Correlations Between Hip Strength and Static Foot and Knee Posture

Transcription

1 J Sport Rehabil. 2006, 15, Human Kinetics, Inc. Correlations Between Hip Strength and Static Foot and Knee Posture John H. Hollman, Kimberly E. Kolbeck, Jamie L. Hitchcock, Jonathan W. Koverman, and David A. Krause Context: Hip-muscle weakness might be associated with impaired biomechanics and postures that contribute to lower extremity injuries. Objective: To examine relationships between hip-muscle strength, Q angle, and foot pronation. Design: Correlational study. Setting: Academic laboratory. Participants: 33 healthy adults. Main Outcome Measures: Maximal isometric hip abduction (Abd), adduction (Add), external-rotation (ER) and internal-rotation (IR) strength; Q angle of the knee; and longitudinal arch angle of the foot. We analyzed Pearson product moment (r) correlation coefficients between the Abd/Add and ER/IR force ratios, Q angle, and longitudinal arch angle. Results: The hip Abd/Add force ratio was correlated with longitudinal arch angle (r =.35, P =.025). Conclusions: Reduced strength of the hip abductors relative to adductors is associated with increased pronation at the foot. Clinicians should be aware of this relationship when examining patients with lower extremity impairments. Key Words: longitudinal arch angle, Q angle, lower extremity malalignment Many lower extremity injuries, particularly overuse injuries, are thought to be related to impaired segmental biomechanics that occur between the pelvis, thigh, leg, and foot. 1-4 Conventional wisdom suggests that the lower extremity acts as a linked segment in which movement at one joint or joint complex requires compensatory movement at the other joints when the foot is on the ground. Two theoretical approaches have been presented to explain the effects of impaired biomechanics on the lower extremity linkages. 2,3 The first is a ground up approach. As an example, patellofemoral mechanics might be altered in people who have excessive or prolonged foot pronation during weight-bearing activities. As described by Tiberio, 2 when a person ambulates the tibia must rotate externally relative to the femur so that knee extension during midstance of the gait cycle can be achieved through the screw-home mechanism. With prolonged or excessive foot pronation during the weight-bearing phase of the gait cycle, the tibia remains internally rotated as the knee begins to extend. To compensate for the internal rotation of the tibia, it is thought that the femur must rotate internally to place the tibia in a position of external rotation relative to the femur. This compensation places the tibiofemoral joint in greater valgus and creates a larger quadriceps angle (Q angle) an angle formed between a line representing the resultant quadriceps force vector and a The authors are with the Mayo Clinic College of Medicine, Rochester, MN Hollman(12) 12 1/17/06, 10:41:18 AM

2 Hip Strength, Static Foot, and Knee Posture 13 line representing the force vector of the infrapatellar ligament which might lead to increased lateral patellofemoral-joint contact pressures. 5 This ground-up theoretical approach is often used to explain atypical lower extremity mechanics that are observed clinically in people with patellofemoral pain, but it is controversial. Two studies have compared foot pronation during the gait cycle in people with and without patellofemoral pain, and neither reported a difference in magnitude or timing of pronation between the groups. 6,7 In contrast, other investigators consider a top down theoretical approach to explaining similar atypical lower extremity mechanics. Proximal factors such as hip-muscle weakness, particularly in the frontal and transverse planes of motion, have been proposed to contribute to abnormal lower extremity mechanics If the hip lacks dynamic stability in either the frontal or the transverse plane during weight-bearing activities, the femur might adduct and/or rotate internally (Figure 1). This medial collapse of the femur described by Powers 3 is accompanied by increased valgus of the tibiofemoral joint, internal rotation of the tibia, and subsequently excessive pronation of the foot. Figure 1 Example of medial collapse of the lower extremity during a unilateral weightbearing activity: (a) adduction and/or internal rotation of the femur accompanied by (b) tibiofemoral valgus, (c) internal rotation of the tibia, and (d) pronation of the foot. 02Hollman(12) 13 1/17/06, 10:41:20 AM

