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Poor alignment between the patella, tibia, and femur has been identified as a primary cause of anterior knee pain. More recently, impaired hip strength has been discussed as a possible reason for the onset of knee pain.
The purpose of this study was to determine if individuals with knee pain had weakness in the hip muscles.
Nineteen females between the ages of 18 and 40, experiencing unilateral knee pain for no greater than four weeks, were examined. Bilateral gluteus maximus and medius strength were measured with a MicroFET hand-held dynamometer.
Strength of the gluteus medius and maximus muscles were significantly less in the extremities of patients experiencing knee pain than the extremity without knee pain.
Given biomechanical relationships between the hip and knee, examining the entire lower kinetic chain should occur when evaluating patients with knee pain. Using impairment-based interventions, such as addressing hip strength in addition to knee pain, may enhance intervention effectiveness. Results of this study provide data that suggest that individuals with knee pain had weak hip muscles.
According to the American Academy of Orthopaedic Surgeons,1 knee pain is a common condition resulting in 19.4 million pain-related visits to a physician's office each year. This number is significantly higher than low back pain (5.9 million visits), hip pain (3.2 million visits), and ankle pain (1.9 million visits). Knee pain impairs function, which can lead to disability.2
Historically, causes of knee pain have been related to a structural deformity or malalignment of the patella and its relationship to surrounding structures. Patella and distal femur malalignment have been linked to anterior knee pain.3 Interventions for such impairments have been to use either surgical or non-surgical strategies to correct the deformity or re-align the patella.4 More recent information has indicated that improper alignment at the knee may originate proximally and that poor force production at the hip, secondary to muscle weakness, could be a factor that ultimately causes stress to the knee.5,6 Juhn7 and Powers8 hypothesized that knee pain could be caused by a maltracking of the patella on the femur. They proposed that instability of the patella can result from unequal activity in the components of the quadriceps femoris muscle8 or diminished hip extensor, rotator, or abduction strength.5 Repetitive contact between the patella and femur, along with maltracking due to muscular imbalance in the hip, can result in anterior knee pain.7
It is important to understand the relationship between the different forces acting on the femur when considering the biomechanics of the lower extremity. A number of forces are exerted on the femur during ambulation, and the muscles acting on the femur play an important role in balancing these forces.9 Duda et al9 suggest an important relationship between the gluteal muscles and femoral neck loading. Femoral neck loading occurs when the muscle contraction exerts a force on the femur. When femoral loading occurs during weight bearing, the proximal and distal ends of the femur are subject to considerable shear forces. Previous research has suggested that hip strength may be a factor on anterior knee pain, therefore, the purpose of this study was to determine if individuals with knee pain had weakness in the hip muscles.
A sample of convenience of 24 females between 18 and 40 years of age were recruited for this study. Fulkerson10 stated that females are more likely to be affected by anterior knee pain and Lephart et al11 demonstrated that biomechanical differences exist between men and women. Given the effects of gender on biomechanics, females were chosen for this study to decrease the impact of a potentially confounding variable.
According to Berk,12 bone growth is complete in most girls by age 16, when the epiphyses close completely. Torry and McCaw13 reported “approximately ninety percent of all persons over the age of 40 will show pathological evidence of osteoarthritis in weight bearing joints.” Based on these findings, females within the range of 18 to 40 years of age were recruited in order to minimize the chance of results being skewed by pubertal changes before the age of 18 or the onset of osteoarthritis after the age of 40.
The inclusion criteria were females with unilateral anterior knee pain (intermittent or constant) for no greater than four weeks. Exclusion criteria included (1) back, hip, or foot pain, (2) bilateral knee pain, (3) prior surgery or trauma to the back, hip, knee, or foot, and (4) medical conditions such as pregnancy, cancer, neurological impairment, bone disease, or stomach ulcer. These exclusion criteria were chosen to ensure subject safety and enable proper positioning.
The Utica College Institutional Review Board approved this research proposal. Upon approval, subjects were recruited using general announcements. All subjects signed an informed consent form prior to participation in this study.
