McMillan, Amy Gross PT, PhD; Phillips, Kali A. PT, DPT; Collier, David N. MD, PhD, FAAP; Blaise Williams, D. S. MPT, PhD
During the past several decades, the prevalence of obesity in children in the United States has been greatly increasing. According to the Centers for Disease Control and Prevention, the percentage of children with obesity has increased from 6.5% to 17.0% in those aged 6 to 11 years and from 5.0% to 17.6% in those aged 12 to 19 years.1 Furthermore, 13% of high school students in the United States were considered obese (OB) in 2007.2 This obesity epidemic has also been shown to persist into adulthood, with children who are OB having a 50% to 70% chance of becoming overweight or OB as adults.3 Based on these reports, it seems that today's young people may, on average, live less healthy and ultimately shorter lives than their parents.4
Children who are OB are at an increased risk for a variety of health conditions, including dyslipidemia, hypertension, glucose intolerance, type 2 diabetes, and sleep disturbances.5,6 These children also experience musculoskeletal issues at an increased rate compared with peers who are healthy weight (HW), including pes planus, abnormal knee alignment (valgus and varus), increased risk for fractures, general musculoskeletal pain (particularly low back and lower extremity [LE]), slipped capital femoral epiphysis, and excessive tibial varum (Blount's disease).5–10 Pediatric obesity may also be associated with an increased risk for musculoskeletal injuries, including ankle sprains and tendonitis.11–13
Misalignment of the LE joints coupled with the excess load placed on these joints in individuals who are overweight may explain some of the increased pain and injuries found in this population.14 A part of most weight management programs for the pediatric population is increased physical activity.
Jumping and landing are common in sports, general play, and physical education activities for children. Jumping as a part of play and exercise is a part of normal motor development,15 and helps improve bone mineral density in children.16 These physical activities involve increased LE weight-bearing, which likely increases the risk for musculoskeletal pain and dysfunction because of excessive forces in misaligned LEs.14 Because of the repetitive, excessive loading on joints that occurs during jumping activities, investigation of the safety of these activities for children who are OB is needed.
In typical children, jumping has been shown to produce ground reaction forces (GRFs) from 2 to 5 times body weight.17 Children who are OB would experience very high GRFs during a jump-landing activity due to their increased mass.18 In addition to the increased compressive and shear forces associated with landing, additional muscular forces will be necessary when landing from a jump around the joints to control the momentum (mass × velocity) of a child who is OB.19 Furthermore, the passive structures surrounding the knee will likely be placed on much greater strain as a result of the increased forces experienced on landing.
Limited research is available on the biomechanical characteristics of drop jump landings in children. However, several groups have reported differences during landings between children and adults.20,21 Swartz et al20 demonstrated that during vertical jump landings children tended to land with greater hip and knee extension angles at initial contact demonstrating a stiffer landing technique when compared with adults. Barber-Westin et al21 studied jump-landing characteristics in young athletes aged 9 to 17 years and found no age or gender differences in limb alignment although there was a trend toward increasing knee valgus angles during landing in all subjects. If, in fact, children land more stiffly and with increased knee valgus, they could be at greater risk for LE injury such as a torn anterior cruciate ligament.22 This risk would likely be further increased in children who are OB, given the increased absolute forces generated during their landings. To date, there is no research available on the LE biomechanics of landing from a jump in children who are OB.
McMillan et al23 reported that boys who were OB demonstrate greater knee abduction (valgus) and greater hip adduction angles, and greater hip abduction moments compared with their counterparts who are HW during the early phases of stance. These variables are associated with a general collapse of the LE during initial stance phase of walking. Differences in the timing of peak joint motions and moments during the later stance phases of gait were also demonstrated. McMillan et al proposed that the LE collapse and the timing differences during stance phase of gait may suggest underlying weakness, instability, and differences in motor control in boys who are OB that may place them at increased risk for musculoskeletal injury. Similar differences in LE biomechanics may occur during landing and will likely be increased due to greater GRFs in jumping as compared with walking. The purpose of this study was to investigate the frontal and sagittal plane LE biomechanics during drop jump landings in boys who were OB and boys who were HW.
