Athletic Training and Sports Health Care

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Original Research 

Evaluation of the Lateral Step-Down Test as a Clinical Assessment of Hip Musculature Strength

Marc F. Norcross, MA, ATC; S. Doug Halverson, MA, ATC; Tandice J. Hawkey, MA; J. Troy Blackburn, PhD, ATC; Darin A. Padua, PhD, ATC

Abstract

Gluteal musculature weakness is associated with lower extremity injury and greater frontal plane knee motion. A method for functionally evaluating hip abduction and external rotation strength by observing knee motion would be beneficial for clinicians. This study evaluated the association between hip abductor and external rotation strength and knee motion. Peak isometric and eccentric hip abduction and external rotation strength were assessed in 47 healthy participants before analysis of knee kinematics during a lateral step-down test. The relationships between each strength measure and frontal plane knee kinematics were evaluated using bivariate Pearson correlation coefficients. Frontal plane knee displacement was not associated with hip musculature strength. Frontal plane knee angle at peak knee flexion was not associated with external rotation or eccentric abduction strength. Isometric abduction strength was negatively correlated with peak knee flexion. Hip musculature strength is not indicative of the magnitude of frontal plane knee motion during the lateral step-down test, and this test is not recommended for clinical assessment of hip muscle strength.

Abstract

Gluteal musculature weakness is associated with lower extremity injury and greater frontal plane knee motion. A method for functionally evaluating hip abduction and external rotation strength by observing knee motion would be beneficial for clinicians. This study evaluated the association between hip abductor and external rotation strength and knee motion. Peak isometric and eccentric hip abduction and external rotation strength were assessed in 47 healthy participants before analysis of knee kinematics during a lateral step-down test. The relationships between each strength measure and frontal plane knee kinematics were evaluated using bivariate Pearson correlation coefficients. Frontal plane knee displacement was not associated with hip musculature strength. Frontal plane knee angle at peak knee flexion was not associated with external rotation or eccentric abduction strength. Isometric abduction strength was negatively correlated with peak knee flexion. Hip musculature strength is not indicative of the magnitude of frontal plane knee motion during the lateral step-down test, and this test is not recommended for clinical assessment of hip muscle strength.

Mr Norcross and Dr Blackburn are from the Neuromuscular Research Laboratory, Mr Halverson is from Campus Health Services, and Dr Padua is from Sports Medicine Research Library, the University of North Carolina at Chapel Hill, Chapel Hill, NC; and Ms Hawkey is from the Department of Athletics, University of California, Los Angeles, Los Angeles, Calif.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Marc F. Norcross, MA, ATC, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; e-mail: norcross@email.unc.edu.

Received: April 29, 2009
Accepted: August 25, 2009

Weakness of the hip abductors and external rotators is associated with a variety of lower extremity injuries as well as low back pain.1–5 Specifically, decreased hip abduction and external rotation strength has been reported in individuals with patellofemoral pain syndrome,1,3,5 whereas lesser hip abductor strength has been observed in patients with iliotibial band friction syndrome.2 Furthermore, a prospective evaluation of collegiate athletes found individuals who suffered a lower extremity or low back injury displayed lesser hip abduction and external rotation strength compared to their uninjured counterparts.4

In addition to strength deficits, a lower extremity kinematic pattern consisting of hip adduction and internal rotation, and knee valgus has been proposed as a risk factor for patellofemoral pain syndrome,3,6,7 iliotibial band friction syndrome,2 and anterior cruciate ligament injury.8,9 As a result, it has been suggested that greater eccentric resistance to hip adduction and internal rotation provided by the hip abductors and external rotators may attenuate this kinematic pattern and result in an associated decrease in knee valgus.2,3,10 This premise is supported by studies reporting greater frontal plane knee motion during running11 and single-leg landing10 in individuals with weak hip musculature. Accordingly, it has been recommended that rehabilitation and prevention programs for these injuries should include evaluation and strengthening of the hip musculature.3,12–14

The lateral step-down test is a clinically applicable method of functional evaluation that potentially may be used to evaluate hip abductor and external rotator strength. This test, which demonstrates moderate inter-tester reliability in patients with patellofemoral pain syndrome (0.67), evaluates patients’ quality of movement by identifying characteristics of lower extremity function, including medial knee deviation or knee valgus.15 As knee valgus is reported to be coupled with hip adduction and internal rotation,9,16 it may be plausible to interpret excessive knee valgus observed during this test as an indication of eccentric weakness of the hip abductors and external rotators. To this effect, Mascal et al5 used an anterior step-down task for evaluative and rehabilitative purposes in 2 case studies on the management of patellofemoral pain syndrome, reporting increases in hip abduction and external rotation strength and decreased knee valgus during the step-down test following a 14-week intervention program.

