Muscle Activity and Flexibility in Individuals With Medial Knee Displacement During the Overhead Squat
Abstract
Functional movement analyses are commonly performed to evaluate for the risk of developing a knee or lower extremity injury. Conclusions are drawn about muscle strength, flexibility, and activation during these screenings based upon observed movement patterns. The purpose of this project was to compare range of motion (ROM), isokinetic strength, and muscle activity in individuals with and without excessive medial knee displacement (MKD) during a double-leg squat. Seventeen control participants and 14 participants with MKD had ankle and hip ROM and hip isokinetic peak torque assessed. Participants also had muscle activity of the gluteus maximus and adductor complex assessed during the descending and ascending phases of 5 consecutive double leg squats. The MKD group demonstrated decreased dorsiflexion ROM with the knee straight (P < .001), and increased adductor activation (P = .02) during the squat. These results indicate that gastrocnemius muscle tightness and increased adductor activity may cause excessive MKD.
Introduction
Lower extremity movement screenings are commonly used to assess dynamic postural alignment1–5 and identify individuals at high risk for injury.6 These screenings are designed to be easily implemented in the clinical setting because they require only visual observation (limited equipment),1,6 can be completed in a short amount of time, and mimic common athletic motions.2 During movement screenings, a clinician can observe an athlete performing a series of movements, such as squats, and watch for abnormal movement patterns or movement dysfunctions. These movement dysfunctions are theorized to be representative of muscle imbalances caused by decreased flexibility, muscle weakness, and unbalanced muscle activation patterns.2 However, much of the information garnered from movement screenings is theoretical and lacks scientific validation.
A movement dysfunction pattern commonly observed during lower extremity movement screenings is excessive medial knee displacement (MKD), or dynamic knee valgus. The position of MKD is associated with both acute7 and chronic knee injuries.8,9 Injury to the anterior cruciate ligament (ACL) is an acute knee injury that can occur as a result of altered biomechanics, resulting in increased knee valgus kinematics and kinetics.7,10 This has been shown in both prospective7 and in vitro10 studies. Patellofemoral pain syndrome (PFPS) is a chronic knee injury that can occur as a result of excessive knee valgus.9 Knee valgus positions can alter the compressive forces between the patella and femur, resulting in abnormal wear patterns and cartilage degeneration.11–13 Although the mechanisms for ACL injuries and PFPS differ, both mechanisms and associated risk factors highlight the importance of proper lower extremity alignment and the prevention of MKD during athletic maneuvers. Therefore, it is important to understand the factors that influence MKD to develop interventions that correct excessive MKD and potentially reduce injury risk.
During clinical screenings, MKD has been identified in an individual when the center point of the patella moves medial to the great toe.14 Medial knee displacement is theorized to originate from factors proximal and distal to the knee joint.15 The hip contributes to MKD through a combination of femoral adduction and internal rotation.7 The hip muscles stabilize and control the femur proximally, but a lack of control may manifest as MKD during activity. Therefore, the presence of MKD during squatting may indicate a lack of neuromuscular control in the hip external rotators or abductors.16,17 The lower leg may also contribute to MKD via tibial adduction and external rotation.7 Tightness or overactivity of the gastrocnemius or soleus muscles may restrict ankle dorsiflexion motion and increase tibial adduction,14,15 leading to MKD.5,18 Both hip and ankle musculature should be evaluated to determine factors associated with MKD.
The purpose of this project was to evaluate differences in hip (activation, flexibility, and isokinetic strength) and ankle musculature (flexibility) characteristics between participants exhibiting excessive MKD and those who do not. We hypothesized that individuals with excessive MKD would exhibit decreased strength, activation, and flexibility deficits at the hip and ankle compared with the control group.
Method
Participants
Seventy participants, recruited from the general university population, attended an initial screening session and were assessed by a single investigator (B.J.V.) for excessive MKD while performing a double-leg squat.2,14 All participants read and signed an informed consent form approved by the University’s Institutional Review Board prior to testing.
