Journal of Applied Biomechanics, 2014, 30, 707-712 http://dx.doi.org/10.1123/jab.2014-0003 © 2014 Human Kinetics, Inc.
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Kinematic Differences Between Those With and Without Medial Knee Displacement During a Single-leg Squat Timothy C. Mauntel,1 Barnett S. Frank,1 Rebecca L. Begalle,2 J. Troy Blackburn,1 and Darin A. Padua1 1University
of North Carolina; 2Illinois State University
A greater knee valgus angle is a risk factor for lower extremity injuries. Visually observed medial knee displacement is used as a proxy for knee valgus motion during movement assessments in an attempt to identify individuals at heightened risk for injury. The validity of medial knee displacement as an indicator of valgus motion has yet to be determined during a single-leg squat. This study compared three-dimensional knee and hip angles between participants who displayed medial knee displacement (MKD group) during a single-leg squat and those who did not (control group). Participants completed five single-leg squats. An electromagnetic motion tracking system was used to quantify peak knee and hip joint angles during the descent phase of each squat. MANOVA identified a difference between the MKD and control group kinematics. ANOVA post hoc testing revealed greater knee valgus angle in the MKD (12.86 ± 5.76) compared with the control (6.08 ± 5.23) group. There were no other differences between groups. Medial knee displacement is indicative of knee valgus motion; however, it is not indicative of greater knee or hip rotation, or hip adduction. These data indicate that clinicians can accurately identify individuals with greater knee valgus angle through visually observed medial knee displacement. Keywords: knee valgus, movement assessment, movement screening, biomechanics Physically active individuals are susceptible to noncontact injuries of the anterior cruciate (ACL)1,2 and medial collateral ligaments (MCL),3–5 and to developing patellofemoral pain syndrome.6,7 Approximately 70% of all ACL injuries are the result of noncontact mechanisms.8,9 The MCL, one of the most commonly injured ligaments, can be injured in isolation or concomitantly with the ACL.10,11 Patellofemoral pain syndrome accounts for up to 25% of all knee injuries seen in sports medicine clinics7 and results in physical activity reduction in 36% of people affected by it.6 These injuries affect an individual physically, mentally, and financially.12 These negative consequences are not limited to immediately following injury, but can affect an individual for their lifetime, as ACL injury and patellofemoral pain syndrome have been linked to increased incidence of osteoarthritis.13–15 The high incidence and short- and long-term consequences of these injuries make it important to identify high-risk individuals before injury so they may be targeted for injury prevention training programs. One potential predisposing factor for noncontact knee injuries is greater knee valgus angle during functional tasks.16–19 When the knee assumes a valgus or varus position during activity it places the joint in a less stable position and makes it more susceptible to injury.20 While greater knee valgus angle observed during jumping tasks has been previously established as a predictor for future knee Timothy C. Mauntel, Barnett S. Frank, and Darin A. Padua are with the Department of Exercise and Sport Science, Sports Medicine Research Laboratory, University of North Carolina, Chapel Hill, NC. Rebecca L. Begalle is with the School of Kinesiology and Recreation, Illinois State University, Normal, IL. J. Troy Blackburn is with the Department of Exercise and Sport Science, Neuromuscular Research Laboratory, University of North Carolina, Chapel Hill, NC. Address author correspondence to Timothy C. Mauntel at [email protected]
injury,16,21 the direct link between knee valgus angle and noncontact knee injuries must be further investigated. The predictability of future knee injury based on knee valgus angle during functional tasks makes it important to be able to identify individuals who display greater knee valgus motion in a clinical setting. Development of a valid clinical screening mechanism to identify individuals at heightened risk of noncontact knee injuries, similar to what has been previously done with jumping tasks,21 is an essential step in reducing the risk associated with noncontact knee injuries. A number of functional movement assessments have been developed to identify at-risk individuals through the observation of faulty lower extremity biomechanics. Traditionally knee valgus angle, visually observed as medial knee displacement, has been a staple among functional movement assessments. The jumplanding21 and overhead squat tasks22 are valid indicators of knee valgus angles as they have been compared with the gold-standard of three-dimensional motion analysis. The single-leg squat is also a commonly observed functional movement assessment23,24 but has not yet been validated against three-dimensional motion analysis. Validation of the single-leg squat is important, as the single-leg squat may be more advantageous for clinicians, especially novices, than the previously validated movement assessment tasks. The single-leg squat is slower than the jump-landing task and more demanding than the overhead squat, allowing for movement errors to be more easily observed. Easier identification of movement errors may allow for an increased ability to identify individuals who are at a greater risk of lower extremity injury. Three-dimensional knee valgus angle is a risk factor and mechanism for noncontact knee injuries.16,20 Valid clinical screening tools for identifying individuals with greater knee valgus angle are needed so high-risk individuals may be targeted with injury prevention programs to correct functional knee valgus motion. Observation of medial knee displacement during functional tasks 707
708 Mauntel et al.
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is commonly described as a screening procedure for identifying individuals who display knee valgus motion;21–23 however, research has not validated this screening procedure during a single-leg squat as a measure of three-dimensional lower extremity kinematics. Therefore the purpose of this study was to compare lower extremity kinematics between individuals who display medial knee displacement and those who do not during a single-leg squat. The primary hypothesis was that individuals who display medial knee displacement would have significantly greater peak knee valgus angles compared with individuals who maintain a neutral knee alignment. If the primary hypothesis was shown to be true, the secondary hypothesis was that individuals who displayed medial knee displacement would also display greater peak knee and hip angles for those motions that contribute to dynamic knee valgus angle (ie, tibial rotation, hip adduction, and hip internal rotation). Determining differences in lower extremity kinematics will allow clinicians to better discriminate high- and low-risk individuals for noncontact knee injuries.
Methods Forty individuals, aged 18–35 years volunteered to participate in this study. All participants self-reported to be in good physical condition and participated in a minimum of 30 minutes of physical activity three times per week for at least the previous six months. Participants were involved in a variety of physical activities including cardiovascular and resistance training. Individuals were excluded if they had a previous surgical operation to the lower extremity or low back, reported current symptoms of injury to the lower extremity or low back, or had an injury to the lower extremity or low back within the past six months that resulted in three or more consecutive days of missed activity. Persons with known neurologic conditions were also excluded. This study was approved by the university’s institutional review board, and each participant signed an institutional review board (IRB) approved informed consent form. Following informed consent, demographic information was collected for each subject: height (cm), mass (kg), age (yrs), and leg dominance (the leg used to kick a soccer ball for maximal distance). Each participant then completed a five minute warm-up on a stationary cycle ergometer at a self-selected pace. Participants wore their own t-shirt and athletic shorts and were barefoot throughout the screening and testing sessions.
