Limited ankle dorsiflexion (DF) range of motion (ROM) has been previously implicated in several knee and ankle disorders, including anterior cruciate ligament (ACL) tears (30), patellofemoral pain (PFP) (18,32), patellar and Achilles tendinopathy (1,15,21), as well as ankle sprains and fractures (19,28). The link between limited DF and ACL tears, PFP, patellar and Achilles tendinopathy may result from its association with several faulty hip and knee kinematics hypothesized to increase the risk of these conditions (4,5,8,9,13,16,17,20,22,23,26,27,31). Similarly, the link between limited DF ROM and ankle sprains and fractures may result from the association between limited DF and decreased dynamic balance (3,10,11). Regardless of the mechanism linking DF ROM with the aforementioned conditions, it seems that the assessment of DF ROM may be warranted in preparticipation screening for the risk of these injuries.
Several considerations, however, need to be addressed before choosing the method of screening for limited DF ROM. First, assessment may be performed in weight bearing (WB) or non–weight bearing (NWB). The decision as to which measurement to use is confounded by the somewhat selective association of WB and NWB DF ROM with several disorders. For example, although WB DF has been previously implicated in the risk of sustaining ankle sprains (19), NWB DF has not (12). Contrarily, although NWB DF has been previously implicated in the risk of developing Achilles tendinopathy and PFP (18, 21), WB DF has not (2,21). Similarly, previous studies also suggest that WB and NWB DF are differentially associated with several hip and knee kinematics purported to increase the risk of PFP and ACL rupture (7,25). Collectively, these findings suggest that it may be more prudent to include both WB and NWB measurements when screening for a wide spectrum of injuries.
Laterality is another consideration in screening for DF ROM. Previous research indicates NWB DF may be greater on the dominant side, whereas WB DF seems greater on the nondominant side of healthy individuals (14,24). This suggests any measurement used may need to be performed bilaterally.
As several different measurements may need to be performed to gain a comprehensive appreciation of the risk of injury associated with limited DF, a quick screening test for this impairment may be valuable. One potential screening test is the overhead squat (OS), which involves deep squatting with the heels in full contact with the floor, while holding a dowel directly overhead (6). Limited DF may hinder the ability to perform a deep squat by prematurely halting forward motion of the tibia. This may necessitate a greater compensatory posterior roll of the femur to complete the squat, which is likely to increase the level of difficulty due to the resultant posterior shift of the center of mass. Although several studies previously assessed at the association between the OS and ankle DF ROM (4,7), these studies primarily focused on the association between DF ROM and quality of movement (4,7). We are unaware, however, of studies assessing the accuracy of the OS in identifying individuals with limited DF ROM. Furthermore, it has been our impression that a successful completion of the OS necessitates an exceptionally high DF ROM, suggesting if the OS was used as an indicator of limited DF, the potential for falsely classifying individuals with limited DF may be high (i.e., high false-positive rate). This impression is somewhat supported by a recent study showing that only a small percentage of healthy individuals were able to complete the OS successfully (29). If the OS could be modified to reduce its level of difficulty, this concern may be lessened.
One possible modification of the OS is to replace the overhead arm component by a forward arm position. A forward arm position is likely to result in a forward shift in the center of mass compared with the overhead position. This may allow the completion of the squat even in the absence of an exceptionally high DF ROM, thereby decreasing the potential for false-positive ratings.
Given the association between DF ROM and several lower extremity disorders, and in attempt to facilitate screening procedures for limited DF ROM, the primary purpose of this study was to assess whether the OS, and its modification, the forward arm squat (FAS), can detect individuals with limited DF ROM. As a secondary purpose, we aimed to determine the interrater reliability of the OS and FAS.
Experimental Approach to the Problem
The OS and FAS were performed on 53 healthy individuals, although bilateral measurements of WB and NWB ankle DF ROM were also taken. In an attempt to provide an acceptable reference standard for limited DF ROM, individuals whose ROM fell below 1 SD from the sample average were considered to have limited DF ROM, whereas all other participants were considered to have nonlimited DF ROM. The sensitivity, specificity, positive, and negative likelihood ratio (LR) of the OS and FAS in detecting individuals with limited DF ROM was calculated.