Participants were divided into groups based on their performance of the double-leg squat test, which is also called the overhead squat test. During the test, participants began in a standardized starting position with their feet shoulder width apart, toes pointed straight ahead, and hands over their head with the elbows extended. Participants were instructed to squat naturally, as if they were sitting in a chair, and performed 5 squats in a row (Figure A). Participants were viewed in real time and had to have excessive MKD in their dominant limb for 3 of the 5 squats. Excessive MKD was defined as the midpoint of the patella moving medial to the great toe (Figure B).5,14 Participants who did not demonstrate MKD were placed in the control group and contacted at a later date to return for testing. Because not every subject who was screened met the MKD inclusion criteria, we continued screening to find an adequate number of participants with MKD. Once we identified a large enough sample for the control group, all additional participants were withdrawn from the study.
(A) Control participant during the overhead squat test. The center of the patella stays over the mid-foot. (B) Medial knee displacement group participant: the center of the patella moves medial to the great toe.
A preliminary power analysis demonstrated that 10 to 15 participants per group would result in 80% power to detect an estimated 15% difference between groups for the primary variables of interest.14 Participants were healthy individuals, 18 to 30 years old, with no previous lower extremity injury in the past 6 months or lower extremity surgery in the past 1 year. The control group consisted of 17 participants, whereas the MKD group consisted of 14 participants; demographic information for both groups is located in Table 1.
| VARIABLE | CONTROL GROUP | MKD GROUP | P |
|---|---|---|---|
| Height (cm) | 166.1±31.1 | 167.2±9.3 | .90 |
| Mass (kg) | 69.6±15.4 | 65.9±9.9 | .45 |
| Age (years) | 23.8±5.8 | 22.4±3.1 | .40 |
| Male, female | 7, 10 | 2, 12 |
Experimental Procedures
Participants attended a single testing session lasting approximately 1 hour, and all participants enrolled in the study completed the testing protocol. Participants wore shorts and a t-shirt and their own shoes during testing. Prior to testing, participants were screened again to verify they still met the MKD criteria. Height (cm) and weight (kg) were measured and recorded, and participants warmed up by riding a stationary cycle for 5 minutes at a self-determined pace. The dominant limb was tested, which was defined as the leg used to kick a soccer ball for maximum distance.
Range of Motion
A manual 12-inch goniometer was used to measure peak joint angles in degrees for passive range of motion (ROM), and the measurement position was similar to previously reported methods.19 Three separate measurements were taken and the mean of these 3 trials was recorded. Moderate to very strong reliability20 was recorded for all ROM variables: hip abduction (intraclass correlation coefficient [ICC] [3,1] = 0.65, standard error of measurement [SEM] = 3.44°); hip external rotation (ICC[3,1] = 0.97, SEM = 2.43°); ankle dorsiflexion, knee straight (ICC[3,1] = 0.96, SEM = 1.59°); and ankle dorsiflexion, knee bent (ICC[3,1] = 0.95, SEM = 1.99°).
Participants were supine for all hip and ankle ROM assessments. For hip abduction, the axis of the goniometer was placed over the anterior superior iliac spine (ASIS), with the stationary arm placed along a line connecting the left and right ASIS and the movement arm placed along the anterior midline of the femur. The test leg was passively abducted until the contralateral ASIS move inferiorly, indicating that the muscles were acting on the pelvis. Hip abduction was tested to assess the tightness of the hip adductors. The reliability of this measurement was lower than all other variables. The plinth used for testing was a standard width (approximately 30 inches) and required some participants to hold the limb off the edge of the table while an investigator made a measurement. Effort was made to ensure that movement of the lower extremity was limited; however, unintended motion may have occurred, which may have influenced the reliability of this measurement. Future studies should perform this measure on the ground or on a platform table that is lower and wider to support the leg throughout the duration of the measurement.
