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The Effects of Lower Extremity Muscle Activation and Passive Range of Motion on Single Leg Squat Performance

Mauntel, Timothy C.; Begalle, Rebecca L.; Cram, Tyler R.; Frank, Barnett S.; Hirth, Christopher J.; Blackburn, Troy; Padua, Darin A.

The Journal of Strength & Conditioning Research: July 2013 - Volume 27 - Issue 7 - p 1813–1823
doi: 10.1519/JSC.0b013e318276b886
Original Research

Mauntel, TC, Begalle, RL, Cram, TR, Frank, BS, Hirth, CJ, Blackburn, T, and Padua, DA. The effects of lower extremity muscle activation and passive range of motion on single leg squat performance. J Strength Cond Res 27(7): 1813–1823, 2013—Knee valgus is a potential risk factor for lower extremity (LE) injuries. Clinical movement screenings and passive range of motion (PROM) measurements may help identify neuromuscular patterns, which contribute to knee valgus. The purpose of this study was to compare LE muscle activation and PROM between subjects who display visual medial knee displacement (MKD) during a single leg squat (SLS) and those who do not. We hypothesized that muscular activation and PROM would differ between the groups. Forty physically active adults (20 controls, 20 MKDs) participated in this study. Subjects completed 10 LE PROM assessments and performed 5 SLS trials while electromyography (EMG) data were collected from 8 LE muscles. Three separate multivariate analysis of variance were used to identify group differences in EMG data, muscle coactivation, and PROM. Results during the SLS indicated hip coactivation ratios revealed smaller gluteus medius to hip adductor (GMed:Hip Add) (p = 0.028) and gluteus maximus to hip adductor (GMax:Hip Add) coactivation ratios (p = 0.007) compared with the control group. Also, the MKD group displayed significantly less passive ankle dorsiflexion with the knee extended (p = 0.047) and flexed (p = 0.034), and greater talar glide motion (p = 0.012). The findings of this study indicate that MKD during a SLS seems to be influenced by decreased coactivation of the gluteal to the hip adductor muscles and restricted dorsiflexion. Therefore, conditioning, rehabilitation, and injury prevention programs should focus on decreasing hip adductor activity, increasing hip abductor and external rotator activity, and increasing ankle dorsiflexion in hopes to decrease the incidence of these injuries.

Sports Medicine Research Laboratory, Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

Address correspondence to Darin A. Padua, dpadua@email.unc.edu.

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Introduction

Noncontact knee injuries are common among physically active populations. In particular, physically active people are susceptible to injury of the anterior cruciate ligament (ACL) (22,27,40), medial collateral ligament (MCL) (19,23,48), and to developing patellofemoral pain syndrome (PFPS) (13,45). Malalignment of the lower extremity (LE) during functional activities is associated with a greater risk of noncontact knee injury (4,21,22,26,30). One of the most common predisposing factors for noncontact knee injuries is greater knee valgus motion (20,29,30,33). During clinical evaluation, it is difficult to distinguish a true valgus episode in which the tibia is abducted in the frontal plane and an episode where valgus results from a combination of hip abduction and internal rotation of the femur and tibia, as this distinction requires 3-dimensional (3D) motion analysis (17,42).

A number of clinically based LE screening tools have been developed for identifying injury risk factors associated with these injuries (5,10,15,28,31,37,38,42,49). The development of LE clinical screening assessments is vital to aid in decreasing the amount of noncontact knee injuries that affect a large portion of physically active people. These screening assessments must be cost and time effective, require minimal space to complete, mimic LE functional tasks, and be well researched and understood. Thorough research of these assessments will allow clinicians to have a better understanding of what the assessment is telling them and in turn how to better develop rehabilitation and treatment plans.

The single leg squat (SLS) has been used to identify faulty LE biomechanics (6), primarily through the observation of medial knee displacement (MKD). Greater MKD during the SLS has been attributed to poor hip muscle strength (15,50,51), but recent studies have shown little to no correlation between the two (6,15). Therefore, other factors such as muscular activation and passive range of motion (PROM) may play an important role in dynamic valgus.