The study was approved by the Ethics Committee of Ariel University, and all participants signed an informed consent form before taking part in any study procedure. Fifty-three (32 women; age range 20–41 years) healthy participants were recruited from a university campus. The mean ± SD age, height, and weight were 26.0 ± 4.0 years, 169.8 ± 9.4 cm, and 66.8 ± 12.8 kg. To be included in the study, participants had to be at least 18 years old, free of any pain in either lower extremity, or in the lumbar spine, and without a history of orthopedic surgery to either lower extremity, or to the lumbar spine.
Two examiners performed all measurements for the study. One examiner with over 16 years of clinical experience in the management of musculoskeletal conditions (AR), whereas the other examiner with over 28 years of teaching experience in the field of kinesiology and neurological rehabilitation (ZK). Before data collection, the 2 examiners met for a 4-hour session to review all data collection procedures.
Data were collected on 2 separate sessions approximately 1 week apart. The FAS and DF ROM measurements were performed during the first session, whereas the OS was completed on the second session. As the scoring of the OS may require 6 repetitions to complete (6), it was decided to separate this test from all other measurements, to minimize any tissue-conditioning effects from repetitive squatting efforts. Separating the testing sessions also served to minimize recall bias during rating of the 2 squat tests.
The FAS and OS were simultaneously rated by both examiners, during the first and second session, respectively, to establish their interrater reliability. Each examiner rated the tests separately, and no discussion was allowed between the examiners during the tests. Weight bearing, followed by NWB DF ROM, was measured by one of the examiners (AR) during the first testing session, immediately after the FAS. To minimize biasing the DF measurements based on the results of the FAS, the scores of the FAS made by the examiner not involved in DF measurements (ZK) were used for data analysis.
The OS was performed as described by Cook et al. (6) Participants wore their own sneakers during the test and assumed the starting position by placing their feet approximately shoulder width apart and aligned in the sagittal plane. Participants then gripped a dowel while maintaining their elbows bent 90° with the dowel overhead. Next, the dowel was pressed overhead by abducting both shoulders and extending the elbows. Participants were then instructed to squat as far as possible while maintaining both heels in contact with the floor and keeping the dowel directly overhead. Each participant was allowed up to 3 repetitions to successfully perform the test. The scoring criteria for the OS are outlined in Table 1. As none of the participants in the study received a score of 0 or 1, scoring was dichotomized to negative (score 3, Figure 1) or positive (score 2). The examiners were first positioned 3 meters to the side of the participant, and after scoring the sagittal-plane criteria, examiners moved in front of the participant to verify frontal-plane knee alignment (Table 1).
Forward Arm Squat
Participants were barefooted during the test and assumed the starting position by placing their feet approximately shoulder width apart and aligned in the sagittal plane. Participants were instructed to squat as far as possible while maintaining both heels in contact with the floor, and both arms reaching forward (shoulders flexed 90° and the elbows extended) (Figure 2). Each participant was allowed up to 3 attempts to complete the test. The 2 examiners were positioned 3 meters to the side of the participant during the test. The test was scored dichotomously as positive (score 1) or negative (score 0). A negative score was assigned when the participant was able to reach full knee flexion with both heels maintaining contact with the floor, and hold this position for 3 seconds. A positive score was assigned when the participant could not reach full knee flexion without lifting one or both heels off the floor (Figure 3), or when the participant could not maintain full knee flexion for at least 3 seconds. Participants with a positive score were asked for the reason they could not perform the test, to ascertain this was not due to pain.
Weight-Bearing Ankle Dorsiflexion Range of Motion
The measurement was performed using a fluid-filled inclinometer with 1-degree increments (MIE Medical Research LTD., Leeds, UK) as previously described (20). The right side was measured first followed by the left. A 50-cm long line was drawn on the floor and a continuous 60-cm long line was drawn on a wall where the test was performed. A 1-cm red sticker was placed bilaterally on the anterior tibia of each participant, 15 cm distal to the tibial tuberosity. Participants placed the tested foot along the floor line so that the line bisected the heel and the second toe was on the line as well. The nontested foot was placed comfortably behind the tested foot, and had to remain in contact with the floor throughout the test (although its heel could be raised during the lunge forward). Participants were asked to lunge forward and bring the patella of the tested side as close as possible to the vertical line drawn on the wall without lifting the heel on the tested side off the floor. Once maximal DF was reached, the examiner placed the inclinometer, which was first zeroed on a fixed vertical reference, over the marked spot on the anterior tibia of the participant. The DF angle was recorded, and the participant returned to the starting position. The procedure was repeated 3 times on each side with the average serving for data analysis. The interrater reliability of this measurement has been previously established as excellent (intraclass correlation coefficient [ICC] [95% CI] 0.95 [0.90–0.98]) (20). Pilot testing on 7 healthy individuals reaffirmed an excellent level of interrater reliability (ICC 0.88–0.97).