For hip internal rotation, the dominant hip and knee were flexed to 90° with the hip in neutral rotation. The axis of the goniometer was the anterior center aspect of the patella, the stationary arm was positioned parallel to the table, and the movement arm was placed along the anterior midline of the tibia. The participant’s hip was passively externally rotated until end range was determined. We tested the motion of hip external rotation to assess the tightness of the hip internal rotators. For ankle dorsiflexion, the axis of the goniometer was placed over the distal lateral malleolus with the stationary arm positioned along the lateral midline of the fibula, and the movement arm was placed parallel to the lateral aspect of the fifth metatarsal. The ankle was passively dorsiflexed until the end range was felt, and measurements were taken with the knee straight and the knee bent. During the knee bent testing, a bolster was placed under the participant’s knee to maintain approximately 30° of knee flexion during testing.
Overhead Squat Task
Prior to performing the overhead squat, surface electromyographic (EMG) sensors were placed over the gluteus maximus and adductor group. Prior to electrode placement, sites were shaven and cleaned with alcohol. The gluteus maximus electrode was placed 20% of the distance between the spinous process of S2 and a point 10-cm distal to the greater trochanter.21 The adductor complex electrode was placed over the muscle belly at the midpoint of the femur.21 All electrode placements were confirmed with isometric muscle testing and checked for cross talk. An 8-channel DelSys Bagnoli EMG System (Boston, Massachusetts) was used to measure muscle activity. DE-2.1 single differential surface electrodes (DelSys) with a contact dimension of 1.0×0.1 cm and a contact spacing of 1.0 cm were placed over the muscle parallel to muscle fibers. A reference electrode was placed over the tibial tuberosity. This signal was amplified by a gain of 10,000 as it passed into the computer and was stored for later analysis.
Participants performed the overhead squat identical to the screening, except squat depth was standardized to 70° of knee flexion. Pilot testing demonstrated that 70° to 80° was a comfortable and natural squat depth for most individuals. A tripod was used to give feedback to participants when they achieved the proper squat depth by touching the posterior aspect of the upper thigh. Squat speed was standardized using a metronome at 66 beats per minute. Participants used 2 beats to descend, 2 beats to ascend, and 1 beat to rest between squats. An electrogoniometer (Biometrics, Ltd, Cwmfelinfach, UK) was used to track knee flexion angle and was used during data reduction.22 Five squats were performed in succession.
Isokinetic Strength Measurements
Concentric and eccentric muscle strength were evaluated using a Biodex System 3 Pro isokinetic dynamometer (Biodex Medical Systems, Shirley, New York), measured in foot-pounds (ft×lbs) of torque. All muscles were tested concentrically and eccentrically at 60° × sec−1. Participants were allowed 3 sub-maximal practice trials to become familiar with the task, followed by 5 maximal repetitions. Participants were allowed 2 minutes of rest between testing to reduce fatigue. The testing procedure accounted for gravity corrections during hip abduction and hip extension testing, given that the test limb was sufficiently close to the horizontal plane. The test limb did not come close enough to the horizontal plane to warrant a gravity correction during the testing of the hip rotators. The data were then analyzed with a customized software program (MatLab version 7.0; Mathworks Inc, Natick, Massachusetts) to determine the peak torque for the hip external rotators, internal rotators, hip abductors, and hip extensors. Peak torque was assessed using the middle 3 trials and normalized to body weight. Reliability and standard error for this isokinetic testing method has been previously demonstrated.23 Specific testing protocols are outlined below.
Hip rotator strength was tested in a seated position with the participant’s hip and knee flexed to 90°. The dynamometer was aligned with the long axis of the femur, and the thigh was stabilized to the chair using straps, with a towel placed between the participant’s knees to discourage femoral adduction. During external rotation, pressure was applied to the medial aspect of the distal tibia and rotation was tested through 20° of motion beginning at 5° of external rotation and ending at 15° of internal rotation. For internal rotation, pressure was applied to the lateral aspect of the distal tibia and the rotation was tested through 20° of motion that began at 5° of internal rotation and ended at 15° of external rotation.
