INTRODUCTION
The effect of obesity on the musculoskeletal system has been described for adults who are overweight, but limited data exist regarding the musculoskeletal health of children who are obese.1–6 Musculoskeletal fitness encompasses parameters such as joint range of motion (ROM), muscle strength, muscle flexibility, balance, and coordination. Impairments of the musculoskeletal system can lead to pain and discomfort and subsequent activity restriction. All of these parameters are of particular interest to physical therapists whose practice and therapeutic interventions aim to reduce physical limitations of clients who are overweight. Identifying physical impairments and removing barriers to activity assist a client who is overweight in participating in the lifestyle changes necessary for overall good health.
ROM parameters are commonly used as indicators and predictors of physical function.7 Joint ROM is influenced by bony structure and the extensibility of soft tissue structures. Restrictions of lower limb soft tissue are physical impairments commonly associated with musculoskeletal conditions.7–10 In addition, impaired muscle flexibility may predict the presence of future musculoskeletal symptoms.11 Previous investigators have observed that children who are obese present with less flexible hamstrings than children of healthy weight12; reduced hip joint ROM, reduced hamstring flexibility, and increased body mass index (BMI) have each been identified as predictors of low back pain.8,13–15 In addition, restricted ROM has been reported to adversely affect standing balance,2 postural adjustment,12 and movement efficiency.16 Furthermore, appropriate muscle strength is essential to ease the loading of joints, and it is thought that in individuals who are overweight, the dampening capability of muscles is impaired because of muscle weakness and the resistance offered by body weight, thus increasing the rate of joint loading.17 Finally, gait studies of children propose that obesity leads to increased postural sway, and that gait and postural adaptations contribute to the development of lower limb varus/valgus deformities.18,19
Purpose
Against the backdrop of previous investigations, the current pilot study was designed to profile the lower limb musculoskeletal health of children who are obese to guide practice and appropriate therapeutic intervention. In addition, the study was designed to identify any relationships that existed between musculoskeletal measures, BMI, and physical activity (PA) level.
METHODS
Subjects
Consecutive patients attending the Outpatient Pediatric Endocrinology Clinic in Adelaide, Meath and National Children's Hospital in Dublin, Ireland, were recruited for this study between January and August of 2006. Children were included in the study if they presented with exogenous obesity, a BMI greater than the 97th percentile for age, and were between 10 and 15 years of age. Subjects were excluded from the study if they had sustained any musculoskeletal injury in the previous 6 months or were unable to take part in the study procedures. Ethical permission was obtained from the Research Ethics Committee of Saint James Hospital/Adelaide, Meath and National Children's Hospital. The study procedure was explained to parents and children, and written informed consent was obtained from parents, who were present with their children at all times during data collection.
Procedures
Musculoskeletal History
A subjective history was taken from children and parents relating to past musculoskeletal events involving the lower limb that required medical treatment. Medical records were reviewed to confirm these events. Eligible participants completed a pain profile and a visual analogue scale relating to reported lower limb complaints.
Demographics and Anthropometry
Patient details such as gender, age, height, and weight were collected. Height (to nearest 0.1 cm) was measured in triplicate using a wall-mounted stadiometer (Holtain Ltd, Crymmych, PENBS, UK). Weight (to the nearest 0.1 kg) was measured in triplicate using an electronic scale (Seca Ltd., Birmingham, UK). Waist circumference was measured (to nearest 0.1 cm) using a measuring tape placed midway between the distal margin of the rib cage and the proximal margin of the iliac crest. BMI was calculated (BMI = weight (kg)/height (m)2), and the BMI standardized deviation score was calculated as recommended by Cole.20 Children were classified as moderately obese (BMI = 25–29.99 kg/m2) or severely obese (BMI > 30 kg/m2).
Physical Activity and Sedentary Levels
Physical activity levels were measured using the Modifiable Activity Questionnaire for Adolescents (MAQA), which yields a reasonable estimate of habitual PA in adolescents and can be used to calculate metabolic equivalents (METs) per hour per week.21 Sedentary time was assessed by measuring screen time, the number of hours spent using a screen per day (eg, watching television, playing video games, and using a cell phone for entertainment).
Joint ROM and Muscle Flexibility
Measures of passive joint ROM of the hip, knee, and ankle were taken using standardized techniques and using a universal goniometer (MedFaxx Incorporated, Wake Forest, North Carolina) and an angle finder for hip rotation (Dasco Pro, Inc, Rockford, Illinois).22 Muscle flexibility was measured by assessing the muscle length of quadriceps, hamstrings, and the gastrocnemius, using the quadriceps angle test, the popliteal angle test, and the gastrocnemius length test. Intramalleolar gap distance was measured in centimeters using calipers (MedFaxx Incorporated) and served as a surrogate measure of genu valgum.