A number of past studies have examined muscular activity during functional tasks through the use of surface electromyography (EMG). One such study suggests lesser gluteus medius activation has been associated with greater knee valgus movement during a SLS (51). Additionally, Palmieri-Smith et al. (43) reported greater peak knee valgus angles during a single-leg forward-jump task in individuals with greater lateral quadriceps and hamstrings activity and lesser peak knee valgus angles in those with greater medial quadriceps activation. The medial gastrocnemius reportedly resists knee valgus moment (35), thus lesser medial gastrocnemius activity may result in greater knee valgus.

Greater PROM of the hamstrings resulting in lesser activation has been suggested to decrease dynamic knee stiffness (4,51) and increase knee valgus. A tight iliotibial band and hip flexors may cause the pelvis to tilt anteriorly, resulting in femoral internal rotation (46). Tightness of the hip adductor and internal rotator muscles and greater PROM of the hip external rotators may allow for greater hip internal rotation and knee valgus during dynamic tasks (7,31). Similarly, tightness of the lateral gastrocnemius and peroneals, may contribute to tibial abduction and external rotation, thus increasing knee valgus (7,31). Additionally, limited posterior talar mobility may limit ankle dorsiflexion (11). This is important to consider because decreased ankle dorsiflexion has been associated with increased MKD during functional tasks (8).

Additional research continues to be needed in the examination of the SLS as a LE functional clinical assessment. The SLS has yet to be validated as a screening tool for identifying noncontact knee injury risk factors (49). Also, if validated, additional research is needed to identify which risk factors can be observed through the SLS. More importantly, further research is needed to identify what the underlying causes of these injury risk factors are because these underlying causes are what should be addressed through individualized rehabilitation programs.

The authors of this study are unaware of any previous researchers who have evaluated the relationships between LE muscle activity and PROM during the SLS. Therefore, the purpose of this study was to compare the LE muscle activation and PROM between subjects who display dynamic knee valgus and those who do not during an SLS. The authors hypothesized that individuals who displayed MKD during the SLS would demonstrate greater LE muscle activity and lesser PROM compared with those who maintained a neutral knee alignment.

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Methods

Experimental Approach to the Problem

Before data collection, all subjects underwent a screening session, which allowed the researchers to determine each subject's group assignment (control or MKD). During the data collection session, LE PROM was measured for each subject. Joint angles for PROM were measured using a digital inclinometer or standard goniometer. Surface EMG was used to record LE muscle activity for each of the muscles examined in this study during 5 SLS trials.

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Subjects

Forty individuals (20 males and 20 females) were selected through a screening process of a larger group of 63 subjects who volunteered to participate in this study. Each subject was assigned to either the “control” group or “medial knee displacement” group based on his/her performance of the SLS test. Each group was composed of 20 subjects (10 males and 10 females) between the ages of 18 and 35 years, recruited from the general student, staff, and faculty population at the University of North Carolina at Chapel Hill. No significant differences were observed between the control (height = 173.1 ± 10.1 cm, mass = 71.0 ± 14.6 kg, and age = 20.2 ± 1.5 years) and MKD (height = 173.8 ± 8.8 cm, mass = 71.8 ± 14.7 kg, and age = 20.2 ± 1.8 years) groups.

All subjects were self-reported to be in good physical condition and physically active, defined as consistent participation in at least 30 minutes of physical activity of their own choosing, 3 times per week for at least 6 months. On average, subjects reported 276.47 ± 188.5 minutes of physical activity per week, including running, elliptical, weight training, aerobics, and other such activities. There was no significant difference observed between the amount of participation in physical activity per week between the two study groups (control = 273.8 ± 190.2 minutes, MKD = 246.0 ± 170.3 minutes); therefore, the authors felt comfortable that physical activity was not a confounding variable.