Hip abductor strength was tested with the participant side-lying with the hip abducted and slightly externally rotated. The trunk and pelvis were not allowed to rotate backward and were stabilized by strapping the participant to the chair. Pressure was applied against the lateral thigh and strength was tested through 0° to 20° of hip abduction ROM. Hip extension strength was tested in a supported, standing position in an attempt to isolate the gluteus maximus and maximize stabilization. The dynamometer axis of rotation was aligned with the anterior superior tip of the greater trochanter.24 The participant stood in front of the Biodex chair, and the seat was raised to the level of the participant’s ASIS. If the chair was unable to reach the participant’s ASIS, the participant was then asked to flex the contralateral knee until the chair was even with the level of the ASIS. The trunk was flexed to 90° with the participant’s chest on the chair and stabilized to the Biodex chair prior to testing using straps. This stabilization prevented accessory trunk motions that might influence strength testing. The knee of the test leg was flexed to 90° to eliminate the contribution of the hamstring group and the stance leg (nontest leg) was flexed at the knee, allowing the participant’s chest to rest comfortably on the chair. The participant actively extended the hip through a ROM that began at 90° of hip flexion and ended at 50° of hip flexion. Pressure was applied against the distal portion of the posterior thigh in the direction of hip flexion.25
Maximal voluntary isometric testing (MVIC) was performed following isokinetic testing. The limb was positioned in neutral for adduction and midrange for hip extension (70° hip flexion). Contractions were held for 5 seconds and repeated 3 times with a 15-second rest period. The mean amplitude during the middle 3 seconds of each trial was used for EMG normalization.
Data Filtering and Reduction
Electromyography and electrogoniometer data were collected using Datapac2K2 (Run Technologies, Mission Viejo, California) and reduced using customized MatLab version 7.0 programs. The electrogoniometer signal was filtered using a low-pass Butterworth filter with a cutoff frequency of 15 Hz. The EMG signal was sampled at 1,000 Hz, corrected for DC bias, band-pass (20–350 Hz; 4th order, zero phase lag Butterworth) and notch (59.5–60.5 Hz) filtered, and smoothed using a 25-ms root-mean-square sliding window.26 All EMG data were normalized as a percentage of maximum voluntary isometric contraction to allow for comparison between participants. The squat was divided into phases based on knee flexion angle. The descent phase of the squat was defined as the initiation of motion to peak knee flexion angle based on the electrogoniometer reading, whereas the ascent phase of the squat was defined as the peak knee flexion angle to the return of the start position. Equipment error during data collection resulted in unusable EMG for 3 participants: 2 in the MKD group and 1 in the control group. Thus, after removing the EMG for these participants, 16 remained in the control group and 12 in the MKD group.
Statistical Analyses
Independent samples t tests were used to evaluate differences in groups for hip external rotation and hip abduction ROM. We used a group (2 levels: control versus MKD) by knee position (2 levels: knee straight versus knee bent) mixed model analysis of variance (ANOVA) to assess ROM differences in the gastrocnemius (knee straight) and soleus (knee bent). We also used group (2 levels) by contraction type (2 levels: concentric versus eccentric) mixed model ANOVAs to analyze differences between groups in concentric and eccentric peak torque for each direction tested. Finally, we used a group by phase (2 levels: ascending versus descending) mixed model ANOVA to determine differences in EMG levels. Tukey post hoc testing was used when necessary. Statistical significance was set a priori at P ⩽ .05 and SPSS version 16.0 software (SPSS Inc, Chicago, Illinois) was used for all analyses.
