Running is one of the most popular forms of exercise with upward of 16 million Americans participating (20). However, runners report an alarmingly high annual injury rate of up to 79.3% (36). Patellofemoral pain (PFP) is the most prevalent injury among runners and is characterized by pain under or around the kneecap (32). Unfortunately, in more than 90% of individuals with PFP, this pain becomes chronic (31). Individuals with chronic PFP may be at greater risk for developing patellofemoral osteoarthritis later in life (34,35). As with many knee injuries, females are twice as likely to experience PFP compared with males (32).
Recent studies have suggested that abnormal hip and knee mechanics are associated with PFP in females (6,23–26,28–30,38–40). These abnormal mechanics include excessive peak contralateral pelvic drop (CPD) (6,38,40), peak hip adduction (HADD) (6,25,38,40), and peak hip internal rotation (HIR) (6,29,30) and decreased peak knee adduction (KADD) (25). In combination, these motions result in dynamic valgus of the knee, resulting in lateral patellar tracking and an increase in the loading forces on the lateral aspect of the patellofemoral joint (3,12,14,23,24). Importantly, these abnormal mechanics appear to be present in females with PFP across a variety of activities, including both running and during a single-leg squat (6,29,30,38,39). Thus, evaluation of single-leg squat mechanics is often used clinically to make extrapolations to running mechanics in individuals with PFP (39).
Preliminary investigations suggest that the gait mechanics of males with PFP may differ from females with PFP. In a mixed-sex cohort of runners with PFP, Dierks et al. (6) reported that the females with PFP ran with increased HADD and internal rotation, consistent with previous literature. However, the few males with PFP in the cohort actually exhibited decreased HADD during running (6). The decreased HADD may indicate that the males ran with greater knee varus. Thijs et al. (33) found that military recruits (predominately male) who developed PFP prospectively demonstrated a more laterally deviated center of pressure during walking than healthy controls. A more lateral center of pressure is associated with a higher external KADD moment during walking, suggesting a more adducted knee (7). Decreased HADD, coupled with a laterally deviated center of pressure, may indicate a varus aligned lower extremity during gait in males with PFP. Compared with a lower extremity in dynamic valgus, a lower extremity in dynamic varus may have distinctly different effects on the mechanics of the patellofemoral joint. Specifically, medial maltracking may result from increasing dynamic varus alignment by either increasing KADD or knee internal rotation (3,12,14). These motions may be influenced proximally by decreasing HADD and/or HIR.
Despite the potential differences in dynamic alignment in individuals with and without PFP, the relationship between static frontal plane alignment of the knee and PFP is unclear. Witvrouw et al. (41) found no prospective evidence supporting the role of static frontal plane alignment of the tibiofemoral joint in the development of PFP in a mixed-sex cohort. In contrast, Milgrom et al. (17) reported that individuals who had greater varus alignment were more than twice as likely to develop PFP than those who had normal or valgus alignment. The reason for this discrepancy between the two studies may be due to differences in subject selection. Indeed, Witvrouw et al. studied a mixed-sex cohort, whereas Milgrom et al. evaluated males enrolled in basic military training. Because it has been established that healthy males have been shown to have greater static knee varus than healthy females, differences in alignment between individuals with and without PFP in Witvrouw et al. may have been masked by the mixed-sex cohort (21).
Previously, static varus alignment has been shown to relate closely to increased dynamic varus alignment, as well as a greater knee external adduction moment during walking in young, healthy males (2). A greater knee external adduction moment would act on the frontal plane of the knee, potentially resulting in an even greater increase in dynamic knee varus during gait. In fact, one of the best predictors of knee external adduction moment is indeed static varus alignment (2). Thus, determination of static alignment in individuals with PFP may be critical in understanding the nature of their dynamic alignment during running and squatting.
Therefore, we sought to compare the lower extremity mechanics and structure of males with PFP to male controls and females with PFP during running and during a single-leg squat. We hypothesized that males with PFP would move with less peak CPD, HADD, and HIR but with greater KADD and higher associated knee external adduction moment than healthy males and females with PFP. We further hypothesized that males with PFP would have greater static varus alignment when compared with healthy males and females with PFP.
