Nearly one in every five individuals experience low back pain at any given time.1 A major concern for individuals with a history of low back pain is that although symptom resolution may occur, the majority will go on to have additional episodes of low back pain.2,3 Future episodes may require medical attention,4 leading to increased healthcare costs associated with low back pain. In the United States alone, total healthcare costs associated with individuals with a history of low back pain have accumulated to approximately $100 billion dollars annually.5
Understanding the direct cause of low back pain is difficult for clinicians due to the vast number of contributing factors associated with this condition.6 Although pain is commonly reported, individuals with a history of low back pain may also present with increased disability7,8 or may avoid specific movements associated with pain.9,10 These subjective outcomes perceived by individuals primarily focus on activities of daily living, physical activity, or work-related activities,7,10 and may contribute to a reduced overall quality of life. However, some individuals may still be able to function at high physical activity levels after experiencing low back pain.3 The concern for these individuals, regardless of their functional level, is that after an initial episode of low back pain they are susceptible to future episodes of pain.2,3
Muscular dysfunction is a contributing factor to episodes of low back pain that can be modified by clinicians.11 Neuromuscular dysfunction associated with low back pain has been previously reported at the trunk at both deeper stabilizing muscles12–16 and larger superficial muscles.17–20 Static endurance testing of the trunk extensors is commonly used to assess muscle function in individuals with a history of low back pain.20–22 This method of testing muscle function is easy to administer, and is sensitive enough to distinguish individuals with low back pain and those who may develop low back pain.22 Previous literature has shown that reduced neuromuscular performance during this task has been influenced by both psychological and perceptual attitudes in individuals with low back pain.23 One limitation to this test is that it only assesses muscle function in the sagittal plane. Therefore, any muscle dysfunction in the frontal plane would not be identified during this test.
Recent literature suggests that the hip may also be a contributing factor to low back pain.11,24 Evidence exists supporting that individuals with a history of low back pain have dysfunction of the hip extensors, including reduced hip extensor strength,25 increased fatigability of the gluteus maximus during an extension task,17 and earlier gluteus maximus activity during gait.19 However, the gluteus medius is the main hip abductor muscle and is important in stabilizing the hip during single limb stance.26 This muscle must be capable of producing forces anywhere between two and five times an individual’s body weight to maintain frontal plane stability at the hip.26,27 Individuals with a history of low back pain may present with reduced hip abduction strength compared to healthy individuals,25,28,29 and therefore may be at risk of increased activity that could lead to increased fatigability30 of these muscles. A 30-second isometric hip abduction task has been shown to effectively reduce gluteus medius surface electromyography median frequency in healthy individuals and could be used to quantify changes in muscle function of the hip abductors.31 Median frequency is often used to assess neuromuscular properties and has been used to indicate muscular fatigue between individuals with and without a history of low back pain.17,20 Due to the forces needed to maintain frontal plane stability, individuals with a history of low back pain may be susceptible to increased fatigue of the hip abductor muscles. Understanding muscle function during a bout of exercise will potentially identify neuromuscular dysfunction of the hip abductors that could contribute to future episodes of low back pain.
Relationships between pain, disability, and fears associated with pain have previously been shown to exist among individuals with a history of low back pain.32 These correlations indicate that as one of the variables increases, the other two also increase. Although these subjective measures may be directly related to each other, their contributions to muscle function are not fully understood. Current evidence supports increased gluteus maximus fatigability17 and non-linear fatigue of the trunk muscle in individuals with a history of low back pain.20 There is evidence to support that subjective outcomes may influence trunk muscle endurance during an extension task,23 and that pain can also be a limiting factor during static endurance testing.33 However, associations between subjective outcomes and hip abductor muscle function are currently unknown. Relationships between hip muscle function and subjective outcomes including pain, disability, or fear avoidance behaviors may allow for the development of clinical recommendations to address potential neuromuscular deficiencies in these individuals.