Results
Range of Motion
Means, standard deviations, P values, effect sizes, and 95% confidence intervals (mean 1 – mean 2 / pooled standard deviation) for the ROM variables are displayed in Table 2. Effect sizes (ES) were calculated using the equation: mean 1 – mean 2 / pooled standard deviation.27 No significant differences were observed for hip abduction (t[1,29] = .93) or external rotation (t[1,29] = –.10) ROM between groups. However, we did observe a group by knee position interaction for ankle dorsiflexion ROM (F[1,26] = 17.4, Tukey HSD = 1.63°). Post hoc testing revealed that the MKD group had less ankle dorsiflexion with the knee straight compared with the control group, indicating gastrocnemius tightness.
| VARIABLE | CONTROL GROUP | MKD GROUP | P | EFFECT SIZE |
|---|---|---|---|---|
| Hip abduction | 36.9±5.3 (34.5–39.2) | 35.3±3.9 (32.7–37.9) | .36 | .35 |
| Hip external rotation | 73.9±17.5 (67.1–80.8) | 74.4±7.0 (66.9–81.9) | .92 | .04 |
| Dorsiflexion, knee straight | 17.9±7.9 (13.5–22.2) | 13.4±7.8 (9.1–17.7)b | < .001 | .59 |
| Dorsiflexion, knee flexed | 19.8±8.4 (15.3–24.4) | 19.8±8.4 (15.3–24.5) | NA | NA |
Isokinetic Muscle Strength
Means, standard deviations, and P values for strength variables are presented in Table 3. No significant group by contraction interaction was observed for hip external rotation (F[1,29] = .006), internal rotation (F[1,29] = .07), extension (F[1,29] = .83), or abduction (F[1,29] = 1.92) peak strength. Similarly, we did not observe group main effects for peak torque for external rotation (control group: 0.460±0.185 ft×lbs/kg; MKD group: 0.414±0.234 ft×lbs/kg; F[1,29] = 1.03, P = .32, ES = .23), internal rotation (control group: 0.625±0.301 ft×lbs/kg; MKD group: 0.616±0.270 ft×lbs/kg; F[1,29] = .01, P =.91, ES = .03), extension (control group: 1.04±0.522 ft×lbs/kg; MKD group: 0.987±0.457 ft×lbs/kg; F[1,29] = .20, P = .66, ES = .11), or abduction (control group: 0.473±0.230 ft×lbs/kg; MKD group: 0.414±0.022 ft×lbs/kg; F[1,29] = .78, P = .38, ES = .36). The lack of a significant group by contraction interaction and group main effect indicates that there was no significant difference in strength between the MKD and control groups for all muscle groups evaluated.
| VARIABLE | CONTROL GROUP | MKD GROUP | INTERACTION FORPVALUE |
|---|---|---|---|
| Hip ER | |||
| Concentric | 0.315±0.03 (0.263–0.368) | 0.272±0.03 (0.214–0.329) | .94 |
| Eccentric | 0.605±0.05 (0.513–0.698) | 0.557±0.05 (0.455–0.659) | |
| Hip IR | |||
| Concentric | 0.444±0.05 (0.353–0.535) | 0.429±0.05 (0.327–0.528) | .79 |
| Eccentric | 0.806±0.06 (0.679–0.933) | 0.804±0.07 (0.664–0.944) | |
| Hip extension | |||
| Concentric | 0.806±0.09 (0.617–0.995) | 0.730±0.10 (0.522–0.938) | .83 |
| Eccentric | 1.28±0.12 (1.0–1.5) | 1.24±0.13 (0.982–1.51) | |
| Hip abduction | |||
| Concentric | 0.438±0.04 (0.358–0.518) | 0.327±0.04 (0.239–0.415) | .18 |
| Eccentric | 0.508±0.06 (0.383–0.633) | 0.501±0.07 (0.364–0.639) |
Muscle Activation Amplitude
No group by phase interactions were observed for the adductor (F[1,26] = 1.60) nor gluteus maximus (F[1,26] =.483). A main effect for group was observed for the adductor (MKD group: 26.94±13.32%MVIC; control group: 16.92±13.32%MVIC; F[1,26] = 6.13, P = .02, ES = .78) but not the gluteus maximus (MKD group: 13.64±8.50%MVIC; control group: 9.66±8.5%MVIC; F[1,26] = 3.25, P = .08, ES = .49) (Table 4).