The data collection protocol and informed consent document were approved by the University of Delaware Human Subjects Research Board. To participate, both written and verbal informed consent was obtained from each volunteer. An a priori power analysis was conducted using data from pilot work for this study. Using the variable with the highest SD, HIR, it was revealed that 20 subjects per group (effect size = 1.04, α = 0.05, β = 0.20) were required to adequately power this study. The subject groups consisted of males with PFP, matched male controls, and females with PFP. Healthy females were not collected; comparisons of the running and squatting mechanics in females with and without PFP are readily available in the literature (6,25,29,30,38).
Runners between the ages of 18 and 40 yr old, currently running at least 10 km·wk−1, able to comfortably run at a 3.35 m·s−1 pace with a nonantalgic gait pattern, and free of any lower extremity surgeries were recruited for the study. Subjects were recruited from the University of Delaware student body, area running clubs, and local races. For participants with PFP, a diagnosis was determined during a musculoskeletal screening session by a licensed physical therapist who is board certified in orthopedics (coauthor RW). All participants with PFP were required to have PFP for at least the previous 3 months before their data collection. PFP was operationally defined as retropatellar or peripatellar pain that was self-rated at least a “3” on a visual analog scale of “0” to “10.” This pain was required to be present during running and at least one other activity. The onset of pain was required to be atraumatic in nature. All volunteers with patellofemoral instability, with other knee diagnoses, or who were otherwise unhealthy were excused. Prior rehabilitation for knee pain was not an exclusion criterion. When knee pain was bilateral, the knee with the highest self-rated pain was analyzed. When pain was equal bilaterally, the most dominant limb (defined as the limb used to kick a soccer ball) was analyzed. Males with PFP were matched with healthy male volunteers and females with PFP based upon average weekly running distance and age.
Qualified volunteers were invited to participate in the study. Subjects first completed the Lower Extremity Functional Scale (LEFS) to assess overall functional status. The LEFS requires subjects to rate the extent that their knee pain limits their ability to perform 20 separate activities. Each answer is based on a scale of “1” to “4,” with “1” corresponding with an inability to perform a task and “4” corresponding with no difficulty performing a task. The LEFS has been shown to be a reliable and valid means to classify functional limitations secondary to PFP (4).
Subjects were then prepared for motion analysis. Thirty retroreflective markers were attached to the pelvis and the affected lower extremity to analyze running and single leg squat kinematics. Subjects wore standardized neutral running shoes (Nike Pegasus; Nike, Inc., Beaverton, OR) to control for the effect of footwear on mechanics. All kinematic and kinetic data were sampled at 200 and 1000 Hz, respectively. Three-dimensional marker coordinates were captured with an 8-camera Vicon Mx system (VICON, Oxford, UK). To establish segment coordinate systems, a standing calibration trial was collected while the subject stood on a force platform (Bertec, Worthington, OH) mounted in the center of the capture volume. Next, a functional hip trial was collected to determine the hip joint center (9). Running kinematic and kinetic data were then collected as subjects traversed a 25-m runway at 3.35 m·s−1 (8 min·mile−1). All subjects were asked during the collection of the running data if the fixed running speed of 3.35 m·s−1 was a comfortable pace. No subjects indicated that the test speed was uncomfortable. Subjects were monitored closely for targeting of the force platform during all running trials. Lastly, single-leg squat data were collected as subjects performed a squat to approximately 60° knee flexion while standing on the center of the force platform. While squatting, subjects were asked to maintain an arm position of approximately 90° of shoulder abduction. Five trials per activity were collected for later analysis. Subjects performed the squatting maneuver to a 1-Hz count to standardize the speed of the squatting maneuver.
