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Reliability of Common Lower Extremity Biomechanical Measures of Children With and Without Obesity

Tucker, Jennifer PT, DPT, PCS; Moore, Megan DPT; Rooy, Julie DPT, MS; Wright, Amy DPT; Rothschild, Carey PT, DPT, OCS, SCS, CSCS; Werk, Lloyd N. MD, MPH

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doi: 10.1097/PEP.0000000000000152
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Obesity is a substantial health concern for Americans. Currently, more than one third of adults and 17.3% of children and adolescents are obese—72 million and 12.5 million people, respectively.1 Medical conditions such as cardiovascular disease and type 2 diabetes mellitus, previously thought to appear only in adults, are emerging as health issues for children and adolescents with obesity.2 Furthermore, children with obesity can suffer from social and psychological consequences in addition to physical impairments.3 Children who are obese are likely to struggle with obesity into adulthood.4 Lack of physical activity is one of many factors contributing to the rise in obesity. In comparison to children of normal weight in the United States, children who are overweight spend more time engaged in sedentary activities.5 Children and adolescents who are obese commonly experience pain in at least 1 joint, which may further affect their level of physical activity.6–8 Musculoskeletal malalignments resulting in alterations of the biomechanics of the lower extremities likely contribute to the joint pain experienced by children with obesity9; however, the supporting data are limited.10,11 Children with obesity, in comparison to children of typical weight, appear to have decreased femoral anteversion at the hips,12 genu valgum at the knees,13–15 and increased pronation of the feet.16–20

Measures commonly used to assess alignment of the lower extremity include the Craig test (CT) for femoral anteversion, the tibiofemoral angle (TFA) for genu valgum, and the Foot Posture Index (FPI-6) for foot pronation/supination. The sit-and-reach test (SRT) provides a measure of general flexibility in children.21

Literature describing the reliability of the CT in the pediatric population is sparse, however, studies of adults support the use of the CT as a quick and inexpensive way to determine femoral anteversion as opposed to other techniques such as radiologic and intraoperative measurements.22,23 Overall, the validity of the CT has been found to be moderate in comparison to magnetic resonance imaging22; interrater reliability has varied from fair to moderate, and intrarater reliability has been shown to be substantial.22–25

When assessing the relationship of the hip to the knee, the TFA is a common clinical measure of genu valgum.26,27 The TFA provides a goniometric measure of the angle of the knee by using bony landmarks. Although validity has not been established in the literature, the TFA is recommended as an accurate measurement of the knee valgus angle.26,28,29 With proper training, measurement of the TFA has moderate to substantial interrater reliability and substantial intrarater reliability.25,28,29

Options for clinical measurement of foot postures include subtalar joint measurement, height of the navicular head, and the FPI-6. The FPI-6 provides a strategy for scoring multiple components of the foot and ankle, resulting in a total score reflective of overall foot posture. Position of the foot is assessed in the frontal, sagittal, and transverse planes. Assessments of the talar head, medial and lateral malleoli, talonavicular joint, medial longitudinal arch, calcaneus, and forefoot are incorporated into the FPI-6. The most recent modification of the FPI-6 was developed to produce appropriate validity and is reliable for assessing static foot positions and predicting dynamic foot positions.30 The FPI-6 has provided clinicians with a quick and reliable assessment tool for use with the pediatric population.31,32 This test has been found to have moderate to substantial interrater reliability and substantial intrarater reliability.31,33,34

In addition to the lower extremity measures described above, measures of flexibility are often helpful when addressing the biomechanical alignment of children who are obese. The SRT is the most common and practical flexibility screen measure, providing a test of gastrocnemius, buttock, hamstring, scapular, lumbar, and thoracic flexibility.35,36 The SRT has significant inter- and intrarater reliability.37

Although the malalignments of hip and knee joints associated with obesity in adults have been described using measures of femoral anteversion, Q-angle, and TFA,38 limited evidence is reported on the reliability on similar measures of hip, knee, and foot alignment in children and adolescents who are obese. This study examines the intra- and interrater reliability of the CT for femoral anteversion, the TFA for knee valgus alignment, the FPI-6 for alignment of the foot, and the SRT for flexibility assessment in children aged 8 to 12 years. In addition, this study provides a comparison of the relative reliability of these measures among children who are nonobese and those who are obese. With the high prevalence of obesity in children, identifying measures for joint alignment and flexibility that have high reliability are critical in the management of this population by pediatric physical therapists.