These case reports coupled with the intertester reliability of the lateral step-down test support the premise that this technique may provide clinicians with a simple method to evaluate the strength of the hip abductors and external rotators during a functional task. The applications of this test could be 3-fold, with the test being used to detect strength deficits, serve as a benchmark for progression during rehabilitation, and identify individuals at risk for lower extremity and low back pathology. Therefore, the purpose of this study was to evaluate the association between knee valgus and hip abduction and external rotation strength during the lateral step-down test. It was hypothesized that individuals displaying lesser hip abduction and external rotation strength would demonstrate greater knee valgus during the lateral step-down test.

Methods

Participants

Forty-seven healthy individuals (22 men, 25 women; mean age, 21.3±2.0 years; mean mass = 71.6±14.8 kg; mean height = 194.3±11.5 cm) volunteered to participate in this investigation. Participants were generally healthy and had no history of lower extremity surgery, anterior cruciate ligament injury, or lower extremity injury within 3 months prior to data collection that prevented participation in activity for more than 1 day. All participants read and signed an approved informed consent document prior to participation.

Experimental Procedures

Isometric and eccentric peak torque of the hip abductors and external rotators of the dominant leg were collected prior to kinematic analysis of the lateral step-down task in all participants. The order of testing for hip abduction and external rotation was randomized. However, the eccentric measure was always performed prior to the isometric measure, with participants performing 5 continuous concentric-eccentric isokinetic repetitions and 3 isometric trials lasting 3 seconds each with 10 seconds of rest between trials. All data were sampled from the dominant leg, defined as the leg used to kick a ball for maximal distance.

Following completion of the strength assessment, participants performed the lateral step-down test, during which knee kinematics of the dominant leg were measured. This test was performed at a standardized tempo of 58 beats per minute via a metronome to standardize movement velocity. Participants began in a single-leg stance on their dominant leg atop a 30-cm high box with the nonstance leg positioned just lateral to the box and the hands placed on the hips. At the first beat of the metronome, participants lowered themselves toward the floor so that their nondominant heel made contact with the ground at the time of the second beat; participants returned to the starting position by the third beat of the metronome (Figure 1). Participants were instructed to maintain level hip position by not reaching for the ground with the nonstance leg. Following proper demonstration of the task by a certified athletic trainer (T.J.H. or S.D.H.), participants were allowed to practice until comfortable before performing 5 separate trials of the lateral step-down test.

Starting (A) and Ground Contact (B) Positions of the Lateral Step-Down Test.

Figure 1. Starting (A) and Ground Contact (B) Positions of the Lateral Step-Down Test.

Instrumentation

Peak torque was measured using the Biodex System 3 Isokinetic Dynamometer (Biodex Medical Systems, Shirley, NY). Hip abductor strength was assessed with participants positioned on their side with their arms crossed at the chest facing the dynamometer so that the dominant leg was positioned on top of the nondominant leg (Figure 2). The nondominant leg and trunk were stabilized by straps to control for trunk rotation, and the axis of the dynamometer was aligned 0.5 inches medial to the anterosuperior iliac spine at the level of the greater trochanter.17 Resistance was applied by the lever arm to the lateral aspect of the thigh just proximal to the lateral femoral condyle. Participants were instructed to exert maximally against the dynamometer in the direction of abduction while keeping the dominant leg straight.

Positioning of Participants During Hip Abduction Isokinetic and Isometric Strength Assessment.

Figure 2. Positioning of Participants During Hip Abduction Isokinetic and Isometric Strength Assessment.