Balance Testing
Standing balance was assessed using timed unipedal static and dynamic measures as recommended by Emery et al.23 Subjects were asked to stand on 1 leg with the opposite knee held at 90° of flexion and with the upper limbs relaxed. Subjects performed a timed (to the nearest 0.1 second) static single-leg stance on a hard floor, with their eyes open followed by eyes closed. A timed dynamic single-leg test was performed standing on foam of uniform density measuring 16.4″ × 20″ × 2.5″ with eyes open and closed. Each test was performed 3 times.
Isokinetic Muscle Strength
The lower limb concentric muscle strength of knee flexors and extensors was measured using isokinetic dynamometry (Biodex System 3, Biodex Corp., Shirley, New York), which yields valid and reliable results. Specialized pediatric attachments and additional seat padding that allowed the lower leg to hang freely from the edge of the seat were used. Test velocities of 60° per second, 90° per second, and 180° per second were employed.
Data Analysis
Anthropometric measures (height and weight) were collected in triplicate, BMI, and the mean value for each of these measures was calculated. The mean results for ROM and balance testing were calculated and raw isokinetic data were normalized to body weight. Thereafter, the mean torque/body weight scores for both lower limbs were computed. All data from measured variables were entered into SPSS for Mac OS X version 11.0.2. Descriptive statistics were used to elucidate the mean and standard deviation for all measures. Kolmogorov-Smirnov Z tests were performed to assess whether data approximated a normal distribution. In addition, correlational tests (bivariate Pearson correlation coefficients) were used to investigate the relationships between variables, and differences between groups were investigated using nonparametric tests (Mann-Whitney U test and Wilcoxon W). An α level of 0.05 was used as the criterion of statistical significance.
RESULTS
Study Cohort
Table 1 describes the participant characteristics. Six children (boys: n = 4; girls: n = 2) were classified as moderately obese and 11 children (boys: n = 3; girls: n = 8) were classified as severely obese. The mean age of children was 12.41 years and the mean BMI was 32.45 kg/m2 (95% confidence interval [CI]: 29.35–36.09 kg/m2).
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TABLE 1: Anthropometric Characteristics of Participants, Mean (95% Confidence Interval)
Previous Orthopedic History and Current Pain
Fifty-three percent of the group (n = 9; 2 boys) had sustained a previous fracture or soft tissue injury of the lower limb that required a hospital attendance in the past (between 6 and 18 months prior to the study) and 72% (6 boys) reported having pain in their lower limbs.
Physical Activity Levels and Sedentary Levels
Children were spending 20.46 ± 16.8 hours per week (boys 17.4 ± 6.6; girls 22 ± 20 hours per week) in habitual PA as measured using the MAQA. Children reported engaging in screen time for 3 ± 1 hours every weekday and for 3.5 ± 2 hours on weekend days.
Joint ROM and Muscle Flexibility
Table 2 presents the results for joint ROM. The mean popliteal angle for the cohort was 43.59° ± 6.61°, and for boys and girls, respectively, were 42.93° ± 8.66° and 44.1° ± 5°. The mean measure of quadriceps length for the cohort was 116.91° ± 12.23°, and for boys and girls, respectively, were 121.93° ± 12.93° and 113.4° ± 11°. The mean length of gastrocnemius for the cohort was 91.41° ± 5.06°, and for boys and girls, respectively, were 92.78° ± 2.64° and 90.45° ± 6.20°.
TABLE 2: Passive Joint Range of Motion for the Lower Limb, Mean (95% Confidence Interval)
Standing Balance
The mean bilateral balance measures for the group were 30.84 ± 33.35 seconds and 13.47 ± 9.66 seconds for static standing balance with the eyes open and closed, respectively. For dynamic balance, the mean values obtained were 22.05 ± 21.06 seconds and 3.09 ± 1.20 seconds with the eyes open and closed, respectively.
Isokinetic Muscle Strength
Mean torque/body weight values measured for knee flexion and extension are described in Table 3.
TABLE 3: Mean (%) Isokinetic Torque/Body Weight for the Knee Joint
The Relationship Between BMI and Musculoskeletal Measures
When measures of BMI were correlated to musculoskeletal indices (Table 4), moderate relationships with statistical significance significant were found for hip flexion ROM (r = −0.65, P < .001), hip abduction ROM (r = −0.65, P < .001), knee flexion ROM (r = −0.69, P < .001), knee flexion strength (r = −0.55, right leg; r = −0.58, left leg, P < .05), and flexibility of quadriceps (r = −0.51, P < .05) and gastrocnemius (r = −0.57, P < .001). Positive correlations were observed between BMI measures and measures of knee hyperextension (r = 0.55, P < .001) and genu valgum (intramalleolar gap [r = 0.67, P < .001]). Nonparametric independent samples tests (Mann-Whitney U tests) revealed that children who were severely obese had less knee flexion (P = .015) than those who were less obese.