Non–weight-Bearing Ankle Dorsiflexion Range of Motion
The measurement was performed using a universal goniometer with 1-degree increments as previously described (20). The right side was measured first followed by the left. The participant assumed a prone lying position with the knee flexed 90°. The examiner manually verified a subtalar neutral position and placed the ankle at end-range DF as determined by a firm end feel. Once full DF was attained, the examiner manually verified that end range was maintained while placing the goniometer to measure the angle between the lateral midline of the lower leg and the rearfoot. The average of 3 measurements on each side was used for data analysis. The interrater reliability of this measurement was previously established as excellent (ICC [95% CI] 0.86 [0.71–0.94]) (20). Similarly, pilot data on 7 healthy individuals reaffirmed an excellent level of interrater reliability (ICC 0.91–0.95).
Descriptive statistics were used to summarize the data with measures of central tendency and dispersion for continuous variables and frequency counts for categorical variables. Differences in WB and NWB DF ROM, between participants with a positive and a negative OS and FAS were assessed using separate Mann-Whitney U-tests. To establish a reference standard against which to assess the accuracy of the OS and FAS in detecting limited DF ROM, participants were classified into a low-DF subgroup if their DF ROM fell below 1 SD from the sample average, whereas all other participants were classified into an average/high-DF subgroup.
For the primary purpose of the study, 4 separate 2 × 2 tables were computed for each squat test (OS or FAS) based on the score of the test (positive vs. negative) and the DF subgrouping (low vs. average/high). The sensitivity, specificity, positive, and negative LRs with their corresponding 95% CI were then computed. For the secondary purpose of the study, percent agreement and kappa coefficient were used to determine the interrater reliability of the OS and FAS. All analyses were made using SPSS version 21 with an a priori level of significance of p ≤ 0.05.
Differences in WB and NWB DF ROM between participants with a positive (score 2) and negative (score 3) OS, as well as a positive (score 1) and negative (score 0) FAS are summarized in Table 2. Independent of the type of DF measurement (WB vs. NWB), or the side tested (right vs. left), participants with a positive OS or FAS demonstrated decreased DF ROM than those with a, respectively, negative test (p < 0.01) (Table 2).
Based on our predetermined cutoff threshold, the right low-WB DF subgroup included individuals with less than 45.4°, the left low-WB DF subgroup included individuals with less than 49.6°, the right low-NWB DF subgroup included individuals with less than 26.3°, and the left low-NWB DF subgroup included individuals with less than 22.4°. The sensitivity, specificity, positive, and negative LRs of the OS and FAS in detecting the various low-DF subgroups are summarized in Table 3. The sensitivity of the OS was 1.00 regardless of the type of DF measurement used, whereas specificity ranged from 0.34 to 0.36. The positive LR of the OS ranged from 1.52 to 1.56, whereas negative LR was 0.00. Sensitivity of the FAS ranged from 0.56 to 0.70, whereas specificity ranged from 0.84 to 0.88. The positive and negative LR of the FAS ranged from 3.49 to 6.02, and 0.34 to 0.53, respectively.
Percent agreement and kappa coefficient (95% CI) for the interrater reliability of the OS were 96.2% and 0.91 (0.79–1.00), respectively. Percent agreement and kappa coefficient (95% CI) for the interrater reliability of the FAS were 92.5% and 0.78 (0.58–0.99), respectively.
The OS and FAS demonstrate an excellent level of interrater reliability when performed on healthy individuals. Both tests are also associated with ankle DF ROM as measured in WB or NWB.
When used to detect limited DF ROM, the OS possessed perfect sensitivity. This suggests that the OS is not likely to falsely classify individuals as having an adequate DF ROM (i.e., a low false-negative rate). Consequently, when the OS is negative, limited DF ROM may confidently be ruled out. The negative LR associated with the OS gives an indication of the magnitude of the shift in probability of having limited DF ROM when the OS is negative. For example, the probability of being classified into the right low-WB DF subgroup in the current sample was 19% (pretest probability). Given a negative OS, and along with its negative LR of 0.00, this probability is essentially reduced to 0%. Nevertheless, because of its low specificity, the OS may frequently classify individuals as having limited DF ROM when, in fact, DF ROM is within normal limits (i.e., a high false-positive rate). This tendency, which is in accord with our original concern with this test, suggests that a positive OS may not be relied on to rule in limited DF ROM.
