Abstract
Background
The single leg squat (SLS) is a functional task used by practitioners to evaluate and treat multiple pathologies of the lower extremity. Variations of the SLS may have different neuromuscular and biomechanical demands. The effect of altering the non-stance leg position during the SLS on trunk, pelvic, and lower extremity mechanics has not been reported.
Purpose
The purpose of this study was to compare trunk, pelvic, hip, knee, and ankle kinematics and hip, knee, and ankle kinetics of three variations of the SLS using different non-stance leg positions: SLS-Front, SLS-Middle, and SLS-Back.
Methods
Sixteen healthy women performed the three SLS tasks while data were collected using a motion capture system and force plates. Joint mechanics in the sagittal, frontal, and transverse planes were compared for the SLS tasks using a separate repeated-measures analysis of variance (ANOVA) for each variable at two analysis points: peak knee flexion (PKF) and 60 ° of knee flexion (60KF).
Results
Different non-stance leg positions during the SLS resulted in distinct movement patterns and moments at the trunk, pelvis, and lower extremity. At PKF, SLS-Back exhibited the greatest kinematic differences (p < 0.05) from SLS-Front and SLS-Middle with greater ipsilateral trunk flexion, pelvic anterior tilt and drop, hip flexion and adduction, and external rotation as well as less knee flexion and abduction. SLS-Back also showed the greatest kinetic differences (p < 0.05) from SLS-Front and SLS-Middle with greater hip external rotator moment and knee extensor moment as well as less hip extensor moment and knee adductor moment at PKF. At 60KF, the findings were similar except at the knee.
Conclusion
The mechanics of the trunk, pelvis, and lower extremity during the SLS were affected by the position of the non-stance leg in healthy females. Practitioners can use these findings to distinguish between SLS variations and to select the appropriate SLS for assessment and rehabilitation.
Level of Evidence
3
Keywords: Females, kinematics, kinetics, lower extremity, single limb squat
INTRODUCTION
An estimated 11,000 persons are treated in emergency departments for injuries related to sports, recreation, and exercise activities each day.1 Furthermore, non-contact lower extremity injuries2,3 comprise the majority of the reported incidents. Although the etiologies of many non-contact lower extremity injuries are likely multifactorial, atypical movement patterns and poor neuromuscular control are likely contributors. Identifying and addressing these modifiable factors may reduce the risk of lower extremity injuries.
Functional screening is one strategy used to identify risk factors for lower extremity injuries. It uses dynamic tasks to assess balance, stability, coordinated movement quality, and dynamic alignment throughout the body.2 A common movement task used for assessment4,5 and intervention6,7 by clinicians is the single leg squat (SLS). The SLS can be used to examine lower extremity alignment and may be helpful in identifying faulty movement patterns of the trunk, pelvis, and lower extremity. Prior studies examining the SLS have demonstrated that there are biomechanical differences between healthy individuals and those with lower extremity injuries, such as patellofemoral pain8-10 (PFP), anterior cruciate ligament (ACL) injuries,11 and hip chondropathy.12 For example, Nakagawa et al.8 found that individuals with PFP performed the SLS with greater ipsilateral trunk lean, contralateral pelvic drop, hip adduction, and knee abduction than those without PFP.
The SLS is performed in a variety of ways in clinical, field, and research settings. Generally, participants are instructed to stand on one leg, squat down as far as possible6,13 or to an approximate degree of knee flexion,7,8 and return to the initial position. The position of the non-stance leg is often not specified.6 Altering the non-stance leg position during the SLS may tax the neuromuscular system differently and result in different movement patterns. It is unknown how changing the non-stance leg position during the SLS influences the mechanics of the trunk, pelvis, and lower extremity. A better understanding of how the position of the non-stance leg affects trunk, pelvic, and stance leg mechanics during the SLS may help clinicians modify the SLS to best match their desired task demands.
The purpose of this study was to compare trunk, pelvic, hip, knee, and ankle kinematics and hip, knee, and ankle kinetics of three variations of the SLS using different non-stance leg positions. It was hypothesized that altering the position of the non-stance leg during the SLS would result in different kinematics and kinetics of the trunk, pelvis, and stance lower extremity.
METHODS
This study used a within-subjects, repeated-measures design in a laboratory setting to examine how changing the position of the non-stance leg affects how the SLS is performed. Trunk, pelvic, and lower extremity kinematics and kinetics in the sagittal, frontal, and transverse planes of three common variations of the SLS were examined. The position of the non-stance leg differed between the three SLS tasks. Data were recorded using a 3-dimensional motion capture system and force plates while the participant performed five trials of each of the SLS tasks.
