Our purpose was to investigate the relationship between hip-abductor muscle strength and frontal-plane knee mechanics. We used a 3-week hip-abductor muscle-strengthening protocol to measure potential changes in strength, biomechanics, and pain for patients experiencing PFPS. Previous authors1–,7
have hypothesized that a primary contributing factor to PFPS is weakness of the hip musculature, including the abductors. However, few researchers have directly investigated this possibility.
In support of the first hypothesis, the PFPS patients exhibited 28.71% reduced maximal isometric hip-abductor muscle strength at baseline, compared with the control group. These results are similar to those of several other studies1–,9,12,15–,18
involving PFPS patients and indicate that weakness of the hip abductors may play a key role in the development of PFPS. We also hypothesized that over the 3-week rehabilitation protocol, hip-abductor muscle strength would increase. In support of this hypothesis, PFPS patients exhibited an average 32.69% improvement in strength. Authors of earlier strengthening studies6,14,30–,32
reported 13% to 51% increases in hip-abductor strength after rehabilitation protocols that ranged from 6 to 14 weeks in length. Thus, the increase in abductor strength reported in the current study is comparable with previous findings.
We hypothesized that over the 3-week rehabilitation protocol, the level of pain experienced by the PFPS patients would decrease. In support of this hypothesis, the level of pain for 14 of the 15 patients decreased 40% over the course of the study. These results are in agreement with those of previous studies1–,9,12,15–,18
whose authors also suggested that hip-abductor muscle weakness is a contributing factor in the development and treatment of PFPS and should be targeted in its treatment. An asset of our investigation is that the rehabilitation protocol consisted of exercises intended solely to increase the strength of the hip abductors. Thus, these results provide further evidence that hip-abductor muscle-strengthening exercises should be considered for preventing musculoskeletal injury and treating PFPS.
Tyler et al32
examined the benefits of a hip-strengthening program on 35 patients diagnosed with PFPS. All volunteers participated in a 6-week intervention that consisted primarily of hip-strengthening exercises. Hip strength improved in 66% of the PFPS patients, which is consistent with our results. Based on a minimum decrease of 1.5 cm in the VAS scores, 21 patients (26 knees) had a successful outcome and 14 patients (17 knees) had an unsuccessful outcome. Interestingly, and in contrast to our results, these authors32
reported that, based on their statistical analysis, improvement in hip-abduction strength was unrelated to PFPS pain and treatment outcome. Thus, other muscles in addition to the hip abductors may be important in treating PFPS.31,32
Future studies involving a more comprehensive hip muscle-strengthening protocol are necessary to answer this question.
In such a short period of time, improvements in muscle strength are largely attributable to changes in neuromuscular activation of muscles and not to changes in muscle-fiber composition or hypertrophy.33,34
have shown that after 10 or 14 days of daily strengthening, an increased number of motor units are recruited, concomitant with greater maximal voluntary contraction. These results support the notion that neural adaptation, not hip-abductor muscle-fiber hypertrophy, was primarily responsible for the increased strength exhibited by the PFPS participants after only 3 weeks of muscle strengthening.
Similar to earlier researchers,2,3,8,9,11
we hypothesized that reduced hip-abductor muscle strength would result in a greater peak knee genu valgum angle when running and, thus, would contribute to the development of PFPS, because the hip abductors would not be able to adequately control hip adduction via eccentric contraction. However, no differences in peak knee genu valgum angle were measured between groups or across time.
To date, few investigators2,6,7,14
have evaluated the relationship between hip-abductor muscle strength and knee kinematics. Our findings are similar to those of Bolgla et al,7
who reported that 18 patients with PFPS had hip weakness but did not demonstrate altered hip or knee kinematics while descending stairs, compared with a control group. Yet our results contrast with those of Mascal et al.6
In their case study, the participant who underwent a biomechanical assessment after a 14-week strengthening protocol demonstrated improved hip strength and reduced hip adduction (a component of knee genu valgum) during a step-down maneuver, compared with baseline values. Our findings also contrast with those of Dierks et al,2
who reported an inverse relationship between decreased hip-abductor muscle strength and increased hip adduction at the beginning and end of a prolonged run for PFPS patients.
The single-subject design and stair-descent method used by Mascal et al6
make comparison of their results with ours difficult. We measured 2-D knee genu valgum angle, which is a combination of thigh adduction and shank abduction. Thus, comparison with studies of 3-D angles is challenging. In addition, Dierks et al2
used a fatigue protocol to determine the association between hip-abductor muscle strength and mechanics, whereas we used a muscle-strengthening protocol. Future studies are therefore necessary to help resolve these conflicting data.