3 14 Hollman et al Regardless of whether one considers a top-down or ground-up approach to examining lower extremity mechanics, it is clear that all segments must be examined carefully during the clinical exam. Although the relationship between static posture and dynamic stability is not entirely clear, 2 of the postural assessments typically conducted in patients with lower extremity overuse injuries include the assessment of subtalar-joint pronation and Q angle at the knee. Excessive foot pronation is associated with increased tibiofemoral valgus during running. 11 The malalignment at the knee associated with excessive foot pronation is, by extension, associated with an increased Q angle and with increased lateral patellofemoral-joint contact pressures. 5 Foot pronation and Q angle seem to be intricately linked and are therefore assessed clinically as static postural measures that might be signs of pathomechanical lower extremity movements that occur during dynamic conditions. Traditionally, many lower extremity injuries including patellofemoral-pain syndrome have been attributed to excessive foot pronation 1 and weakness of the muscles that support the medial longitudinal arch of the foot. 12 Although less attention has been paid to the hip muscles than to muscles of the lower leg and foot, some authors have speculated that hip weakness might have a role in lower extremity malalignment and in the etiology of lower extremity overuse injuries such as patellofemoral-pain syndrome, 4,8-10 iliotibial-band syndrome, 13 and even foot and ankle problems. 14 Ireland et al 4 recently provided empirical evidence that young women with patellofemoral pain have 26% less hip-abduction strength and 36% less hip external-rotation strength than age-matched control subjects without patellofemoral pain. Nicholas et al 14 reported a correlation between ankle and foot problems and impaired ipsilateral hip-abduction and -adduction strength. Neither study 4,14 examined lower extremity posture. Based on biomechanical principles and given the presumed postural impairments that occur with medial collapse of the femur, one could reasonably assume that impaired strength of the hip abductors and external rotators would be associated with an increased Q angle at the knee and increased pronation at the foot. To our knowledge, this relationship has not been tested empirically. The purpose of this study, therefore, was to examine the relationship between hip-muscle strength, Q angle, and foot pronation. We think it is the imbalance between opposing muscle forces that often leads to atypical mechanics. We therefore examined force-production capability at the hip in terms of the abductionto-adduction (Abd/Add) force ratio and the external-rotation to internal-rotation (ER/IR) force ratio. We hypothesized that the Abd/Add and ER/IR force ratios at the hip would be statistically correlated with Q angle and foot-pronation postures during static stance. Subjects Methods Thirty-three healthy, active volunteers (11 men and 22 women), age 22 to 31 years (mean 24.5 ± 2.6 years), participated in the study. Brief medical histories were obtained, and volunteers were excluded from participating in the study if they reported having had any lower extremity injury or surgery in the year before 02Hollman(12) 14 1/17/06, 10:41:22 AM

