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Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent ab
c
ab
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Qipeng Song , Bing Yu , Cui Zhang , Wei Sun
& Dewei Mao
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Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China b
Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
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c
University of North Carolina at Chapel Hill, Chapel Hill, USA Published online: 21 Mar 2014.
To cite this article: Qipeng Song, Bing Yu, Cui Zhang, Wei Sun & Dewei Mao (2014) Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent, Research in Sports Medicine: An International Journal, 22:2, 172-184, DOI: 10.1080/15438627.2014.881823 To link to this article: http://dx.doi.org/10.1080/15438627.2014.881823
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Research in Sports Medicine, 22:172–184, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 1543-8627 print/1543-8635 online DOI: 10.1080/15438627.2014.881823
Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent
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QIPENG SONG Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China; Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
BING YU University of North Carolina at Chapel Hill, Chapel Hill, USA
CUI ZHANG, WEI SUN, and DEWEI MAO Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China; Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
This study investigates the effects of backpack weight on posture, gait pattern, and ground reaction forces for children with obesity in an attempt to define a safe backpack weight limit for them. A total of 16 obese (11.19 ± 0.66 years of age) and 21 normal body weight (11.13 ± 0.69 years of age) schoolboys were recruited. Two force plates and two video cameras were used. Multivariate analysis of variance with repeated measures was employed. Obese children showed increased trunk and head forward inclination angle, gait cycle duration and stance phase, decreased swing phase, and increased ground reaction force in the medial-lateral and anteriorposterior directions when compared with male children with a normal body weight. The changes were observed even with an empty backpack in comparison with normal body weight children
Received 31 July 2013; accepted 8 December 2013. Project of Shandong Education Department, grant ID: J08LE13. Natural Science Foundation of Shandong Province, grant ID: ZR2010CM064. Address correspondence to Dewei Mao, Shandong Sports Science Research Center, Jinan, China. Email: [email protected] 172
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and a 15% increase in backpack weight led to further instability and damage on their already strained bodies. KEYWORDS children with obesity, gait and posture, stair descent
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INTRODUCTION Carrying a backpack is an activity in daily life for students, especially for elementary, middle school, and high school students. Many studies have been conducted to investigate the effects of backpack weight on children. These studies reported that excessive backpack weight result in back pain, muscle soreness, numbness, shoulder pain, and muscle fatigue (Johnson, Knapik, & Merullo, 1995; Pascoe, Pascoe, Wang, Shin, & Kim, 1997). Many studies also investigated the effects of carrying weight on ground reaction force (GRF) (Stacoff, Diezi, Luder, Stussi, & Quervain, 2005), gait pattern (Hong & Brueggemann, 2000; Hong & Cheung, 2003), body posture (Grimmer, Dansie, Milanese, Pirunsan, & Trott, 2002; Hong & Brueggemann, 2000; Hong & Chueng, 2003; Li, Hong, & Robinson, 2003; Singh & Koh, 2009; Stacoff et al., 2005), and electromyography (Hong, Li, & Fong, 2008) during level walking. However, only few of these studies revealed the biomechanical averages of carrying a backpack during stair walking. Stair walking requires an impulsive force equivalent to 155% of the body weight while level walking requires 120% of the body weight (Dowling, Steele, & Baur, 2004). Hong and Li (2005) investigated the influence of different schoolbag weight on GRF during stair walking. The results of their experiment showed that a load equal to 15% of the body weight induces significant increases in maximum peak force, stance, and double support duration for the backpack condition. However, only vertical reaction forces have been measured, and only normal body weight subjects are recruited in their respective works. Previous studies on the effects of backpack weight on gait were mainly focused on individuals with normal body weight. Our literature review did not show any study on the effects of backpack weight on posture, gait patterns, and ground reaction forces of children with obesity. Obesity results in a significant disadvantage in movement and discomfort in simple daily activities such as walking and stair-climbing (Hills & Parker, 1991) because large proportions of their body weight do not contribute to their movement performance. A good understanding of the effects of backpack weight on these children’s posture, gait, and ground reaction forces will provide significant information for preventing musculoskeletal system injuries and disorders among these children. Stair descent results in increased GRF in comparison to level walking and stair ascent. Andriacchi, Andersson, Fermier, Stern, and Galante (1982) found that the maximum external knee flexion moment in stair descent was 2.7 times that in stair ascent. Hong and Li (2005) reported that the maximum peak force in stair descent with a backpack of 15% body weight was1.89 times greater than that in stair ascent, while the ground reaction force was three times of
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that in stair ascent. Tripping is also more common during stair descent than in stair ascent, because people usually descend a flight of stairs with their center of pressure near the edge of the stairs (Beaulieu, Pelland, & Robertson, 2008). Many studies have attempted to establish safe weight carrying limits for children, and the guidelines vary among countries. Load carrying limits were set as 10% of the body weight in New Zealand (Whittfield, Legg, & Hedderley, 2005); 10% to 12% in India (Malhotra & Gupta, 2007); 15% in Hong Kong (Hong & Li, 2008), Australia (Chansirinukor, Wilson, Grimmer, & Dansie, 2001), and Singapore (Singh & Koh, 2009); and 20% in Turkey (Seven, Akalan, & Yucesoy, 2008). These load carrying limits were set for subjects with normal body weight in standing or level walking, and thus may not be appropriate for children with obesity in stair descent. The purpose of this study was to determine the effects of backpack weight on posture, gait pattern, and ground reaction forces of male obese children in stair descent. We hypothesized that increasing backpack weight would increase trunk and head forward inclination angle, gait cycle duration, and ground reaction force during stair descent. We also hypothesized that male children with obesity would increase their trunk and head forward inclination angle, gait cycle durations and would exert greater GRF in the mediallateral and anterior-posterior directions and less GRF in the vertical direction in comparison to children with normal body weight.
