Patellofemoral pain syndrome (PFPS) is a common cause of knee pain among physically active individuals and represents approximately 25% of all knee conditions treated in sports medicine outpatient clinics (1). It has been proposed that abnormal hip motion may contribute to PFPS (31). In the frontal plane, excessive hip adduction and/or knee abduction during weight-bearing activities increase the knee valgus angle and, consequently, the lateral force that acts on the patella and lateral patellofemoral joint stress (18). Because the incidence of PFPS in females is more than two times higher than in males (4), most of the studies have only evaluated hip kinematics in females with and without PFPS (2,25,35,38,39). However, males do experience PFPS and have been shown to present differences in hip kinematic patterns when compared with females during functional and sports-related weight bearing activities (16,21,23,40). Thus, it is important to investigate if males and females with PFPS present differences in their hip and knee frontal plane motions to design more specific interventions for PFPS.
Although some authors reported increased hip adduction during functional weight-bearing activities (9,25,38,39), other studies have not reported the same findings in individuals with PFPS (2,8,35). It is noteworthy that hip adduction may be the result of adduction of the femur relative to the pelvis, the pelvis dropping on the contralateral side, or a combination of both. However, only one study evaluated contralateral pelvic drop in females with PFPS (38). Although biomechanical studies have suggested performing weight-bearing exercises at knee flexion angles lower than 48°–50° (11,36), researchers have yet to describe the frontal plane mechanics of the trunk, pelvis, hip, and knee at these lower knee flexion angles. It is not known if there is increased hip adduction and/or knee abduction in PFPS subjects at smaller knee flexion angles, which could increase the patellofemoral joint stress during functional activities. Thus, more studies are necessary to evaluate the pelvis, hip, and knee frontal plane kinematics at various knee flexion angles during weight-bearing activities in subjects with PFPS.
Ipsilateral trunk lean has been described as a common compensation for hip abductor weakness because this maneuver moves the ground reaction force vector closer to the hip joint center and thus decreases the demand on the hip abductor muscles (9,35). Dierks et al. (9) and Souza and Powers (35) reported that the increased hip abduction shown by some of the individuals with PFPS evaluated in their studies during weight-bearing activities could be a consequence of a compensatory movement of the trunk toward the stance limb. However, none of the cited studies evaluated trunk kinematics. In contrast, ipsilateral trunk lean can cause the ground reaction force vector to pass laterally with respect to the knee joint center, creating a valgus moment at the knee (17). Therefore, it is important to investigate if males and females with PFPS present increased ipsilateral trunk lean during functional activities because this has the potential to affect the patellofemoral joint mechanics.
It has been hypothesized that hip abductor weakness and/or altered neuromuscular control may be associated with poor control of femoral adduction during weight bearing activities in individuals with PFPS (18,31,32). Previous studies have reported diminished hip abductor strength in sedentary individuals and female athletes with PFPS when compared with pain-free controls (6,20,24), but no study compared isometric hip abductor torque between males and females with PFPS. Previous studies have evaluated the magnitude of gluteus medius (GM) activation during functional activities in subjects with PFPS as compared with the controls (28,29,34,35,41). However, the studies cited only evaluated the overall activity of the GM or its activation in the stance phase during weight-bearing activities. Also, GM activation has not been assessed at the different knee flexion angles during the eccentric and concentric phases of the functional activity.
Given the limited studies focusing on sex differences in the trunk, pelvis, hip, and knee mechanics and hip muscle activation at various knee flexion angles during weight-bearing activities in subjects with PFPS, the purpose of the present study was to compare ipsilateral trunk lean, contralateral pelvic drop, hip adduction, knee abduction excursion, and GM activity at 15°, 30°, 45°, and 60° of knee flexion during the downward and upward phases of the stepping maneuver in males and females with and without PFPS. Isometric hip abductor torque was also compared among the groups cited. It was hypothesized that males and females with PFPS would present increased trunk, pelvis, hip, and knee frontal plane motion as the knee flexion increased and also diminished activation of the GM when compared with the control subjects. In addition, participants with PFPS would exhibit lower isometric hip abductor torque when compared with the pain-free controls. It was also hypothesized that the alterations in the kinematics, strength, and neuromuscular variables cited previously would be more evident in females than in males.