Subjects
Sixteen healthy, young women (mean ± standard deviation (SD); age, 23.1 ± 1.9 years; height, 1.65 ± 0.08 m; mass, 63.1 ± 8.0 kg) participated in this study. To be included, participants had to have no current or recent (within the last two months) history of back or lower extremity pain or injury lasting more than two weeks. All participants were informed of the benefits and risks of the study and signed an informed consent form approved by Boston University's Institutional Review Board prior to participation.
Instrumentation
Three-dimensional kinematic data of the trunk, pelvis, and lower extremity were recorded at 100 Hz using a 10-camera motion capture system (Vicon Motion Systems Ltd., Centennial, CO). Ground reaction force data were collected using the force plates in a split-belt instrumented treadmill (Bertec Corporation, Columbus, OH) sampling at 1000 Hz.
Procedures
Forty-two retroreflective markers were placed on the participant's trunk, pelvis, and lower extremities based on the marker placement of a previous study.14 A standing static calibration trial was recorded to create a subject-specific model. Markers on the medial knees and ankles were removed after the calibration trial to allow for freer movement.
For the three SLS tasks, participants were asked to stand on the treadmill force plates with each foot on a plate. Participants were instructed to stand on one leg, maintain their non-stance leg in one of three positions (Figure 1), and squat as low as possible in a controlled manner while keeping their arms at or out to their sides. The three non-stance leg positions were (1) the non-stance leg extended out front (SLS-Front) (Figure 1A), (2) the non-stance foot held in line with the ankle of the stance leg (SLS-Middle) (Figure 1B), and (3) the non-stance knee flexed 90 ° while maintaining a vertical thigh position (SLS-Back) (Figure 1C). Similar non-stance leg positions have been used in previous SLS studies. For example, SLS-Front was used by Crossley et al.,13 SLS-Middle was used by Mauntel et al.,15 and SLS-Back was used by Graci et al.16 These three non-stance leg positions likely represent most of the variation in non-stance leg positioning across clinicians. Verbal feedback was given to help participants maintain a consistent speed while completing the SLS in a smooth, fluid motion. Five trials of each SLS task were collected. A trial was recollected if the participant lost her balance, did not position the non-stance leg correctly, or performed the SLS in a jerky or non-continuous manner. The order of the SLS tasks was block randomized. Both legs were tested; however, preliminary analysis showed no side differences between the squats performed on the left and right legs. Thus, the left stance leg was arbitrarily chosen for analysis of the hip, knee, and ankle data in this study.
Figure 1.
Three single leg squat (SLS) tasks: (A) SLS-Front, (B) SLS-Middle, and (C) SLS-Back.
Data Processing
Kinematic marker data were labeled using Vicon Nexus (Version 1.8.5). Kinematic and kinetic data were processed using commercially available software (Visual3D, C-Motion, Rockville, MD). Marker trajectories were filtered using a low-pass, fourth-order Butterworth filter with a cutoff frequency of 6 Hz. Hip, knee, and ankle joint angles were calculated with respect to the proximal segment. Pelvic and trunk segment angles were determined with reference to the lab coordinate system. All joint angles were calculated using a Visual3D hybrid model with a CODA pelvis17 and a right-handed Cardan X-Y-Z (mediolateral, anteroposterior, vertical) rotation sequence.18 The model consisted of eight rigid segments: a trunk, a pelvis, right and left thighs, right and left shanks, and right and left feet. Ground reaction force data were low-pass filtered using a fourth-order Butterworth filter with a cutoff frequency of 10 Hz. Internal joint moments were calculated based on kinematic marker positions and ground reaction force data. Trunk, pelvic, hip, knee, and ankle angles as well as hip, knee, and ankle moments of the stance leg in all three planes were identified at peak knee flexion (PKF) using custom MATLAB code (The MathWorks, Inc., Natick, MA). In addition, all variables were extracted when the stance leg first reached 60 ° of knee flexion (60KF). Sixty degrees of knee flexion was selected because it corresponded to the approximate knee flexion angle achieved or minimum knee flexion angle required or targeted in previous SLS studies.7,8,15,19,20 In addition, a consensus panel of five physical therapists in a previous study13 agreed that a SLS must be performed to at least 60 º of knee flexion to be clinically rated as “good.”