Screening Session Following the warm-up, each participant underwent a screening session to determine if they met inclusion criteria for the study and group assignment. During the screening session participants completed five consecutive single-leg squat trials to a depth of 60° of knee flexion (mechanical block set to contact the gluteal muscle mass at 60° of knee flexion) while being visually observed by the primary investigator. Each participant stood on the dominant leg, with the toes facing forward. The non-weight-bearing leg was flexed 90° at the knee and 45° at the hip, the hands were placed on the hips, and the head and eyes faced forward. A metronome set at a frequency of 60 beats per minute was used to control the movement velocity of the squatting task. The participant descended for two beats of the metronome until the gluteals touched the mechanical block and then returned to the starting position in two beats. The methodology for the single-leg squat was adapted from the methods recommended by the National Academy of Sports
Medicine.25 Participants were barefoot throughout the screening and testing sessions, as shoes could have altered foot pronation and supination and subsequently altered the proximal segments and joints. This methodology allowed for an accurate portrayal of the individuals’ true biomechanics while minimizing the influence of external sources. Furthermore, had the participants not been barefoot, shoes could have confounded the results of the study, as not all participants would have worn the same shoes. Participants were assigned to the medial knee displacement group (MKD; n = 20, age = 20.2 ± 1.8 y, height = 173.8 ± 8.8 cm, mass = 71.8 ± 14.7 kg; Figure 1) if in at least three of five trials the midpoint of the patella moved medially to the great toe during the single-leg squat.22,23 Participants were assigned to the control group (n = 20, age = 20.2 ± 1.5 y, height = 173.1 ± 10.1 cm, mass = 71.0 ± 14.6 kg; Figure 2) if in at least three of five trials the knee remained in line with the hip and ankle joints throughout the single-leg squat. Real-time visual observation of patella location and subsequent group assignment was completed by the primary investigator. The primary investigator is a certified athletic trainer with experience and training in clinical assessments of lower extremity kinematics. Furthermore, a frontal plane view of all single-leg squat trials was recorded with a standard video camera; all videos were exported to a computer which allowed for the videos to be slowed down and stopped so patellar alignment and group assignment could be confirmed. These methods have previously been shown by Padua et al22 to be capable of discerning between individuals with visually observed medial knee displacement and those who maintain neutral knee alignment during a double-leg squat; these visually observed differences were confirmed with three-dimensional kinematic data. Participants did not receive feedback or coaching concerning technique, other than what constituted a successful trial. For a successful trial the participant: (1) maintained proper testing position; (2) squatted until the gluteals contacted the mechanical block; (3) completed the task at the appropriate rate; (4) did not touch down with the nondominant foot; (5) did not touch the legs together; (6) maintained the heel in contact with the ground; and (7) completed the task in a fluid motion. Participants were allowed as many practice trials as needed to perform the task successfully. Qualifying participants completed the testing session at a later date. A total of 23 participants were excluded from the study after undergoing the screening session for one or more of the following reasons: (1) they demonstrated a knee varus position during the single-leg squat, defined as the midpoint of the patella moving laterally to the base of the fifth metatarsal in at least three of five trials; (2) the positioning of their knee during the single-leg squats did not allow them to be clearly assigned to one of the two study groups; or (3) when the subject returned for the testing session they no longer met the criteria for the originally assigned group. These participants were excluded from the study to not confound the study results (Figure 3). The majority of the 23 excluded participants were excluded because they displayed a knee varus position during the single-leg squat or were unable to be clearly assigned to one of the two study groups. The authors are confident that participants were not mistakenly excluded from the study based on the participant’s screening session performance. The primary investigator, who completed all screening and testing sessions, had almost perfect agreement26 (Kappa = .86) for intersession group assignment.
Testing Session Participants returned to the laboratory for the testing session within one week of the screening session. They again completed the five
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Kinematic Differences During a Single-leg Squat 709
Figure 1 — Medial knee displacement (MKD) group participant.
Figure 2 — Control group participant.
Figure 3 — Participant screening and group assignment. MKD = medial knee displacement.
minute warm-up and had their group assignment confirmed through the same screening process as previously described. During the testing session a Motion Star electromagnetic motion tracking system (Ascension Technologies, Inc., Burlington, VT) was used to track three-dimensional lower extremity kinematics during the descent phase of the single-leg squat (ie, the time from the start of knee flexion motion to the point of greatest knee flexion). Electromagnetic sensors were placed over the sacrum, lateral aspect of the thigh, and the anteromedial aspect of the proximal tibia and secured with double-sided tape, prewrap, and athletic tape. A segment-linkage model of the pelvis and dominant lower extremity was generated by digitizing the anterior superior iliac spines,
femoral epicondyles, and malleoli. The location of the hip joint center was approximated using the Bell method.27 Joint centers for the knee and ankle were defined as the midpoints of the femoral epicondyles and malleoli, respectively. Three-dimensional coordinate data were collected at a sampling rate of 100 Hz and filtered using a fourth-order low-pass Butterworth filter at a cutoff frequency of 20 Hz. PASW Statistics for Windows (Version 18.0; SPSS Inc., Chicago, IL) was used to calculate a single MANOVA to compare peak sagittal, frontal, and transverse plane knee and hip joint angles during the descent phase of the single-leg squat between the MKD and control groups. Post hoc one-way ANOVAs were performed to
710 Mauntel et al.
compare the MKD and control groups for each independent variable. Statistical significance was set a priori at α ≤ .05.
Results The MANOVA analysis revealed a significant difference between the MKD and control group kinematics (Wilks’s Lambda = .519, F8,31 = 3.477, P = .006, η2 = .492). Follow-up one-way ANOVAs revealed significantly greater peak knee valgus (F1,38 = 14.831, P < .000) in the MKD group (mean = 12.86; 95% CI = 11.07, 14.65) compared with the control group (mean = 6.08; 95% CI = 4.46, 7.70). There were no significant differences between groups for peak knee flexion, knee or hip external or internal rotation, hip flexion, or hip adduction (Table 1).