TABLE 4: Correlations Between Musculoskeletal Measures, Body Mass Index (BMI), and Physical Activity
The Relationship Between PA and Musculoskeletal Measures
A positive relationship was observed between PA and muscle strength (peak torque/body weight) for knee flexion/body weight at 60° per second (r = 0.76, P < .001), 90° per second and 180 per second (r = 0.62, r = 0.55, respectively, P < .05). Further positive correlations were observed between PA and static balance (r = 0.64, right leg; P < .05, left leg) for eyes closed and (r = 0.70, P < .001) for eyes open.
Significant correlations were observed between sedentary activity measured by screen hours per weekday and peak torque/body weight for knee flexion (r = −0.59 at 60° per second.; r = −0.60 at 90° per second and r = −0.49 at 180° per second, P < .05) and extension at 60° per second. (r = −0.50, P < .05) and 180° per second (r = −0.62, P < .001). Children who reported spending more than 2 hours engaging in screen time per day had significantly lower knee extension strength at 90° per second (P = .007) and 180° per second (P = .003) than those children who engaged in less than 2 hours of screen time per day.
DISCUSSION
The current pilot study was designed to profile the lower limb musculoskeletal health of children who are obese in order to guide practice and appropriate therapeutic intervention. In addition, the study was designed to identify any relationships that existed between musculoskeletal measures, BMI, and PA level.
It was observed that half of the group had previous lower limb injury, with more than 40% having sustained a fracture of the lower limb. Currently, in Ireland, no national data are available pertaining to the incidence of musculoskeletal injury in children and therefore comparison to normative data is not possible. As the study did not include a control group, previous reports of greater musculoskeletal injury in children who are obese cannot be supported. In this study, lower limb pain was reported by 72% of the group. Without the inclusion of a control group, it is unknown whether children who are obese report more musculoskeletal pain than their peers who are leaner; however, the work by Bell et al24 observed a greater likelihood of musculoskeletal pain in children who are overweight and obese compared with lean controls. Children who were obese were 4.09 times more likely to report pain than controls (odds ratio, P < .05). Similarly, Krul et al25 observed more self-report musculoskeletal problems in adolescents (12–17 years) who were obese when compared with counterparts who were lean (odds ratio = 1.69; P < .05). As pain may act as a barrier to the lifestyle changes necessary to facilitate effective obesity management, it is vital that musculoskeletal discomfort is screened during the assessment of children with obesity. Previous work has identified knee pain as the most common symptom reported by children who are overweight.2,24 In the current study, foot pain (53%) was the most commonly reported symptom followed by knee pain (12%). The effect of childhood obesity on foot function warrants attention as flattening of the medial longitudinal arch may place greater strain on soft tissue structures of the medial lower limb and thus increase the potential for musculoskeletal injury. Krul et al observed greater self-reported ankle and foot problems (odds ratio = 1.89; 95% CI: 0.85, 4.17) compared with hip and knee problems (odds ratio = 1.70; 95% CI: 0.80, 3.58) in children who were overweight and obese compared with controls who were lean.25 Although Krul and colleagues did not objectively assess participants, the results indicate an increased prevalence of lower limb discomfort in children who are obese. In addition, the authors reported that children aged 12 to 17 years who were overweight and obese consulted a family physician with lower limb complaints more frequently than peers who were lean (odds ratio = 1.92, 95% CI: 1.05, 3.51; P < .05). Significant associations have been observed between obesity and low back pain, lower limb pain, genu valgum, knee hyperextension, and tight quadriceps.26 Obesity may have a negative effect on the osteoarticular health of children through the promotion of biomechanical changes in the lumbar spine and lower limbs, and for this reason musculoskeletal examination in the assessment of children who are overweight is recommended. The results of this pilot study confirm that musculoskeletal discomfort should be screened in children who are obese.
Activity was measured and results indicated that children were spending more than 20 hours per week in habitual PA. Therefore, it would appear that the study cohort was reaching recommended guidelines of 60 minutes per day of moderate activity. These results should be interpreted with caution, as the MAQA is a self-report questionnaire and may not accurately reflect the actual amount or intensity of activity performed. Children in the study reported engaging in screen time more than the recommended cutoff level of 2 hours per day.
Another objective of this study was to profile the objective lower limb musculoskeletal fitness of the cohort. Normative data for joint ROM and flexibility measures in children are limited. Therefore, whether joint ROM was reduced in this cohort is unclear. Work by Bell et al24 compared lower limb ROM in children who were overweight and obese with counterparts who were lean and reported no observed differences.24 The authors did not describe the methods used to assess ROM and as such these results should be considered with caution. Our results suggested that children who are obese have less gastrocnemius flexibility than reported normative values,26 and as reduced gastrocnemius flexibility can affect ankle dorsiflexion and balance, this finding should be investigated in future controlled trials. Goulding et al18 suggest that boys who are overweight have significantly impaired balance compared with controls of healthy weight. Future research is recommended to investigate the influence of obesity on balance.