Twelve male subjects between the ages of 10 and 12 years participated in this study. Subjects were referred from the local community and from a HW clinic. In children 2 to 20 years of age, body mass index (BMI) classifications were based on gender and age; a BMI between the 85th and 94th percentile for age and gender (BMI-for-age) was considered overweight, whereas a BMI-for-age ≥95th percentile was considered OB.1 For this study, subjects were classified as HW if their BMI for age was <85th percentile (n = 6), and as OB if their BMI-for-age was ≥95th percentile (n = 6). None of the subjects had a history of musculoskeletal, neurological, or metabolic conditions that would affect their jump-landing performance. Given the small number of subjects in this initial study, only boys were included to decrease potential variability in the data because of gender differences. Group characteristics are shown in Table 1. The OB group was significantly heavier for their height and age compared with the HW group, as is noted by their BMI Z score (number of standard deviations above or below the mean for their age/gender) and their actual mass in kilograms. All participants and their parents gave written informed assent/consent before participating in the study. The study was approved by the University and Medical Center Institutional Review Board of East Carolina University.
Kinematic data were collected during all trials using a 6-camera Qualysis Motion Analysis System (Glastenbury, CT) sampled at 120-Hz. Force data were sampled at 960 Hz with an Advanced Mechanical Technology Inc, (AMTI; Watertown, MA) force platform. Kinematic data were filtered at 12 Hz with a fourth-order zero lag Butterworth filter, and kinetic data were filtered similarly at 50 Hz.
Drop landing data collection was performed for each subject during 1 session in the Human Movement Research Laboratory, Department of Physical Therapy at East Carolina University. Twenty-two retroreflective markers were attached to each subject's right LE and pelvis with hypoallergenic tape as reported previously.23 Subjects started in standing on a 6-inch platform with their right leg extended out in front of them above the force plate (Fig. 1A). Subjects were instructed to drop and land on 2 feet with only the right foot landing on the force platform (Fig. 1B). Ten acceptable trials were collected for each subject. An acceptable trial was defined as when the subject dropped down off of the step to land on 2 feet simultaneously with only the right foot landing on the force platform.
Marker and force data were processed using Visual 3-D (C-Motion, Inc., Germantown, MD) to obtain joint angle and joint moment data. For the purpose of data analysis, the landing cycle was defined as the time from the moment of landing (first contact with the force plate) to 0.5 seconds after the moment of landing. Timing variables were calculated as a percentage of this total landing cycle. Joint excursion was measured as the amount of joint motion change from initial ground contact to peak angle. Joint moments were normalized to each subject's height and body mass.
Data from acceptable trials were averaged for each subject. Group averages were then calculated and used for statistical analyses. Student t tests were used to determine differences between the 2 groups on the variables of interest. A Bonferroni correction24 was performed to bring the corrected significance level to p ≤ 0.01.
Kinematic variables of interest included initial positions, peak positions, timing of peak positions, and total joint excursions in both the sagittal and frontal planes at the rearfoot, knee, and hip. Kinetic variables of interest were peak moments and timing of peak moments in both the sagittal and frontal planes for the rearfoot, knee, and hip.
The data in the tables that follow represent average values of individual points chosen for each variable (peak motion or moment, or timing of these peaks) for all subjects in a group. Data in the graphs represent average (±1 SD) motion or moment over the entire landing cycle for all subjects in a group.
At initial contact, subjects who were OB landed in significantly more knee valgus than peers who were HW, who landed in a slight varus position (OB = −11.2° [±4.9], HW = 2.1° [±5.3], p = 0.000, Fig. 2B). The OB group also landed in significantly more hip adduction as compared with the HW group who landed in a slightly abducted hip position (OB = 3.9° [±5.4], HW = −4.3° [±4.8], p = 0.01, Fig. 2C) Frontal plane rearfoot position and sagittal plane rearfoot, knee, and hip position were not significantly different at initial ground contact (Table 2).