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Discussion The single-leg squat is a common functional movement assessment used to identify individuals displaying high risk biomechanical patterns that may place them at greater risk of injury.23–25 The most important finding of this study is that visual observation of medial knee displacement is able to distinguish peak knee valgus angles between individuals during a single-leg squat. Surprisingly, there were no other frontal or transverse plane kinematic differences between groups. Based on these findings medial knee displacement during a single-leg squat is a valid functional movement assessment for identifying individuals with greater knee valgus angle, but visual observation of medial knee displacement during a single-leg squat does not indicate differences in hip adduction, hip rotation, or tibial rotation. Thus, observation of medial knee displacement during a single-leg squat may not be able to identify all of the multiplanar factors commonly associated with noncontact knee injuries.17,21 On average, there was 6° more peak knee valgus in the MKD group compared with the control group, which represents a large difference between groups (effect size = 1.23). This difference of approximately 6° is similar to the difference in knee valgus motion observed by Hewett et al16 between females who sustained an ACL injury and those who did not. Similarly, Padua et al21 found greater knee valgus motion in individuals who scored poorly on the Landing Error Scoring System (LESS), a clinical assessment of lower extremity kinematics during double-leg landing. While the previous studies examined biomechanics during doubleTable 1 Peak knee and hip joint angles presented as means ± SD and effect sizes MKD Group
Control Group
Mean ± SD
Mean ± SD
Effect Size
Knee flexion
62.05 ± 3.20
64.32 ± 9.57
0.35
Knee ER
3.88 ± 8.28
4.22 ± 8.09
0.04
Knee IR
8.28 ± 8.41
5.71 ± 8.11
0.31
Knee valgus*
12.86 ± 5.76
6.08 ± 5.23
1.23
Hip flexion
30.72 ± 13.57
35.29 ± 14.54
0.33
Hip adduction
3.46 ± 5.26
5.35 ± 10.08
0.23
Hip ER
7.25 ± 8.76
5.71 ± 7.47
0.19
Hip IR
6.35 ± 8.36
4.09 ± 8.39
0.17
Note. ER, external rotation; IR, internal rotation. *Indicates significance at α ≤ .05.
leg jumping tasks, it is likely the difference in knee valgus angle observed in the current study may also be clinically meaningful. Identifying individuals at increased risk of injury could potentially direct development of individualized injury prevention and rehabilitation programs to better combat the high incidence and negative consequences of knee injury. The presence of medial knee displacement does not appear to be indicative of greater knee or hip rotation, or hip adduction, and thus the secondary hypothesis was proven incorrect. This finding was surprising as it is contradictory to previous literature.21 The lack of a significant difference in knee and hip rotation and hip adduction between the MKD and control groups may be influenced by the multiplanar nature of knee valgus motion. Knee valgus angle is not isolated to frontal plane motion between the femur and tibia that results in medial joint space opening (true knee valgus).28 Rather, dynamic knee valgus angle is the combination of tibial rotation, hip adduction, and hip internal rotation as the foot is fixed on the ground.29 These motions may result in a greater knee valgus angle in those with visual presence of medial knee displacement; however, the relative magnitudes of knee and hip rotation and hip adduction may be quite different between individuals. For example, two individuals may display the same peak knee valgus angle, yet one individual may achieve this angle with a large amount of hip adduction and minimal tibial rotation, while the second individual achieves this valgus angle with minimal hip adduction and a large amount of tibial rotation. This theory is supported by our data. The amount of variation observed for peak knee valgus angle is small compared with the other motions contributing to it (knee rotation, hip adduction, hip internal rotation). The coefficient of variation (CV = standard deviation / mean) was calculated for each peak lower extremity angle to allow for a scaled comparison of the variation for each dependent variable. The CV for knee valgus was 0.45 and 0.86 for the MKD and control groups, respectively. The CVs for both study groups ranged from 1.02–2.16 for knee rotation, 1.52–1.88 for hip adduction, and 1.32–2.05 for hip internal rotation. The CVs for motions contributing to knee valgus angle were 118–482% larger than the CV for peak knee valgus angle. Large amounts of variation in the data can minimize the differences observed between groups and suppress the statistical significance. In this study it is also unlikely there were clinically relevant differences between the groups for motions contributing to knee valgus angle as the effect size for each contributing motion ranged from 0.04–0.31. While no differences were observed in the motions contributing to knee valgus angle, the main finding of this study is still important as greater knee valgus angle, which can be identified through visually observed medial knee displacement, has been identified as a knee injury risk factor.16,17,20 While the presence of medial knee displacement is an indicator of greater knee valgus motion, the ability of single-leg squat performance to predict future injury is not yet known. The potential exists for the single-leg squat to be developed into a reliable movement assessment similar to the LESS. This is supported by previous authors who have shown a clinician-friendly tool (the Star Excursion Balance Test) can be implemented to identify individuals at increased risk of lower extremity injury.30 The anterior reaching component of the Star Excursion Balance Test is similar to the single-leg squat; when the individual is required to extend the nonstance limb directly in front of the body, they will flex the knee and hip of the stance limb to maintain balance and achieve a further reaching distance. Although there are no prospective data to support single-leg squat performance as a predictor of injury, it
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Kinematic Differences During a Single-leg Squat 711
could be developed into an injury risk screening tool as it shares similar movement patterns with the previously validated Star Excursion Balance Test.30 A movement assessment combining the balance errors and postural control observed during the Star Excursion Balance Test with movement errors observed during the single-leg squat, especially knee valgus angle, may prove to be a superior clinical movement assessment compared with previously validated assessments. The ability of visually observed medial knee displacement during a single-leg squat to identify kinematic risk factors for lower extremity injury could greatly impact clinical practice as medial knee displacement has been linked with neuromuscular control abnormalities22,23 that have been linked to ACL and MCL injuries and patellofemoral pain syndrome. Fortunately, these abnormalities are modifiable risk factors which can be altered through injury prevention programs.31 If individuals with neuromuscular abnormalities can be identified via the single-leg squat, these abnormalities may be corrected during injury prevention and rehabilitation programs,31,32 potentially reducing the risk of injury. The major limitation of this study is that only one dynamic task was used. Additional differences in biomechanics may have been identified between groups had additional tasks been used. This is especially true if more dynamic and demanding tasks had been used, such as a jump-landing or drop-jump. In addition, kinematic data were not collected from the foot or ankle, and additional differences in lower extremity kinematics may have been observed had these data been collected. The ability of the single-leg squat to predict future injury was not assessed in this study, and while the single-leg squat can identify individuals who exhibit a known lower extremity injury risk factor, the single-leg squat’s ability to predict injury is yet to be determined. The single-leg squat is commonly used by sports medicine professionals to identify individuals who display biomechanical patterns which may place them at greater risk of lower extremity injury.23–25 Visual observation of medial knee displacement is able to discriminate between individuals with significant differences in peak knee valgus angle during a single-leg squat. No other biomechanical differences existed between groups. The single-leg squat may not be sensitive enough to identify all of the multiplanar factors which contribute to noncontact knee injury. Furthermore, while the single-leg squat is able to identify a known lower extremity injury risk factor (knee valgus),16 this motion may not be the same as knee valgus collapse, the motion commonly described at the time of ACL injury.17 The use of the single-leg squat as a clinical movement assessment could greatly impact clinical practice as it may be easier for novice raters to identify movement errors as it is a slow, controlled task that may exacerbate movement errors. However, until the single-leg squat is more formally developed into a movement assessment capable of identifying multiplanar lower extremity injury risk factors, clinicians should continue to use the single-leg squat in conjunction with other movement assessments.