Statistical Analysis
Kinematic variables of interest were trunk, pelvic, and left hip, knee, and ankle angles in all three planes, at the two analysis points, PKF and 60KF. Kinetic variables of interest included left hip, knee, and ankle moments normalized to body mass in all three planes at PKF and 60KF. For each variable, a separate one-way repeated-measures analysis of variance (ANOVA) was used to compare the three SLS tasks at each of the two analysis points. The Mauchly's Test of Sphericity was used to check for violations of sphericity. The Greenhouse-Geisser correction was applied if needed. A resulting significant main effect was followed by post-hoc Bonferroni-corrected paired t-tests to identify significant pairwise differences between the SLS tasks. Cohen's d was used to compute the effect size (ES) of each pairwise comparison. As per Cohen's21 suggestion, effect sizes of 0.2, 0.5, and 0.8 were interpreted as small, medium, and large changes, respectively. All statistical analyses were performed using SPSS, Version 20.0 (IBM Corporation, Armonk, NY). The alpha level was set at 0.05 for all tests.
RESULTS
The three variations of the SLS resulted in kinematic and kinetic differences at the trunk, pelvis, hip, knee, and ankle at PKF and 60KF (Tables 1 and 2). The Greenhouse-Geisser correction was applied to ankle eversion and ankle plantar flexor moment at PKF and ankle dorsiflexion and ankle inversion moment at 60KF. Results of the post-hoc tests and effect sizes of the pairwise differences between the SLS tasks are provided (Tables 3 and 4).
Table 1.
Descriptive statistics and main effect comparisons of kinematic variables of interest at 60 ° of knee flexion and peak knee flexion for the three single leg squat tasks.
| Mean ± Standard Deviation | ANOVA | ||||
|---|---|---|---|---|---|
| Joint angle (°) | SLS-Front | SLS-Middle | SLS-Back | F2,30 | p |
| Trunk flexion | |||||
| 60KF | 13.0 ± 9.9 | 19.3 ± 11.7 | 17.6 ± 11.1 | 10.7 | <0.001 |
| PKF | 18.5 ± 12.6 | 24.3 ± 14.4 | 22.9 ± 12.7 | 5.9 | 0.007 |
| Trunk ipsilateral flexion | |||||
| 60KF | 1.1 ± 2.1 | 1.4 ± 2.9 | 2.4 ± 2.5 | 3.7 | 0.036 |
| PKF | 0.3 ± 2.1 | 1.5 ± 2.9 | 2.5 ± 2.2 | 10.2 | <0.001 |
| Trunk backward rotation | |||||
| 60KF | 2.7 ± 4.5 | 2.8 ± 5.1 | 1.5 ± 4.1 | 1.7 | 0.194 |
| PKF | 1.1 ± 5.4 | 1.5 ± 5.3 | 0.4 ± 4.5 | 0.9 | 0.413 |
| Pelvic anterior tilt | |||||
| 60KF | 6.6 ± 8.7 | 16.9 ± 8.9 | 23.5 ± 9.3 | 103.9 | <0.001 |
| PKF | 5.6 ± 9.7 | 17.4 ± 8.4 | 26.8 ± 9.2 | 146.5 | <0.001 |
| Pelvic drop | |||||
| 60KF | −3.9 ± 2.0 | −2.0 ± 2.1 | 1.8 ± 2.3 | 55.5 | <0.001 |
| PKF | −3.1 ± 3.1 | −1.2 ± 3.5 | 5.3 ± 2.4 | 64.3 | <0.001 |
| Pelvic backward rotation | |||||
| 60KF | 1.9 ± 3.9 | 3.1 ± 2.9 | 2.5 ± 4.2 | 0.9 | 0.430 |
| PKF | −0.9 ± 4.8 | 0.4 ± 3.5 | −0.6 ± 5.2 | 1.0 | 0.396 |
| Hip flexion | |||||
| 60KF | 35.3 ± 11.3 | 47.4 ± 12.4 | 51.6 ± 11.4 | 64.9 | <0.001 |
| PKF | 47.1 ± 13.9 | 59.8 ± 16.1 | 64.6 ± 12.4 | 58.0 | <0.001 |