| VARIABLE | CONTROL GROUP | MKD GROUP | GROUP X PHASEPVALUE |
|---|---|---|---|
| Gluteus maximus | |||
| Descending | 7.34±3.42 (4.85–9.83) | 10.78±6.30 (7.90–13.66) | .49 |
| Ascending | 11.98±4.54 (8.29–15.67) | 16.50±9.68 (12.24–20.76) | |
| Adductors | |||
| Descending | 16.09±8.39 (10.55–21.63) | 25.00±13.37 (18.60–31.40) | .22 |
| Ascending | 17.75±8.41 (12.15–23.35) | 28.87±13.57 (22.41–35.34) |
Discussion
Our investigation compared isokinetic concentric and eccentric strength, ROM, and muscle activation between groups that were created based on knee alignment during a double-leg squat. We used the double-leg squat task to identify those individuals who displayed MKD. Our hypotheses were partially supported, and the most important finding was that the MKD group had decreased ankle dorsiflexion and increased adductor activation levels compared with the control group.
Restricted ankle dorsiflexion ROM with the knee straight indicates that the gastrocnemius is tight and restricting ankle motion. Tightness in the gastrocnemius muscle may have played a role in MKD during the overhead squat. Normal ankle dorsiflexion ROM is approximately 10° during the stance phase of gait.28 During more functional tasks, such as sit-to-stand, stair climbing, and sport-specific activities, the requirement for ankle dorsiflexion ROM can increase to approximately 25°.29 Knee flexion angle ranges from approximately 64° to 101° during these activities.30
The double-leg squat used in our study is most likely related to the functional activities that require greater dorsiflexion. Although both groups appear to have limited ROM, the MKD group demonstrated approximately 25% less motion than did the control group (control group: 17.9°; MKD group: 13.4°). This finding was also associated with a moderate effect size (ES = .59).27 Thus, a lack of ankle motion seen in the MKD group could explain their inability to perform the task with proper alignment.
We theorized that increased frontal plane motion might be a compensatory mechanism for reduced sagittal plane motion. We controlled squat depth (70°) and required participants to keep their heels on the ground. Knee, hip, and ankle sagittal plane motion would be required for a participant to achieve the standardized squat depth. However, the MKD group had restricted ankle dorsiflexion ROM available, and medial knee displacement may be required as a compensatory mechanism for the participant to complete the squat task.
Previous research used a similar, visually based, grouping methodology and found that MKD is associated with restrictions in dorsiflexion flexibility.5,14 Bell et al14 studied individuals with excessive MKD during a double-leg squat and found that the MKD group demonstrated reduced plantarflexion strength (P = .007) and tended to have limited soleus ROM (P = .06). In contrast to our results, Bell et al14 observed that ankle ROM tended to be restricted by the soleus, rather than the gastrocnemius. The differences between these results are likely explained by the testing position. Bell et al14 used a foam roller placed under the distal lower leg to help maintain knee extension while measuring gastrocnemius flexibility, whereas we measured gastrocnemius length with the knee straight on a plinth. The lack of standardizing the starting position of the straight knee dorsiflexion measurement may have led to the difference in results of the 2 studies.
In further support of our results, Rabin and Kozol5 used a lateral step down and rated women based on their quality of movement. Participants were visually rated on a variety of variables, including the presence of dynamic knee valgus. Women with a moderate movement score had decreased ankle dorsiflexion ROM, compared with those rated as good. Although the demands are somewhat different, the lateral step down is similar to the double-leg squat. However, when combined with the results from this study, independent studies5,14 have now verified that individuals with visually identified movement impairments have tight posterior ankle musculature.