Peak eccentric torque was recorded during the middle 3 of 5 continuous concentric-eccentric repetitions at 60°/second through a range of 0° to 20° of hip abduction. Isometric peak torque was then assessed 3 times for 3 seconds each with 10 seconds of rest between trials. Participants’ positioning remained the same as in isokinetic testing but with the hip fixed at 10° of abduction. A separate reliability study established high intrasession reliability for eccentric isokinetic intraclass correlation coefficient (ICC) = 0.85, SEM = 11.34) and isometric (ICC = 0.95, SEM = 6.41) hip abduction peak torque measures.

Hip external rotation strength was assessed with participants seated with the hip and knee flexed to 90°. Participants sat with the arms crossed at the chest, the thigh and trunk stabilized by straps, and a rolled towel placed between the knees (Figure 3) to act as a fulcrum for external rotation and to prevent compensatory hip adduction, which could have altered the peak external rotation strength measured. The axis of rotation of the dynamometer was aligned with the long axis of the femur, and resistance was applied by the lever arm immediately superior to the medial malleolus.

Positioning of Participants During Hip External Rotation Isokinetic and Isometric Strength Assessment.

Figure 3. Positioning of Participants During Hip External Rotation Isokinetic and Isometric Strength Assessment.

Peak eccentric torque was recorded during the middle 3 of 5 continuous concentric-eccentric repetitions at 60°/second through a range of 15° of hip internal rotation and 5° of hip external rotation. Isometric peak torque was assessed 3 times for 3 seconds each with 10 seconds of rest between trials. Participants’ positioning remained the same as in isokinetic testing but with the hip fixed in neutral rotation (0°). Intrasession reliability for eccentric isokinetic (ICC = 0.94, SEM = 3.61) and isometric (ICC = 0.93, SEM = 3.20) hip external rotation peak torque measures were established during pilot testing prior to data collection.

Knee kinematics were assessed during the lateral step-down test using an electromagnetic motion capture system (Flock of Birds, Ascension Technology Corp, Burlington, Vt). Electromagnetic tracking sensors were positioned directly on the skin of the anteromedial shank, lateral thigh, and sacrum over areas of minimal muscle mass to reduce motion artifact. Global and segment axis systems were established with the positive X axis designated as forward/anteriorly, the positive Y axis as leftward/medially, and the positive Z axis as upward/superiorly. The dominant lower extremity was modeled by digitizing the ankle, knee, and hip joint centers. Ankle and knee joint centers were defined as the midpoint of the digitized medial and lateral malleoli, and the medial and lateral femoral condyles, respectively. The hip joint center was estimated using a least squares method described by Leardini et al.18 The Motion Monitor motion analysis system (Innovative Sports Training, Chicago, Ill) was used for model generation and calibration as well as data acquisition.

Data Sampling and Reduction

Peak eccentric isokinetic and isometric torque values for hip abduction and external rotation were gravity corrected, normalized to the product of participants’ height and mass, and averaged across trials, resulting in 4 unitless expressions of relative strength:

  • Eccentric hip abduction (Ecc ABD).
  • Isometric hip abduction (Iso ABD).
  • Eccentric hip external rotation (Ecc ER).
  • Isometric hip external rotation (Iso ER).

Kinematic data were sampled at 50 Hz and filtered using a zero-phase lag 4th-order Butterworth low-pass filter with a cutoff frequency of 12 Hz. Kinematic angles were calculated as the reference frame of the shank relative to the reference frame of the thigh using Euler angle sequences rotated in an order of flexion-extension (Y-axis), valgus-varus (X-axis), and internal-external rotation (Z-axis). By convention, knee flexion was indicated by positive values, and knee valgus was indicated by negative values.

Values for knee varus-valgus and flexion-extension were obtained at initial descent, defined as the time when the knee flexion angle exceeded 10° and at peak knee flexion. In addition, the peak knee varus-valgus angle during this time period was calculated. All kinematic angle values were identified by computer algorithms used via custom software (LabVIEW, National Instruments, Austin, Tex). The variables of interest for statistical analysis were frontal plane knee angle at peak knee flexion (FPA) and frontal plane displacement (FPD) during the lateral step-down test. Frontal plane displacement was defined as the difference between the peak frontal plane knee angle during the test and this same angle at initial descent.

