a 0 to 100% scale,^{4, 46} the normalized EMG data in our study can be interpreted based on the classification system proposed by Escamilla et al¹⁶ and Reiman et al³⁹ where 0% to 20% MVIC indicates low muscle activity, 21% to 40% MVIC indicates moderate muscle activity, and 41% to 60% MVIC indicates high muscle activity. Using this classification, gluteus maximus activity was low except for the stance limb in the squat posture, when it was moderate. Activation of the gluteus medius was moderate in both limbs in the squat posture but only for the stance limb in the upright standing posture. TFL activity level, however, was moderate bilaterally in the squat posture and high bilaterally in the upright standing posture. It has been suggested that moderate activity is necessary for improvements in strength¹ while lower activity levels may result in improved muscle endurance¹⁶ or neuromuscular re-education.¹⁵ Therefore, the levels of muscle activity measured in this study would indicate that resisted side-stepping addresses gluteal endurance or neuromuscular control more than strength. However, the band resistance level used in this study was purposefully kept low, using a medium resistance band with the majority of our participants, to minimize the effects of fatigue while testing multiple conditions. It is assumed that the use of greater elastic resistance would increase the level of muscle activation if strengthening is desired.

While clinicians typically focus on muscle activity levels during these exercises, it is also important to consider joint movement. As expected, knee, hip, and trunk flexion were greater in the squat posture than in the upright standing posture. Our mean hip and trunk flexion angles were substantially higher than those previously reported by Cambridge et al.⁶ This difference could be due to differences in task performance; however, based on their figures, our participants were in a similar amount of hip and trunk flexion. The difference could also be due to differences in the models used for angle calculations.

During resisted side-stepping, the trailing (stance) limb had greater hip abduction excursion than the leading (moving) limb. The greater hip abduction on the stance limb seems to occur as weight is shifted over to the leading limb after it contacts the ground. This knowledge is particularly important when treating patients, such as those with FAI, who have concurrent decreased hip abduction range of motion^{29, 31, 38} and decreased hip abduction strength.⁸

There are limitations due to the design of this study. We selected only healthy asymptomatic individuals to participate in this study to allow us to investigate muscle activity without pain affecting the results.²⁴ Second, we did not measure the level of resistance provided by the band. However, the band was not adjusted between stepping direction or posture, and the same step distance was used, making it likely that the resistance level was the same between testing condition for a given individual. We also purposefully kept the level of resistance low. As we were testing multiple conditions, the low level of resistance helped to lessen the potential effect of muscle fatigue during testing. Overall, caution should be used in interpreting the MVIC values. Muscle activity was only analyzed during the concentric phase and was only measured from a single location on each muscle. Only a single position was used for elicitation of MVIC, contributing to the high variability of normalized EMG. Additionally, given the complex architecture of the gluteal muscles, the normalized EMG may not accurately reflect level of muscle activation. Lastly, we did not provide cues to the participants during side-stepping regarding trunk position or cadence of movement. The goal was to capture typical performance with minimal instruction.

CONCLUSION

The findings of this study indicate that during resisted side-stepping, the hip abductors on the stance limb are more active than the hip abductors on the moving limb. In the squat posture, the activity of the gluteal muscles is increased, while the activity of the TFL is reduced compared to the upright standing posture. Overall, the highest gluteal muscle activation is obtained in the squat position while side-stepping away from the target hip. The hip abduction excursion of the stance hip is greater than the moving hip. These findings can help guide exercise prescription.

Key Points.

Findings: During resisted side-stepping, the muscle activity in the hip abductors is greater in the stance limb than in the moving limb. The activity of the gluteal muscles is increased, while that of the TFL is reduced, in the squat posture compared to the upright standing posture. The stance hip has greater hip abduction excursion than the moving hip.

Implications: This information can guide the clinician when selecting the direction and posture to be used during resisted side-stepping for a patient with hip abductor weakness.

Caution: This study was conducted in healthy asymptomatic participants without known hip abductor weakness and without any potential muscle activation or joint motion impairments related to disability or injury.