The purpose of this study was to compare muscle function of the hip abductors and surrounding muscles at the beginning and end of a continuous bout of hip abduction exercise between individuals with and without a history of low back pain. We compared changes in median frequency and the amount of variance of hip abduction torque explained by changes in median frequency and muscle activation of the individual muscles. Additionally, we sought to determine the association between changes in hip abduction torque during a 30-second isometric contraction with patient-reported disability and fear avoidance beliefs. By assessing frontal plane muscle function during a 30-second contraction, we hoped to identify potential neuromuscular dysfunction that may contribute to future episodes of low back pain, thus leading to the development of intervention strategies to reduce the onset of recurrent episodes. We hypothesized that individuals with a history of low back pain will have greater gluteus medius median frequency shifts during the 30-second hip-abduction exercise. We also hypothesized that changes in gluteus medius muscle activity would explain the most variance during the 30-second hip abduction exercise in healthy individuals, whereas the addition of surrounding muscles would explain the variance in those with a history of low back pain. Finally, we hypothesized that an increase in reported fear avoidance behavior and disability would coincide with a greater change in hip abduction torque during the 30-second hip abduction exercise.
The current study was a clinical laboratory study. The dependent variables were hip abduction torque, median frequency, and root mean square (RMS) surface electromyography (EMG) of the gluteus medius, gluteus maximus, lumbar paraspinal, biceps femoris, and vastus lateralis muscles, self-reported disability, and fear avoidance. The independent variables were group (low back pain and healthy) and time (onset of exercise and end of exercise). Approval for this project was granted through the university institutional review board and all participants provided informed consent prior to testing procedures.
Twenty-four individuals (16 female, 8 male; age: 23 ± 3 years, height: 172.30 ± 11.20 cm, mass: 69.21 ± 14.05 kg, body mass index: 23.24 ± 3.52) completed the hip abduction exercise previously described as part of a larger study (Table 1).34 Eligible participants were between the ages of 18 and 40 years with a self-reported history of low back pain and current pain levels less than 30 mm on a 100-mm visual analog scale for the low back pain group, and 12 healthy individuals without a lifetime history of low back pain for comparison. None of the participants reported any history of disc injury or fracture to the spine, radiating symptoms down the leg, scoliosis of the spine, lower extremity ligamentous reconstruction surgeries, lower extremity injuries in the past 6 months, or were currently pregnant.
Means and Standard Deviations for Participant Demographics
Surface EMG was collected using 35-mm Ag-AgCl round disposable electrodes (EL503; Biopac Systems, Inc., Goleta, CA) at 2,000 Hz, whereas hip abduction torque was collected with a Biodex System 3 isokinetic dynamometer (Biodex Medical Systems, Inc., Shirley, NY) at 125 Hz during the side-lying hip abduction exercise. Both surface EMG and torque were collected simultaneously with a 16-bit data acquisition system (MP150; Biopac Systems, Inc.) and input impedance of 1.0 MΩ. Surface EMG was collected with a noise voltage of 0.2 µV, common mode rejection ratio of 110 dB and amplified with a gain of 1,000 (EMG100C; Biopac Systems Inc.). Disability was measured using the Oswestry Disability Index7,8 and reported as a percentage that ranged from 0, indicating no disability, to 100, indicating complete disability. The Fear Avoidance Beliefs Questionnaire was used to measure fear avoidance behaviors,10 consisting of 16 questions related to how physical activity and work-related activities affect back pain. Fear avoidance scores were reported as the total score for all items, with higher values indicating more fear avoidance beliefs associated with low back pain. AcqKnowledge software version 4.2 (Biopac Systems Inc.) was used for data analysis.
After initial consent and screening, participants completed the Oswestry Disability Index and Fear Avoidance Beliefs Questionnaire followed by 10 minutes on a stationary bicycle. The surface EMG placement consisted of preparing the skin by shaving the area of hair, rubbing the skin with a surface like fine sandpaper, and cleaning with isopropyl alcohol. Individual electrodes were placed in-line to the muscle fiber orientation with a standard 2-cm distance between electrodes. Lumbar paraspinal electrodes were placed on active muscle tissue verified via palpation during active trunk extension approximately 3 to 4 cm lateral to the L5 spinous process.35 Gluteus maximus electrodes were positioned at a point one-third the distance from the greater trochanter and second sacral vertebrae,36 and one-third the distance from the iliac crest and greater trochanter for the gluteus medius muscle.31,36 Vastus lateralis electrodes were attached 10 cm proximal from the patella,35,37 and between the ischial tuberosity and popliteal fossa for the biceps femoris muscle, while avoiding the dynamometer arm.38 Electrode placement for each muscle was verified against manual resistance to confirm placement and minimize potential cross talk of surrounding musculature.