Based on the PFPS data reported by Hamill et al15
and others, we hypothesized that over the 3-week rehabilitation protocol, stride-to-stride knee-joint variability would increase as pain-free status and more typical movement patterns were restored. However, at baseline we measured a marked increase in variability for the PFPS group compared with the control group. Moreover, we measured reductions in stride-to-stride knee-joint variability after the strengthening protocol.
Inspection of reveals that the PFPS group adopted a more consistent stride-to-stride kinematic pattern after the rehabilitation protocol. From a clinical perspective, it is reasonable to assume that restoring a more consistent and predictable movement pattern, concomitant with increased muscle strength and reduced pain, would be expected after such an exercise regime. By providing the knee joint with more consistent (ie, less variable) movement patterns on a stride-to-stride basis, it is possible that a more optimal environment is established, allowing for tissue healing and pain resolution. Additional clinical studies are necessary to answer these questions.
It is important to compare the methods used by Hamill et al15
with ours. They investigated intersegment coupling variability, whereas we assessed variability within a single joint. They theorized that increased movement variability from 2 lower extremity segments represents a healthy population and is necessary to help prevent injury. This theory can still be applied to our results because increased intersegmental coupling between the shank and thigh may have produced our observed reduction in single-joint motion. More research will address this question.
At the posttest, compared with their baseline values, all 15 PFPS patients increased muscle strength and demonstrated at least a 4-cm (33%) VAS drop in pain; in fact, 4 patients were pain free at 3 weeks. Reduced stride-to-stride knee-joint variability was seen in 13 of 15 patients. At the end of the study, we provided all patients with a more comprehensive rehabilitation program, including recommendations for stretching and strengthening exercises that focused on the low back, hip, knee, and ankle musculature. Anecdotally, we followed up after an additional 3 weeks of rehabilitation and learned that all patients were pain free and had returned to their preinjury running regimes.
Several limitations of this study are apparent. First, the biomechanical measures used a 2-D camera system, and knee motion occurs in 3 dimensions. Thus, using a 3-D motion system would provide more comprehensive data regarding the changes in pelvic and lower limb mechanics that occur as a result of muscle strengthening. However, previous authors27
have shown this 2-D approach to be valid and moderately reliable for side-step and side-jump maneuvers, and data from our laboratory show this 2-D measure to be valid and highly reliable compared with 3-D measures during treadmill running. The control group exhibited no change in peak knee genu valgum angle over the 3 weeks, results similar to those of earlier investigations.7,14,22
For example, Snyder et al14
reported 0.4° and 1.4° changes in knee-abduction and hip-adduction excursion values, respectively, after a 6-week strengthening protocol. Moreover, the 0.66° and 0.43° differences we measured for the PFPS and control groups, respectively, are similar to those reported by Ferber et al,22
who investigated 3-D kinematics for test-retest reliability in healthy runners. These authors reported mean differences of 0.64° in peak knee adduction and 1.64° in peak hip adduction over 2 days. Thus, even though we used a 2-D analysis, our results are comparable with those of previous 3-D studies.
Second, because we had a standard camera placement and did not position it at a specific height from the ground relative to the participant's height or knee location (or both), perspective error was a possibility. However, our volunteers were all of the same approximate height, and given that our day-to-day values were similar to those of an earlier investigation,22
we are confident that this concern was minimized. Next, the investigator was aware of the participant's group allocation (PFPS or control), and only the PFPS group performed the exercises, perhaps leading to bias during testing and analysis. However, using a standardized protocol for data collection and analysis for each measure reduced this bias. Also, future research involving a PFPS group that did not perform the exercises would be helpful to better understand how the exercises and subsequent increases in hip-abductor muscle strength influenced movement mechanics. In addition, the groups were not matched for number of participants or sex. However, no differences in age, height, or mass were measured between the groups, and all study participants were recreational runners, indicating that the groups were similar. The volunteers performed a simple running task that lasted a relatively brief period of time, and all ran at the same speed. This procedure may not have been strenuous enough to reveal changes in lower extremity mechanics, so future studies involving running to fatigue, similar to the protocol of Dierks et al,2
may be beneficial. Furthermore, the running speed chosen was a comfortable pace for all participants and was similar to their own regular running paces on a treadmill and compared with previous studies. Next, the number of patients and length of the rehabilitation protocol were limited. A study with a larger control population matched for age, sex, mass, and mileage or a longer and more comprehensive rehabilitation protocol (or both) may reveal different results, especially with respect to changes in knee genu valgum mechanics. Lastly, we measured isometric muscle-force output, which is not a direct measure of hip-abductor muscle strength and does not necessarily reflect the dynamic concentric and eccentric muscle contractions involved in running.