| Hip adduction | |||||
| 60KF | 4.9 ± 3.7 | 5.9 ± 3.4 | 9.4 ± 4.5 | 30.3 | <0.001 |
| PKF | 9.5 ± 4.8 | 10.7 ± 3.9 | 14.9 ± 5.2 | 47.0 | <0.001 |
| Hip external rotation | |||||
| 60KF | 9.4 ± 7.2 | 10.3 ± 7.8 | 10.8 ± 7.5 | 2.4 | 0.111 |
| PKF | 9.1 ± 7.9 | 9.3 ± 8.5 | 12.0 ± 7.9 | 9.7 | 0.001 |
| Knee flexion | |||||
| 60KF | 60.3 ± 0.2 | 60.2 ± 0.1 | 60.3 ± 0.2 | 3.1 | 0.060 |
| PKF | 82.0 ± 7.0 | 79.2 ± 8.7 | 75.1 ± 6.1 | 15.5 | <0.001 |
| Knee abduction | |||||
| 60KF | 8.6 ± 7.2 | 8.4 ± 7.0 | 8.0 ± 6.8 | 1.3 | 0.291 |
| PKF | 9.9 ± 7.8 | 8.8 ± 7.9 | 7.4 ± 7.9 | 15.4 | <0.001 |
| Knee internal rotation | |||||
| 60KF | 6.4 ± 9.5 | 6.2 ± 9.3 | 5.5 ± 9.7 | 2.4 | 0.110 |
| PKF | 3.1 ± 10.1 | 3.1 ± 8.8 | 2.7 ± 9.5 | 0.3 | 0.776 |
| Ankle dorsiflexion | |||||
| 60KF | 25.1 ± 5.4 | 24.1 ± 5.7 | 26.3 ± 5.2 | 12.2* | 0.001 |
| PKF | 31.5 ± 5.5 | 29.3 ± 5.8 | 30.5 ± 5.6 | 8.2 | 0.001 |
| Ankle eversion | |||||
| 60KF | 8.9 ± 3.6 | 8.3 ± 2.7 | 9.1 ± 3.6 | 3.3 | 0.049 |
| PKF | 10.4 ± 4.8 | 9.9 ± 3.6 | 10.0 ± 4.5 | 1.2† | 0.314 |
| Ankle abduction | |||||
| 60KF | 8.7 ± 5.4 | 8.5 ± 4.3 | 9.3 ± 4.7 | 1.5 | 0.234 |
| PKF | 10.5 ± 5.6 | 10.6 ± 5.1 | 10.8 ± 4.8 | 0.2 | 0.836 |
Abbreviations: 60KF = 60 ° of knee flexion; ANOVA = analysis of variance; PKF = peak knee flexion; SLS = single leg squat
Note: Bolded text indicates a significant difference (p<0.05).
Greenhouse-Geisser correction applied: F1.4,21.3
Greenhouse-Geisser correction applied: F1.5,21.8
Table 2.
Descriptive statistics and main effect comparisons of kinetic variables of interest at 60 ° of knee flexion and peak knee flexion for the three single leg squat tasks.
| Mean ± Standard Deviation | ANOVA | ||||
|---|---|---|---|---|---|
| Normalized internal moment (Nm/kg) | SLS-Front | SLS-Middle | SLS-Back | F2,30 | p |
| Hip extensor | |||||
| 60KF | 0.64 ± 0.34 | 0.74 ± 0.41 | 0.44 ± 0.30 | 19.8 | <0.001 |
| PKF | 1.05 ± 0.41 | 1.11 ± 0.54 | 0.79 ± 0.40 | 13.9 | <0.001 |
| Hip abductor | |||||
| 60KF | 0.76 ± 0.12 | 0.75 ± 0.14 | 0.74 ± 0.14 | 1.0 | 0.383 |
| PKF | 0.89 ± 0.14 | 0.87 ± 0.14 | 0.86 ± 0.16 | 1.4 | 0.268 |
| Hip external rotator | |||||
| 60KF | 0.09 ± 0.13 | 0.20 ± 0.12 | 0.24 ± 0.10 | 42.7 | <0.001 |
| PKF | 0.11 ± 0.16 | 0.26 ± 0.14 | 0.28 ± 0.11 | 27.6 | <0.001 |
| Knee extensor | |||||
| 60KF | 1.16 ± 0.24 | 1.20 ± 0.25 | 1.34 ± 0.22 | 22.4 | <0.001 |
| PKF | 1.52 ± 0.28 | 1.51 ± 0.30 | 1.59 ± 0.26 | 8.0 | 0.002 |
| Knee adductor | |||||
| 60KF | 0.14 ± 0.20 | 0.14 ± 0.20 | 0.17 ± 0.23 | 2.1 | 0.135 |
| PKF | 0.34 ± 0.23 | 0.28 ± 0.24 | 0.26 ± 0.25 | 7.3 | 0.003 |
| Knee internal rotator | |||||
| 60KF | 0.27 ± 0.05 | 0.30 ± 0.05 | 0.30 ± 0.05 | 5.9 | 0.007 |
| PKF | 0.33 ± 0.07 | 0.34 ± 0.06 | 0.35 ± 0.05 | 1.0 | 0.367 |
| Ankle plantar flexor | |||||
| 60KF | 0.68 ± 0.21 | 0.54 ± 0.21 | 0.57 ± 0.23 | 13.8 | <0.001 |
| PKF | 0.87 ± 0.23 | 0.71 ± 0.26 | 0.72 ± 0.24 | 16.1* | <0.001 |
| Ankle inversion | |||||
| 60KF | 0.26 ± 0.10 | 0.23 ± 0.08 | 0.24 ± 0.09 | 4.2† | 0.044 |