METHODS
Subjects.
All the subjects were recruited as a result of advertisement (flyers posted in physical therapy clinics, athletic health clubs, and common areas in the university). A total of 111 subjects were evaluated by a licensed physical therapist to screen for the inclusion and exclusion criteria. The consecutive subjects satisfying the criteria were enrolled and subsequently reported to the Federal University of São Carlos for testing. The subjects signed a written informed consent form, and the study was approved by the Federal University of São Carlos Ethics Committee for Human Investigations.
Eighty recreational athletes were evaluated in this cross-sectional study, divided into four groups, with 20 subjects per group: a females-with-PFPS group (female PFPS), a pain-free female age-matched control group (female controls), a males-with-PFPS group (male PFPS), and a pain-free male age-matched control group (male controls). A recreational athlete was defined as anyone participating in aerobic or athletic activity at least three times per week. The physical activity was evaluated with the self-administered International Physical Activity Questionnaire short form (7,30) (Table 1).
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TABLE 1: Characteristics and anthropometric measurements of the subjects (mean ± SD).
The a priori sample size was calculated on the basis of the hip adduction data from a pilot study with five subjects per group (four groups) (using α = 0.05, β = 0.20, and an expected difference between groups of 5°). On the basis of the results, 18 subjects per group were required to adequately power the study for the variables of interest.
All the participants were between 18 and 35 yr. The males and females with PFPS recruited for this study met the following criteria: 1) insidious onset of symptoms unrelated to a traumatic event; 2) presence of peri- or retropatellar knee pain during at least two of the following functional activities, i.e., stair ascent or descent, running, kneeling, squatting, prolonged sitting, jumping, isometric quadriceps contraction, and palpation of the medial and/or lateral facet of the patella; and 3) reports of pain for more than 3 months of duration. The control subjects were selected if they had no history of knee injury or pain. The exclusion criteria for all groups included 1) previous history of knee surgery; 2) history of back, hip, or ankle joint injury or pain; 3) patellar instability; 4) signs or symptoms of meniscal or knee ligament involvement; and 5) any neurological involvement that would affect gait.
Procedure.
Eleven subjects reported bilateral symptoms, so in these individuals, the most affected lower extremity was tested. The corresponding limb of the age-matched control participant was tested.
Before testing, each participant completed a 5-min warm-up on a treadmill at 1.66 m·s−1. The EMG signals of the GM were recorded at 2000 Hz, detected using surface electrode DE-3.1 sensors (Delsys, Inc., Boston, MA) and amplified by a Bagnoli™ eight-channel system (Delsys, Inc.). The EMG signals were digitized using a 12-bit analog-to-digital board synchronized with the motion analysis data. Before electrode placement, the skin was shaved, abraded, and cleaned with isopropyl alcohol. The surface electrodes were applied to the skin in parallel to the midmuscle belly of the GM (15). The GM electrode was placed one-half of the distance between the iliac crest and the greater trochanter.
The EMG data obtained during the stepping maneuver were normalized by the maximal voluntary isometric contraction (MVIC). The participants carried out one practice trial before collection of three 5-s MVICs for each muscle, resting for 30 s between trials (3). The handheld dynamometer (Lafayette Instruments, Lafayette, IN) was used to simultaneously measure the strength generated during each MVIC (3). The subjects had to carry out three measurements with a variability of less than 10%; otherwise, another trial was carried out (3). The GM MVIC was recorded with the participant lying on his/her side with the evaluated limb placed in a neutral position by placing pillows between the lower extremities (3). To ensure that the legs were kept at a consistent distance apart across all subjects, 0° of hip abduction was measured with respect to a line connecting the anterior superior iliac spines. An adjustable nylon strap placed just proximal to the iliac crest and secured firmly around the underside of the table was used to stabilize the subject’s trunk. The handheld dynamometer was positioned over the lateral femoral condyle and a second nylon strap secured around the distal thigh and the examination table to resist hip abduction. Strong verbal encouragement was given throughout testing.
