The purpose of this study was to compare running mechanics between children classified as overweight or obese (OW/OB) and children classified as having HW. We hypothesized that the OW/OB group would display increased vertical loading, joint moments, and frontal plane joint angles and excursions and decreased sagittal plane joint angles and excursions compared with the HW group. Based on our results, there appear to be several kinematic and kinetic differences in running mechanics between these two groups of children, confirming many of our predictions.
Even with both groups running the trials at the same speed, the OW/OB children displayed shorter step lengths and spent more time in the stance phase compared with the HW group of children. These findings are consistent with previous research studies reporting that obese children display shorter step lengths during walking and running and spend more time in the stance phase during running compared with HW children.[7,31] Huang and colleagues suggest that obese children use a shorter step length during walking to avoid the higher metabolic cost needed to transport excess body mass. However, the obese children in their study also displayed significantly slower walking velocities compared with the HW children, which likely explains their shorter step lengths. With no group differences in running speeds for our study, it is unknown if shorter steps would result in decreased or increased energy expenditure.
Another possible explanation for the group differences in step length and stance time may be the decreased range of motion and poor balance observed in obese children. Compared with HW children, obese children demonstrate significantly lower joint ranges of motion with the largest differences in the lower extremity joints. The OW/OB group in our study did display significantly lower hip flexion angles during the first part of stance as well as smaller knee flexion excursions compared with the HW group. Although we did not directly measure joint range of motion in our study, it is possible that OW/OB children ran with shorter steps due to a limited range of motion at the knee and hip joints. Regarding balance, previous studies have demonstrated that children have greater postural instability compared with their HW counterparts. Colné and colleagues explain that overweight children experience greater difficulty in controlling the fall of their center of gravity during gait. Colné et al. go on to explain that using shorter step lengths and spending more time in stance may help overweight children better control the fall of the center of gravity during each step helping them to maintain equilibrium during gait. Our findings indicate that the OW/OB children in this study ran with shorter steps and spent more time in the stance phase. If these changes are due to limited range of motion and/or impaired balance, clinicians may need to incorporate flexibility and balance training into the daily activities of OW/OB children. More research is needed to definitively determine whether these differences were due to limited range of motion and/or postural instability of the OW/OB group.
Differences in Joint Kinematics
Differences in the sagittal plane joint angles and excursions occurred exclusively at the hip and knee joints. As predicted, the OW/OB group of children displayed greater knee and hip flexion excursions compared with the HW group of children. These differences occurred primarily in the early portion of the stance phase (Table 2 and Table 3). Although this is the first study to examine differences in joint angles and excursions of OW/OB children during running, our results are consistent with previous studies reporting that obese children display decreased hip and knee angles and excursions during the stance phase of walking.[8,13] The decreased knee and hip flexion during early stance is suggested to be indicative of a more rigid gait pattern that could potentially result in greater vertical loading on the body. Qualitatively, approximately half of the children classified as HW (n = 12) displayed hip flexion during early stance with the other half displaying hip extension. In comparison, only one child classified as OW/OB displayed hip flexion during early stance. The lack of flexion during early stance may be due to a decreased range of motion at the lower extremity joints, which may be related to the increased amount of adiposity. If the OW/OB children are unable to reach a higher degree of flexion during high-impact activities, such as running, their body may not absorb as much shock as the body of someone who goes into greater flexion.