Maximal voluntary isometric contractions were then obtained for each individual muscle during active contractions against manual resistance. Resistance was applied against the torso for the lumbar paraspinals and the distal portion of the lever arm for each lower extremity muscle. Each muscle was tested against resistance three times, with each trial lasting 5 seconds. Participants were placed prone with the legs extended on a standard plinth for lumbar paraspinal activation, and flexed at the knee 90° for the gluteus maximus and biceps femoris positions. Individuals were seated at the end of the plinth with the knee flexed 90° for the vastus lateralis and in a side-lying position with the legs straight and in line with the trunk and pelvis for the gluteus medius muscle.
Hip Abduction Exercise
Participants were then secured to a dynamometer in side-lying hip abduction position (Figure 1) with the isokinetic dynamometer secured just proximal to the vastus lateralis electrodes. The center of the hip was positioned as the axis of rotation and the limb were placed in zero degrees of abduction. Foam bolsters were secured to the posterior trunk and pelvis with an adjustable strap. Participants completed a 30-second bout of continuous isometric hip abduction exercise at maximal effort with verbal encouragement by pushing their limb to the ceiling. The surface EMG preparation and hip abduction exercise were repeated on the contralateral limb following a 15-minute resting period.
Positioning for side-lying hip-abduction exercise.
Hip abduction torque was recorded during a 2-second interval at the second to fourth and 26th to 28th seconds of contraction, filtered (15-Hz low pass) and normalized to body mass (Nm/kg). The surface EMG data were band-pass filtered (range: 10 to 500 Hz) and the amount of muscle activity was processed using a 10-sample moving average RMS algorithm and normalized to the respective maximal voluntary isometric contractions of each individual muscle. Median frequency was obtained during a 2-second epoch and transformed into the frequency domain using a fast Fourier transformation to measure changes in frequencies across exercise. Slopes for both hip abduction torque and surface EMG data were calculated by obtaining rate of change of the line for each measure over the change in time. Lumbar paraspinals surface EMG was averaged bilaterally, whereas each limb was analyzed independently for all individuals. Hip abduction slope was averaged between limbs to assess relationships with subjective disability and fear avoidance scores.
We performed a 2 (group) × 2 (time) repeated measures analysis of variance for median frequency characteristics of the gluteus medius, gluteus maximus, vastus lateralis, biceps femoris, and lumbar paraspinals muscles. A forward multiple regression analysis was used to determine the variance explained between the RMS and median frequency slopes of each muscle and change in hip abduction torque. Pearson’s r correlation coefficients were used to identify relationships between averaged slopes in hip abduction with both disability and fear avoidance behaviors. As an exploratory analysis, we also conducted separate forward multiple regressions between the slopes in hip abduction torque and surface EMG measures in individuals with and without a history of low back pain. Additionally, we ran a separate correlation for the change in hip abduction torque for individuals with a history of low back pain with disability and fear avoidance beliefs. All statistical analyses were performed using SPSS software version 20.0 (SPSS, Inc., Chicago, IL) with an a priori alpha level for significance set at a P value of .05 or less.
There was a significant reduction in median frequency for the gluteus medius muscle across all individuals (main effect for time: F(1,46) = 106.50, P < .01) after the 30-second hip abduction exercise (Figure 2). No group differences were observed for gluteus medius median frequency (F(1,46) = 0.73, P = .40, 1-β = .13) or group by time interactions (F(1,46) = 0.67 P = .42, 1-β = .13) between individuals with and without a history of low back pain (Figure 2).
Median frequency changes during the 30-second hip abduction exercise for (A) gluteus medius and gluteus maximus muscles and (B) vastus lateralis, biceps femoris, and lumbar paraspinals muscles. LBP = low back pain
A significant decline in median frequency was also observed for the gluteus maximus (F(1,46) = 74.77, P < .01), biceps femoris (F(1,46) = 12.45, P < .01), and lumbar paraspinals (F(1,46) = 18.13, P <.01), but not the vastus lateralis (F(1,46) = 2.734, P = .11, 1-β = .37), during the 30-second hip abduction exercise. No group differences (P > .05, 1-β < .35) or group by time interactions (P > .05, 1-β < .19) were observed for any of the additional muscles (Figure 2).
Relationships Between Changes in Hip Abduction Torque and Surface EMG
Following a 30-second bout of exercise, the RMS slopes of the gluteus medius and gluteus maximus muscles combined to account for approximately 50% of the variance in the slope for hip abduction torque in all participants (Table 2). When separating individuals by history of low back pain, the RMS slope of the gluteus medius muscle explained 32% of the variance in the hip abduction slope in those without a history of low back pain. The gluteus medius muscle in isolation explained 58% of the variance in hip abduction slope, whereas the combination of the gluteus medius and gluteus maximus muscles explained approximately 67% of the variance in the slope of the hip abduction torque in those with a history of pain (Table 2). Neither the RMS changes of the vastus lateralis, biceps femoris, or lumbar paraspinals muscles (P > .05) nor changes in median frequency (P > .05) predicted variability in changes in hip abduction torque among all individuals or those with or without a history of low back pain (Table 2).