| PKF | 0.31 ± 0.13 | 0.28 ± 0.11 | 0.29 ± 0.12 | 2.7 | 0.080 |
| Ankle adductor | |||||
| 60KF | 0.08 ± 0.05 | 0.09 ± 0.06 | 0.10 ± 0.07 | 9.2 | 0.001 |
| PKF | 0.12 ± 0.08 | 0.11 ± 0.07 | 0.13 ± 0.06 | 2.7 | 0.080 |
Abbreviations: 60KF = 60 ° of knee flexion; ANOVA = analysis of variance; PKF = peak knee flexion; SLS = single leg squat
Note: Bolded text indicates a significant difference (p<0.05).
Greenhouse-Geisser correction applied: F1.5,22.1
Greenhouse-Geisser correction applied: F1.3,19.9
Table 3.
Post-hoc pairwise comparisons and effect sizes of kinematic variables of interest at 60 ° of knee flexion and peak knee flexion for the three single leg squat tasks.
| SLS-Front vs. SLS-Middle | SLS-Middle vs. SLS-Back | SLS-Front vs. SLS-Back | ||||
|---|---|---|---|---|---|---|
| Joint angle | p | ES | p | ES | p | ES |
| Trunk flexion | ||||||
| 60KF | 0.006 | −0.58 | 0.355 | 0.15 | 0.016 | −0.44 |
| PKF | 0.036 | −0.43 | 1.000 | 0.10 | 0.057 | −0.35 |
| Trunk ipsilateral flexion | ||||||
| 60KF | 1.000 | −0.10 | 0.265 | −0.36 | 0.054 | −0.54 |
| PKF | 0.141 | −0.45 | 0.231 | −0.39 | <0.001 | −0.99 |
| Trunk backward rotation | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | --- | --- | --- | --- | --- | --- |
| Pelvic anterior tilt | ||||||
| 60KF | <0.001 | −1.17 | <0.001 | −0.73 | <0.001 | −1.88 |
| PKF | <0.001 | −1.30 | <0.001 | −1.08 | <0.001 | −2.25 |
| Pelvic drop | ||||||
| 60KF | 0.002 | −0.91 | <0.001 | −1.72 | <0.001 | −2.63 |
| PKF | 0.056 | −0.57 | <0.001 | −2.18 | <0.001 | −3.02 |
| Pelvic backward rotation | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | --- | --- | --- | --- | --- | --- |
| Hip flexion | ||||||
| 60KF | <0.001 | −1.02 | 0.004 | −0.35 | <0.001 | −1.44 |
| PKF | <0.001 | −0.84 | 0.009 | −0.34 | <0.001 | −1.33 |
| Hip adduction | ||||||
| 60KF | 0.188 | −0.30 | 0.001 | −0.88 | <0.001 | −1.10 |
| PKF | 0.111 | −0.28 | <0.001 | −0.91 | <0.001 | −1.08 |
| Hip external rotation | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | 1.000 | −0.03 | 0.006 | −0.33 | 0.006 | −0.37 |
| Knee flexion | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | 0.165 | 0.36 | 0.024 | 0.54 | <0.001 | 1.05 |
| Knee abduction | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | 0.114 | 0.14 | 0.019 | 0.17 | <0.001 | 0.32 |
| Knee internal rotation | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | --- | --- | --- | --- | --- | --- |
| Ankle dorsiflexion | ||||||
| 60KF | 0.260 | 0.18 | <0.001 | −0.40 | 0.067 | −0.23 |
| PKF | 0.009 | 0.38 | 0.014 | −0.20 | 0.343 | 0.18 |
| Ankle eversion | ||||||
| 60KF | 0.323 | 0.21 | 0.136 | −0.26 | 1.000 | −0.04 |
| PKF | --- | --- | --- | --- | --- | --- |
| Ankle abduction | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | --- | --- | --- | --- | --- | --- |
Abbreviations: 60KF = 60 ° of knee flexion; ES = effect size; PKF = peak knee flexion; SLS = single leg squat
Note: Bolded text indicates a significant difference (p<0.05).