Subjects were excluded if they had any history of a surgical procedure to the LE or low back and/or reported an injury to the LE or low back within the 6 months before participation that resulted in an inability to participate in physical activity for 3 consecutive days. Persons with a known neurologic condition resulting in decreased balance and/or proprioception and knowingly pregnant females were also excluded. Twenty-three subjects were excluded from the study after undergoing the screening process for one of the following reasons: (a) they demonstrated a knee varus position during their SLS screening procedure, (b) the positioning of their knee during the SLS trials was too inconsistent and did not allow them to be clearly placed into one of the two study groups, or (c) when the subject returned for the testing session and he/she no longer fit into his/her originally assigned group based on his/her performance during the screening session. These subjects were excluded from the study, to not confound the study results. Before data collection, this study underwent review and was approved by the institutional review board of the University of North Carolina at Chapel Hill; each subject was informed of the risks of participating in this study and signed an informed consent form.

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Procedures

Screening Session

After a 5-minute warm-up on a stationary cycle ergometer at a self-selected pace, each subject underwent a screening protocol to determine group assignment. Height (centimeters), mass (kilogram), and leg dominance (the leg that would be used to kick a soccer ball for maximal distance) were recorded. The screening protocol consisted of 5 consecutive SLS trials to a preset depth of 60° angle of knee flexion (mechanical block set to touch gluteus maximus) while being visually observed by one of the researchers. Subjects were instructed to stand on their dominant leg, with their toes facing forward, their non–weight-bearing leg flexed at the knee to 90° and 45° angles at the hip, their hands placed on the hips, and their head and eyes facing forward. To control their movement velocity, subjects were instructed to descend for 2 beats of the metronome (set at 60 beats per minute), until their gluteals touched the mechanical block, then to return to the starting position in 2 beats.

Subjects were placed in the control group if in at least 3 of 5 trials the knee remained in line with the hip and ankle joints throughout the SLS (Figure 1). Subjects were placed in the MKD group if in at least 3 of 5 trials the midpoint of the patella moved medial to the great toe during the SLS (Figure 2). Subjects were not informed of what group they were placed in, to avoid possibly influencing their performance on future SLS trials during the testing session. Selected study subjects went through the following testing procedures at a later date.

Figure 1

Figure 1

Figure 2

Figure 2

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Lower Extremity Passive Range of Motion

Lower extremity PROM was measured for each subject in a counterbalanced order. Joint angles for PROM of the hip external rotators, hip internal rotators, hamstrings, iliotibial band, iliopsoas, and femoral anteversion were measured using a digital inclinometer (Saunders Group, Inc. Chaska, MN, USA), whereas those for the hip adductors and plantar flexors were measured using a standard goniometer. Intrasession and intrarater reliability for the primary investigator on each PROM testing procedure were calculated with intraclass coefficients (ICCs) and standard errors of the measurement (SEMs) for each range of motion measurement (ICC3,1 range, 0.64–0.93; SEM range, 0.68°–7.45° angle) (Table 1). The investigator passively moved the associated joint through its range of motion from neutral position to the point of first resistance, or until the subject vocalized discomfort. Three trials were recorded for each PROM measurement and averaged for statistical analysis. All PROM testing procedures are described further in Table 2 (18,24,36,39,44). For the posterior talar glide measurement, the subject was seated with the legs off the end of a table so that the knees were in a flexed position. One researcher established subtalar neutral position then applied a posteriorly directed force to the talus until a capsular end-feel was detected. Once this point was reached, a second researcher measured the angle of the tibia, with respect to horizontal, with a digital inclinometer placed perpendicular to the length of the tibia (24). The researcher assessing PROM was not blinded to each subject's group assignment, as it would be clear during data collection to which group the subject had been placed in regardless of blinding during the screening session.