After the MVIC testing, the participants were instrumented with the electromagnetic tracking sensors. The three-dimensional joint kinematics of the trunk, pelvis, hip, and knee were measured using the Flock of Birds® (miniBIRD®) hardware (Ascension Technology Corporation, Burlington, VT) integrated with the MotionMonitor™ software (Innovative Sports Training, Inc., Chicago, IL). Five electromagnetic sensors were placed on the sternum, the sacrum, the distal lateral thighs, and the anteromedial aspect of the proximal tibia. The kinematic data were collected at a sampling rate of 90 Hz. Each sensor was placed over an area of the least muscle mass to minimize potential sensor movement and was secured using double-sided tape and adhesive tape (3M™ Transpore™ Medical Tape; St. Paul, MN).
Before dynamic testing, the medial and lateral malleoli and femoral epicondyles were digitized to determine the ankle joint center and knee joint center, respectively. The hip joint center was estimated using the functional approach described by Leardini et al. (22), with the data being collected as the subjects moved the hip into a minimum of 14 different static positions, representing positive and negative rotations around all three axes (isolated and combined hip flexion, extension, adduction, abduction, and internal and external rotation). The trunk angle was designated by the sternal sensor, the sacral sensor, and the respective hip joint center. When the sensors were digitized, a static trial was taken to determine the resting angles of the trunk, pelvis, hip, and knee. The subjects were instructed to stand on their evaluated lower limb. The contralateral lower limb was positioned with slight hip flexion and full knee extension, and their arms were crossed over their chest. The static standing trial was registered in this position and used to determine the lower limb anatomical position. This static measurement was used as the neutral alignment for each participant, with subsequent measurements referring to this position.
The subjects were then given the opportunity to practice the stepping maneuver. Standing on their evaluated lower limb on a step, the participants were instructed to lower themselves until their contralateral heel was touching the floor (downward phase of the maneuver) and to return to the starting position (upward phase of the maneuver) (39). A digital metronome was used to control the stepping task rate (15 steps per minute), and the height of the step was normalized to the subject’s height (10% of total body height) (35). A trial was considered valid if the subject performed the stepping maneuver at 15 steps per minute without losing his or her balance. If the trial was not considered valid, an additional trial was performed. The data from three trials of the stepping maneuver were collected for analysis with 1-min rest intervals between trials.
Data analysis.
All the kinematic data were filtered using a fourth-order zero-lag low-pass Butterworth filter at 6 Hz (39). The Euler angles were calculated using the joint coordinate system definitions recommended by the International Society of Biomechanics (14,42) using the MotionMonitor™ software. The kinematic variables of interest consisted of the maximum excursion of ipsilateral trunk lean, the contralateral pelvic drop, hip adduction, and knee abduction at 15°, 30°, 45°, and 60° of knee flexion during the downward and upward phases of the stepping maneuver. If the exact knee flexion angle was not achieved during the dynamic test, an upper bound of 0.10° was used. For example, if the 15° knee flexion angle was not achieved, the first knee flexion angle value between 15° and 15.1° was used for analysis. The trunk, pelvic, hip, and knee kinematic variables represented the movement excursions, which were calculated by subtraction of the peak values acquired during the cited knee flexion angles from those recorded in the static standing position. There was no significant difference in maximum excursion of knee flexion during the step-down maneuver among the groups evaluated. The evaluator assessed a priori test–retest reliability of the kinematic measurements. Intraclass correlation coefficients (3, 1) and SEM for the kinematic measurements evaluated in eight subjects on two occasions separated by 3–5 d were 0.79 (0.07°) for ipsilateral trunk lean, 0.94 (1.15°) for contralateral pelvic drop, 0.91 (1.97°) for hip adduction, and 0.95 (1.80°) for knee abduction.