Statistical Analysis

The relationships between each strength variable (Ecc ABD, Iso ABD, Ecc ER, and Iso ER), and FPA and FPD, respectively, were evaluated using bivariate Pearson correlation coefficients following the removal of outliers. In total, the data for 3 participants were removed from various analyses when their value on a dependent variable involved in the correlation was more than 2.5 standard deviations from the mean. This resulted in slightly different sample sizes and composition across analyses but maintained the assumption of normality for the statistical models. All analyses were conducted using SPSS 16.0 software (SPSS Inc, Chicago, Ill). Statistical significance was established a priori with the alpha level set at P ≤ .05.

Results

There were no significant relationships between Ecc ABD and FPA (r = 0.103, P = .496, n = 46) and FPD (r = 0.216, P = .149, n = 46); Ecc ER and FPA (r = 0.052, P = .734, n = 45) and FPD (r = 0.050, P = .746, n = 45); Iso ER and FPA (r = −0.179, P = .228, n = 47) and FPD (r = −0.187, P = .208, n = 47); and Iso ABD and FPD (r = −0.132, P = .381, n = 46) (Figures 47). However, Iso ABD was significantly and negatively correlated with FPA (r = −0.372, P = .011, n = 46), indicating that participants with greater isometric hip abduction strength demonstrated greater knee valgus ([−] angular convention) motion (Figure 4). Descriptive statistics for all outcome variables are presented in the Table.

Scatterplot Showing Normalized Isometric (Iso ABD) and Eccentric (Ecc ABD) Hip Abduction Strength and FPA ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ABD (r = −0.372, P = .011*), and the Broken Line Represents Ecc ABD (r = 0.103, P = .496). *Correlation Is Significant at p ≤ 0.05.

Figure 4. Scatterplot Showing Normalized Isometric (Iso ABD) and Eccentric (Ecc ABD) Hip Abduction Strength and FPA ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ABD (r = −0.372, P = .011*), and the Broken Line Represents Ecc ABD (r = 0.103, P = .496). *Correlation Is Significant at p ≤ 0.05.

Scatterplot Showing Normalized Isometric (Iso ER) and Eccentric (Ecc ER) Hip External Rotation Strength and FPD ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ER (r = −0.187, P = .208), and the Broken Line Represents Ecc ER (r = 0.050, P = .746).

Figure 7. Scatterplot Showing Normalized Isometric (Iso ER) and Eccentric (Ecc ER) Hip External Rotation Strength and FPD ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ER (r = −0.187, P = .208), and the Broken Line Represents Ecc ER (r = 0.050, P = .746).

Descriptive Statistics for FPA and FPDa, and Normalized Isometric and Eccentric Hip Abduction and External Rotation Strength

Table: Descriptive Statistics for FPA and FPD, and Normalized Isometric and Eccentric Hip Abduction and External Rotation Strength

Discussion

These findings show eccentric strength of the hip abductors and eccentric and isometric strength of the hip external rotators are not related to frontal plane knee motion during the lateral step-down test. Isometric hip abduction strength was weakly correlated with frontal plane knee angle at peak knee flexion. Individuals with greater isometric hip abduction strength demonstrated greater knee valgus with Iso ABD strength explaining approximately 14% of the variation in FPA (r2 = 0.138). However, the low strength of the association in the direction contrary to what would be expected from a biomechanical framework makes the clinical significance of this result unclear.

Prior to comparing our results to previous research, it is important to note how the use of either 2-dimensional (2-D) or 3-dimensional (3-D) methods of motion analysis changes the strict interpretation of the measured “knee valgus” motion. Although 3-D motion analysis yields pure frontal plane knee angles, 2-D motion analysis results in dynamic knee valgus, or medial knee displacement, angles in which the specific contributions of femoral adduction and internal rotation, and knee valgus motions to this composite angle are not known. As a result, it is difficult to directly compare the magnitudes of the frontal plane knee angles reported in the literature. Therefore, it is necessary to compare studies based on the premise that during closed kinetic chain activities, knee valgus, femoral adduction, and femoral internal rotation are associated,9,16 and increased movement into these positions would result in greater measured 2-D dynamic knee valgus or 3-D pure knee valgus angles.