Table 1

Table 1

Table 2

Table 2

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Single Leg Squat Task

The subjects then completed the same procedure for the SLS task as they did during the screening session. The subject was instructed as to what constituted a successful trial, specifically, the subject: (a) maintained proper testing position throughout the entire motion, (b) squatted until the gluteals came in contact with the mechanical block, (c) completed the task at the appropriate rate, (d) did not touch down with the nondominant foot, (e) did not touch the legs together, (f) maintained the heel in contact with the ground, and (g) the task was completed in a fluid motion. The subjects were given as many practice trials as needed to perform the task successfully. No additional feedback, coaching, or other instructions were provided to the subject concerning technique because capturing the subject's true movement patterns was vital to this study. EMG and kinematic data were collected simultaneously for 5 successful SLS trials with 1-minute rest given between trials.

A Motion Star (Ascension Technologies, Inc., Burlington, VT, USA) electromagnetic motion tracking system was used to track 3D LE kinematics. These data were used to identify the start position, the point of greatest knee flexion, and the end position of the SLS. Electromagnetic sensors were placed over the sacrum, lateral aspect of the thigh, and the anteromedial aspect of the proximal tibia and were secured with double-sided tape, prewrap, and athletic tape. A segment-linkage model of the pelvis and dominant LE was generated by digitizing the left and right anterior-superior iliac spines, medial and lateral femoral epicondyles, and medial and lateral malleoli. The location of the hip joint center was approximated using the Bell method (2). Joint centers for the knee and hip were defined as the midpoints of the medial and lateral femoral epicondyles and malleoli, respectively. Three-dimensional coordinate data were collected at a sampling rate of 100 Hz.

A surface EMG system (Bagnoli-8; Delsys, Inc., Boston, MA, USA) was used to record LE muscle activity of the gluteus maximus, gluteus medius, hip adductors, medial hamstrings, biceps femoris, vastus medialis, vastus lateralis, and medial gastrocnemius. Before electrode application, the same researcher identified and marked each electrode site with a felt-tip marker. Electrode sites were shaved, abraded, and cleaned with isopropyl alcohol to reduce skin impedance. Electrode placement was confirmed with manual muscle testing and observation of the muscle activity on an oscilloscope. A maximal voluntary isometric contraction (MVIC) was then performed for each muscle group (hip extensors, hip abductors, hip adductors, hamstrings, quadriceps, and plantar flexors) as previously described in the literature (1,34) to provide EMG normalization criteria. A handheld dynamometer (CSD300; Chatillon, Largo, FL, USA) was used to quantify the peak force for each trial, recorded in Newtons. Three 5-second MVICs were performed for each muscle with 1-minute rest between trials. All EMG data were collected at 1,000 Hz.

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Data Processing and Reduction

Kinematic data were filtered using a fourth-order low-pass Butterworth filter at a cutoff frequency of 14.5 Hz. EMG data were passively demeaned, bandpass (10–350 Hz), and notch (59.5–60.5 Hz) filtered, and smoothed using a 25-ms root-mean-square sliding window function.

EMG and kinematic data were evaluated during the descent phase of the SLS, as defined as the time from the initiation of movement to peak knee flexion. Three-dimensional joint angles were identified at the start of the trial when the subject was in the initial position, at the point when the subject reached peak knee flexion, and once the subject returned to the initial position. Mean EMG amplitude during the descent phase of the SLS was normalized to MVIC activation averaged over the largest 1-second mean interval during each MVIC trial averaged across the 3 trials. All MVIC trials were normalized to each subject's weight, by dividing the subject's MVIC value by his/her weight. Muscle coactivation ratios were calculated for gluteus medius activation and hip adductor muscle activation by dividing the mean gluteus medius activity by the mean hip adductor activity (GMed:Hip Add). In addition, coactivation ratios were calculated for gluteus maximus activity and hip adductor muscle activation by dividing the mean gluteus maximus activity by the mean hip adductor activation (GMax:Hip Add). A ratio resulting in 1.0 would indicate completely balanced muscular activation; ratios resulting in values greater than 1.0 indicate greater activation of the muscle in the numerator (GMed and GMax) compared with the muscle in the denominator (Hip Add).