Two cohorts of participants were studied: cohort 1 (n = 25) included children with a body mass index (BMI) of 15% to 85%, termed “children who are nonobese”; cohort 2 (n = 25) included children with a BMI 95% or more, termed “children who are obese.” Children meeting the inclusion criterion of age 8 to 12 years were recruited for study participation between January and June 2012. A convenience sample of participants who were not obese was recruited from the community surrounding the University of Central Florida (Orlando, FL) by faculty and local clinicians. A convenience sample of children with obesity (BMI ≥95%) were recruited from among the patients receiving care in a Nemours Children's Hospital pediatric multidisciplinary weight management program known as the Nemours Healthy Choices Clinic. Possible participants were screened to exclude children with known orthopedic conditions that could affect lower extremity measures (eg, Blount disease and slipped capital femoral epiphysis). Parents provided the participant's age, sex, race, and ethnicity. The majority of participants who selected “other” as their race, or who chose not to answer, reported their ethnicity as Hispanic; therefore, race and ethnicity categories were collapsed. Participants misclassified as nonobese or obese were excluded before data analysis. Both the University of Central Florida and Nemours Institutional Review Boards approved study procedures. Parents and/or legal guardians gave written consent and participants gave assent before the measures were taken.


Height and Weight

Measurements were recorded for each participant using standardized methods, and BMI values were calculated. Among children, BMI percentiles are used to determine obesity classification on the basis of the Centers for Disease Control and Prevention growth charts.39

Craig Test

Participants were asked to lie prone with a pillow placed under the stomach slightly superior to the hip to maintain neutral lumbopelvic positioning. With the hip in neutral and the left knee passively flexed to 90°, the left lower leg was gently rotated to produce hip internal and external rotation until the greater trochanter was parallel to the floor and measurement determined with an inclinometer.40,41 A rater performed this measurement on the left leg, returned the leg to its starting position, before repeating the measure 2 additional times, and then repeated the process on the right leg. Each rater recorded 3 measurements for each extremity.

Tibiofemoral Angle

Participants stood barefoot with their feet hip-distance apart (approximately 6 inches). Goniometric measurement was performed as follows: (1) the axis of the goniometer was placed at the center of the patella, (2) the proximal arm was aligned along the thigh in line with the anterior superior iliac spine of the pelvis, and (3) the distal arm was aligned with the lower leg in line with the midpoint between the malleoli. The goniometric angle was recorded for each participant's left leg. Three measures were taken and recorded and then the process was repeated on the right leg.26,42

Foot Posture Index 6

Participants stood barefoot with their feet hip-distance apart (approximately 6 inches). The foot was assessed using an index of 6 components incorporating both the rear and forefoot.30 The 6 components included (1) talar head palpation, (2) symmetry of curves above and below the lateral malleolus, (3) inversion/eversion of the calcaneus, (4) prominence in the region of the talonavicular joint, (5) medial arch height, and (6) forefoot abduction/adduction.30 For each participant's left foot, a rater performed this assessment 3 times and then repeated the process on the right foot.

Sit-and-Reach Test

Before testing, a yard stick was secured to the floor and a 12-inch strip of tape was placed perpendicular to the yardstick at the 15-inch mark. Participants were positioned in long sitting on the floor with their legs extended in a V shape on either side of the yardstick and their feet 12 inches apart with their heels touching either end of the tape strip. Participants then placed their right hand over left and slowly stretched forward with maximal effort and distance recorded.36,43 The SRT was performed by each participant just 3 times, with all 3 raters recording their own observed measures. This procedure reduced the excessive repetition of the SRT, which could result in a stretch effect.


Training of Raters

Three raters participated in three 2-hour training sessions covering the physical measurement techniques to be used for clinical assessment of each participant's lower extremities. The raters included an experienced pediatric physical therapist and 2 doctor of physical therapy students. The training consisted of two 3-hour workshops for the measures of the CT, TFA, FPI-6, and SRT with proctored practice under the supervision of a physical therapist with board certification in orthopedics.

Data Collection

After completing 3 sets of each measure, each participant repeated the sequence with the second and then the third rater. Each rater used individual data collection sheets and was blinded to previously recorded results. All measures were collected in a private treatment area.