Table 4.
Post-hoc pairwise comparisons and effect sizes of kinetic variables of interest at 60 ° of knee flexion and peak knee flexion for the three single leg squat tasks.
| SLS-Front vs. SLS-Middle | SLS-Middle vs. SLS-Back | SLS-Front vs. SLS-Back | ||||
|---|---|---|---|---|---|---|
| Normalized internal moment | p | ES | p | ES | p | ES |
| Hip extensor | ||||||
| 60KF | 0.322 | −0.26 | <0.001 | 0.84 | 0.002 | 0.63 |
| PKF | 1.000 | −0.13 | <0.001 | 0.68 | 0.001 | 0.64 |
| Hip abductor | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | --- | --- | --- | --- | --- | --- |
| Hip external rotator | ||||||
| 60KF | <0.001 | −0.92 | 0.137 | −0.36 | <0.001 | −1.32 |
| PKF | <0.001 | −1.00 | 1.000 | −0.20 | <0.001 | −1.27 |
| Knee extensor | ||||||
| 60KF | 0.425 | 0.05 | <0.001 | −0.28 | <0.001 | −0.24 |
| PKF | 1.000 | −0.18 | 0.003 | −0.57 | 0.014 | −0.78 |
| Knee adductor | ||||||
| 60KF | --- | --- | --- | --- | --- | --- |
| PKF | 0.094 | 0.23 | 0.745 | 0.11 | 0.002 | 0.34 |
| Knee internal rotator | ||||||
| 60KF | 0.036 | −0.55 | 1.000 | 0.02 | 0.029 | −0.56 |
| PKF | --- | --- | --- | --- | --- | --- |
| Ankle plantar flexor | ||||||
| 60KF | <0.001 | 0.66 | 0.845 | −0.11 | 0.016 | 0.53 |
| PKF | 0.001 | 0.69 | 1.000 | −0.07 | 0.005 | 0.64 |
| Ankle inversion | ||||||
| 60KF | 0.129 | 0.31 | 0.615 | −0.10 | 0.181 | 0.21 |
| PKF | --- | --- | --- | --- | --- | --- |
| Ankle adductor | ||||||
| 60KF | 0.357 | −0.15 | 0.007 | −0.17 | 0.005 | −0.33 |
| PKF | --- | --- | --- | --- | --- | --- |
Abbreviations: 60KF = 60 ° of knee flexion; ES = effect size; PKF = peak knee flexion; SLS = single leg squat
Note: Bolded text indicates a significant difference (p<0.05).
Kinematics
At PKF, the repeated-measures ANOVAs revealed significant main effects (p < 0.05) for kinematic variables of interest in the sagittal and frontal planes at the trunk, pelvis, hip, and knee, in the transverse plane at the hip, and in the sagittal plane at the ankle for the three SLS tasks and post-hoc analyses were performed. When the data were analyzed at 60KF, the differences across SLS tasks largely persisted except at the knee.
Trunk
SLS-Front resulted in less trunk flexion compared to SLS-Middle (ES = −0.43, p = 0.036) and less ipsilateral trunk flexion relative to the stance leg compared to SLS-Back (ES = −0.99, p < 0.001) at PKF. At 60KF, trunk flexion in SLS-Front was less than both SLS-Middle (ES = -0.58, p = 0.006) and SLS-Back (ES = −0.44, p = 0.016), but there was no difference in ipsilateral trunk flexion. In the transverse plane, there were no trunk differences at PKF or 60KF. ES for significant trunk differences ranged from small to large (ES = −0.99−-0.43).