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Statistical Analyses

EMG and kinematic data were averaged over the 5 trials of the SLS, and PROM and MVIC data were averaged from the 3 trials of each measurement for use in statistical analyses. Three separate multivariate analysis of variance (MANOVAs) were used to compare peak EMG amplitude, EMG coactivation ratios, and PROM between the MKD and control groups. One-way ANOVA was used to evaluate significant MANOVAs post hoc. Six separate independent-samples t-tests were run to compare the normalized MVIC means for each muscle group between study groups. Additionally, 4 separate independent-samples t-tests were used to compare the means for the averaged height, weight, age, and amount of time participating in physical activity per week measures between the control and MKD groups. Statistical significance was set a priori at α < 0.05.

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Results

All MANOVA statistical analyses examining group differences for EMG amplitude, EMG coactivation ratios, and PROM measurements met the assumptions for Homogeneity of Covariance. For statistically significant MANOVAs, follow-up one-way ANOVA analyses were run to examine the main effect for group for the EMG coactivation ratios and PROM measurements; all follow-up ANOVAs met the assumptions for Homogeneity of Variance.

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Subject Descriptive Statistics

No significant differences were observed between height (T 38 = −0.250, p = 0.804), weight (T 38 = −0.184, p = 0.855), age (T 38 = 0.000, p = 1.00), or minutes of physical activity per week (T 38 = 0.486, p = 0.630), between the control and MKD groups.

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Maximal Voluntary Isometric Contraction

Means, SDs, and 95% confidence intervals for all normalized MVIC values are presented in Table 3. No significant differences were observed between groups for any of the normalized values for the 6 muscle groups tested with MVIC trials: hip extensors (T 38 = −0.840, p = 0.460), hip abductors (T 38 = −0.897, p = 0.376), hip adductors (T 38 = −0.885, p = 0.382), hamstring group (T 38 = 0.172, p = 0.865), quadriceps group (T 38 = 0.733, p = 0.468), and plantar flexors (T 38 = −0.718, p = 0.477).

Table 3

Table 3

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EMG Amplitude

Means, SDs, and 95% confidence intervals for all EMG amplitude measures are presented in Table 4. No significant differences were observed between the normalized EMG activation of the 2 groups (Wilk Lambda = 0.742, F 10,29 = 1.301, p = 0.280, η2 = 0.49). While not significant, it is worth noting that the group difference in hip adductor muscle activation trended toward significance (F 1,38 = 3.059, p = 0.089). Specifically, individuals in the MKD group tended to display greater hip adductor EMG activation (MKD = 20.1 ± 14.0, control = 13.3±9.8). One MKD subject's hamstring EMG value was identified as a statistical outlier, thus this subject was excluded from statistical analyses.

Table 4

Table 4

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EMG Coactivation Ratios

Means, SDs, and 95% confidence intervals for calculated coactivation ratios are presented in Table 5. A significant multivariate main effect for group was observed for the coactivation ratios (Wilks' Lambda = 0.822, F 10,29 = 4.003, p = 0.027, η2 = 0.68). Follow-up one-way ANOVAs revealed significant differences for the gluteus medius:hip adductor ratio (F 1,38 = 5.187, p = 0.028) and the gluteus maximus:hip adductor ratio (F 1,38 = 8.201, p = 0.007). Specifically, individuals in the MKD group displayed significantly lower gluteus medius:hip adductor (2.4 ± 1.1 vs. 4.5 ± 3.9) and gluteus maximus:hip adductor (1.1 ± 0.62 vs. 2.4 ± 1.8) ratios compared with the control group, indicating greater hip adductor EMG activation compared with gluteal EMG activation in the MKD group when compared with the control group.