The raw EMG signals were band-pass filtered at 35–500 Hz, and a 60-Hz notch filter was applied. The data were full-wave rectified, and a moving 75-ms average window smoothing algorithm was used to generate a linear envelope (35), with the maximum amplitude across the MVICs representing 100% activity (3). Because of the considerable variability of the EMG data, the GM EMG data were used for each knee flexion angle ± 0.5° evaluated and expressed as a percentage of the MVIC. For example, to evaluate the EMG data at 15° of knee flexion of the downward phase of the stepping maneuver, the average of the GM EMG data from 14.5° to 15.5° of knee flexion of the eccentric phase was expressed as a percentage of the EMG during the MVIC. The kinematic and EMG data were reduced using custom MATLAB software (The MathWorks, Natick, MA). The average of the three trials for the kinematic and EMG variables was used for the statistical analysis.
For the isometric hip abductor torque, the dynamometer measurement in kilograms was converted to newtons (kilograms × 9.81) to achieve a unit of force (12). Newtons were then converted to torque (force (N) × action length (m)) (12). The measured length between the greater trochanter and the lateral epicondyle of the femur was used as the action length. The average of the peak torque values obtained during three trials was used for the analysis. All the torque (N·m) data were normalized for body weight and height (12).
Statistical analysis.
All the statistical analyses were carried out using SPSS statistical software (version 17.0; SPSS, Inc., Chicago, IL). Descriptive values (means, SD) were first obtained for each variable, and these data were then analyzed with respect to their statistical distribution, variance homogeneity, and sphericity using the Shapiro–Wilk W test, Levene test, and Mauchly test, respectively. The kinematic and EMG data were compared between the female and male patellofemoral pain subjects and pain-free controls using separate three-way (sex × group × angles) mixed-model ANOVAs with the knee flexion angles as the repeated measures. The hip abductor torque was compared among the groups using a two-way (sex × group) ANOVA. For all the ANOVA tests, significant main effects were reported if there were no significant interactions. If a significant interaction was found, the individual effects were analyzed separately. Bonferroni-adjusted t-tests were used to assess pairwise comparisons. The α level was set at 0.05.
RESULTS
Kinematics.
Overall, significant within-subjects angle × sex interaction (P = 0.02–0.001) and angle × group interaction (P < 0.001 for all variables) were found for ipsilateral trunk lean, contralateral pelvic drop, hip adduction, and knee abduction during the stepping task. Also, significant sex (P = 0.019–0.001) and group (P = 0.002–0.001) between-subjects main effects (no interaction) were reported for all the kinematic variables evaluated.
Ipsilateral trunk lean.
Females demonstrated significantly greater ipsilateral trunk lean when they achieved 60° of knee flexion in the downward phase up to 15° of knee flexion in the upward phase during the stepping task, as compared with males (P = 0.022–0.001) (Fig. 1A). The subjects with PFPS showed significantly greater ipsilateral trunk lean than the controls for all the angles evaluated, except for 15° and 30° of knee flexion in the downward phase of the task (P < 0.001) (Fig. 2A).
FIGURE 1: Mean ± SD of ipsilateral trunk lean (A), contralateral pelvic drop (B), hip adduction (C), and knee abduction (D) excursions at 15°, 30°, 45°, and 60° of knee flexion, in the downward (Dow) and upward (Upw) phases of the stepping maneuver in male and female groups. *Females significantly greater than males (P < 0.05).
FIGURE 2: Mean ± SD of ipsilateral trunk lean (A), contralateral pelvic drop (B), hip adduction (C), and knee abduction (D) excursions at 15°, 30°, 45°, and 60° of knee flexion, in the downward (Dow) and upward (Upw) phases of the stepping maneuver in PFPS and control groups. *PFPS significantly greater than controls (P < 0.05).
Contralateral pelvic drop.