Our second primary finding was that the MKD group demonstrated increased adductor activity during the squat indicating a hip muscle imbalance. Theoretically, overactivity of the adductor in the MKD group may be the result of 2 possibilities: the adductor muscles could be pulling the femur into adduction, creating the MKD, or the participants may be using the adductor muscles to control hip extension during the squat. Of note, peak adductor isokinetic strength was not different between groups (main effect: P = .38), which indicates that hip strength may not be an issue but rather neuromuscular control may be an important factor in controlling knee position during the squat.
Women demonstrate significantly more adduction during single-leg squats31 and are more likely to sustain knee injuries that are associated with frontal plane knee motion, including patellofemoral pain32 and non-contact ACL injury.33 These results suggest hip musculature strength is not a causative factor in MKD. Bell et al14 also concluded that a group of individuals with clinically assessed MKD had adequate hip strength using a handheld dynamometer. In addition, Thijs et al34 found no correlation between hip muscle strength and a group of individuals that moved into dynamic knee valgus during a tennis lunge. Our results add to the current debate about the role that the hip muscle strength and neuromuscular control play in proximal control of the knee and knee injuries.35
We provided our participants standardized instructions and did not inform them about the MKD criteria during the screening process. We purposefully chose to not give them feedback or instructions to observe the preferred movement pattern selected by the participants.5 An individual who has never received proper squatting instruction may develop these muscle imbalances over time, which results in MKD. In theory, correcting these imbalances by lengthening tight muscles and inhibiting overactive muscles may help reduce MKD. Providing verbal instructions or augmented feedback has been shown to alter lower extremity biomechanics during higher demand tasks, such as landing.36 Therefore, providing feedback may also assist modifying MKD, but it is not clear whether this type of feedback would be a successful short- or long-term intervention for individuals with MKD and the double-leg squat.
Limitations
We did not verify foot type in this study, but it is possible that a tight gastrocnemius could pull the calcaneus into eversion and pronation.37 Increase pronation can encourage tibial internal rotation which may increase MKD.9 The gastrocnemius could affect MKD by acting on the knee after the arch flattens and pronation ceases. Therefore, more research is needed to determine whether the gastrocnemius muscle itself can cause knee MKD or it if only contributes to increased MKD when already present. We also did not control tibial position, as we wanted to keep our methodology as clinical relevant as possible and restricting tibial motion is not common in the clinical setting.
Another limitation to this study is that the vast majority of our participants were women. The inability to identify men who fit the inclusion criteria for the MKD group is similar to that reported by Bell et al.14 This makes sense given that women have been shown to be at greater risk of developing patellofemoral pain,38 increased risk for noncontact ACL injuries,7 and have been shown to have greater hip adduction.39 We were not attempting to perform a gender study; rather, we wanted to focus on the presence of MKD and therefore we included individuals in order as they qualified for our study. Future research should attempt to balance groups by gender or examine gender and MKD as separate variables.
We tried to be as inclusive as possible when studying the relationship between MKD and lower extremity musculature. However, the lack of comprehensive lower extremity EMG is also a limitation of this study. Although we found differences in ankle ROM, we did not assess ankle muscle activation or strength. Also, the quadriceps play a major role in squatting and, to date, no research has examined quadriceps strength or activation in people with MKD. Furthermore, exploration into the best predictors of MKD and development of strategies that will prevent MKD need to be further understood.
Implications for Clinical Practice
These results indicate that gastrocnemius muscle tightness and increased adductor activity may cause excessive MKD. The double-leg squat test is a time-efficient tool that clinicians can use to screen participants for dynamic postural alignment problems, such as MKD. Our most important findings were that the MKD group had decreased ankle dorsiflexion caused by the gastrocnemius muscle and increased adductor activation compared with the control group. We found no differences in hip strength between groups, indicating that neuromuscular control, and not strength, may be the most important factor in controlling knee motion during a squat. If a participant presents with MKD, an exercise program should be implemented that focuses on improving ankle dorsiflexion and decreasing hip adductor activity. The most common methods to implement these changes may be via static stretching, myofasical release, and therapeutic exercise with proper technique. However, the most effective means of achieving these goals still needs to be investigated in future research.
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