In addition to differences found at initial contact, significant differences were found for the timing of peak dorsiflexion (OB = 44% [±9.2], HW = 28.6% [±3.1], p = 0.003, Fig. 3A) and knee flexion (OB = 34.5% [±6.1], HW = 23.3% [±2.8], p = 0.002, Fig. 3B). Several other kinematic variables also approached significance during the landing phase (Table 2).
Overall, boys in the OB group landed in and maintained a more abducted knee position throughout landing, with very few subjects reaching a neutral or adducted position during the landing recovery (Fig. 2B). A similar trend was found at the hip with the OB group landing in and maintaining a more adducted position throughout the landing phase with mean hip position approaching, but not reaching, neutral (Fig. 2C). Overall, the OB group reached their kinematic peaks later during the landing phase in both the sagittal (Fig. 3) and frontal (Fig. 2) planes as compared with the HW group. Total excursion measurements were similar in both groups for all joints in both the sagittal and frontal planes (Table 2).
Significant differences were found in timing of peak knee extension moment, timing of peak hip abduction moment, and peak hip abduction moment (Table 3). With the exception of hip adduction, the HW and OB groups had similar peak LE moments in both the sagittal and frontal planes. The OB group, however, consistently reached these peak moments later in the landing phase (Figs. 4, 5).
The purpose of this study was to investigate the sagittal and frontal plane mechanics when landing from a jump in boys who were OB compared with those who were HW. In the sagittal plane, no differences were found in the motion or moments of joints during the landing. However, the 2 groups had significantly different timing of several peak motions and moments. Differences in timing were also observed in the frontal plane, although only the timing of peak knee abduction moment reached statistical significance.
Differences in timing of peak motions and moments may be evidence for a lack of stability in individuals who are OB when landing from a jump. Individuals who are OB must control greater forces and more momentum when moving due to their increased mass.17 The OB group reached their motion and moment peaks later during the landing cycle, suggesting that they required more time to control and stabilize their mass when landing from a jump. This greater time required to stabilize may place individuals who are OB at a greater risk for injury because any unsteadiness during a landing could lead to a fall or compensation with excessive and abnormal movements at distal joints in an attempt to regain proximal stability.25
This delayed timing of peak movements may offer a partial explanation for the compromised balance seen in children who are OB.6,26 Impaired balance could contribute to altered methods of motor control in the OB group when landing, which may place the OB individual at a greater risk for injury. Based on these findings, children who are OB require more time to perform the same movements during a landing, which may be indicative of decreased stability and control.
Although a similar landing strategy was used by subjects in both the OB and HW groups, there were several notable differences between the groups both at initial contact and throughout the landing cycle. At initial ground contact, a difference in LE alignment was observed. The boys who were OB landed in a position of significantly more knee abduction and hip adduction. This collapsed LE position is consistent with the collapse seen in stance phase of gait in these same subjects.23 This collapse may be the result of a functional weakness in the hip abductor muscles of the stance limb, which should be eccentrically controlling hip adduction. Although the OB subjects did present with greater hip adduction, they also exhibited significantly greater hip abduction moments relative to HW subjects. Our clinical findings are inconsistent with these data, as youth who are OB typically have weak hip abductors when assessed with manual muscle testing and/or functional strength testing (ie, single-limb partial squats). Further evaluation of anthropometrics and the calculations of moments is necessary to understand these differences.
During walking, the LE collapse seen in subjects who were OB continued throughout the stance phase of the gait cycle, and it was expected that the same would occur during a drop landing. However, after initial contact, both the HW and OB groups moved toward hip abduction and knee adduction, a position consistent with the position of the knee and hip during a squat.27 This is an interesting and unexpected landing strategy. Given the LE collapse seen during walking, we expected an even larger collapse to occur during landing. The drop landing performed in this study was from a 6-inch height, which may have been too low to sufficiently stress the system and cause significant collapse, especially because subjects landed on both lower extremities. With a larger drop and/or a single limb landing, subjects may have exhibited a more collapsed LE position and perhaps other compensations.