Females only showed increased contralateral pelvic drop during the upward phase of the stepping task, when compared with males (P = 0.027–0.001) (Fig. 1B). The PFPS subjects revealed greater contralateral pelvic drop during all the angles evaluated, except for 15° and 30° of knee flexion in the downward phase, when compared with the controls (P = 0.034–0.001) (Fig. 2B).
Hip adduction.
Females presented greater hip adduction for all the angles evaluated when compared with males (P = 0.02–0.001) (Fig. 1C). Also, the subjects with PFPS revealed increased hip adduction at the angles evaluated when compared with the controls (P = 0.021–0.001), except for 15° and 30° of knee flexion during the downward phase of the stepping task (Fig. 2C).
Knee abduction.
Females demonstrated significantly greater knee abduction during the entire downward and upward phases of the stepping task, when compared with males (P = 0.007–0.001) (Fig. 1D). The subjects with PFPS showed significantly greater knee abduction than the controls at all the angles evaluated (P = 0.013–0.001) (Fig. 2D).
GM EMG.
Significant within-subjects angle × sex (P < 0.001) and angle × group (P = 0.014) effects were found. Also, significant sex (P = 0.001) but no group (P = 0.050) between-subject main effects (no interaction) were observed. Females showed significantly greater GM activation, at all the angles evaluated, except for 15° and 30° of knee flexion in the upward phase of the stepping task when compared with males (P = 0.008–0.001) (Fig. 3A). Subjects with PFPS only showed significantly less GM activation at 60° of knee flexion for both the downward and upward phases as compared with the controls (P = 0.015 and 0.005, respectively) (Fig. 3B).
FIGURE 3: Mean ± SD of GM activation expressed as a percentage of the MVIC at 15°, 30°, 45°, and 60° of knee flexion in the downward (Dow) and upward (Upw) phases of the stepping maneuver in male and female groups (A) and in PFPS and control groups (B). *Significant difference (P < 0.05).
Isometric hip abductor torque.
Females showed less hip abductor torque than males (P < 0.001), and subjects with PFPS generated less isometric hip abductor torque than the controls (P < 0.001). There was significant sex × group interaction with respect to hip abductor torque (P = 0.03), and post hoc analysis revealed less isometric hip abductor torque in females with PFPS when compared with the other groups (P < 0.001) (Fig. 4).
FIGURE 4: Box plots for differences in isometric hip abductor torque in males and females with PFPS and the controls. In each box plot, the black line represents the median, and the extremities of the box are the 25th and 75th percentiles of the data. The lines extend to ±1.5 SD. *Females with PFPS significantly lower than the controls and males with PFPS.
DISCUSSION
The purpose of the present study was to compare the trunk, pelvis, hip, and knee frontal plane kinematics and the GM EMG activity at 15°, 30°, 45°, and 60° of knee flexion during the downward and upward phases of a stepping maneuver between males and females with and without PFPS. There was no significant sex × group interaction for the kinematic and EMG variables, meaning that differences between genders did not depend on pain and vice versa. Overall, the female and the PFPS groups showed greater ipsilateral trunk lean, contralateral pelvic drop, hip adduction, and knee abduction during the step-down maneuver, when compared with the male and control groups, respectively. These kinematic findings were accompanied by altered activation of the GM and decreased capacity to generate isometric hip abductor torque by females and subjects with PFPS when compared with the males and controls, respectively. The females with PFPS showed less isometric hip abductor torque when compared with the other groups. The kinematic, EMG, and torque alterations cited should be considered when designing treatment protocols for male and female patients with PFPS.
It had been suggested that ipsilateral trunk lean might act as a compensatory mechanism for hip abductor weakness to diminish hip adduction by elevating the contralateral pelvis during weight-bearing activities (9,35). In contrast, ipsilateral trunk lean may increase the knee abduction moment by increasing the ground reaction force that passes laterally to the knee (17,19). Excessive hip adduction and knee abduction could produce detrimental effects on the patellofemoral joint (31,32). To the authors’ knowledge, this was the first study to evaluate ipsilateral trunk lean, contralateral pelvic drop, hip adduction, knee abduction, and GM EMG activity at various knee flexion angles during a stepping maneuver in males and females with and without PFPS. It is important to acknowledge the alterations in kinematic and hip muscular activity at various knee flexion angles during the downward and upward phases of the maneuver to design more specific and effective treatment protocols for PFPS patients.