References 1. Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes. A prospective study. Am J Sports Med. 1999;27(6):699–706. PubMed 2. Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and preventing noncontact anterior cruciate ligament injuries: a review of the Hunt Valley II meeting, January 2005. Am J Sports Med. 2006;34(9):1512– 1532. PubMed doi:10.1177/0363546506286866
3. Fetto JF, Marshall JL. Medial collateral ligament injuries of the knee: a rationale for treatment. Clin Orthop Relat Res. 1978; (132):206–218. PubMed 4. Griffith CJ, LaPrade RF, Johansen S, Armitage B, Wijdicks C, Engebretsen L. Medial knee injury: Part 1, static function of the individual components of the main medial knee structures. Am J Sports Med. 2009;37(9):1762–1770. PubMed doi:10.1177/0363546509333852 5. Wijdicks CA, Griffith CJ, LaPrade RF, et al. Medial knee injury: Part 2, load sharing between the posterior oblique ligament and superficial medial collateral ligament. Am J Sports Med. 2009;37(9):1771–1776. PubMed doi:10.1177/0363546509335191 6. Stathopulu E, Baildam E. Anterior knee pain: a long-term follow-up. Rheumatology (Oxford). 2003;42(2):380–382. PubMed doi:10.1093/ rheumatology/keg093 7. Devereaux MD, Lachmann SM. Patello-femoral arthralgia in athletes attending a Sports Injury Clinic. Br J Sports Med. 1984;18(1):18–21. PubMed doi:10.1136/bjsm.18.1.18 8. Hewett TE, Myer GD, Ford KR. Anterior cruciate ligament injuries in female athletes: Part 1, mechanisms and risk factors. Am J Sports Med. 2006;34(2):299–311. PubMed doi:10.1177/0363546505284183 9. Boden BP, Griffin LY, Garrett WE, Jr. Etiology and prevention of noncontact ACL injury. Phys Sportsmed. 2000;28(4):53–60. PubMed doi:10.3810/psm.2000.04.841 10. Grood ES, Noyes FR, Butler DL, Suntay WJ. Ligamentous and capsular restraints preventing straight medial and lateral laxity in intact human cadaver knees. J Bone Joint Surg Am. 1981;63(8):1257–1269. PubMed 11. LaPrade RF. The medial collateral ligament complex and the posterolateral aspect of the knee. American Academy of Orthopaedic Surgeons; 1999:327–340. 12. Freedman KB, Glasgow MT, Glasgow SG, Bernstein J. Anterior cruciate ligament injury and reconstruction among university students. Clin Orthop Relat Res. 1998; (356):208–212. PubMed doi:10.1097/00003086-199811000-00028 13. Svoboda SJ, Harvey TM, Owens BD, Brechue WF, Tarwater PM, Cameron KL. Changes in serum biomarkers of cartilage turnover after anterior cruciate ligament injury. Am J Sports Med. 2013;41(9):2108– 2116. PubMed doi:10.1177/0363546513494180 14. Lohmander LS, Ostenberg A, Englund M, Roos H. High prevalence of knee osteoarthritis, pain, and functional limitations in female soccer players twelve years after anterior cruciate ligament injury. Arthritis Rheum. 2004;50(10):3145–3152. PubMed doi:10.1002/art. 20589 15. Utting MR, Davies G, Newman JH. Is anterior knee pain a predisposing factor to patellofemoral osteoarthritis? Knee. 2005;12(5):362–365. PubMed doi:10.1016/j.knee.2004.12.006 16. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med. 2005;33(4):492–501. PubMed doi:10.1177/0363546504269591 17. Ireland ML. Anterior cruciate ligament injury in female athletes: epidemiology. J Athl Train. 1999;34(2):150–154. PubMed 18. Noehren B, Barrance PJ, Pohl MP, Davis IS. A comparison of tibiofemoral and patellofemoral alignment during a neutral and valgus single leg squat: an MRI study. Knee. 2012;19(4):380–386. PubMed doi:10.1016/j.knee.2011.05.012 19. Laprade RF, Wijdicks CA. The management of injuries to the medial side of the knee. J Orthop Sports Phys Ther. 2012;42(3):221–233. PubMed doi:10.2519/jospt.2012.3624 20. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased ham-
Downloaded by York Univ Libraries on 09/17/16, Volume 30, Article Number 6
712 Mauntel et al. string torques. Am J Sports Med. 1996;24(6):765–773. PubMed doi:10.1177/036354659602400611 21. Padua DA, Marshall SW, Boling MC, Thigpen CA, Garrett WE, Jr, Beutler AI. The landing error scoring system (LESS) is a valid and reliable clinical assessment tool of jump-landing biomechanics: The JUMP-ACL study. Am J Sports Med. 2009;37(10):1996–2002. PubMed doi:10.1177/0363546509343200 22. Padua DA, Bell DR, Clark MA. Neuromuscular characteristics of individuals displaying excessive medial knee displacement. J Athl Train. 2012;47(5):525–536. PubMed 23. Mauntel TC, Begalle RL, Cram TR, et al. The effects of lower extremity muscle activation and passive range of motion on single leg squat performance. Journal of strength and conditioning research / National Strength & Conditioning Association. 2013;27(7):1813-1823. 24. Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics and electromyographic activity between men and women during the single-legged squat. Am J Sports Med. 2003;31(3):449–456. PubMed 25. National Academy of Sports Medicine.. NASM Essentials of Corrective Exercise Training. 1st ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2010. 26. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977;33(1):159–174. PubMed doi:10.2307/2529310
27. Bell AL, Pedersen DR, Brand RA. A comparison of the accuracy of several hip center location prediction methods. J Biomech. 1990;23(6):617–621. PubMed doi:10.1016/0021-9290(90)90054-7 28. Quatman CE, Hewett TE. The anterior cruciate ligament injury controversy: is “valgus collapse” a sex-specific mechanism? Br J Sports Med. 2009;43(5):328–335. PubMed doi:10.1136/bjsm.2009.059139 29. Ageberg E, Bennell KL, Hunt MA, Simic M, Roos EM, Creaby MW. Validity and inter-rater reliability of medio-lateral knee motion observed during a single-limb mini squat. BMC Musculoskelet Disord. 2010;11:265. PubMed doi:10.1186/1471-2474-11-265 30. Plisky PJ, Rauh MJ, Kaminski TW, Underwood FB. Star Excursion Balance Test as a predictor of lower extremity injury in high school basketball players. J Orthop Sports Phys Ther. 2006;36(12):911–919. PubMed doi:10.2519/jospt.2006.2244 31. Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med. 2006;34(3):490–498. PubMed doi:10.1177/0363546505282619 32. Hirth CJ. Clinical movement analysis to identify muscle imbalances and guide exercise. Athl Ther Today. 2007;12(4):10–14.