Relationships Between Surface Electromyography Slope and Hip Abduction Torque
Associations Between Changes in Hip Abduction Torque and Subjective Outcomes
A moderate negative relationship was observed between hip abduction slope and disability reported among all individuals (r = −.402, P = .05), but not for total scores for fear avoidance (r = −.382, P = .07). No significant correlations were observed for either disability (r = −.293, P = .36) or fear avoidance (r = −.229, P = .48) among individuals with a history of low back pain independently.
The results of this study found that changes in gluteus medius median frequency are similar between individuals with and without a history of low back pain following a 30-second bout of continuous hip abduction exercise. However, individuals with a history of low back pain may use additional surrounding muscles to explain changes in hip abduction torque as a result of hip abduction exercise. These findings partially support our hypothesis in that muscle function differs between individuals with and without a history of low back pain during a hip abduction task. Additionally, a negative relationship was observed between the hip abduction slope and patient-reported disability among all individuals. This indicated that individuals who reported higher disability levels had smaller changes in hip abduction torque. This finding is contradictory to our hypothesis that individuals with a history of low back pain would have higher levels of disability and fear avoidance behavior and greater changes in hip abduction torque following a side-lying hip abduction exercise.
The current findings are consistent with previous studies showing a downward shift in median frequency of the gluteus medius muscle during a 30-second hip abduction exercise (Figure 2).31 These shifts in median frequency occur from higher frequencies of fast-twitch fibers to lower frequencies of slow twitch fibers,31 and may indicate the presence of neuromuscular fatigue. Previous studies have used downward median frequency shifts of 15% to quantify muscular fatigue after exercise.39 The current hip abduction exercise was successful in reducing median frequency of the hip abductors with an average reduction of 19% ± 11% in median frequency of the gluteus medius muscle. Although this shift from higher to lower frequencies was observed among all individuals, no group differences were observed as a result of exercise. Earlier findings have shown increased gluteus maximus fatigability through changes in median frequency in individuals with a history of low back pain during an extension endurance exercise.17
One possible explanation why no group differences were observed in the current study may be the duration of the exercise task. Recent findings have indicated that median frequency slopes of the lumbar extensors are similar between individuals with and without a history of low back pain during the initial 30 seconds of a modified Sorensen test.20 Individuals without a history of low back pain maintain similar median frequency slopes during the initial and final 30 seconds of the task duration, but those with a history of low back pain presented with shallower slopes across the final 30 seconds of exercise.20 Therefore, a longer isometric hold until task failure or an aerobic fatigue model may be better suited to detect differences in hip muscle function between individuals with and without a history of low back pain. According to the results of the current study, hip abduction fatigability during a single bout of isometric exercise is not different between those with and without a history of low back pain.
Although the median frequency response to a 30-second hip abduction exercise was similar between groups, the variance in hip abduction torque explained by gluteal muscle activation was different between individuals with and without a history of low back pain (Table 2). Variance in the hip abduction slope was explained by the RMS activation of the gluteus medius muscle in healthy individuals, indicating the change in the amount of gluteus medius muscle activity accounted for the change in hip abduction torque. This would be expected because the gluteus medius is the primary hip abductor muscle.26 In individuals with a history of low back pain, muscle activation of the gluteus maximus explained an additional 8% of the variance (R2 = .67) in the hip abduction slope along with the gluteus medius during a side-lying exercise. The addition of the gluteus maximus muscle may be a potential strategy in these individuals to maintain hip abduction torque. Similar adaptations of larger surrounding muscles have been observed in individuals with a history of low back pain incorporating larger surrounding muscles during functional tasks.19,40,41
The addition of the gluteus maximus activation observed in the current findings may compensate for muscles that are dysfunctional and may attempt to provide a protective response to the lumbo-pelvic region. Increased gluteus maximus activation has previously been reported during gait and may be associated with a more guarded gait pattern.19 This increased demand of surrounding musculature may reduce the efficiency of the task and could place individuals at additional risk for injury following exercise. Therefore, implementing an isometric side-lying hip abduction exercise task may be able to identify abnormal muscle coordination between individuals with and without a history of low back pain.