Table 5

Table 5

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PROM Measurements

Means, SDs, and 95% confidence intervals for all PROM measures are presented in Table 6. A significant multivariate main effect for group was observed for PROM measurements (Wilks' Lambda = 0.555, F 10,29 = 2.321, p = 0.038, η2 = 0.83). Follow up one-way ANOVAs revealed significant group differences in dorsiflexion range of motion with the knee extended (F 1,38 = 4.203, p = 0.047), dorsiflexion range of motion with the knee flexed (F 1,38 = 4.857, p = 0.034), and posterior talar glide (F 1,38 = 7.040, p = 0.012). Specifically, the MKD group displayed significantly less dorsiflexion PROM with the knee extended (5.5 ± 5.4 vs. 8.8 ± 4.7) and flexed (9.5 ± 6.2 vs. 14.2 ± 7.3), and greater posterior talar glide (29.8 ± 4.8 vs. 25.7 ± 5.0) in comparison to the control group. No other significant differences were observed.

Table 6

Table 6

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Power and Effect Size

Observed power (range, 0.050–0.797) and effect size (range, 0.01–1.07) for each PROM and EMG measure were calculated and are presented in Table 7. This information will be discussed to describe the clinical significance of these findings.

Table 7

Table 7

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Discussion

To the authors' knowledge, this is the first study to identify differences in LE muscle activation and PROM between individuals presenting with MKD and those who do not during a SLS. In summary, the findings of this study revealed that the MKD group had lesser coactivation ratios compared with the control group, indicating that these individuals use a more hip adductor dominant activation strategy compared with control subjects. This study also found the MKD group displayed less dorsiflexion PROM and greater posterior movement of the talus compared with the control group. However, no other PROM measurements or any isometric strength measures were different between groups. These combined results suggest the combination of altered hip adductor and gluteal musculature coactivation and less ankle dorsiflexion ROM may contribute to dynamic knee valgus during a SLS.

No statistically significant differences in LE EMG activation were observed between groups. Previous research comparing muscle activation between sexes during SLS has identified greater activation of the rectus femoris (51) and lesser activation of the gluteus medius (25) in females, both of which are believed to contribute to increased risk of noncontact ACL injury. Padua et al. (41) reported 34% greater hip adductor activity in individuals displaying MKD during a double leg squat task compared with a control group that did not. Similar to this finding, the MKD group in the present investigation demonstrated 34% greater hip adductor activity during the SLS (not statistically significant; p = 0.089). In addition, there were no differences found between groups for the activation of the gluteus medius and gluteus maximus muscles, also similar to Padua et al. (41,47).

The relative coactivation between the gluteus medius and gluteus maximus with the hip adductors may contribute to MKD. Greater hip adductor activity that is not balanced by gluteal activity may permit greater hip adduction and internal rotation (41). This notion is supported by our finding that the GMed:Hip Add and GMax:Hip Add coactivation ratios in the MKD group were smaller than those in the control group. The coactivation ratio of GMed:Hip Add for the MKD group was 2.4 while the control group's ratio was 4.5 (effect size = 0.84); similarly, the ratios for the MKD and control groups for the GMax:Hip Add coactivation ratio were 1.1 and 2.4, respectively (effect size = 1.07). Larger coactivation ratios indicate that the GMed or GMax were more active relative to the Hip Add. Conversely, smaller coactivation ratios indicate greater reliance on the Hip Add muscles. The findings of this study indicate the MKD group placed greater reliance on their Hip Add musculature compared with the control group. It is generally thought that MKD may be caused by insufficient gluteal muscle strength (6) or activation (25). The current findings may help refine this theory and suggest that MKD may be caused by greater reliance on the Hip Add muscles rather than inadequate strength or activation of the gluteal musculature. The authors of this study believe that increased Hip Add muscles relative to GMed and GMax activation played a role in facilitating visual MKD during the SLS.