The International Knee Documentation Committee (IKDC) Subjective Knee Form was used to measure participants' knee pain and function in order to further define the sample. This form is knee specific, rather than disease specific, and provides data relative to knee symptoms and function during activities of daily living, sports, and work activities.14 Irrgang et al14 studied a group of 533 patients with a variety of knee pathologies to determine the reliability and validity of the IKDC Subjective Knee Form. The results of the study revealed a test-retest reliability coefficient of 0.94 and an internal consistency coefficient alpha of 0.92.
The MicroFET hand-held dynamometer (Noggan Health Industry; South Draper, UT) was used to measure muscle strength on all subjects. A reliability study was performed to determine the testers with the highest intrarater reliability for the hand-held dynamometer in order to maximize reliability. The reliability study took place on a sample of female subjects in the Utica College Clark Athletic Center, Physical Therapy Clinical Laboratory. Intrarater reliability was determined by testing gluteus medius and gluteus maximus muscles bilaterally on five female subjects without knee pain, each measured three times. Data were analyzed using Pearson product-moment coefficient of correlation in order to decipher the two researchers with the greatest interrater reliability. Intrarater reliability for the tester chosen to perform all measurments was r = 0.87 and 0.93 for the gluteus medius and gluteus maximus, respectively, which is consistent with high reliability reported previously by Click et al (0.95).15
Once recruited, and after the consent form was signed, participants completed an intake form for demographic and subjective information about their knee pain. After reviewing the intake forms, subjects that met inclusion criteria completed the IKDC Subjective Knee Form, answering questions regarding performance during activity. Participants did not need to carry out physical activity while completing this form.
Participants underwent bilateral gluteus medius and maximus muscle strength testing using the MicroFET hand-held dynamometer. Each researcher tested bilateral gluteus medius and maximus muscles using the positions outlined by Bohannon16 for the gluteus medius and Kendall et al17 for the gluteus maximus. Hip extension force (Figure 1) was measured in prone on the posterior aspect of the thigh, proximal to the knee, with the knee at a ninety-degree angle.17 Hip abduction force (Figure 2) was measured in supine with the knee extended and the hip in neutral relative to extension, abduction, and rotation. Force was measured at the lateral surface of the thigh, just proximal to the lateral condyle of the femur.16
The unaffected extremity was measured first. For hip extension, stabilization was given at the lumbar spine and for hip abduction it occurred at the lateral aspect of the contralateral extremity. Subjects performed three trials of one repetition each, with five to seven second holds, for both the gluteus maximus (Figure 1) and medius (Figure 2). Twenty to thirty seconds of rest were provided between each trial as recommended by Bohannon.18 Time for muscle contraction and rest periods was measured with a stopwatch.
Descriptive data for the average of the three measurements of the gluteus maximus and medius muscles included means and standard deviations. One dependent t-test was used to compare the strength differences in the extremity with pain and the extremity with no pain for the gluteus maximus and one t-test for the gluteus medius muscle. In order to adjust for possible inflation of the 0.05 alpha level due to multiple t-tests, the significance was adjusted (0.05/2) to the accepted alpha level of p < 0.025
Of the 24 subjects volunteering for the study, 19 subjects satisfied the inclusion and exclusion criteria and completed the study (mean age = 24.0 years; SD = 4.8; range = 18-35 years). The mean IKDC Subjective Knee Form score was 66.12 (SD = 15.8; range = 39.09 to 89.66). Of the subjects tested, 52.6% had an affected left knee and 47.4% had an affected right knee. College students comprised 57.9% of the subjects tested.
The difference of 4.39% change (SD = 5.33) between the strength scores for the unaffected gluteus medius and affected gluteus medius was significant (df = 18; t = 3.49; p<0.025). In addition, the difference of 1.9% change (SD = 1.26) between the strength scores for the unaffected gluteus maximus and affected gluteus maximus was also significant (df = 18; t = 6.47; p<0.025).