Negative relationships between the slope in hip abduction torque and perceived disability were observed among all individuals, with higher reported disability scores experiencing smaller changes in hip abduction torque. This result is similar to previous findings reporting a negative relationship between a timed endurance test and disability measures in individuals with chronic low back pain.42 However, the current findings indicate that this relationship existed when including all participants, and not when looking at individuals with a history of low back pain in isolation. This may be due to the small number of individuals with a history of low back pain in the current study and should be examined further to see whether a similar relationship exists in a larger sample of individuals with a history of low back pain. We did not see a significant correlation between changes in hip abduction torque and fear avoidance measures. This may be because we recorded a total fear avoidance score, rather than the individual contributions of physical activity and work-related fears associated with low back pain. We decided to include a total score to provide a overall association between fear avoidance beliefs and pain in the current sample. Future research is needed to determine whether a relationship exists between fear avoidance behaviors and hip muscle function in these individuals.
To the authors’ knowledge, this was the first study that compared median frequency changes of the hip abductors between individuals with and without a history of low back pain during a continuous side-lying hip abduction exercise. As a result, we may be under-powered to determine whether true differences exist between groups on our outcome measures as reported by our 1-β values. The current findings may allow researchers to perform sample size estimations for further studies comparing gluteus medius muscle function between these two populations. Although the 30-second hip abduction exercise may be a limitation to this study, we did see a decline in median frequency values in all individuals (Figure 2). However, the ability to identify neuromuscular changes in individuals with a history of low back pain at low levels of fatigue may help clinicians understand how recurrent episodes of low back pain occur. The current findings were able to detect changes prior to a recurrent episode of low back pain, and may provide clinicians with additional information to help prevent additional episodes of pain.
The current sample of individuals with a history of low back pain was a younger cohort reporting both low pain values and disability scores (Table 1). Thus, this group may not be a clinically disabled population and limits the generalizability of our results. A more homogenous group of individuals with a history of low back pain, a more clinically disabled group, or a population experiencing higher pain levels than the current sample of individuals with a history of low back pain may present with more observable changes than healthy individuals. However, a subpopulation of individuals with a history of low back pain that perceive a functional disability but are not actively seeking care is of concern because these individuals may have physical activity limitations or may place themselves at risk for future episodes of pain resulting in further disability. It should also be noted that the individuals with a history of low back pain included in the current study also had an increased body mass index compared to healthy controls. Previous literature has suggested that increased body mass can contribute to lower back muscle endurance,43,44 and obesity has been identified as a factor that can affect muscular endurance times.45 Although the current individuals with a history of low back pain responded similarly to the current hip abduction fatigue task, future studies should attempt to match participants by body mass index to control potential confounding from this factor.
Implications for Clinical Practice
Current findings indicate that young active individuals with a history of low back pain perceive impairment and report fear avoidance behaviors. Although not clinically disabled, these individuals may perceive functional limitations, which could affect their lifestyle due to pain. Although median frequency changes were similar between groups, the contribution of changes in gluteus maximus RMS activation explained additional variance in the change in hip abduction torque between groups during a 30-second exercise. This increase in surrounding muscles during a hip abduction task may act as a protective response in these individuals in response to previous episodes of low back pain. Altered neuromuscular responses to exercise could provide an understanding of potential adaptive strategies in these individuals and identify potential neuromuscular alterations among people with a history of low back pain.
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Means and Standard Deviations for Participant Demographics
|PARAMETER||HEALTHY (n = 12)||LOW BACK PAIN (n = 12)||P|
|Age (years)||22 (3)||24 (4)||.12|
|Height (cm)||175.90 (8.94)||168.70 (12.42)||.12|
|Mass (kg)||65.81 (8.84)||72.61 (17.59)||.25|
|BMI (kg/m2)||21.28 (2.40)||25.19 (3.44)||< .01a|
|Disability (%)||1.3 (2.5)||17.3 (11.2)||< .01a|
|FABQ-T||0 (0)||18 (8)||< .01a|
|Pain (mm)||0 (0)||10.58 (8.81)||< .01a|
Relationships Between Surface Electromyography Slope and Hip Abduction Torque
| GMed RMS slope||.435||< .01|
| GMax RMS slope||.498||.063||.02|
| GMed RMS slope||.323||< .01|
|Low back pain|
| GMed RMS slope||.583||< .01|
| GMax RMS slope||.666||.083||.03|