In agreement with previous studies, the present study revealed that females presented less capacity to generate isometric hip abductor torque (23,40), accompanied by greater hip adduction and knee abduction (16,21,33) at all the angles evaluated in the stepping maneuver. GM activation was increased during almost the whole maneuver, except for 15° and 30° of knee flexion in the upward phase. Increased contralateral pelvic drop and ipsilateral trunk lean only occurred later during the stepping maneuver, from 60° of knee flexion up to the end of the maneuver. Because of the important role of the GM in controlling the hip in the frontal plane, it is possible that females showed greater activation of the GM to compensate for the reduced hip musculature strength. It also seems that the increased ipsilateral trunk lean acted as a compensatory mechanism for the reduced hip abductor strength because it was more evident toward the end of the stepping maneuver, when the females carried out the greatest hip adduction excursion. However, this greater frontal plane trunk motion may have contributed to an increase in the knee abduction load. Thus, hip abductor strengthening and motor control training should be considered when implementing rehabilitation or knee injury prevention programs to avoid excessive frontal plane movements of the trunk, pelvis, hip, and knee during functional tasks, especially in females.
The PFPS subjects showed greater knee abduction at all the angles evaluated in the stepping maneuver when compared with the control group. From 45° of knee flexion in the downward phase and up to the end of the stepping maneuver, the PFPS group showed increased hip adduction, contralateral pelvic drop and ipsilateral trunk lean when compared with the control group. Previous studies also reported increased knee valgus (8), hip adduction (9,25,38,39), and contralateral pelvic drop (38) during functional weight-bearing activities in subjects with PFPS when compared with the controls. The results of the present study suggested that, in the initial phase of PFPS rehabilitation, it might be prudent to carry out functional exercises with a small knee flexion angle (≤30°) to avoid the kinematic alterations that could lead to excessive lateral patellofemoral stress and exacerbate the pain. Although previous studies recommended rehabilitation weight-bearing exercises with knee flexion angle below 48°–50° to minimize patellofemoral joint stress (11,36), these studies failed to take into account any possible effects of the increased hip adduction or knee abduction on the lateral patellofemoral joint stress. The PFPS subjects managed to control excessive trunk, pelvis, and hip frontal plane motions during the first 30° of knee flexion in the stepping maneuver, but once they lost control of the excessive motion of these joints at 45° of knee flexion, they carried out increased ipsilateral trunk lean, contralateral pelvic drop, and hip adduction excursions up to the end of the maneuver as compared with the controls. The stepping maneuver is a common exercise used during functional training in the treatment of PFPS, so special attention should be given in improving the downward control of the trunk, pelvis, hip, and knee frontal plane motions when performing with more than 30° of knee flexion to avoid excessive patellofemoral joint stress.
In the PFPS subjects, the previously cited kinematic differences were accompanied by lower GM activation at 60° of knee flexion during the stepping maneuver as compared with the controls. One possible explanation for the lower GM activation in the PFPS group may have been the increased ipsilateral trunk lean, which could have reduced the demand of the hip abductor during the stepping maneuver. It is important to note that the PFPS subjects failed to increase their GM activity at the same knee flexion angle that they carried out the greatest excursion of contralateral pelvic drop, hip adduction, and knee abduction, when compared with the control group. Our results are in agreement with a previous study (34) that also found decreased GM activation in PFPS subjects during a stepping task compared with controls but in contrast with others that reported no difference between groups (28,29,35,41). Differences in the methodology used to assess GM activation may have accounted for the different results among studies. In addition, despite the lack of statistical difference (P = 0.051), the altered GM activity pattern (Fig. 3B) was more evident in females than males with PFPS (mean ± SD %MVIC at 60° of knee flexion: female PFPS = 31.22% ± 6.10% vs female controls = 52.51% ± 8.34%; male PFPS = 24.35% ± 5.21% vs male controls = 27.45% ± 6.81%). It is possible that a difference greater than 20% found in the GM activation between the females with PFPS and controls could be important clinically and affect the hip motion control in the frontal plane. On the basis of the present results, improving the GM motor control could be beneficial when treating PFPS patients, especially females, during functional activities with 60° or more of knee flexion angle.