Lesser ankle dorsiflexion ROM in the MKD group supports our hypotheses. Based on these findings the authors believe decreased dorsiflexion ROM may be a large contributor to MKD during functional tasks. The MKD group was observed to have 37.5% and 33.1% less dorsiflexion ROM with the knee extended (effect size = 0.65) and flexed (effect size = 0.70), respectively. Similar differences in PROM have been observed between groups displaying MKD and a neutral knee position during a double leg squat (3,47). Limited dorsiflexion has been proposed to contribute to excessive rearfoot pronation and, in turn, result in compensatory increases in LE internal rotation (14), which may contribute to dynamic knee valgus (32). Similarly, Cortes et al. (8) reported that subjects had significantly greater knee valgus angle at initial contact and decreased dorsiflexion motion after landing when they performed rear-foot landings compared with self-preferred landing styles (8). Based on these combined findings it seems that restricted ankle dorsiflexion ROM may be an important factor contributing to MKD across a variety of functional tasks.

Decreased dorsiflexion PROM may be because of decreased extensibility of the gastrocnemius/soleus complex and/or restricted posterior talar glide on the tibia (11). Posterior glide of the talus within the talocrural joint is a necessary accessory motion to permit full ankle dorsiflexion PROM. The authors originally hypothesized decreased ankle dorsiflexion PROM in the MKD group may be because of restricted posterior talar glide motion. However, this hypothesis was not supported as MKD subjects demonstrated greater posterior talar glide compared with the control subjects (effect size = 0.84). This finding suggests that decreased ankle dorsiflexion ROM in the MKD group was most likely because of decreased extensibility of the gastrocnemius/soleus complex and not restricted posterior talar glide. However, the sensitivity of the posterior talar glide test has been questioned based on research demonstrating weak associations between the posterior talar glide test with open and closed kinetic chain measures of ankle dorsiflexion ROM (9). Thus, future research is needed to better understand the underlying mechanism contributing to decreased dorsiflexion PROM in the MKD group.

Each subject stood so that the foot was fixed on the ground with the toes pointing forward, and we assured the heel remained in contact with the ground throughout each trial. As the subject lowered the body to the required 60° angle of knee flexion, the ankle was required to move through dorsiflexion. Limited dorsiflexion ROM would inhibit the tibia from moving forward over the foot and may have caused MKD subjects to compensate for this lack of motion. The authors speculate that MKD subjects compensated for a lack of sagittal plane ankle motion by increasing frontal and/or transverse plane motion at the foot and up through the kinetic chain. Individuals may have compensated by going into more pronation of the foot, eversion of the talus, and internal rotation of the tibia (14) thus creating the visual appearance of MKD. Future research investigating the 3D kinematics of the foot and lower leg is needed to better understand if these compensatory motions actually do occur in those individuals displaying MKD.

There were no group differences observed in hip or knee PROM measurements, but the MKD group displayed limited ankle dorsiflexion ROM. The authors, therefore, propose that the imbalance that was observed in Hip Add to gluteal activation stems from a neuromuscular compensation as a result of the limited dorsiflexion ROM. Limited dorsiflexion PROM creates an abnormal axis of rotation of the tibia on the talus, resulting from altered arthokinematics which limit roll and glide between the joint surfaces. This abnormal rotation applies abnormal stresses on the tissues that have been suggested to produce altered proprioceptive input, which in turn causes the motor control system to adapt (12). The authors propose one such altered motor control response is using more frontal and transverse plane motion when sagittal plane motion is restricted, resulting in the leg being pulled actively inward. One possible neuromuscular mechanism used to achieve this is altering the adductor and gluteal coactivation ratios.

In conclusion, the findings of this study indicate that dorsiflexion range of motion measurements are lesser in subjects displaying MKD compared with those who do not. The authors believe this limited dorsiflexion may result in compensatory movements in the ankle and lower leg, resulting in foot pronation and tibial internal rotation. Greater levels of hip adductor activity without an associated increase in gluteus medius and/or gluteus maximus activity may increase femoral adduction and internal rotation; potentially increasing MKD during dynamic tasks (41). MKD is suggested to be a biomechanical factor associated with ACL injury, MCL injury, and PFPS. Rehabilitation and injury prevention programs that increase dorsiflexion range of motion, decrease hip adductor activity, and increase hip abductor and external rotator activity may decrease the incidence of these injuries.