4 Hip Strength, Static Foot, and Knee Posture 15 their participation in the study. Subjects signed a consent form approved by the institutional review board. Procedures Subjects were asked to identify their preferred stance leg, operationally defined in this study as the preferred plant leg during kicking. All measurements were taken from the preferred stance leg of each individual. A MicroFET 2 handheld dynamometer (Hoggan Health Industries, Inc, West Jordan, Utah) was used to quantify strength of the hip abductors, adductors, external rotators, and internal rotators. For the purpose of this study, strength was operationally defined as maximal voluntary isometric force-production capability. The MicroFET 2 dynamometer has been previously shown to have high interrater and intrarater reliability for quantifying muscle-force-production capability in lower extremity muscles in patients with pathological conditions. 15,16 Strength was tested isometrically with a break test held for 5 seconds. Positions for testing were based on manual-muscle-test standards established by Daniels and Worthingham, 17 and all manual muscle tests were performed by the same investigator. The order of testing was randomized. Each test was performed 3 times with approximately 30 seconds of rest between repetitions. The maximum value of the 3 tests was recorded. Ten subjects selected at random returned for repeat testing 48 to 72 hours after the first testing session to establish an estimate of intrarater reliability. A second investigator measured Q angle, a measure of frontal-plane knee posture defined as the acute angle between a line connecting the anterior superior iliac spine (ASIS) and the midpoint of the patella and a line connecting the tibial tubercle with the same reference point on the patella. 18 Although impaired strength at the hip might presumably have a greater effect on Q angle during single-legstance activities, we measured Q angle while subjects were in relaxed bilateral stance with their feet approximately shoulder-width apart. Quadriceps and gluteal activation during single-leg stance would likely have reduced the Q angle 19 and might have reduced the measurementʼs reliability. The investigator palpated the ASIS, midpoint of the patella, and tibial tubercle and placed a small sticker at each of the bony prominences. Each subject held the end of a string at the ASIS while the investigator held the distal end of the string taut at the midpoint of the patella. The same investigator then used a universal goniometer (NexGen Ergonomics, Inc, Point Claire, Quebec, Canada), which was aligned proximally from the midpoint of the patella with the guide string and was aligned distally with the tibial tubercle, to measure Q angle. Ten subjects selected at random returned for repeat testing 48 to 72 hours after the first testing session to establish an estimate of intrarater reliability. A third investigator measured longitudinal arch angle. Longitudinal arch angle measured in bilateral stance has high intrarater reliability (ICC =.90) 20 and is highly correlated (r =.97) with the dynamic longitudinal arch angle during midstance of gait, 21 and for those reasons we chose to use it in this study as a measure of foot pronation. The longitudinal arch angle is measured as the obtuse angle formed by the lines between the center of the medial malleolus and the navicular tubercle and a line between the center of the navicular tubercle and the first metatarsal head (Figure 2). We measured longitudinal arch angle while subjects were in relaxed 02Hollman(12) 15 1/17/06, 10:41:24 AM

5 16 Hollman et al Figure 2 Longitudinal arch angle was measured using the method described by Jonson and Gross. 20 An obtuse angle less than 140 indicates excessive pronation. bilateral stance, using the method described by Jonson and Gross. 20 The investigator palpated each of the bony landmarks, placed a small sticker on each, and measured the longitudinal arch angle with a universal goniometer. Ten subjects selected at random returned for repeat testing 48 to 72 hours after the first testing session to establish an estimate of intrarater reliability. Data Analysis Intrarater reliability coefficients for muscle strength, Q angle, and longitudinal arch angle were estimated with the intraclass correlation coefficient (ICC 3,1 ) described by Shrout and Fleiss. 22 Descriptive data (means and standard deviations) were calculated. Relationships between hip-muscle performance, Q angle, and longitudinal arch angle were quantified with Pearson product moment (r) correlation coefficients (α =.05). All muscle-performance data were normalized to body weight. Because agonist antagonist force ratios can characterize muscle imbalance about a joint, 23 we calculated the Abd/Add and ER/IR force ratios to be included in the analysis. We hypothesized that the Abd/Add and ER/IR force ratios would be positively correlated with the longitudinal arch angle and negatively correlated with Q angle; that is, a reduction in relative hip-abduction and external-rotation strength would be associated with a decreased longitudinal arch angle and an increased Q angle. Data were analyzed with SPSS 10.0 statistical software (SPSS Inc, Chicago, Ill). Results Intrarater reliability coefficients for force-production-capability, Q-angle, and longitudinal-arch-angle measurements are presented in Table 1. Although the reliability coefficient for hip-adduction force-production capability was relatively low 02Hollman(12) 16 1/17/06, 10:41:25 AM