Along with the similarities noted during the landing cycle several differences between groups were also observed. The HW group achieved a more abducted hip position, whereas the OB group's mean peak hip abduction was positive but very close to neutral (0.36° ± 6.4), indicating that on average subjects in the OB group never reached a truly abducted position. This is likely due to the fact that the OB group began the landing phase in significantly more hip adduction, and although movement was toward abduction, most never got to an abducted position. A similar finding occurred at the knee with OB subjects not reaching a fully adducted knee position but starting in more initial knee abduction. Total knee excursion was greater in the OB group, although not significantly so. Excess frontal plane motion at the knee could be an indicator of instability and a possible risk factor for injury in this population.
Along with landing in a more abducted position at the knee, the OB group also had less knee flexion in the sagittal plane. Less knee flexion is associated with a stiffer landing and may explain the extra frontal plane motion in the OB subjects.28 Stiffer landings coupled with excessive valgus and weak hip musculature may place the OB group at a higher risk for injuries during a landing compared with the HW group.22 Further investigations are needed to confirm these hypotheses.
The groups compared in this initial study were very different in terms of BMI. Future studies will investigate biomechanical differences in subjects with BMIs in the intermediate range (between the 85th and 95th percentile) to determine whether effects of increased body mass are linear (changing gradually with increased mass) or whether there is a threshold of BMI at which biomechanical changes in movement occur.
The drop height in this study was 6 inches, which may not have been challenging enough to elicit significant differences in the variables we measured. Typical drop heights in the literature range from 10 to 25 inches. For this initial investigation, we erred on the side of caution, particularly for the children who were OB, and chose a smaller drop height. Future studies will assess landing strategies in this population using a variety of jumps and heights.
Positioning of the markers used for motion analysis may also be a limitation for the data from the OB group. When using reflective markers, the goal is to place them over bony prominences so that movement of the markers is comparable with movement of the bones. Accurate location of bony landmarks is difficult for subjects who are OB, though we believe that the marker placements did represent the subject's bony structure. Soft tissue movement associated with a jumping task was also troublesome when analyzing joint movement.
Generalizability of the results of this study is limited because of the small group size. Further research will be necessary to more fully investigate the mechanics of landings in children who are OB.
The results of this study provide evidence that children who are OB have significant differences in frontal and sagittal plane biomechanics when landing from a jump compared with children who are HW. These biomechanical differences could be placing children who are OB at greater risk for knee injuries when they engage in jump-landing activities. Physical therapists can play a crucial role in identifying children at greatest risk for injury during weight-bearing activities, eg, those who are OB and have LE misalignment and proximal muscle weakness. By providing a thorough musculoskeletal evaluation, impairments can be identified and addressed, and appropriate physical activity can be recommended. As they become more active, children who are OB may benefit from interventions, such as proximal hip strengthening, balance training, foot orthotics, and knee/ankle bracing or taping, to improve LE alignment and control and decrease their risk for injury. Children with severe LE alignment issues and/or significant joint instability may benefit from performing more non–weight-bearing exercises such as swimming or cycling. All of these interventions will require investigation to fully understand their effectiveness for this patient population. Increasing physical activity is an important aspect of a weight management program; however, activity should be monitored by a physical therapist to ensure safety and decrease injury risks. Pain or injuries occurring during physical activity will only perpetuate the cycle of childhood obesity by discouraging children from becoming more active. It is important that physical therapists become involved, and they become active members of a team of professionals who care for children who are OB.
This study provides the first evidence of biomechanical differences between children who are OB and children who are HW during drop jump landings. Given these results, all children who are OB should undergo a thorough musculoskeletal evaluation before beginning a physical activity program to decrease their risk for LE injury. Guidelines based on mechanical findings should be developed for safe participation in landing activities.
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