The following limitations should be considered when interpreting the findings of this study. First, these findings are limited to a single-leg squatting task. Future research should look at whether findings carry over to when individuals perform more challenging dynamic tasks (i.e., jump-landing or cutting maneuvers). Also, these findings are limited to healthy physically active individuals who display visual MKD during an SLS and those who did not, thus they may not be applicable to injured populations or the population(s) at the greatest risk of LE injury. However, our population was similar to other populations in the literature (3,16,41,47). The subjects self-reported that they were healthy and physically active; the researchers had no means of checking the validity of these statements. Also, the researchers did not control for what physical activity, if any, was completed by the subject immediately preceding screening and testing sessions. Similarly, the time of day, the nutrition, and the hydration levels of the subjects were not controlled for during data collection sessions. These variables are important when assessing muscle function and could be additional limitations to this study.

Inherent limitations exist with the use of surface EMG. The assumption was made based on previous literature that EMG signal amplitudes represent levels of muscle activity. Cross talk may occur with the placement of the EMG surface electrodes on the skin and may not give a true reading of the underlying muscle activity. However, the researchers minimized the potential for error by using standard methods of applying the electrodes, sufficiently securing the electrodes to prevent movement, and checking the output of the electrodes before data collection to ensure proper placement. Finally, interpretation of the results was based on EMG signals normalized to maximal isometric voluntary activity (MVIC). Another assumption was made that all participants gave their maximal effort during the MVIC measurements and during the SLS; this would affect the normalized percentages used during the statistical analyses. In addition, other LE muscles not investigated in this study could be involved in dynamic control during a SLS.

Another potential limitation of this study is that the researcher who completed the measures of PROM was not blinded to the subject's group assignment. Also these measurements required the researcher to subjectively identify the point of first resistance felt in the muscle as the limb was moved through its PROM. Similarly, the measure of posterior talar glide was dependent on the researcher's ability to subjectively determine subtalar neutral position and the end feel/restriction in motion as the knee was moved into flexion. However, the researcher responsible for all PROM measurements established himself to have good reliability and precision with this measure (ICC = 0.93, SEM = 1.2° angle). Therefore, the authors do not believe these limitations were meaningful sources of error in the data. Future research should look at a more sensitive measure of quantifying restricted posterior talar glide as a possible factor limiting dorsiflexion ROM. Use of an ankle arthrometer to quantify posterior talar displacement and stiffness has been described in previous literature and may be a good tool for future research investigating factors associated with MKD (9).

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Practical Applications

Clinical screenings may be used by clinicians to help identify athletes at increased risk for LE injuries. These screening tools may also help identify which underlying factors contribute to the faulty movement patterns exhibited during the screening. The information gained from clinical screenings should then be used by the clinician to aid in the development of conditioning, rehabilitation, and injury prevention programs.

Specifically, the findings of this study suggest smaller Gluteal:Hip Add coactivation ratios and lesser dorsiflexion PROM in individuals displaying MKD may have important implications for knee injury prevention and rehabilitation programs. Individuals displaying smaller Gluteal:Hip Add coactivation ratios may benefit from rehabilitation exercises focused on increasing gluteus medius and gluteus maximus activation. These individuals may also benefit from inhibiting the hip adductors through stretching and inhibition techniques, such as self-myofascial release, which allow the muscle to relax to enhance stretching (31). Similar treatments could be used to increase gastrocnemius and soleus PROM and allow for greater ankle dorsiflexion. Ultimately, clinical screenings may aid in the development of better individualized prevention and treatment plans and decrease the incidence of noncontact knee injuries.

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Keywords:

knee valgus; dorsi flexion; hip adductor; gluteus medius; gluteus maximus

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