Unlike the OS, the FAS exhibited lower levels of sensitivity and higher levels of specificity. Therefore, although the FAS may not be relied on to rule out limited DF ROM, the high specificity associated with this test makes it unlikely to falsely classify individuals as having limited DF ROM (low false-positive rate). Consequently, a positive FAS may help confirm limited DF ROM. Accordingly, given a pretest probability of 19% for being classified into the right low-WB DF subgroup, and given a positive LR of 5.35, a positive FAS will shift the posttest probability to approximately 60%. This level of probability may justify a more thorough assessment of DF ROM, or alternatively, may directly justify the prescription of corrective exercises.
The somewhat opposite diagnostic qualities of the OS and FAS may make them good complementing tests in screening for limited DF ROM. Because of its superior sensitivity, the OS may best be performed first, and if negative, may confidently rule out limited DF ROM. However, when the OS is positive, testing should proceed with the more specific FAS, which if positive, may indicate the need for further assessment or direct intervention to increase DF ROM. We believe that this 2-step process, which should take no longer than 1 minute to complete, may save valuable time, while simultaneously contribute to accurate identification of individuals with limited DF ROM.
The apparent greater level of difficulty of the OS compared with FAS likely explains its much lower rate of success (28% vs. 77%). Because of its more challenging nature, the OS likely possesses greater sensitivity, as for the most part, only individuals with a fairly high DF ROM were able to complete the test properly. In fact, the average DF ROM among participants with a negative OS test was greater by almost 1 SD from the sample average. By contrast, the less challenging nature of the FAS likely contributed to its greater specificity, as the average DF ROM among participants with a positive FAS, was lower by almost 1 SD from the sample average.
The cutoff thresholds for classifying individuals into the low-DF subgroups were based on the distribution of scores in the current sample, rather than on a predetermined threshold. This method was preferred, as even slight differences in measurement methodology are likely to result in differences in ROM value. By contrast, providing a normal distribution of DF ROM, the 1 SD threshold should result in classifying only individuals in the lowest quintile of the sample into the low-DF subgroup. This, we believe, is an acceptable definition for low-DF ROM. Furthermore, if the cutoff thresholds of the current investigation were used on individuals from 4 previous studies using the same DF measurement techniques (20,22,23,25), then with the exception of right NWB DF, only individuals in the lower quartile of the sample would qualify for the low-DF ROM subgroup (20,22,23,25). Finally, the threshold for the right low-WB DF subgroup was remarkably similar to that previously suggested to increase the risk of patellar tendinopathy (15), whereas the threshold for the left low-NWB DF subgroup was very similar to that previously suggested to increase the risk of Achilles tendinopathy (21). Collectively, these findings suggest that the reference standards used in the current investigation are likely to be valid.
Two previous studies reported findings in agreement with the current investigation (4,7). Dill et al. (7) used 3-dimensional motion analysis to demonstrate decreased sagittal-plane knee and ankle motion during the OS among individuals with limited clinically measured WB, but not NWB DF ROM. (7) Our findings suggest that visually detected differences in the performance of the OS are associated with clinically measured WB, as well as NWB DF ROM. The fact that we measured NWB DF with the knee flexed, as opposed to knee extended as measured by Dill et al. (7), likely contributed to the significant difference in NWB DF detected in the current investigation. Bell et al. (4) found a trend toward decreased NWB DF ROM among individuals who demonstrated a medial knee collapse during the OS, that was later corrected when the test was repeated using a 2-inch heel lift (4). The fact that we incorporated sagittal-plane motion criteria to rate the OS (wand position, trunk, femur, and tibial alignment) likely contributed to the clearer differences in NWB DF in the current investigation. Furthermore, compared with these previous 2 studies (4,7), our investigation also established the accuracy of the OS, as well as the FAS, in detecting limited DF ROM, which contributes to its clinical applicability.