Several group differences were also detected in the frontal plane. The OW/OB group displayed significantly greater inversion at FS. However, no group differences in the ankle eversion angle were detected at FZmax, which occurs close to midstance. Subsequently, the OW/OB children displayed significantly greater eversion excursions at the ankle during the first part of stance (P = 0.01). Furthermore, an examination of the steeper slope for ankle inversion/eversion in Figure 2 suggests that the OW/OB may have also had greater eversion velocity. Excessive heel eversion excursions and eversion velocity has previously been linked to various running-related injuries, including tibial stress fractures, patellofemoral pain syndrome, and Achilles tendonopathy.[34–36]
As predicted, the OW/OB group displayed greater knee abduction than the HW group of children. Consistent with previous findings, the OW/OB spent the entirety of stance in an abducted knee position. In comparison, the HW group began in a slightly abducted knee position, quickly transitioned to a knee adducted position before returning to a knee abducted position for the last 25% of the stance phase (Figure 2). Increased knee abduction angles during dynamic movements have been identified as a mechanism for noncontact ACL injuries and have been associated with increased strain on the ACL.[37,38] In addition, increased knee abduction angles during running have been reported to increase contact forces on the lateral patellofemoral joint. The increased knee abduction angles exhibited by the OW/OB during running may potentially increase their risk of developing patellofemoral pain syndrome and ACL injuries. In comparing the frontal plane knee excursions, the HW group displayed greater knee adduction excursions during the first part of stance compared with the OW/OB groups. However, the total knee excursions were similar across groups. These findings can partially be explained by comparing the knee abduction/adduction curve in Figure 2. During the first 5%–10% of stance, the OW/OB group displays a small knee abduction excursion before starting to abduct, whereas the HW group starts adducting at the knee at FS. As predicted, no group differences were detected for frontal plane hip angles and hip joint excursion throughout stance. Our findings are consistent with other studies who also did not find differences in frontal plane hip angles and excursions between obese and HW children during walking.[9,12,40]
Differences in Ground Reaction Forces and Joint Moments
As expected, several of the ground reaction force and joint moment variables of interest were significantly greater in the OW/OB compared with HW group. Unscaled ground reaction forces and joint moments accounted for most of these group differences. With few exceptions, ground reaction forces and joint moments scaled to body mass were not significantly different across groups. However, it is important to note that scaling ground reaction forces and joint moments to body mass does not consider that increases in plantar and joint surface areas are not proportionate to increases in body mass. It has been reported that a 49% increase in body mass in children is associated with a 20% increase in their foot contact area during running. Ding and colleagues reported that a 48% increase in body mass was associated with only an 8% increase in tibial plateau surface area. Furthermore, it has also been reported that overweight children have reduced bone mineral density and reduced bone strength compared with HW children. Findings from these studies suggest that even if ground reaction forces and joint moments scaled to body mass are similar across groups, because of greater mass, OW/OB children may experience greater loading on less dense bones and across relatively smaller surface areas. Although plantar and joint surface areas and bone density were not evaluated in this study, examining the unscaled ground reaction forces and joint moments can provide greater insight into the loads experienced by children during running.
Ground Reaction Forces
All unscaled GRF variables of interest were significantly greater in the OW/OB children. These results confirm our hypothesis that OW/OB children would display greater vertical loading during running compared with HW children. Of major concern is our finding that VIP, average vertical loading rate, and instantaneous vertical loading rate were over 40% higher in the OW/OB children compared with the HW children. These findings are consistent with Rubinstein et al. who reported the greatest difference in FZmax between overweight and HW children is in the heel region of the foot during early stance. Both retrospective and prospective studies have linked increased vertical loading during early stance to several overuse running injuries, including tibial stress fractures, patellofemoral pain syndrome, iliotibial band syndrome, plantar fasciitis, and other soft tissue injuries.[42–44] A prospective study of 242 runners reported that those with high loading rates were at a three times greater risk of developing a running-related injury compared with those with low loading rates. Of further concern is that even with significantly greater surface area of the foot in contact with the ground, OW/OB children display significantly greater peak pressures on the plantar surface of their feet during running.[3,7,20] Increased vertical loading and foot pressure for obese children has been linked to increased foot discomfort, pain, and injury. In addition, higher plantar pressure of overweight children is inversely associated with physical activity levels. These findings are problematic considering running is a component in many of the moderate- to vigorous-intensity activities recommended for children by the World Health Organization and the U.S. Department of Health and Human Services.
FZmax was the only GRF scaled to body mass that displayed a significant group difference. Compared with the OW/OB children, the HW children demonstrated significantly greater FZmax scaled to body weight. Anecdotally, we observed that many of the HW children appeared to run with a more bounding/up and down motion compared with the OW/OB children. Considering that FZmax represents the push off in the vertical direction, a greater FZmax would likely result in greater vertical displacement of the body's center of mass. However, follow-up analysis revealed no group differences for vertical center of mass displacement scaled to height (HW = 5.4% ± 1.0% body height, OW/OB = 5.3% ± 1.3% body height, P = 0.77). At this time, it is unclear why the FZmax scaled to body mass is greater in the HW group. However, unlike the other ground reaction force variables that occur during the loading phase of stance, FZmax occurs during the push-off phase of stance and has not been found to be a significant predictor of injury. The lower rate of loading that commonly precedes FZmax likely explains why it has not previously been linked to injury.
As predicted, several significant group differences were detected with the OW/OB children displaying greater peak joint moments compared with the HW children. Our results are consistent with Briggs et al., who reported unscaled peak knee moments in the frontal and sagittal planes are significantly greater in obese adolescents (age 12–18 yr) during both walking and jogging. Similarly, Shultz and colleagues reported overweight children display higher unscaled hip, knee, and ankle joint moments in the frontal and sagittal planes during walking. In a study by Gushue et al., unscaled ankle plantarflexion and knee abduction moments during walking were also significantly greater in overweight children.