Statistical Analysis

The key outcomes of interest were the intrarater and interrater reliability among 3 raters assessing 2 cohorts (nonobese and obese) of participants. Twenty-five participants were determined sufficient to detect an intraclass correlation coefficient (ICC) of 0.7 on the basis of a repeated-measure analysis of variance assuming a medium effect size of 0.30 with power of 80. Data were analyzed using SPSS Version 19 (IBM, Armonk, NY).

Descriptive statistic Means were calculated for each rater's scores from each participant. The continuous variables for each measurement (CT, TFA, FPI-6, and SRT) were then compared using ICCs. Data for calculation of ICCs were pooled from left and right lower extremity measurements. For intrarater reliability, an ICC(3,k) and a 95% confidence interval were calculated, and for inter-rater reliability, an ICC(2,k) and 95% confidence interval were calculated for each clinical test. The intrarater reliability among all 3 raters for the TFA, FPI-6, and SRT was classified on the basis of the Shrout classification,44 indicating substantial reliability with ICC values more than 0.81, moderate by values 0.61 to 0.80, fair by values 0.41 to 0.60, slight by values 0.11 to 0.40, and no reliability by values less than 0.10. The total scores of the FPI-6 were considered continuous data and calculated with ICCs consistent with a previous reliability study.31 One participant had missing data from 1 rater for the SRT. To replace these missing data, the mean of the other 2 raters' measurements was calculated. In addition, the data were further analyzed using the analysis of variance to determine the differences between groups.



Fifty participants were recruited, but 4 meeting exclusion criteria (BMI >85%) in the cohort of those without obesity were removed before analysis. Analyses relied on data obtained from 46 participants, including 21 participants who were not obese and 25 participants with obesity. The 2 cohorts were similar in age and height (Table 1). The most frequent race reported among participants who were not obese was white, and participants with obesity most frequently reported Hispanic as their race (76% vs 12%; P = .002).

Baseline Characteristics


Three sets of 3 measurements were obtained for each of the measures and a mean value among raters determined (Table 2). The intrarater reliability among all 3 raters for the TFA, FPI-6, and SRT was substantial according to the Shrout classification.44 The intrarater reliability values are listed in Table 3.

Measures of Alignment and General Flexibility of Lower Extremities
Intrarater Reliability

Of all the measures, the CT showed the least consistency among raters. Interrater reliability was slight (ICC = 0.372) for the CT among the participants who were not obese and fair (ICC = 0.527) for participants with obesity, or fairly and slightly reliable, respectively, according to the Shrout classification.44 Interrater reliability for the TFA measurement was moderate for measurements obtained from the participants who were not obese (ICC = 0.657) and was substantial among raters measuring the participants with obesity (ICC = 0.781). Substantial interrater reliability (ICC = 0.834) was found for FPI-6 measurements of the participants with obesity and moderate interrater reliability for the scores reported by raters for participants who were not obese (ICC = 0.788). The ICC values for the SRT consistently showed substantial reliability among raters for both the participants without and with obesity (ICC = 0.997, 0.999; Table 4).

Interrater Reliability

Comparing the rater measurements recorded for participants without and with obesity, the between-group comparison revealed significant differences (P < .05) in the following measures: TFA (right, P = .003; left, P = .00); prominence of the talonavicular joint, a component of the FPI-6 (right, P = .006; left, P = .003); and the medial arch, a component of the FPI-6 (right, P = .007).


Intrarater reliability was found to be substantial for the CT, TFA, FPI-6, and SRT. The loss of 4 subjects in the cohort without obesity could have limited the ability to detect a medium effect size in ICC among raters. The interrater reliability analysis, however, demonstrated moderate to substantial reliability for all measures except the CT. The variability between raters in their CT findings was consistent with previous reports24; for children both with and without obesity, as CT had slight reliability in intra- and interrater assessments.

The data analysis revealed substantial intrarater reliability, which is consistent with the literature reviewed.24,26,28,33–35,38 In reviewing the methodology and results, we noted that the lack of rater blinding may have influenced the high intrarater reliability values. Although raters were not blinded to their individual measurements, each rater attempted to treat each measure separately and recorded their findings on separate data collection forms.

The CT showed the most variability between raters in both groups of participants. Previous researchers have demonstrated interrater reliability that varied from fair to substantial for this measurement and reported findings consistent with our results.24,25 Accurate and reproducible palpation of the greater trochanter in its most lateral position can be challenging. The subjective nature of palpation may result in a greater amount of variability between raters. The reference standard for femoral anteversion requires radiographic imaging,45 which exposes children to radiation, can be costly, and may not be readily accessible to physical therapists. Further investigation for an alternative noninvasive measurement of femoral alignment is needed.