Previous authors have studied the effect that muscles have on the function of the skeletal system. An19 stated that the mechanical behavior of the skeletal system was directly related to the muscles that were acting on it. Duda et al9 reported that other structures have to adapt if the muscles are not able to balance forces acting on the skeletal system. The present study suggested that persons with knee pain had weak hip muscles. Therefore, it is possible that weak musculature around the proximal end of the femur may alter the forces acting on the knee and lead to compensatory stress at the distal aspect of the femur. These findings were consistent with previous theories on the effects that muscles have on the skeletal system.9, 19
Previous authors have linked hip pathology to problems in other areas of the biomechanical chain, including the back, knee, and foot. 3,6,20–27 Results of these studies further support the need to evaluate the entire biomechanical chain when a problem occurs in one area of the chain, and the results of the present study support this theory, as well. As the results of this study suggested, a relationship appears to exist between knee pain and weak gluteal musculature and clinical interventions focused on restoring proximal weakness may be warranted.
The gluteus maximus is one of the muscles thought to play a role in controlling alignment of the lower extremity and is identified as an important muscle to consider when treating anterior knee pain. The gluteus maximus posteriorly rotates the pelvis and controls limb activities during rotational movements.28 Weakness of the gluteus maximus could, therefore, alter the rotational forces on the femur during activities and possibly affect the knee joint.
Powers et al29 provided evidence for a biomechanical theory that the entire lower extremity chain should be considered during patient management. Using kinematic magnetic resonance imaging these authors observed that, during weight bearing conditions, femoral internal rotation was associated with lateral patellar displacement. The authors found that during knee extension in the closed kinetic chain a measurable femoral internal rotation occurred. These findings are consistent with the idea that forces in an area other than the site of pain may be associated with the problem.
Previous authors have supported the relationship between gluteal weakness and knee pain. Ireland et al6 tested abduction and external rotation strength of 15 females, between the ages of 12 and 21, who had patellofemoral pain and 15 non-symptomatic subjects matched for age, gender, and body weight. Subjects with knee pain demonstrated less strength in hip abduction and external rotation. Although the research of these authors is valuable, limitations existed that should be identified. All subjects experienced pain for longer than three months, which presents a potential significant confounding variable because it is possible that gluteal weakness, from disuse or guarding, was a result of knee pain. The present study differed in that subject recruitment was limited to subjects who had an onset of pain for less than 28 days; therefore, reducing the potential confounding nature of this variable.
Tyler et al30 suggested that subjects with anterior knee pain responded favorably to interventions that included hip abduction, adduction, flexion, and extension progressive resistive strengthening strategies. Similar to Ireland et al,6 Tyler et al30 included only subjects that had experienced anterior knee pain for four weeks or more.
As the evidence mounts for a relationship between hip weakness and knee pain; causality has not been established. The methodology of Ireland et al,6 Tyler et al30 and the present study do not allow for causality to be inferred. Implications for patient/client management, however, should be considered. Patients may benefit from these findings by receiving a comprehensive examination, as well as interventions for deficits along the entire lower extremity kinetic chain during treatment.
A small age range hindered the study. Given that females with knee pain were obtained through a sample of convenience from the college community and the surrounding area, this study's age distribution was skewed. The majority of subjects were students in the age range of 18 to 24. For these reasons, study results cannot be considered to represent the general population.
A control group was not considered at the time of data collection. It was not within the scope of the study given limitations on study time and resources. The study could be enhanced by including a control group in future investigations.
Future study on this topic should involve a broader age range in order to illustrate a better representation of the general population with knee pain. A study comparing males and females could also be conducted to compare differences in gluteal strength and knee pain between the genders.
Results of our study suggested that perhaps intervention for weak hip musculature could be useful when treating patients with knee pain. These results, therefore, provide a seed of evidence that could lead to further study. Studying the effectiveness of hip strengthening to address deficits in a randomized controlled manner is a possible outgrowth of this work. Considering the relationship that hip strength has on the entire biomechanical chain, it may also prove beneficial to examine the most reliable means of strengthening the abductors and adductors of the hip.
The results of this study suggested that individuals with knee pain had weak hip muscles. Results may have significant clinical implications consistent with the foundational components of physical therapy practice. Physical therapist examination and intervention strategies that consider the entire lower extremity kinetic chain were supported by the study results.
The authors wish to thank St. Luke's Memorial Hospital Center in Utica, New York for the kind use of the MicroFET dynamometer and Deborah Marr OTR, ScD and Peter Pawson PT, PhD for the advisement provided during the initial phases of this project. This work contains data and text originally written as a Master's thesis presented to the faculty of Utica College, December 2003.