Pelvis
At both analysis points, participants exhibited the greatest anterior pelvic tilt in SLS-Back and the least in SLS-Front, with SLS-Middle in-between (ES = -2.25−-0.73, p < 0.001). Greater contralateral pelvic drop relative to the stance leg (represented as positive on Tables 1 and 3) was observed in SLS-Back than SLS-Front and SLS-Middle at both PKF and 60KF (p < 0.001). The SLS-Front and SLS-Middle actually displayed contralateral pelvic hike at both analysis points. In addition, at 60KF, the contralateral pelvic hike was greater in SLS-Front than SLS-Middle (p = 0.002). There were no differences observed in the transverse plane for the pelvis at PKF or 60KF. ES for significantly different pelvic variables in the sagittal and frontal planes were generally large (ES = −3.02−-0.73).
Hip
Hip flexion was greatest in SLS-Back, followed by SLS-Middle, and least in SLS-Front at PKF (p ≤ 0.009) and 60KF (p ≤ 0.004). Hip adduction was greater in SLS-Back than SLS-Front and SLS-Middle at both analysis points (p < 0.001). Hip external rotation was greater in SLS-Back than SLS-Front and SLS-Middle (p = 0.001) at PKF, but not at 60KF. These differences at the hip ranged from small to large (ES = −1.44‐−0.33).
Knee
At PKF, SLS-Back exhibited less knee flexion (p ≤ 0.024) and abduction (p ≤ 0.019) compared to SLS-Front and SLS-Middle. These differences were small to large (ES = 0.17−1.05). There were no differences at the knee in the transverse plane at PKF or in any of the three planes at 60KF.
Ankle
SLS-Middle demonstrated less ankle dorsiflexion than both SLS-Front (ES = 0.38, p = 0.009) and SLS-Back (ES = -0.20, p = 0.014) at PKF. There were no differences in ankle eversion or abduction at PKF. At 60KF, SLS-Middle had less ankle dorsiflexion than SLS-Back (ES = -0.40, p ≤ 0.001). There was a significant main effect between the three SLS tasks for ankle eversion (p = 0.049) at 60KF, but pairwise comparisons revealed no differences. No difference was found in ankle abduction at 60KF. ES for significantly different ankle variables were generally small (ES = -0.40−0.38).
Kinetics
Differences in hip, knee, and ankle kinetics for the three SLS tasks were observed at both PKF and 60KF.
Hip
At both PKF and 60KF, the hip extensor moment was moderately greater in SLS-Front (ES = 0.63−0.64, p ≤ 0.002) and SLS-Middle (ES = 0.68−0.84, p < 0.001) compared to SLS-Back. There was no difference in the hip abductor moment for the three SLS tasks at either analysis point. The hip external rotator moment was smaller in SLS-Front compared to SLS-Middle (p < 0.001) and SLS-Back (p < 0.001) at both analysis points. These differences in the transverse plane were large (ES = −1.32‐−0.92).
Knee
At both analysis points, the knee extensor moment was greater in SLS-Back compared to SLS-Front (p ≤ 0.014) and SLS-Middle (p ≤ 0.003). These differences were small at 60KF (ES = −0.28‐−0.24), but moderate at PKF (ES = −0.78−-0.57). The knee adductor moment was greater in SLS-Front compared to SLS-Back at PKF (ES = 0.34, p = 0.002), but not at 60KF. The knee internal rotator moment was moderately smaller in SLS-Front compared to SLS-Middle (p = 0.036) and SLS-Back (p = 0.029) at 60KF, but not at PKF.
Ankle
SLS-Front had a moderately greater plantar flexor moment than SLS-Middle (ES = 0.66−0.69, p ≤ 0.001) and SLS-Back (ES = 0.53−0.64, p ≤ 0.016) at both PKF and 60KF. The ankle inversion moments for the three SLS tasks were not different at PKF. Although there was a significant main effect between the three SLS tasks for the ankle inversion moment (p = 0.023) at 60KF, post-hoc tests revealed no pairwise differences. While there was no difference in the ankle adductor moment at PKF, the ankle adductor moments for SLS-Front (ES = −0.33, p = 0.005) and SLS-Middle (ES = −0.17, p = 0.007) were smaller than for SLS-Back at 60KF.