Our study has several limitations. First, we only measured DF ROM; and thus, we cannot determine the possible effects of other variables potentially influencing squatting ability, such as hip or knee ROM. Be that as it may, none of the participants in the study had a history of surgery in either lower extremity, a current complaint of pain in either lower extremity, or any pain specifically during the performance of either of the squat tests. Second, although WB DF among participants was fairly similar to that in 3 previous investigations (20, 22, 23), NWB DF ROM was somewhat greater than that previously described (18,20,23). This is most likely due to the manual assistance provided by the examiner during the NWB measurement. Previous experience with the NWB measurement suggests that some individuals experience difficulty in actively maintaining end-range DF long enough to allow measurement. As, in the context of its association with squatting, passive DF ROM seems to be more meaningful, we elected to assess this range passively by manually verifying and maintaining the end-range position. Third, participants wore their own sneakers during the OS while performing the FAS barefooted. As the OS was originally described with shoes (6), we decided to replicate this in the current investigation. Furthermore, as the OS may involve standing on a 2 × 6-inch board, we believed barefoot conditions would have been uncomfortable. Nevertheless, as direct observation of the heels is the most accurate way to ascertain their position, the FAS was performed without shoes. In any event, we do not believe the different footwear conditions had a substantial effect on our findings. The shod condition could have only served to facilitate squatting during the OS, and given the low success rate in this test, we believe such an effect was likely to be minimal. Finally, the examiner performing the DF ROM measurements was not completely blinded to the scores of the FAS, as he also observed and rated this test to establish interrater reliability.
The OS and FAS are reliable tests associated with WB and NWB DF ROM. When used to detect limited DF ROM, the OS possesses excellent sensitivity but inadequate specificity, whereas the FAS possesses inadequate sensitivity and fairly high specificity. These somewhat contrasting qualities suggest that the 2 tests may best be used to complement one another in screening for limited DF ROM. Accordingly, a negative OS can confidently be used to rule out a limited DF ROM. However, given a positive OS, testing should proceed with the FAS, which if positive, can more confidently rule in limited DF ROM.
1. Backman LJ, Danielson P. Low range of ankle dorsiflexion predisposes for patellar tendinopathy in junior elite basketball players: A 1-year prospective study. Am J Sports Med 39: 2626–2633, 2011.
2. Barton CJ, Bonanno D, Levinger P, Menz HB. Foot and ankle characteristics in patellofemoral pain syndrome: A case control and reliability study. J Orthop Sports Phys Ther 40: 286–296, 2010.
3. Basnett CR, Hanish MJ, Wheeler TJ, Miriovsky DJ, Danielson EL, Barr JB, Grindstaff TL. Ankle dorsiflexion range of motion influences dynamic balance in individuals with chronic ankle instability. Int J Sports Phys Ther 8: 121–128, 2013.
4. Bell DR, Padua DA, Clark MA. Muscle strength and flexibility characteristics of people displaying excessive medial knee displacement. Arch Phys Med Rehabil 89: 1323–1328, 2008.
5. Boling M, Padua D. Relationship between hip strength and trunk, hip, and knee kinematics during a jump-landing task in individuals with patellofemoral pain. Int J Sports Phys Ther 8: 661–669, 2013.
6. Cook G, Burton L, Hoogenboom B. Pre-participation screening: The use of fundamental movements as an assessment of function—Part 1. N Am J Sports Phys Ther 2006 1: 62–72. 2006.
7. Dill KE, Begalle RL, Frank BS, Zinder SM, Padua DA. Altered knee and ankle kinematics during squatting in those with limited weight-bearing-lunge ankle-dorsiflexion range of motion. J Athl Train 49: 723–732, 2014.
8. Fong CM, Blackburn JT, Norcross MF, McGrath M, Padua DA. Ankle-dorsiflexion range of motion and landing biomechanics. J Athl Train 46: 5–10, 2011.
9. Hewett TE, Myer GD, Ford KR, Heidt RS Jr, Colosimo AJ, McLean SG, van den Bogert AJ, Paterno MV, Succop P. Biomechanical measures of neuromuscular and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am J Sports Med 2005 33: 492–501. 2005.
10. Hoch MC, Andreatta RD, Mullineaux DR, English RA, Medina McKeon JM, Mattacola CG, McKeon PO. Two-week joint mobilization intervention improves self-reported function, range of motion, and dynamic balance in those with chronic ankle instability. J Orthop Res 31: 1798–1804, 2012.
11. Hoch MC, McKeon PO. Joint mobilization improves spatiotemporal postural control and range of motion in those with chronic ankle instability. J Orthop Res 29: 326–332, 2011.
12. Kaufman KR, Brodine SK, Shaffer RA, Johnson CW, Cullison TR. The effect of foot structure and range of motion on musculoskeletal overuse injuries. Am J Sports Med 27: 585–593, 1999.