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Kinematic Differences Between Those With and Without Medial Knee Displacement During a Single-leg Squat Timothy C. Mauntel,1 Barnett S. Frank,1 Rebecca L. Begalle,2 J. Troy Blackburn,1 and Darin A. Padua1 1University
of North Carolina; 2Illinois State University
A greater knee valgus angle is a risk factor for lower extremity injuries. Visually observed medial knee displacement is used as a proxy for knee valgus motion during movement assessments in an attempt to identify individuals at heightened risk for injury. The validity of medial knee displacement as an indicator of valgus motion has yet to be determined during a single-leg squat. This study compared three-dimensional knee and hip angles between participants who displayed medial knee displacement (MKD group) during a single-leg squat and those who did not (control group). Participants completed five single-leg squats. An electromagnetic motion tracking system was used to quantify peak knee and hip joint angles during the descent phase of each squat. MANOVA identified a difference between the MKD and control group kinematics. ANOVA post hoc testing revealed greater knee valgus angle in the MKD (12.86 ± 5.76) compared with the control (6.08 ± 5.23) group. There were no other differences between groups. Medial knee displacement is indicative of knee valgus motion; however, it is not indicative of greater knee or hip rotation, or hip adduction. These data indicate that clinicians can accurately identify individuals with greater knee valgus angle through visually observed medial knee displacement. Keywords: knee valgus, movement assessment, movement screening, biomechanics Physically active individuals are susceptible to noncontact injuries of the anterior cruciate (ACL)1,2 and medial collateral ligaments (MCL),3–5 and to developing patellofemoral pain syndrome.6,7 Approximately 70% of all ACL injuries are the result of noncontact mechanisms.8,9 The MCL, one of the most commonly injured ligaments, can be injured in isolation or concomitantly with the ACL.10,11 Patellofemoral pain syndrome accounts for up to 25% of all knee injuries seen in sports medicine clinics7 and results in physical activity reduction in 36% of people affected by it.6 These injuries affect an individual physically, mentally, and financially.12 These negative consequences are not limited to immediately following injury, but can affect an individual for their lifetime, as ACL injury and patellofemoral pain syndrome have been linked to increased incidence of osteoarthritis.13–15 The high incidence and short- and long-term consequences of these injuries make it important to identify high-risk individuals before injury so they may be targeted for injury prevention training programs. One potential predisposing factor for noncontact knee injuries is greater knee valgus angle during functional tasks.16–19 When the knee assumes a valgus or varus position during activity it places the joint in a less stable position and makes it more susceptible to injury.20 While greater knee valgus angle observed during jumping tasks has been previously established as a predictor for future knee Timothy C. Mauntel, Barnett S. Frank, and Darin A. Padua are with the Department of Exercise and Sport Science, Sports Medicine Research Laboratory, University of North Carolina, Chapel Hill, NC. Rebecca L. Begalle is with the School of Kinesiology and Recreation, Illinois State University, Normal, IL. J. Troy Blackburn is with the Department of Exercise and Sport Science, Neuromuscular Research Laboratory, University of North Carolina, Chapel Hill, NC. Address author correspondence to Timothy C. Mauntel at [email protected]
injury,16,21 the direct link between knee valgus angle and noncontact knee injuries must be further investigated. The predictability of future knee injury based on knee valgus angle during functional tasks makes it important to be able to identify individuals who display greater knee valgus motion in a clinical setting. Development of a valid clinical screening mechanism to identify individuals at heightened risk of noncontact knee injuries, similar to what has been previously done with jumping tasks,21 is an essential step in reducing the risk associated with noncontact knee injuries. A number of functional movement assessments have been developed to identify at-risk individuals through the observation of faulty lower extremity biomechanics. Traditionally knee valgus angle, visually observed as medial knee displacement, has been a staple among functional movement assessments. The jumplanding21 and overhead squat tasks22 are valid indicators of knee valgus angles as they have been compared with the gold-standard of three-dimensional motion analysis. The single-leg squat is also a commonly observed functional movement assessment23,24 but has not yet been validated against three-dimensional motion analysis. Validation of the single-leg squat is important, as the single-leg squat may be more advantageous for clinicians, especially novices, than the previously validated movement assessment tasks. The single-leg squat is slower than the jump-landing task and more demanding than the overhead squat, allowing for movement errors to be more easily observed. Easier identification of movement errors may allow for an increased ability to identify individuals who are at a greater risk of lower extremity injury. Three-dimensional knee valgus angle is a risk factor and mechanism for noncontact knee injuries.16,20 Valid clinical screening tools for identifying individuals with greater knee valgus angle are needed so high-risk individuals may be targeted with injury prevention programs to correct functional knee valgus motion. Observation of medial knee displacement during functional tasks 707
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is commonly described as a screening procedure for identifying individuals who display knee valgus motion;21–23 however, research has not validated this screening procedure during a single-leg squat as a measure of three-dimensional lower extremity kinematics. Therefore the purpose of this study was to compare lower extremity kinematics between individuals who display medial knee displacement and those who do not during a single-leg squat. The primary hypothesis was that individuals who display medial knee displacement would have significantly greater peak knee valgus angles compared with individuals who maintain a neutral knee alignment. If the primary hypothesis was shown to be true, the secondary hypothesis was that individuals who displayed medial knee displacement would also display greater peak knee and hip angles for those motions that contribute to dynamic knee valgus angle (ie, tibial rotation, hip adduction, and hip internal rotation). Determining differences in lower extremity kinematics will allow clinicians to better discriminate high- and low-risk individuals for noncontact knee injuries.
Methods Forty individuals, aged 18–35 years volunteered to participate in this study. All participants self-reported to be in good physical condition and participated in a minimum of 30 minutes of physical activity three times per week for at least the previous six months. Participants were involved in a variety of physical activities including cardiovascular and resistance training. Individuals were excluded if they had a previous surgical operation to the lower extremity or low back, reported current symptoms of injury to the lower extremity or low back, or had an injury to the lower extremity or low back within the past six months that resulted in three or more consecutive days of missed activity. Persons with known neurologic conditions were also excluded. This study was approved by the university’s institutional review board, and each participant signed an institutional review board (IRB) approved informed consent form. Following informed consent, demographic information was collected for each subject: height (cm), mass (kg), age (yrs), and leg dominance (the leg used to kick a soccer ball for maximal distance). Each participant then completed a five minute warm-up on a stationary cycle ergometer at a self-selected pace. Participants wore their own t-shirt and athletic shorts and were barefoot throughout the screening and testing sessions.