6 Hip Strength, Static Foot, and Knee Posture 17 (ICC 3,1 =.54), the reliability coefficients for hip abduction, external rotation, and internal rotation were much higher (ICC 3,1 ranging from.75 to.91). The ICC 3,1 values for Q angle and longitudinal arch angle were both.87. Descriptive data are provided in Table 2. The correlation coefficients between the Abd/Add force ratio, ER/IR force ratio, Q angle, and longitudinal arch angle were generally low (Table 3). A statistically significant correlation (r =.35, P =.025) was found, however, between the Abd/Add force ratio and longitudinal arch angle (Figure 3). None of the other correlations was statistically significant at α =.05. Table 1 Intrarater Reliability Coefficients (ICC 3,1 ) and Standard Errors of Measurement (SEM) for Hip-Muscle Strength, Q Angle, and Longitudinal Arch Angle Variable ICC 3,1 SEM Force production (N) abduction adduction external rotation internal rotation Q angle ( ) Longitudinal arch angle ( ) Table 2 Descriptive Statistics (mean ± SD) for the Variables Measured Variable Men (n = 11) Women (n = 22) Height (cm) ± ± 7.1 Mass (kg) 89.1 ± ± 11.8 Peak abduction force (%BW) 34.3 ± ± 8.3 Peak adduction force (%BW) 24.8 ± ± 5.3 Peak external-rotation force (%BW) 24.1 ± ± 5.1 Peak internal-rotation force (%BW) 26.0 ± ± 6.2 Abduction to adduction force ratio 1.4 ± ± 0.2 External-rotation to internal-rotation force ratio 1.0 ± ± 0.2 Q angle ( ) 14.7 ± ± 5.4 Longitudinal arch angle ( ) ± ± Hollman(12) 17 1/17/06, 10:41:27 AM

7 18 Hollman et al Table 3 Pearson Product Moment (r) Correlation Coefficients Between Abduction to Adduction (Abd/Add) Force Ratio, External-Rotation to Internal-Rotation (ER/IR) Force Ratio, Q-Angle, and Longitudinal Arch Angle Abd/Add force ratio ER/IR force ratio Q angle Longitudinal arch angle Abd/Add force ratio * ER/IR force ratio Q angle.11 Longitudinal arch angle *P <.05. Figure 3 Correlation between the hip abduction to adduction force ratio and longitudinal arch angle of the foot. The correlation (r =.35) is statistically significant (P =.025). Discussion The role of impaired hip strength in the etiology of lower extremity injury, particularly impairments of the muscles controlling frontal- and transverse-plane hip motions, has received more attention in recent years. This relationship, although based on clinical observations and sound biomechanical reasoning, has been 02Hollman(12) 18 1/17/06, 10:41:28 AM