DISCUSSION
Although the SLS is often used in clinical assessments and rehabilitation, prior studies have not established if and how changing the non-stance leg position affects the way the SLS is performed. The aim of this study was to examine how participants perform the SLS when instructed to maintain the position of the non-stance leg in three ways and compare trunk, pelvic, and lower extremity kinematics and kinetics of the three variations of the SLS. The results confirmed the hypothesis, showing that the three SLS variations elicited distinct kinematic and kinetic demands throughout the trunk, pelvis, and lower extremity. The primary findings of this study were (1) SLS-Back had the most kinematic differences from SLS-Front and SLS-Middle, (2) SLS-Back had a smaller hip extensor moment than SLS-Front and SLS-Middle, (3) SLS-Back had a greater knee extensor moment than SLS-Front and SLS-Middle, and (4) the largest effects of changing the non-stance leg position were found at the hip and pelvis.
While previous studies have reported SLS kinematics, they often lacked specific details about the SLS procedure including the non-stance leg position.6 The current findings demonstrated that changing the position of the non-stance leg during the SLS had significant biomechanical effects throughout the trunk, pelvis, and stance lower extremity. Based on effect sizes, altering the position of the non-stance leg during the SLS had the greatest effects on pelvic kinematics, followed by hip kinematics. Overall, the largest effect sizes were observed between SLS-Front and SLS-Back for both pelvic anterior tilt and pelvic drop. At the pelvis, the difference between SLS-Front to SLS-Middle was larger for pelvic anterior tilt than pelvic drop, while the reverse was true when comparing SLS-Middle to SLS-Back. SLS-Front compared to SLS-Middle and SLS-Back showed large effect sizes for hip flexion, while SLS-Back compared to SLS-Front and SLS-Middle resulted in large effect sizes for hip adduction. Large effect sizes were also found between SLS-Front and both SLS-Middle and SLS-Back for hip external rotator moment. These findings suggest that hip and pelvic kinematics are most likely to be affected by changing the non-stance leg position.
Limited information about hip, knee,9 and ankle kinetics and ankle kinematics during the SLS is available in the literature. The current kinetic results provide practitioners with an indication of how different muscle groups are being stressed by the SLS variations. SLS-Back had a higher knee extensor moment than SLS-Front and SLS-Middle; thus, it may be a more appropriate SLS task when assessing the strength of the quadriceps. When assessing the strength of the gluteal muscles or hamstrings, SLS-Front and SLS-Middle may be more challenging than SLS-Back because they have higher hip extensor moments. Practitioners can use the hip, knee, and ankle kinetics of the SLS variations, along with their joint angles, to inform their selection of SLS tasks in order to create exercise plans that strengthen targeted muscles, are progressively challenging, or are more appropriate for individual patients. For example, the increased hip flexion and adduction during the SLS-Back may make it less appropriate for patients with femoroacetabular impingement (FAI) as hip flexion and adduction often elicits symptoms.22,23 Similarly, the decreased trunk flexion in the SLS-Front may be less appropriate for patients with anterior cruciate ligament (ACL) injuries since ACL forces and strains are higher when squatting with minimal forward trunk lean than with moderate forward trunk lean.24
This study had several limitations. People with lower extremity pain, such as those with PFP,8-10 may use different movement strategies to perform the SLS variations than the asymptomatic participants in this study. Thus, caution should be taken when generalizing these results to symptomatic populations. While kinematic differences by sex have been reported for the SLS,8,9,16,25 this study focused on females only. Females have higher rates of PFP and noncontact ACL injury than males, so practitioners may encounter more female patients that may benefit from using different SLS tasks to address some of the biomechanical impairments including greater ipsilateral trunk lean, contralateral pelvic drop, hip adduction, and knee abduction identified by Nakagawa et al.8 Future studies should examine if these findings apply to males and if there are sex differences for the three SLS variations. While more complex, multi-segment trunk models26 exist, the trunk was modeled as a single rigid segment as commonly used in other SLS studies.16,27 Additionally, leg dominance was not considered in this analysis because preliminary data analysis of each leg revealed consistent findings for both sides. Finally, while instructions were given to control the position of the non-stance leg, none were given regarding the trunk, pelvis, and stance leg position during the three SLS tasks. This was done in order to more closely resemble how instructions for the SLS are given during clinical or field testing and to observe what modifications the individual naturally made following a change in non-stance leg position.
CONCLUSION
The results of the current study indicate that changing the position of the non-stance leg during the SLS results in different trunk, pelvic, and lower extremity biomechanics. Practitioners can use these results to better understand the biomechanical differences between the SLS variations and determine if certain variations may be more appropriate for individual patients, because not all SLS are equal. Applying these findings to athletes and patients may improve functional testing, as well as strength and rehabilitation paradigms.
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