Screening Session Following the warm-up, each participant underwent a screening session to determine if they met inclusion criteria for the study and group assignment. During the screening session participants completed five consecutive single-leg squat trials to a depth of 60° of knee flexion (mechanical block set to contact the gluteal muscle mass at 60° of knee flexion) while being visually observed by the primary investigator. Each participant stood on the dominant leg, with the toes facing forward. The non-weight-bearing leg was flexed 90° at the knee and 45° at the hip, the hands were placed on the hips, and the head and eyes faced forward. A metronome set at a frequency of 60 beats per minute was used to control the movement velocity of the squatting task. The participant descended for two beats of the metronome until the gluteals touched the mechanical block and then returned to the starting position in two beats. The methodology for the single-leg squat was adapted from the methods recommended by the National Academy of Sports
Medicine.25 Participants were barefoot throughout the screening and testing sessions, as shoes could have altered foot pronation and supination and subsequently altered the proximal segments and joints. This methodology allowed for an accurate portrayal of the individuals’ true biomechanics while minimizing the influence of external sources. Furthermore, had the participants not been barefoot, shoes could have confounded the results of the study, as not all participants would have worn the same shoes. Participants were assigned to the medial knee displacement group (MKD; n = 20, age = 20.2 ± 1.8 y, height = 173.8 ± 8.8 cm, mass = 71.8 ± 14.7 kg; Figure 1) if in at least three of five trials the midpoint of the patella moved medially to the great toe during the single-leg squat.22,23 Participants were assigned to the control group (n = 20, age = 20.2 ± 1.5 y, height = 173.1 ± 10.1 cm, mass = 71.0 ± 14.6 kg; Figure 2) if in at least three of five trials the knee remained in line with the hip and ankle joints throughout the single-leg squat. Real-time visual observation of patella location and subsequent group assignment was completed by the primary investigator. The primary investigator is a certified athletic trainer with experience and training in clinical assessments of lower extremity kinematics. Furthermore, a frontal plane view of all single-leg squat trials was recorded with a standard video camera; all videos were exported to a computer which allowed for the videos to be slowed down and stopped so patellar alignment and group assignment could be confirmed. These methods have previously been shown by Padua et al22 to be capable of discerning between individuals with visually observed medial knee displacement and those who maintain neutral knee alignment during a double-leg squat; these visually observed differences were confirmed with three-dimensional kinematic data. Participants did not receive feedback or coaching concerning technique, other than what constituted a successful trial. For a successful trial the participant: (1) maintained proper testing position; (2) squatted until the gluteals contacted the mechanical block; (3) completed the task at the appropriate rate; (4) did not touch down with the nondominant foot; (5) did not touch the legs together; (6) maintained the heel in contact with the ground; and (7) completed the task in a fluid motion. Participants were allowed as many practice trials as needed to perform the task successfully. Qualifying participants completed the testing session at a later date. A total of 23 participants were excluded from the study after undergoing the screening session for one or more of the following reasons: (1) they demonstrated a knee varus position during the single-leg squat, defined as the midpoint of the patella moving laterally to the base of the fifth metatarsal in at least three of five trials; (2) the positioning of their knee during the single-leg squats did not allow them to be clearly assigned to one of the two study groups; or (3) when the subject returned for the testing session they no longer met the criteria for the originally assigned group. These participants were excluded from the study to not confound the study results (Figure 3). The majority of the 23 excluded participants were excluded because they displayed a knee varus position during the single-leg squat or were unable to be clearly assigned to one of the two study groups. The authors are confident that participants were not mistakenly excluded from the study based on the participant’s screening session performance. The primary investigator, who completed all screening and testing sessions, had almost perfect agreement26 (Kappa = .86) for intersession group assignment.
Testing Session Participants returned to the laboratory for the testing session within one week of the screening session. They again completed the five
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Kinematic Differences During a Single-leg Squat 709
Figure 1 — Medial knee displacement (MKD) group participant.
Figure 2 — Control group participant.
Figure 3 — Participant screening and group assignment. MKD = medial knee displacement.
minute warm-up and had their group assignment confirmed through the same screening process as previously described. During the testing session a Motion Star electromagnetic motion tracking system (Ascension Technologies, Inc., Burlington, VT) was used to track three-dimensional lower extremity kinematics during the descent phase of the single-leg squat (ie, the time from the start of knee flexion motion to the point of greatest knee flexion). Electromagnetic sensors were placed over the sacrum, lateral aspect of the thigh, and the anteromedial aspect of the proximal tibia and secured with double-sided tape, prewrap, and athletic tape. A segment-linkage model of the pelvis and dominant lower extremity was generated by digitizing the anterior superior iliac spines,
femoral epicondyles, and malleoli. The location of the hip joint center was approximated using the Bell method.27 Joint centers for the knee and ankle were defined as the midpoints of the femoral epicondyles and malleoli, respectively. Three-dimensional coordinate data were collected at a sampling rate of 100 Hz and filtered using a fourth-order low-pass Butterworth filter at a cutoff frequency of 20 Hz. PASW Statistics for Windows (Version 18.0; SPSS Inc., Chicago, IL) was used to calculate a single MANOVA to compare peak sagittal, frontal, and transverse plane knee and hip joint angles during the descent phase of the single-leg squat between the MKD and control groups. Post hoc one-way ANOVAs were performed to
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compare the MKD and control groups for each independent variable. Statistical significance was set a priori at α ≤ .05.
Results The MANOVA analysis revealed a significant difference between the MKD and control group kinematics (Wilks’s Lambda = .519, F8,31 = 3.477, P = .006, η2 = .492). Follow-up one-way ANOVAs revealed significantly greater peak knee valgus (F1,38 = 14.831, P < .000) in the MKD group (mean = 12.86; 95% CI = 11.07, 14.65) compared with the control group (mean = 6.08; 95% CI = 4.46, 7.70). There were no significant differences between groups for peak knee flexion, knee or hip external or internal rotation, hip flexion, or hip adduction (Table 1).