8 Hip Strength, Static Foot, and Knee Posture 19 described in the professional literature primarily through theoretical descriptive reports 2 and clinical commentaries. 3 With the exception of a few studies that documented hip-strength impairments in patients with distal lower extremity injuries, 4,13,14 there is little empirical evidence to support the concept that medial collapse of the lower extremity during weight bearing, with subsequent postural impairments described by Powers, 3 is associated with impaired strength in the frontal- and transverse-plane hip muscles. We therefore sought to explore the relationship between hip-muscle strength, standing knee posture, and standing foot posture in the present study. The correlation between the hip Abd/Add ratio and longitudinal arch angle (r =.35, P =.025) can be interpreted as a fair relationship. 24 Furthermore, results indicate that variance in the hip Abd/Add force ratio accounts for approximately 12% (r 2 =.12) of the variance in longitudinal arch angle. The longitudinal arch angle provides a reliable measure of foot pronation 20 and explains 94% of the variance in pronation measured dynamically during midstance of gait. 21 It is interpreted such that smaller obtuse angles are indicative of increased subtalar-joint pronation. The hip Abd/Add force ratio quantifies hip-abduction strength relative to hip-adduction strength, and deviations from expected values 25 can indicate a muscle imbalance in the frontal plane. Our results therefore indicate that a reduction in hip-abduction strength relative to adduction strength is associated with increased subtalar-joint pronation in standing. For example, 5 subjects in the present study had longitudinal arch angles that were less than or equal to 140, the threshold level described by Johnston and Gross 26 as being indicative of excessive pronation. Each of those 5 individuals had an Abd/Add force ratio that was lower than the sampleʼs mean ratio of 1.5 (Figure 3). We can only speculate, but because there appears to be a strong relationship between the static measure of longitudinal arch angle and dynamic pronation, 21 it is conceivable that the individuals with lower Abd/Add force ratios (weak abductors relative to adductors) might also be those who demonstrate excessive pronation during gait. Although we did not test the hypothesis directly, our results support the concept that medial collapse of the lower extremity adduction and internal rotation of the femur accompanied by knee valgus, tibia internal rotation, and increased foot pronation during weight-bearing activity is associated with reduced hip-abductor strength. The design of the present study limits our ability to assume that hip strengthening will produce a change in foot pronation, and we cannot assert that hip strengthening will be beneficial for treating lower extremity injuries. The results, however, reinforce the concept, from a biomechanical perspective, that hip strengthening might be beneficial for either preventing or rehabilitating lower extremity injury. Ireland et al 4 reported that young women with patellofemoral-pain syndrome have less hip-abduction strength and less hip external-rotation strength than age-matched control subjects. Although it was not related to the purpose of their study, the authors suggested that exercise to address impaired strength of the hip abductors and external rotators might be beneficial for people with patellofemoral-pain syndrome. Similarly, in a study of runners, Fredericson et al 13 reported that people with iliotibial-band syndrome have impaired hip-abduction strength relative to symptom-free control subjects. Furthermore, the authors reported that after 6 to 8 weeks of rehabilitation focusing on gluteus medius strengthening, over 90% of subjects had returned to pain-free running. Presumably, it is the control of hip 02Hollman(12) 19 1/17/06, 10:41:29 AM

9 20 Hollman et al stability in the frontal and transverse planes that minimizes medial collapse of the lower extremity during repetitive weight-bearing activities and thereby minimizes stresses to soft tissues that cause patientsʼ symptoms. According to general guidelines for interpreting reliability coefficients, the measures we obtained for Q angle and longitudinal arch angle were good and had a narrow range of scores through which one would expect a retest to produce, as indicated by the small standard errors of measurement (Table 1). 24 The measurement of abduction, external-rotation, and internal-rotation strength can also be interpreted as having good reliability according to the guidelines described by Portney and Watkins, 24 although the measurement of adduction might be interpreted as having moderate reliability. Adduction-strength testing yielded a standard error of measurement that exceeded 30 N. We think that the reliability was lower because of the difficulty in manually stabilizing the nontest leg while simultaneously providing sufficient resistance to induce a maximal effort in the test leg of healthy, active subjects. Subsequent to the completion of the present study, we learned that alternative stabilization and testing procedures produce greater reliability for hipadduction strength testing, 27 and they should perhaps be incorporated into future studies that quantify hip strength. Most of the data measured in the present study are consistent with similar variables measured in previously published reports. Hip-abduction-force values are within 1 SD of normative values published by Bohannon. 28 The Abd/Add force ratios (1.4 and 1.5 for men and women, respectively) and ER/IR force ratios (1.0 and 0.9 for men and women, respectively) obtained in the present study are similar to isometric Abd/Add torque ratios (1.3 and 1.2 for men and women, respectively) and ER/IR torque ratios (0.9 and 0.8 for men and women, respectively) calculated from data published by Cahalan et al. 25 Mean Q-angle values, although slightly greater in the present study, are within 1 SD of normative values published by Horton and Hall. 29 Likewise, the mean longitudinal arch angles measured in the present study are also within 1 SD of normative values published by Jonson and Gross. 20 These similarities, when considered with the intrarater reliability coefficients we obtained, suggest that the variables measured in the present study are valid. Nevertheless, interpreting the results of the present study is limited by several factors. First, no patients with lower extremity pathology or complaints of lower extremity symptoms were included in the study. All of the subjects were healthy, active young adults, and therefore the sample lacked individuals who might have otherwise had clinically significant strength impairments at the hip or postural impairments at the knee or foot. Only 5 of the subjects had a longitudinal arch angle less than or equal to 140, the threshold level described by Johnston and Gross 26 as being indicative of excessive pronation. The correlation between hip strength and foot pronation might have been stronger had we recruited more subjects with excessive pronation. Second, although reliability of hip-muscle-strength, Q-angle, and longitudinal-arch-angle measurements was generally good, having met or exceeded reliability coefficients published elsewhere, 20,28,30 reliability for hip-adduction forceproduction capability was somewhat lower than in other published reports. 31 The low reliability for assessing hip-adduction strength might have affected the Abd/Add force-ratio measurements we analyzed. Third, the medial-collapse concept posed in the articleʼs introduction is generally considered a response to hip weakness that occurs in dynamic single-leg-stance situations. The relationship between hip-muscle 02Hollman(12) 20 1/17/06, 10:41:31 AM