The measurement of tibial mechanical axis was obtained as per Barrios et al. (2). For this measure, each subject was asked to assume side by side stance with even weight distribution (Fig. 1). The proximal arm of a caliper inclinometer (Isomed, Portland, OR) was aligned with the most prominent aspect of the tibial tuberosity. The distal arm was aligned with the center of the neck of the talus, yielding an angle in respect to vertical. Three trials were collected. The tibial mechanical axis was recorded to the nearest degree, and the mean of the three trials was used for eventual analysis. This technique has been shown to have a high correlation to full-length radiographs quantifying the mechanical axis of the lower extremity (r = 0.80) (10).
All kinematic and kinetic data were filtered with an 8- and 50-Hz, low-pass, fourth-order, zero-lag Butterworth filter, respectively. Three-dimensional (3-D) joint and segment angles were calculated with Visual 3-D software (C-Motion, Bethesda, MD) using an X, Y, Z Euler angle rotation sequence. Internal joint moments were calculated using segment inertial properties as per Dempster et al. (5). Internal joint moments were normalized to body mass and height. Internal knee abduction moment was negated to represent external KADD moment.
Only the stance phase of running was analyzed. For single-leg squat data, the event was defined as beginning when knee flexion was initiated and concluding when 60° of knee flexion was reached. For each participant, single-leg squat mechanics were analyzed at the index of mean peak knee flexion calculated from their running data. During running, peak KADD and external KADD moment typically occur at peak knee flexion. Therefore, this subject-specific index for the single-leg squat was chosen to facilitate direct comparisons between the two tasks. By evaluating at the same knee flexion angle, the feasibility of using the single-leg squat as a clinical screen for running mechanics could be evaluated. Customized software (LabVIEW 8.0; National Instruments, Austin, TX) was used to extract the discrete variables of interest from five individual curves for the motion files. Means and SDs of these values were calculated. Finally, individual mean curves were time normalized and ensemble curves were created for display of the group mean data.
The variables of interest were HADD, HIR, CPD, KADD, external KADD moment, and tibial mechanical axis. To better understand the contributors to KADD, we performed a post hoc assessment of tibial and femoral segment data during the two tasks. To conduct the post hoc analysis, the tibial and femoral segments were referenced to the laboratory coordinate system during running (indexed to the primary outcome variable peak KADD) and squatting (indexed to individual peak knee flexion angle during running). Statistical analyses were conducted using SPSS (IBM, Chicago, IL). Normality of the data was assessed with a Shapiro–Wilk test. Two separate ANOVAs (group 3 × activity) were conducted to compare differences in mechanics and structure. Statistical significance was determined at α = 0.05, and a trend was defined as an α ≤ 0.10.
A total of 54 qualified subjects (18 per group) participated in this study. Although originally powered for 60 subjects, we opted to halt data collection once statistical power had been reached in the main outcome variables. The three groups were not statistically different in regards to age and mileage (Table 1). However, body mass index (BMI) was significantly higher in males with PFP compared with control males and females with PFP. Interestingly, males with PFP were considered overweight, as per World Health Organization guidelines (42). In addition, females with PFP had a significantly greater level of chronicity of PFP than males with PFP. However, there were no differences in pain and LEFS scores between males and females with PFP.
During running, several differences were detected between the three groups (Table 2). Contrary to our hypotheses, males with PFP ran with greater peak CPD than healthy male controls (P = 0.002, F = 11.881) (Fig. 2A). Interestingly, no differences were detected in peak HADD (P = 0.394, F = 0.745) and peak HIR (P = 0.557, F = 0.351) (Fig. 2B, C) between the injured and healthy male groups. However, males with PFP ran with greater peak KADD (P = 0.029, F = 5.224) and greater peak external KADD moment (P = 0.041, F = 4.501) than male controls, as hypothesized (Fig. 3A, B). Peak KADD was greater in the males with PFP (P = 0.018, F = 6.136), and peak HADD was less than females with PFP (P = 0.000, F = 34.319). Despite these differences, both males and females with PFP ran with similar peak CPD (P = 0.19, F = 1.772).