13. Kulig K, Loudon JK, Popovich JM, Pollard CD, Winder BR. Dancers with achilles tendinopathy demonstrate altered lower extremity takeoff kinematics. J Orthop Sports Phys Ther 41: 606–613, 2011.
14. Macedo LG, Magee DJ. Differences in range of motion between dominant and nondominant sides of upper and lower extremities. J Manipulative Physiol Ther 31: 577–582, 2008.
15. Malliaras P, Cook JL, Kent P. Reduced ankle dorsiflexion range may increase the risk of patellar tendon injury among volleyball players. J Sci Med Sport 9: 304–309, 2006.
16. Malloy P, Morgan A, Meinerz C, Geiser C, Kipp K. The association of dorsiflexion flexibility on knee kinematics and kinetics during a drop vertical jump in healthy female athletes. Knee Surg Sports Traumatol Arthrosc 23: 3550–3555, 2015.
17. Nakagawa TH, Moriya ETU, Maciel CD, Serrao FV. Trunk, pelvis, hip, and knee kinematics, hip strength, and gluteal muscle activation during a single-leg squat in males and females with and without patellofemoral pain syndrome. J Orthop Sports Phys Ther 42: 491–501, 2012.
18. Piva SR, Goodnite EA, Childs JD. Strength around the hip and flexibility of soft tissues in individuals with and without patellofemoral pain syndrome. J Orthop Sport Phys Ther 35: 793–801, 2005.
19. Pope R, Herbert R, Kirwan J. Effects of ankle DF range and pre-exercise calf muscle stretching on injury risk in army recruits. Aust J Physiother 44: 165–177, 1998.
20. Rabin A, Kozol Z. Measures of range of motion and strength among healthy women with differing quality of lower extremity movement during the lateral step-down test. J Orthop Sports Phys Ther 40: 792–800, 2010.
21. Rabin A, Kozol Z, Finestone AS. Limited ankle dorsiflexion increases the risk for mid-portion achilles tendinopathy in infantry recruits: A prospective cohort study. J Foot Ankle Res 7: 48, 2014.
22. Rabin A, Kozol Z, Moran U, Efergan A, Geffen Y, Finestone AS. Factors associated with visually assessed quality of movement during a lateral step-down test among individuals with patellofemoral pain. J Orthop Sports Phys Ther 44: 937–946, 2014.
23. Rabin A, Kozol Z, Spitzer E, Finestone A. Ankle dorsiflexion among healthy men with different qualities of lower extremity movement. J Athl Train 49: 617–623, 2014.
24. Rabin A, Kozol Z, Spitzer E, Finestone AS. Weight-bearing ankle dorsiflexion range of motion—Can side-to-side symmetry be assumed. J Athl Train 50: 30–35, 2015.
25. Rabin A, Portnoy S, Kozol Z. The association of ankle dorsiflexion range of motion with hip and knee kinematics during the lateral step down test. J Orthop Sports Phys Ther 46: 1002–1009, 2016.
26. Sigward SM, Susumu O, Powers CM. Predictors of frontal plane knee excursion during a drop land in young female soccer players. J Orthop Sports Phys Ther 38: 661–667, 2008.
27. Souza RB, Powers CM. Differences in hip kinematics, muscle strength, and muscle activation between participants with and without patellofemoral pain. J Orthop Sports Phys Ther 39: 12–19, 2009.
28. Tabrizi P, McIntyre WMJ, Quesnel MB, Howard AW. Limited dorsiflexion predisposes to injuries of the ankle in children. J Bone Joint Surg Br 82: 1103–1106, 2000.
29. Teyhen DS, Shaffer SW, Lorenson CL, Halfpap JP, Donofry DF, Walker MJ, Dugan JL, Childs JD. The functional movement screen: A reliability study. J Orthop Sports Phys Ther 42: 530–540, 2012.
30. Wahlstedt C, Rasmussen-Barr E. Anterior cruciate ligament injury and ankle dorsiflexion. Knee Surg Sports Traumatol Arthrosc 23: 3202–3207, 2015.
31. Williams B, Zambardino JA, Banning VA. Transverse-plane mechanics at the knee and tibia in runners with and without a history of achilles tendonopathy. J Orthop Sports Phys Ther 38: 761–767, 2008.
32. Witvrouw E, Lysens R, Bellemans J, Cambier D, Vanderstraeten G. Intrinsic risk factors for the development of anterior knee pain in an athletic population. A two-year prospective study. Am J Sports Med 28: 480–489, 2000.