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Discussion The single-leg squat is a common functional movement assessment used to identify individuals displaying high risk biomechanical patterns that may place them at greater risk of injury.23–25 The most important finding of this study is that visual observation of medial knee displacement is able to distinguish peak knee valgus angles between individuals during a single-leg squat. Surprisingly, there were no other frontal or transverse plane kinematic differences between groups. Based on these findings medial knee displacement during a single-leg squat is a valid functional movement assessment for identifying individuals with greater knee valgus angle, but visual observation of medial knee displacement during a single-leg squat does not indicate differences in hip adduction, hip rotation, or tibial rotation. Thus, observation of medial knee displacement during a single-leg squat may not be able to identify all of the multiplanar factors commonly associated with noncontact knee injuries.17,21 On average, there was 6° more peak knee valgus in the MKD group compared with the control group, which represents a large difference between groups (effect size = 1.23). This difference of approximately 6° is similar to the difference in knee valgus motion observed by Hewett et al16 between females who sustained an ACL injury and those who did not. Similarly, Padua et al21 found greater knee valgus motion in individuals who scored poorly on the Landing Error Scoring System (LESS), a clinical assessment of lower extremity kinematics during double-leg landing. While the previous studies examined biomechanics during doubleTable 1 Peak knee and hip joint angles presented as means ± SD and effect sizes MKD Group
Control Group
Mean ± SD
Mean ± SD
Effect Size
Knee flexion
62.05 ± 3.20
64.32 ± 9.57
0.35
Knee ER
3.88 ± 8.28
4.22 ± 8.09
0.04
Knee IR
8.28 ± 8.41
5.71 ± 8.11
0.31
Knee valgus*
12.86 ± 5.76
6.08 ± 5.23
1.23
Hip flexion
30.72 ± 13.57
35.29 ± 14.54
0.33
Hip adduction
3.46 ± 5.26
5.35 ± 10.08
0.23
Hip ER
7.25 ± 8.76
5.71 ± 7.47
0.19
Hip IR
6.35 ± 8.36
4.09 ± 8.39
0.17
Note. ER, external rotation; IR, internal rotation. *Indicates significance at α ≤ .05.
leg jumping tasks, it is likely the difference in knee valgus angle observed in the current study may also be clinically meaningful. Identifying individuals at increased risk of injury could potentially direct development of individualized injury prevention and rehabilitation programs to better combat the high incidence and negative consequences of knee injury. The presence of medial knee displacement does not appear to be indicative of greater knee or hip rotation, or hip adduction, and thus the secondary hypothesis was proven incorrect. This finding was surprising as it is contradictory to previous literature.21 The lack of a significant difference in knee and hip rotation and hip adduction between the MKD and control groups may be influenced by the multiplanar nature of knee valgus motion. Knee valgus angle is not isolated to frontal plane motion between the femur and tibia that results in medial joint space opening (true knee valgus).28 Rather, dynamic knee valgus angle is the combination of tibial rotation, hip adduction, and hip internal rotation as the foot is fixed on the ground.29 These motions may result in a greater knee valgus angle in those with visual presence of medial knee displacement; however, the relative magnitudes of knee and hip rotation and hip adduction may be quite different between individuals. For example, two individuals may display the same peak knee valgus angle, yet one individual may achieve this angle with a large amount of hip adduction and minimal tibial rotation, while the second individual achieves this valgus angle with minimal hip adduction and a large amount of tibial rotation. This theory is supported by our data. The amount of variation observed for peak knee valgus angle is small compared with the other motions contributing to it (knee rotation, hip adduction, hip internal rotation). The coefficient of variation (CV = standard deviation / mean) was calculated for each peak lower extremity angle to allow for a scaled comparison of the variation for each dependent variable. The CV for knee valgus was 0.45 and 0.86 for the MKD and control groups, respectively. The CVs for both study groups ranged from 1.02–2.16 for knee rotation, 1.52–1.88 for hip adduction, and 1.32–2.05 for hip internal rotation. The CVs for motions contributing to knee valgus angle were 118–482% larger than the CV for peak knee valgus angle. Large amounts of variation in the data can minimize the differences observed between groups and suppress the statistical significance. In this study it is also unlikely there were clinically relevant differences between the groups for motions contributing to knee valgus angle as the effect size for each contributing motion ranged from 0.04–0.31. While no differences were observed in the motions contributing to knee valgus angle, the main finding of this study is still important as greater knee valgus angle, which can be identified through visually observed medial knee displacement, has been identified as a knee injury risk factor.16,17,20 While the presence of medial knee displacement is an indicator of greater knee valgus motion, the ability of single-leg squat performance to predict future injury is not yet known. The potential exists for the single-leg squat to be developed into a reliable movement assessment similar to the LESS. This is supported by previous authors who have shown a clinician-friendly tool (the Star Excursion Balance Test) can be implemented to identify individuals at increased risk of lower extremity injury.30 The anterior reaching component of the Star Excursion Balance Test is similar to the single-leg squat; when the individual is required to extend the nonstance limb directly in front of the body, they will flex the knee and hip of the stance limb to maintain balance and achieve a further reaching distance. Although there are no prospective data to support single-leg squat performance as a predictor of injury, it
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Kinematic Differences During a Single-leg Squat 711
could be developed into an injury risk screening tool as it shares similar movement patterns with the previously validated Star Excursion Balance Test.30 A movement assessment combining the balance errors and postural control observed during the Star Excursion Balance Test with movement errors observed during the single-leg squat, especially knee valgus angle, may prove to be a superior clinical movement assessment compared with previously validated assessments. The ability of visually observed medial knee displacement during a single-leg squat to identify kinematic risk factors for lower extremity injury could greatly impact clinical practice as medial knee displacement has been linked with neuromuscular control abnormalities22,23 that have been linked to ACL and MCL injuries and patellofemoral pain syndrome. Fortunately, these abnormalities are modifiable risk factors which can be altered through injury prevention programs.31 If individuals with neuromuscular abnormalities can be identified via the single-leg squat, these abnormalities may be corrected during injury prevention and rehabilitation programs,31,32 potentially reducing the risk of injury. The major limitation of this study is that only one dynamic task was used. Additional differences in biomechanics may have been identified between groups had additional tasks been used. This is especially true if more dynamic and demanding tasks had been used, such as a jump-landing or drop-jump. In addition, kinematic data were not collected from the foot or ankle, and additional differences in lower extremity kinematics may have been observed had these data been collected. The ability of the single-leg squat to predict future injury was not assessed in this study, and while the single-leg squat can identify individuals who exhibit a known lower extremity injury risk factor, the single-leg squat’s ability to predict injury is yet to be determined. The single-leg squat is commonly used by sports medicine professionals to identify individuals who display biomechanical patterns which may place them at greater risk of lower extremity injury.23–25 Visual observation of medial knee displacement is able to discriminate between individuals with significant differences in peak knee valgus angle during a single-leg squat. No other biomechanical differences existed between groups. The single-leg squat may not be sensitive enough to identify all of the multiplanar factors which contribute to noncontact knee injury. Furthermore, while the single-leg squat is able to identify a known lower extremity injury risk factor (knee valgus),16 this motion may not be the same as knee valgus collapse, the motion commonly described at the time of ACL injury.17 The use of the single-leg squat as a clinical movement assessment could greatly impact clinical practice as it may be easier for novice raters to identify movement errors as it is a slow, controlled task that may exacerbate movement errors. However, until the single-leg squat is more formally developed into a movement assessment capable of identifying multiplanar lower extremity injury risk factors, clinicians should continue to use the single-leg squat in conjunction with other movement assessments.