Isometric hip abductor torque was lower in females with PFPS when compared with all the other groups. Although it was previously suggested that increased ipsilateral trunk lean might compensate hip abductor weakness (9,35), the PFPS subjects showed increased contralateral pelvic drop, hip adduction, and knee abduction during the stepping maneuver when compared with the controls. In addition, the increased ipsilateral trunk lean may have contributed to the increased knee abduction excursion (17,19) at 45° of knee flexion up to the end of the maneuver. Recently, Thijs et al. (37) suggested that isometric hip muscle strength might not be a predisposing factor for PFPS in female runners. However, several studies reported a reduced capacity to generate isometric hip abductor torque in subjects with PFPS (6,20,24). Thus, decreased strength of the hip musculature may be present in individuals after the development of PFPS but may not predispose individuals to injury. In addition, previous studies demonstrated the beneficial effects of hip abductor muscle strengthening on the pain and function of PFPS subjects (10,13,27). Therefore, it could be potentially beneficial to include hip abductor strengthening in PFPS treatment, especially when treating females with PFPS.
The authors recognize there were some limitations in this study. Because a cross-sectional design was used, it was not possible to establish cause-and-effect relationships. Myer et al. (26) identified an increased knee abduction moment as a risk factor for PFPS. Thus, further prospective studies are necessary to draw definite conclusions about the role of the trunk, pelvis, hip, and knee kinematics and GM muscular activation on PFPS. It must be acknowledged that previous studies demonstrated the importance of the hip and knee transverse plane motion (5,25,35), gluteus maximus EMG activity (35), and hip rotator strength (6,20,24) in those with PFPS. The present study only focused on the frontal plane kinematics, EMG, and hip torque; thus, future studies should evaluate rotation at the hip and knee and gluteus maximus EMG activity with various knee flexion angles and during a variety of functional and sports-related activities to design more specific injury prevention programs and treatment protocols.
CONCLUSIONS
Females carried out increased hip adduction and knee abduction during the downward and upward phases of the stepping maneuver when compared with males. Ipsilateral trunk lean and contralateral pelvic drop only increased from 60° of knee flexion and up to the end of the maneuver. These kinematic results were accompanied by increased GM activity and reduced isometric hip abductor torque as compared with males. The PFPS subjects demonstrated greater knee abduction excursion at all the knee flexion angles evaluated during the stepping maneuver when compared with pain-free controls. Moreover, the PFPS subjects showed increased hip adduction, contralateral pelvic drop, and ipsilateral trunk lean from 45° of knee flexion of the downward phase and up to the end of the maneuver as compared with the controls. The GM activity decreased at 60° of knee flexion angle at which the greatest frontal plane excursion of the trunk, pelvis, hip, and knee occurred in the PFPS group as compared with the control group. Females with PFPS demonstrated less capacity to generate isometric hip abductor torque than males with PFPS and the controls. When treating or designing knee injury prevention protocols, it is recommended that the trunk, pelvis, hip, and knee frontal plane kinematics and the GM EMG activity at specific knee flexion angles be addressed. Hip abductor strengthening and motor control training should be considered when treating PFPS, especially in females.
The first author of this article was financially supported with a scholarship from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior scholarship, Process number 1016/11-3), and the second author was financially supported with a scholarship from the Fundação de Amparo à Pesquisa do Estado de São Paulo (Proc number 2010/07756-5).
The authors have no conflict of interest to report.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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