During the single leg squat, injured males demonstrated greater KADD (P = 0.021, F = 5.855) than their healthy counterparts (Table 3). However, external KADD moment was similar between both male groups. Males with PFP squatted with greater KADD (P = 0.000, F = 23.279) than females with PFP who squatted with an abducted knee. As hypothesized, males with PFP squatted with less HADD than their injured male counterparts (P = 0.007, F = 8.097).
Surprisingly, there were no differences in the tibial mechanical axis when comparing males with PFP to either healthy males or to females with PFP (Table 2). Thus, although peak KADD during the two tasks was greater in the males with PFP, the mechanical axis of the tibia was not. Although there was no difference in peak femoral adduction (running: P = 0.10, F = 2.931; squatting: P = 0.46, F = 0.559), the males with PFP ran with greater tibial adduction than the controls (P = 0.05, F = 4.114) (Tables 2 and 3). Similarly, a trend toward greater tibial adduction in males with PFP during the single-leg squat was noted (P = 0.09, F = 2.879). In contrast, females with PFP ran and squatted with greater femoral adduction (P < 0.000, F = 19.535, P = 0.001, F = 9.138, respectively) when compared with the males with PFP. Interestingly, there was not a significant difference in tibial adduction between males and females with PFP during running (P = 0.09, F = 2.971) and squatting.
We sought to determine whether males with PFP run and squat differently than healthy males and females with PFP. The results of this study suggest that males with PFP run and squat with greater KADD, knee external adduction moment, and CPD than healthy males. In contrast, males with PFP ran and squatted with less HADD compared with females with PFP. These differences in mechanics suggest that males and females with PFP may require different interventions.
Our findings suggest that males with PFP run and squat in excessive dynamic knee varum. Increasing knee varum will likely lead to a decrease in the quadriceps angle (23). The quadriceps angle is defined as the angle formed by two lines: a) a line drawn between the anterior superior iliac spine and the midpoint of the patella and b) a line drawn between the tibial tuberosity and the midpoint of the patella (23). Excessively decreasing the quadriceps angle has been shown to result in medial tracking of the patella while increasing medial patellofemoral joint stress (3,12,14). In contrast, increasing the quadriceps angle (as seen in females with PFP) has been shown to increase lateral translation of the patella while increasing lateral patellofemoral joint stress (3,12,14). Both decreasing and increasing the quadriceps angle can have a detrimental effect on the articular contact area of the patellofemoral joint (3,12,14). In contrast to increasing the quadriceps angle, decreasing the quadriceps angle appears to decrease contact area to an even greater extent as the patella shifts to the smaller medial aspect of the trochlea (3). Thus, decreasing the quadriceps angle can have an even larger effect on patellofemoral joint stresses than increasing the quadriceps angle an equal amount (3,12). Thus, even the small differences noted in KADD between males with PFP and healthy males may have a large effect on patellofemoral joint stress. Chronically high patellofemoral stress may lead to overloading of the articular cartilage and subchondral bone, ultimately resulting in the pain associated with PFP (3,8).
No differences were found between groups for the structural measure of tibial mechanical axis. This was somewhat surprising, especially considering the differences in dynamic alignment. The post hoc analysis of segmental motion revealed greater tibial adduction in males with PFP when compared with healthy males (and a trend when compared with females with PFP). This suggests that the greater peak KADD exhibited by males with PFP was more related to dynamic rather than static alignment. This is encouraging because mechanics are modifiable, whereas structure is not easily changed without surgical intervention. During the single leg squat males with PFP demonstrated only a trend of increased dynamic tibial adduction, but a significantly more vertical femur compared with females with PFP. These mechanics may speak to the influence of different pelvic widths between sexes, necessitating increased femoral adduction in the females and resulting in a less adducted knee. We found a significant difference in CPD, yet no differences in HADD between males with and without PFP. This discrepancy is likely explained by the trend of decreased adduction of the femoral segment in the laboratory coordinate system, indicating a more vertical femur in males with PFP. Interestingly, an increase in CPD has been associated with higher external KADD moments in the medial knee osteoarthritis population during walking (11,15). Thus, the excessive CPD exhibited by males with PFP may contribute to the higher knee external adduction moment, resulting in the higher KADD seen in males with PFP. Although not assessed in the present study, variations in step width may also have an effect on the knee external adduction moment during running. For instance, a crossover gait pattern results in elevated frontal plane moments of the knee in healthy runners (16). Future investigations should investigate if males with PFP run with crossover gait pattern.