10 Hip Strength, Static Foot, and Knee Posture 21 strength and lower limb alignment might be more prominent in single-leg stance than in bilateral stance. Because we measured Q angle and longitudinal arch angle in bilateral stance, this might explain the low correlations observed in the study. A fourth limitation of the study is that the study design does not establish a causal relationship between changes in hip strength and changes in lower extremity posture. Results of our study establish an association between the Abd/Add force ratio and foot pronation. The results also provide some support for the medial-collapse concept of the lower extremity in which lack of hip control in the frontal plane is potentially associated with impaired posture distally. Because of our study design we cannot, however, state that lack of dynamic stability in the frontal plane causes medial collapse. Future studies of a similar nature should examine the relationship between hip-muscle strength and lower extremity postures in patients with lower extremity impairments and perhaps should examine postural malalignments in single-leg stance as compared with bilateral stance. Prospective studies should be conducted to better investigate the role of reduced hip strength in the etiology of lower extremity overuse injuries. In addition, studies should be conducted to examine the effectiveness of hip-muscle strengthening for a variety of lower extremity impairments that might be influenced by medial collapse of the lower extremity during weightbearing activity. Quantifying the relationships between static posture, strength, and dynamic motion will enhance our understanding of lower extremity function and might improve our collective ability to prevent lower extremity injury and to rehabilitate after injury. Conclusion Results of the present study indicate that reduced isometric strength of the hip abductors relative to the hip adductors is associated with increased pronation at the foot. Results provide support for the concept that medial collapse of the lower extremity during weight-bearing activity described as adduction and internal rotation of the femur accompanied by knee valgus, tibia internal rotation, and increased foot pronation might be related to hip-strength imbalance in the frontal plane. Clinicians should be aware of this relationship when examining patients with lower extremity impairments. References 1. Krivickas LS. Anatomical factors associated with overuse sports injuries. Sports Med. 1997;24: Tiberio D. The effect of excessive subtalar joint pronation on patellofemoral mechanics: a theoretical model. J Orthop Sports Phys Ther. 1987;9: Powers CM. The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Orthop Sports Phys Ther. 2003;33: Ireland ML, Willson JD, Ballantyne BT, Davis IM. Hip strength in females with and without patellofemoral pain. J Orthop Sports Phys Ther. 2003;33: Csintalan RP, Schulz MM, Woo J, McMahon PJ, Lee TQ. Gender differences in patellofemoral joint biomechanics. Clin Orthop. 2002;402: Hollman(12) 21 1/17/06, 10:41:32 AM