Greater inconsistency has been reported for the CT on participants who were obese because excess adipose tissue could increase the difficulty in palpating the greater trochanter.25 However, in this study, greater variability was found in the cohort of participants who were not obese in comparison with participants with obesity. The cohort of participants who were not obese displayed notable differences in body types. Ranges of height and weight were substantially greater for the cohort of participants who were not obese compared with the cohort of participants with obesity. The participants who were obese showed less variability in height and weight and therefore presented with a more consistent body type. For example, the raters may have demonstrated more consistency in their interpretation of the most lateral position of the greater trochanter because of a practice effect from measuring participants of similar body composition. An alternative explanation for the greater interrater reliability for measurements of the participants who are obese is the effect of performance bias and/or training effect. The data of the majority of participants with obesity were collected later in the study.

In an attempt to reduce bias, blinding was maintained among raters. Each rater took the individual measurements separately from the other raters with the exception of the SRT. After completing the other measurements individually, the raters performed the SRT concurrently. This procedure reduced the excessive repetition of the sit and reach, which could result in a stretch effect that could in turn have affected the reliability. Despite the measurements being recorded at the same time, each rater took care to stand apart from one another and recorded their measurement on an individual data form in an attempt to avoid biases and maintain blinding. Although not formally randomized, there was no explicit order among raters.

As noted in Table 1, the proportion of participants that were male and distribution of race/ethnicity differed among the nonobese and participants with obesity. This is likely the result of referral patterns to a weight management program and prevalence of obesity among Hispanic children in the local community. Differences in the cohorts' demographics may have affected our measures; however, the focus on determining the intrarater reliability and interrater reliability of biomechanical measurements would not be affected.

The forces applied to their lower extremities affect postural alignment of children who are obese. Decreased values of femoral anteversion and increased values of the TFA in children who are obese have been documented in previous studies.10–12,15 However, among the participants with obesity in this study, values from the CT were within the high normal range. This may be related to our small sample size or to other characteristics of the participant population. The effect of increased body weight on foot alignment is less clear. The literature is somewhat conflicted on the morphology of feet in children who are obese and whether their feet are indeed flat or appear so because of excess fat. Researchers consistently recognize the correlation of flat feet and obesity.16–20 Mickle et al18 suggest that the fat pads of children who are obese do not differ significantly, but rather the feet often appear flat because of a collapse of the arch itself. We chose the FPI-6 measure because it assesses different components of the foot and ankle. Our findings of a flattened arch are consistent with previous findings.18

Although the reliability and mean values of the measurements were consistent with those reported in the literature, the use of the techniques together for assessing lower extremity alignment in children has not been well documented. Determining the effect of increased body mass on the alignment of the lower extremities requires the use of different measurement techniques for each joint, and measures that have demonstrated intra- and interrater reliability. Taylor et al15 state that “efforts should be made to encourage health care providers' recognition of the orthopedic complications of excess weight so that interventions can be investigated.”

Children with obesity may present with atypical postural alignment, placing them at risk for pain, gait abnormality, and impaired functional exercise capacity. Regular assessment of biomechanical alignment as part of a multidisciplinary team evaluation of children with obesity will contribute to a comprehensive clinical evaluation. Future research is needed to examine the relationship between biomechanical alignment, pain, and tolerance of physical activity in children who are obese. Insight into the impairments associated with obesity will help health care professionals develop effective interventions for promoting physical activity and overall health of children who are obese.


This study demonstrates that a series of lower extremity measurements and a measure of general flexibility can be completed with moderate reliability on children with obesity aged 8 to 12 years. The CT was found to be a less reliable measure. The values provide insight into the effect of excess weight on the alignment of the lower extremities.


We thank Patrick Pabian, PT, DPT, OCS, SCS, of the Program in Physical Therapy at the University of Central Florida and Stephen Frick, MD, of Nemours Children's Hospital Department of Orthopedics for their assistance editing this article. We thank the staff of the Nemours Children's Hospital Division of General Pediatrics whose clinical practice and patient population were featured in this study.


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child; femoral anteversion; foot; lower limb; obesity; postural alignment; reproducibility of results

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