The increased joint moments exhibited by the OW/OB children in our study may partially be explained by their more rigid running pattern. As mentioned previously, only one of the children classified as OW/OB displayed hip flexion during early stance. In addition, knee flexion excursions were also significantly lower for the OW/OB children. The more rigid gait pattern accompanied by greater unscaled ground reaction forces likely contributed to the greater unscaled joint moments observed in the OW/OB children.
Greater joint moments at the hip, knee, and ankle during walking and running suggest greater joint loading, increased risk of malalignments, and increased risk of joint injury and/or joint damage for children classified as OW/OB. A common joint injury observed in obese children is slipped capital femoral epiphysis. Researchers suggest that increased hip extensor moments and hip abduction angles will, respectively, increase the compressive and shear forces on the capital femoral growth plate potentially, resulting in femoral neck fractures or slipped capital femoral epiphysis.[18,46] Unscaled hip abduction was significantly greater in the OW/OB group of our study potentially increasing their risk for developing these hip injuries. Higher than normal frontal plane joint moments at the hip and knee have also been linked to the development of genu valgum and tibia vara, malalignments often observed in obese individuals. Although knee alignment was not directly evaluated in this study, children classified as OW/OB displayed an abducted knee position throughout the stance phase (Figure 2). This finding would be consistent with someone who displays genu valgum. Subsequently, the abducted knee position and greater knee abductor moments displayed by the OW/OB group in our study may indicate increased lateral compartment loading of the tibial femoral joint. For OW/OB children who may be predisposed to varus alignment, increased knee adductor moments may also be problematic. Increased knee adductor moments have been linked to increased compressive loads on the medial compartment of the tibial femoral joint and may result in tibia vara. Although tibia vara is not as common as genu valgum among obese children, both malalignments have been linked to uneven loading at the tibiofemoral joint. In addition, higher than normal frontal plane joint moments at the hip and knee during running have also been linked to increased joint loading at the patellofemoral joint and the development of both patellofemoral pain syndrome and knee osteoarthritis.
Consistent with previous literature, we also observed significantly higher unscaled ankle plantarflexion and inversion moments in children classified as OW/OB.[9,12] Ankle inverter moments are necessary to help slow down and control ankle eversion during early stance of running. Subsequently, the OW/OB children in our study may have increased inversion moments to help control their increased ankle eversion displacement during early stance. Increased plantarflexion moments observed in the OW/OB children in this study are likely explained by the need to propel a greater body mass forward. Unfortunately, increased plantarflexion moments during running have been linked to increased loads on the Achilles tendon resulting in the increased risk of developing Achilles tendinopathy. Shultz and colleagues suggest that increased plantarflexion moments during gait may also increase peak pressure under the forefoot and metatarsal heads, leading to increased risk of metatarsal stress fractures and general foot pain. As such, increased plantarflexion and inversion moments during running may place increased loads on the structures of the foot and ankle that place OW/OB children at an increased risk for developing these injuries.
Although several group differences were detected for the unscaled joint moments in our study, there were only a few group differences detected for the joint moments scaled to body weight. Interestingly, all but one significant group difference for joint moments scaled to body weight indicated greater joint moments in children classified as OW/OB. These findings further emphasize the impact that excessive weight has on increased joint moments and loading during running.
One limitation of this study was the lack of physical activity and sedentary time data from participants. Because a child's daily activity, or lack thereof, can greatly influence how their body adapts to daily loading, knowing the child's activity level could give further insight into how running, regardless of weight, may impact the lower extremity joints. Another potential limitation to this study is using a standard running speed across all participants. While using a given running speed can minimize the impact of different running speeds on differences in running mechanics, a given running speed may not represent the typical running mechanics of children who may prefer a slower or faster running speed. It is possible that OW/OB children may choose running speeds that result in running mechanics that are more similar to the HW children. Future studies that include self-selected running speeds of OW/OB and HW children may provide additional information on potential differences between these two groups. Another potential limitation is skin movement artifact caused by excess adipose tissue. To minimize the impact of movement artifact and to improve accuracy of our kinematic data, we used the recommended methods of previous researchers who evaluated 3D kinematics in obese children[9,12,13,17] by having the same researcher place markers on all participants, using rigid marker clusters and using the spherical fit model to calculate functional hip joint centers. Lastly, because of the cross-sectional design of this study, a direct causal relationship between excess body mass and running mechanics cannot be determined. Longitudinal studies that examine the impact of body mass on running mechanics throughout adolescence and into adulthood are needed to provide additional insight.
Med Sci Sports Exerc. 2021;53(10):2101-2110. © 2021 American College of Sports Medicine