Our findings are consistent with a number of previous studies examining the relationship between hip strength and frontal plane knee kinematics during dynamic tasks that imposed similar demands. Thijs et al19 evaluated hip strength in individuals performing a single-leg forward lunge and found no significant differences in peak hip abduction (Cohen’s d effect size [d] = 0.36) or external rotation (d = 0.30) strength between a group that moved into dynamic knee valgus during the lunge compared to a group that moved into dynamic knee varus as measured using 2-D video analysis. In addition, they found no significant relationships between hip abductor and external rotator strength and the magnitude of frontal plane knee motion in either group. Bell et al20 found that participants who displayed medial knee displacement during an overhead squat test exhibited greater normalized isometric hip external rotation strength than participants who did not demonstrate medial knee displacement using 2-D video analysis. However, they also found no differences between groups in isometric hip abduction strength (d = 0.34), and concluded hip musculature weakness did not contribute significantly to frontal plane knee motion during this functional task. Lawrence et al21 also showed no significant difference in the frontal plane knee angle of participants with high and low hip isometric external rotation strength (d = 0.21) during a single-leg drop landing using 3-D motion analysis.

Two studies have reported a significant relationship between hip musculature strength and frontal plane knee angle that are inconsistent with our results. Willson et al22 demonstrated a significant association between greater isometric hip external rotation strength and lesser frontal plane projection angle (r = 0.40), defined as the angle between lines drawn between the anterosuperior iliac spine to the middle of the tibiofemoral joint and the mid-tibiofemoral joint to the middle of the ankle mortise, derived using 2-D kinematic analysis. Claiborne et al23 evaluated hip strength and frontal plane knee motion in individuals performing a single-leg squat using 3-D analysis. Peak knee valgus and frontal plane knee motion, defined as the difference between peak knee valgus during the squat and standing frontal plane knee angle, were not significantly correlated with either eccentric hip abduction (r = −0.249) or external rotation peak torque (r = −0.356). However, concentric hip abduction was significantly correlated with the magnitude of frontal plane knee motion (r = −0.365). The authors concluded greater hip abduction strength resulted in decreased motion in the valgus direction during a single-leg squat. It is important to note, however, that greater knee valgus is associated with greater hip adduction.9,16 As a result, it is expected that eccentric, not concentric, force production from the hip abductors would be important in limiting knee valgus motion as these muscles would be lengthened during hip adduction motion. Therefore, it remains unknown how concentric strength of the hip abductors would influence knee valgus motion.

Although the findings of this study are contrary to our hypotheses, we propose the following possible explanation for the obtained results. The lateral step-down test used in this study may not have placed a sufficient functional demand on our participant sample to elicit the response suggested by earlier research. Previous studies on healthy individuals that identified lesser hip strength in participants displaying greater frontal plane knee motion involved more demanding tasks such as running11 and single-leg landing from a jump.10 It may be that the single leg step-down test used in our study, similar to the forward lunge19 and single-leg squat,23 was not demanding enough dynamically and allowed participants to use compensatory mechanisms they would not be able to use in a more difficult task. For instance, participants with greater strength may have been able to go into greater knee valgus knowing they had the requisite strength to return from that position, whereas the task may have been slow enough for those with lesser strength to make a compensatory knee varus movement to successfully complete the task. These potential compensatory mechanisms help to illustrate the multifactorial nature of the strategies used to complete a dynamic task, which makes it difficult to draw conclusions regarding individual performance and just a limited number of those factors.

Nonetheless, the results suggest the use of the lateral step-down test is not indicative of hip abduction or external rotation strength in asymptomatic individuals. Furthermore, based on the results of this study, it is unclear how this test may be applied to a pathologic population. Although hip abductor1–3,5 and external rotator1,3,5 weakness has been demonstrated in individuals with pathologies associated with increased frontal plane motion, such as patellofemoral pain syndrome and iliotibial band friction syndrome, it is unknown whether this weakness would manifest in increased frontal plane motion during the lateral step-down test or potentially be obscured via the same compensatory mechanisms that were likely present in this study.