References 1. Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes. A prospective study. Am J Sports Med. 1999;27(6):699–706. PubMed 2. Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and preventing noncontact anterior cruciate ligament injuries: a review of the Hunt Valley II meeting, January 2005. Am J Sports Med. 2006;34(9):1512– 1532. PubMed doi:10.1177/0363546506286866
3. Fetto JF, Marshall JL. Medial collateral ligament injuries of the knee: a rationale for treatment. Clin Orthop Relat Res. 1978; (132):206–218. PubMed 4. Griffith CJ, LaPrade RF, Johansen S, Armitage B, Wijdicks C, Engebretsen L. Medial knee injury: Part 1, static function of the individual components of the main medial knee structures. Am J Sports Med. 2009;37(9):1762–1770. PubMed doi:10.1177/0363546509333852 5. Wijdicks CA, Griffith CJ, LaPrade RF, et al. Medial knee injury: Part 2, load sharing between the posterior oblique ligament and superficial medial collateral ligament. Am J Sports Med. 2009;37(9):1771–1776. PubMed doi:10.1177/0363546509335191 6. Stathopulu E, Baildam E. Anterior knee pain: a long-term follow-up. Rheumatology (Oxford). 2003;42(2):380–382. PubMed doi:10.1093/ rheumatology/keg093 7. Devereaux MD, Lachmann SM. Patello-femoral arthralgia in athletes attending a Sports Injury Clinic. Br J Sports Med. 1984;18(1):18–21. PubMed doi:10.1136/bjsm.18.1.18 8. Hewett TE, Myer GD, Ford KR. Anterior cruciate ligament injuries in female athletes: Part 1, mechanisms and risk factors. Am J Sports Med. 2006;34(2):299–311. PubMed doi:10.1177/0363546505284183 9. Boden BP, Griffin LY, Garrett WE, Jr. Etiology and prevention of noncontact ACL injury. Phys Sportsmed. 2000;28(4):53–60. PubMed doi:10.3810/psm.2000.04.841 10. Grood ES, Noyes FR, Butler DL, Suntay WJ. Ligamentous and capsular restraints preventing straight medial and lateral laxity in intact human cadaver knees. J Bone Joint Surg Am. 1981;63(8):1257–1269. PubMed 11. LaPrade RF. The medial collateral ligament complex and the posterolateral aspect of the knee. American Academy of Orthopaedic Surgeons; 1999:327–340. 12. Freedman KB, Glasgow MT, Glasgow SG, Bernstein J. Anterior cruciate ligament injury and reconstruction among university students. Clin Orthop Relat Res. 1998; (356):208–212. PubMed doi:10.1097/00003086-199811000-00028 13. Svoboda SJ, Harvey TM, Owens BD, Brechue WF, Tarwater PM, Cameron KL. Changes in serum biomarkers of cartilage turnover after anterior cruciate ligament injury. Am J Sports Med. 2013;41(9):2108– 2116. PubMed doi:10.1177/0363546513494180 14. Lohmander LS, Ostenberg A, Englund M, Roos H. High prevalence of knee osteoarthritis, pain, and functional limitations in female soccer players twelve years after anterior cruciate ligament injury. Arthritis Rheum. 2004;50(10):3145–3152. PubMed doi:10.1002/art. 20589 15. Utting MR, Davies G, Newman JH. Is anterior knee pain a predisposing factor to patellofemoral osteoarthritis? Knee. 2005;12(5):362–365. PubMed doi:10.1016/j.knee.2004.12.006 16. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med. 2005;33(4):492–501. PubMed doi:10.1177/0363546504269591 17. Ireland ML. Anterior cruciate ligament injury in female athletes: epidemiology. J Athl Train. 1999;34(2):150–154. PubMed 18. Noehren B, Barrance PJ, Pohl MP, Davis IS. A comparison of tibiofemoral and patellofemoral alignment during a neutral and valgus single leg squat: an MRI study. Knee. 2012;19(4):380–386. PubMed doi:10.1016/j.knee.2011.05.012 19. Laprade RF, Wijdicks CA. The management of injuries to the medial side of the knee. J Orthop Sports Phys Ther. 2012;42(3):221–233. PubMed doi:10.2519/jospt.2012.3624 20. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased ham-
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712 Mauntel et al. string torques. Am J Sports Med. 1996;24(6):765–773. PubMed doi:10.1177/036354659602400611 21. Padua DA, Marshall SW, Boling MC, Thigpen CA, Garrett WE, Jr, Beutler AI. The landing error scoring system (LESS) is a valid and reliable clinical assessment tool of jump-landing biomechanics: The JUMP-ACL study. Am J Sports Med. 2009;37(10):1996–2002. PubMed doi:10.1177/0363546509343200 22. Padua DA, Bell DR, Clark MA. Neuromuscular characteristics of individuals displaying excessive medial knee displacement. J Athl Train. 2012;47(5):525–536. PubMed 23. Mauntel TC, Begalle RL, Cram TR, et al. The effects of lower extremity muscle activation and passive range of motion on single leg squat performance. Journal of strength and conditioning research / National Strength & Conditioning Association. 2013;27(7):1813-1823. 24. Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics and electromyographic activity between men and women during the single-legged squat. Am J Sports Med. 2003;31(3):449–456. PubMed 25. National Academy of Sports Medicine.. NASM Essentials of Corrective Exercise Training. 1st ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2010. 26. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977;33(1):159–174. PubMed doi:10.2307/2529310
27. Bell AL, Pedersen DR, Brand RA. A comparison of the accuracy of several hip center location prediction methods. J Biomech. 1990;23(6):617–621. PubMed doi:10.1016/0021-9290(90)90054-7 28. Quatman CE, Hewett TE. The anterior cruciate ligament injury controversy: is “valgus collapse” a sex-specific mechanism? Br J Sports Med. 2009;43(5):328–335. PubMed doi:10.1136/bjsm.2009.059139 29. Ageberg E, Bennell KL, Hunt MA, Simic M, Roos EM, Creaby MW. Validity and inter-rater reliability of medio-lateral knee motion observed during a single-limb mini squat. BMC Musculoskelet Disord. 2010;11:265. PubMed doi:10.1186/1471-2474-11-265 30. Plisky PJ, Rauh MJ, Kaminski TW, Underwood FB. Star Excursion Balance Test as a predictor of lower extremity injury in high school basketball players. J Orthop Sports Phys Ther. 2006;36(12):911–919. PubMed doi:10.2519/jospt.2006.2244 31. Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med. 2006;34(3):490–498. PubMed doi:10.1177/0363546505282619 32. Hirth CJ. Clinical movement analysis to identify muscle imbalances and guide exercise. Athl Ther Today. 2007;12(4):10–14.