Impaired muscle strength and subsequent functional limitation in children who are obese has been reported.28 We have presented data regarding the isokinetic strength of children who are obese, but these data should be interpreted with caution because of the lack of information regarding the pubertal status of our sample. Because of the influence of lower limb muscle strength on developing peak bone mass and bone strength, it is recommended that future studies investigate whether children who are obese have reduced muscle strength compared with children matched for pubertal status, gender, age, and height.
The final objective of this study was to examine the relationships between BMI, PA, and musculoskeletal measures. Inverse associations were observed between BMI and lower limb ROM. These findings have not been reported elsewhere. A reasonable assumption, however, is that in an individual who is obese, joint excursion would likely be limited by excess deposits of subcutaneous adipose tissue. The finding that children who were severely obese had less knee flexion ROM than those who were moderately obese supports this hypothesis. Similarly, such limitation of ROM might affect the flexibility of lower limb musculature. Using the femur as a lever, the hamstrings influence pelvic tilt, which is particularly important in the growing child, where muscle tightness can affect posture, gait, and low back discomfort.12 In this study, significant negative relationships were observed between body composition and lower limb flexibility, supporting previous findings.12,26 Whether flexibility is impaired in children who are obese compared with children who are of healthy-weight requires further investigation, and such study should also assess the functional implication of these impairments.
Our results suggested that children who are obese might present with lower limb misalignment. The significant positive relationship between body composition and knee hyperextension concurs with previous work,26 in which a greater incidence of knee hyperextension in children who were obese compared with those of healthy weight was observed. Considering that knee hyperextension may influence proprioception and the peak joint moments associated with joint loading,29,30 future research should examine whether children who are obese and present with knee hyperextension may be at greater risk of injury compared with controls. The positive relationship observed between BMI and genu valgum has also been described by Shim et al,31 who reported greater intramalleolar distance in a cohort of children with Prader-Willi syndrome who were obese compared with those who were not obese. We do not currently understand whether being overweight during childhood negatively affects developing joints. Considering the associations between bony anomalies (Blount's disease and slipped upper femoral epiphysis) and childhood obesity,27 further study is warranted to investigate the effect of obesity on joint loading, ligamentous stability, and bony development.
We observed a negative association between BMI and knee flexion strength. Given the evidence to date regarding the gait abnormalities observed in children who are obese, future investigation should study the relationship between body composition and the effect of strength indices on functional capacity. Our results suggested that children who were more physically active had greater strength and that a positive relationship existed between PA levels and standing balance. It cannot be determined from the literature what level of PA is necessary for optimal balance development in children, but it is reasonable to assume that for neuromuscular capabilities to develop fully, threshold levels of physical challenge and external pertubation are required. Children reporting more screen time had lower levels of standing balance and greater strength than their contemporaries who were less sedentary. To date, no studies have reported a negative association between screen time and muscle strength of children, and therefore this finding should be investigated further. Our results indicate that there may be a positive relationship between BMI and musculoskeletal impairment in children who are obese. In addition, results suggest an inverse relationship between PA level and musculoskeletal impairment.
Although this pilot study adds to the current evidence regarding the effect of obesity on children's musculoskeletal health, it was greatly limited by a small heterogeneous sample. Participants taking part in the study were not classified according to Tanner stage of maturity, and in addition, the girls in the sample were older than the boys. Therefore, the results should be interpreted with caution as pubertal status may have influenced measures (particularly in the case of muscle strength). Further investigation is warranted using a randomized controlled design to ensure that no inherent differences between the groups are confounding the study. Given the limitations of the study, it is nevertheless recommended that children who are obese undergo a full musculoskeletal assessment as part of their general medical assessment and that physical therapy is considered as part of standard care. The presence of such musculoskeletal impairments as those described in this small study may adversely affect the time spent in PA by children who are obese. As increasing PA is a cornerstone of obesity treatment, examining the effect of such musculoskeletal impairments on PA level is warranted. It is recommended that physical therapists assess, monitor, and treat musculoskeletal impairments associated with childhood obesity where appropriate.
CONCLUSION
This small pilot study investigated the presence of musculoskeletal impairments in obese children and explored the relationships between body composition, PA, and musculoskeletal measures. The results suggest that children who are obese may present with musculoskeletal impairments of the lower limb. It is warranted that children who are obese have a thorough musculoskeletal assessment to identify such impairments and so that clinicians can prescribe suitable therapeutic exercise to reduce these impairments.
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