Intervention studies have suggested that dynamic hip and knee mechanics can be modified through neuromuscular reeducation programs, such as gait retraining (1,18,22,27,37). Neuromuscular treatment programs aimed at reducing KADD and knee external adduction moment may show promise in the treatment of PFP in male runners. For instance, cueing medial thrusting of the thigh or providing real-time feedback on the knee external adduction moment has been shown to reduce both KADD and the knee external adduction moment during walking in individuals (1,27,37). In contrast, cueing a reduction of excessive HADD has been shown to reduce abnormal hip mechanics and pain in female runners with PFP (22).
It was interesting to note that mechanics were not entirely consistent within the male and female groups. In fact, 3 out of 18 males with PFP ran with excessive HADD and decreased KADD, similar to the group means of females with PFP. In contrast, 4 out of 18 females with PFP ran with reduced HADD and increased KADD, similar to the group means of males with PFP. Thus, although there were generally mean differences found between male and females with PFP, the mechanics of PFP during running are not exclusively sex related. This finding highlights the importance of individualized gait analysis in the evaluation of patients with PFP.
During the single-leg squat, males with PFP demonstrated greater KADD than healthy males or than females with PFP. Despite this difference, external KADD moments were similar between groups. Previously, females who would go on to develop PFP have been reported to demonstrate a higher knee external abduction moment during jump landing when compared with healthy females (19). Thus, group differences in external KADD moment may only be present in activities that have higher ground reaction forces such as running (as seen in this study) or jumping. Future investigation of differences in jump landing mechanics in males with and without PFP and females with PFP may be warranted. Regardless, KADD was remarkably consistent across the two tasks for males with and without PFP. On the basis of this finding, it appears that the single-leg squat may be a useful screening tool to assess dynamic varus mechanics in male runners with and without PFP.
We found that our males with PFP were generally heavier than the male controls. Matching the controls to the males with PFP for BMI would have resulted in an overweight reference group. Thus, all joint moments were normalized to body mass. Certainly, an increase in body mass may have a negative impact on loading of the patellofemoral joint. However, a recent meta-analysis found that BMI is not associated prospectively with PFP (13). The cross-sectional design of the present study precludes any inference of causation. However, the elevated BMI in males with PFP may provide a hindrance to recovery from the injury in this cohort. Further study may be necessary in the relationship of excessive BMI and PFP in male runners.
Limitations of this current study should be noted. First, this study is cross-sectional in design. Therefore, care should be taken to infer causation from these findings. For instance, the increased KADD noted in males with PFP may represent a movement strategy to reduce knee pain. Future investigations should have a prospective design to further investigate the mechanics in males with PFP found in this study. We also collected each subject’s running mechanics at a prescribed running speed. Although each subject attested to being comfortable running at the fixed running speed, running mechanics may collected at their preferred running speed. Finally, we are only able to infer patellofemoral joint mechanics on the basis of segmental motions of the thigh and lower leg. Therefore, kinematic imaging techniques, such as biplane fluoroscopy, are required to quantify the 3-D kinematics of the patellofemoral joint during running and other functional tasks in individuals with and without PFP.
In this cross-sectional study, males with PFP ran and squatted with increased KADD. In contrast, females with PFP ran and squatted with greater HADD and less KADD. These sex-specific mechanics suggest that males and females with PFP may need differing interventions.
This work was supported in part by a Promotional of Doctoral Studies scholarship from the Foundation for Physical Therapy, Drayer Physical Therapy Institute, American College of Sports Medicine Biomechanics Interest Group Research Award, and National Institutes of Health 1 S10 RR022396.
The authors thank the subjects for their participation in this study.
There are no conflicts of interest among any of the authors of this manuscript.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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