11 22 Hollman et al 6. Messier SP, Davis SE, Curl WW, Lowery RB, Pack RJ. Etiologic factors associated with patellofemoral pain in runners. Med Sci Sports Exerc. 1991;23: Powers CM, Chen PY, Reischl SF, Perry J. Comparison of foot pronation and lower extremity rotation in persons with and without patellofemoral pain. Foot Ankle Int. 2002;23: Fulkerson JP, Arendt EA. Anterior knee pain in females. Clin Orthop. 2000;372: Fulkerson JP. Diagnosis and treatment of patients with patellofemoral pain. Am J Sports Med. 2002;30: Sommer HM. Patellar chondropathy and apicitis, and muscle imbalances of the lower extremities in competitive sports. Sports Med. 1988;5: McClay I, Manal K. A comparison of three-dimensional lower extremity kinematics during running between excessive pronators and normals. Clin Biomech. 1998;13: Schulhofer SD, Oloff LM. Flexor hallucis longus dysfunction: an overview. Clin Podiatr Med Surg. 2002;19: Fredericson M, Cookingham CL, Chaudhari AM, Dowdell BC, Oestreicher N, Sahrmann SA. Hip abductor weakness in distance runners with iliotibial band syndrome. Clin J Sport Med. 2000;10: Nicholas JA, Strizak AM, Veras G. A study of thigh muscle weakness in different pathological states of the lower extremity. Am J Sports Med. 1976;4: Hedengren E, Knutson LM, Haglund-Akerlind Y, Hagelberg S. Lower extremity isometric joint torque in children with juvenile chronic arthritis. Scand J Rheumatol. 2001;30: Kwoh CK, Petrick MA, Munin MC. Inter-rater reliability for function and strength measurements in the acute care hospital after elective hip and knee arthroplasty. Arthritis Care Res. 1997;10: Hislop HJ, Montgomery J. Daniels & Worthinghamʼs Muscle Testing: Techniques of Manual Examination. Philadelphia, Pa: WB Saunders Co; Neumann DA. Kinesiology of the Musculoskeletal System: Foundations for Physical Rehabilitation. St Louis, Mo: Mosby, Inc; Lathinghouse LH, Trimble MH. Effects of isometric quadriceps activation on the Q-angle in women before and after quadriceps exercise. J Orthop Sports Phys Ther. 2000;30: Jonson SR, Gross MT. Intraexaminer reliability, interexaminer reliability, and mean values for nine lower extremity skeletal measures in healthy naval midshipmen. J Orthop Sports Phys Ther. 1997;25: McPoil TG, Cornwall MW. Use of the longitudinal arch angle to predict dynamic foot posture in walking. J Am Podiatr Med Assoc. 2005;95: Shrout PE, Fleiss JL. Intraclass correlation: uses in assessing rater reliability. Psychol Bull. 1979;86: Kannus P. Isokinetic evaluation of muscular performance: implications for muscle testing and rehabilitation. Int J Sports Med. 1994;15: Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Practice. Upper Saddle River, NJ: Prentice Hall Health; Cahalan TD, Johnson ME, Liu S, Chao EY. Quantitative measurements of hip strength in different age groups. Clin Orthop. 1989;246: Johnston LB, Gross MT. Effects of foot orthoses on quality of life for individuals with patellofemoral pain syndrome. J Orthop Sports Phys Ther. 2004;34: Schlagel SJ, Stember BM, Zoeteway JE, Krause DA. Intra- and inter-rater reliability in hip abductor and adductor strength testing: a comparison of lever length and method of support. Paper presented at: Minnesota American Physical Therapy Association Spring Conference 2005, Brooklyn Park, Minn, April Hollman(12) 22 1/17/06, 10:41:34 AM

12 Hip Strength, Static Foot, and Knee Posture Bohannon RW. Reference values for extremity muscle strength obtained by hand-held dynamometry from adults aged 20 to 79 years. Arch Phys Med Rehabil. 1997;78: Horton MG, Hall TL. Quadriceps femoris muscle angle: normal values and relationships with gender and selected skeletal measures. Phys Ther. 1989;69: Tomsich DA, Nitz AJ, Threlkeld AJ, Shapiro R. Patellofemoral alignment: reliability. J Orthop Sports Phys Ther. 1996;23: Mens JM, Vleeming A, Snijders CJ, Ronchetti I, Stam HJ. Reliability and validity of hip adduction strength to measure disease severity in posterior pelvic pain since pregnancy. Spine. 2002;27: Hollman(12) 23 1/17/06, 10:41:37 AM