Limitations and Future Research

Future research is necessary to assess the utility of the lateral step-down test to evaluate hip abductor and external rotator strength in a patient population actively experiencing symptoms related to chronic lower extremity pathologies. Our study was limited in that it evaluated healthy individuals who may have exhibited strength and neuromuscular control mechanisms that are not congruent with patients currently experiencing symptoms. Evaluation of an adapted version of the lateral step-down test from a greater height or at a greater velocity so as to increase the dynamic demands placed on a healthy population also is warranted for its possible use in identifying individuals at risk for lower extremity and low back injuries.

In addition, because all participants performed eccentric strength testing prior to isometric strength testing as we considered the eccentric measure to be the most important, we cannot rule out the possibility that testing order may have impacted the strength measures. Furthermore, although our methods allowed for assessment of maximal torque production capacity of the hip abductors and external rotators, it is unclear to what extent this capacity was used during the lateral step-down test. It is probable that muscle activation (ie, % maximal effort) in combination with strength of the gluteus maximus, gluteus medius, and associated hip musculature is more important in explaining dynamic control of frontal plane knee motion than absolute strength alone. In other words, the fact that individuals possess a high level of strength does not necessitate that they use that strength during a given task. Future research should incorporate electromyography to determine the relative activation levels and contributions of these muscle groups during this task.

Conclusion

These findings indicate the use of the lateral step-down test in a healthy population to draw conclusions related to hip abductor and external rotator strength is not justified. There appear to be no clinically significant relationships between isometric and eccentric strength of the hip abductors and external rotators and frontal plane knee motion during this test.

References

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  19. Thijs Y, Van Tiggelen D, Willems T, De Clercq D, Witvrouw E. Relationship between hip strength and frontal plane posture of the knee during a forward lunge. Br J Sports Med. 2007;41:723–727. doi:10.1136/bjsm.2007.037374 [CrossRef]
  20. Bell DR, Padua DA, Clark MA. Muscle strength and flexibility characteristics of people displaying excessive medial knee displacement. Arch Phys Med Rehabil. 2008;89:1323–1328. doi:10.1016/j.apmr.2007.11.048 [CrossRef]
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Scatterplot Showing Normalized Isometric (Iso ABD) and Eccentric (Ecc ABD) Hip Abduction Strength and FPD ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ABD (r = −0.132, P = .381), and the Broken Line Represents Ecc Abd (r = 0.216, P = .149).

Figure 5. Scatterplot Showing Normalized Isometric (Iso ABD) and Eccentric (Ecc ABD) Hip Abduction Strength and FPD ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ABD (r = −0.132, P = .381), and the Broken Line Represents Ecc Abd (r = 0.216, P = .149).

Scatterplot Showing Normalized Isometric (Iso ER) and Eccentric (Ecc ER) Hip External Rotation Strength and FPA ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ER (r = −0.179, P = .228), and the Broken Line Represents Ecc ER (r = 0.052, P = .734).

Figure 6. Scatterplot Showing Normalized Isometric (Iso ER) and Eccentric (Ecc ER) Hip External Rotation Strength and FPA ([−] Valgus, [+] Varus) with Associated Simple Linear Regression Trend Lines. The Solid Line Represents Iso ER (r = −0.179, P = .228), and the Broken Line Represents Ecc ER (r = 0.052, P = .734).

Descriptive Statistics for FPA and FPDa, and Normalized Isometric and Eccentric Hip Abduction and External Rotation Strength

MEAN±SD
FPA7.28°±8.70°
FPD5.66°±0.23°
Eccentric hip abduction0.053±0.180
Isometric hip abduction0.052±0.015
Eccentric hip external rotation0.038±0.009
Isometric hip external rotation0.030±0.007
Authors

Mr Norcross and Dr Blackburn are from the Neuromuscular Research Laboratory, Mr Halverson is from Campus Health Services, and Dr Padua is from Sports Medicine Research Library, the University of North Carolina at Chapel Hill, Chapel Hill, NC; and Ms Hawkey is from the Department of Athletics, University of California, Los Angeles, Los Angeles, Calif.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Marc F. Norcross, MA, ATC, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; e-mail: norcross@email.unc.edu

10.3928/19425864-20091019-06

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