METHODS Subjects A total of 16 school boys with obesity and 21 school boys with normal body weight between 10 to 12 years of age from Shimuyuan Primary School in Jinan, China were recruited as subjects. The mean age, standing height, body weight, and body mass index (BMI) were 11.19 ± 0.66 years, 148.85 ± 14.07 cm, 75.26 ± 15.99 kg and 29.78 ± 4.10 kg/m2, respectively, for children with obesity, and 11.13 ± 0.69 years, 152.22 ± 6.95 cm, 48.13 ± 9.52 kg, and 20.03 ± 3.08 kg/m2, respectively, for children with normal body weight. A BMI of 26 kg/m2 or higher was defined as obesity for 12-year-old male children (Cole, Bellizzi, Flegal, & Dietz, 2000). The use of human subjects in this study was approved by the Internal Review Board of Shandong Sport University. Written consent was obtained from each subject’s parents before data collections.
Testing Protocol Each subject was asked to complete one testing session per day for a total of four testing sessions. In each session, the subject was asked to carry a twostrap backpack with both shoulders in a given backpack weight (0, 10, 15, or 20% of body weight). The order of backpack weight conditions was
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randomized using a Latin Square method. In each session, the subject was allowed to have a warm up to get used to ascending and descending the stairs. They were then asked to walk with the backpack for 400 m at a selfselected speed as a level walking test before the stair descent test. In the stair descent test, the subject was asked to perform five successful stair descent trials. A successful stair descent trial was defined as a trial in which the subject continuously descended from the top of the staircase and then continuously walked for 10 m with all data successfully collected. The subject had a 1 minute break between trials. After each trial, subjects had 1 minute to rest. The subject used the same pair of shoes for all testing sessions.
Data Collection A staircase with six steps (Figure 1) was built for the data collection in this study. The step dimensions were 17.0 cm (riser) × 29 cm (tread). A hand rail was set on the right side of the descent. Two force plates (Kistler, 9287BA and 9281CA, Switzerland) were embedded in the third and fourth steps of the descent to collect GRF data at a sample rate of 1000 Hz. Two high-speed video cameras were placed 12 m from the walking track with an angle of 90° to record movements of level walking and stair descent for each subject at a sample rate of 50 frames/second. Six landmarks were manually digitized from video records (Left and right shoulder, left and right hip, forehead and lower jaw). Three-dimensional (3-D) coordinates of the six landmarks were reduced from digitized two-dimensional coordinates. The 3-D coordinates were filtered through a low-pass recursive Butterworth digital filter with a cut-off frequency of 6 Hz.
FIGURE 1 Laboratory staircase, force plates 1 and 2 were embedded in the third and fourth steps of the staircase, respectively.
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Data Reduction Trunk and head inclination angles were reduced from filtered 3-D coordinate data. Trunk inclination angle was defined as the angle between trunk longitudinal axis and horizontal plane projected to the sagittal plane. The longitudinal axis of the trunk is defined as the vector from the mid-point of the line connecting hip joint centers to the mid-point of the line connecting shoulder joint centers. A positive trunk inclination angle means the trunk is leaning forward. The head inclination angle was defined as the angle between the line connecting digitized forehead and lower jaw landmarks and the horizontal plane projected to the sagittal plane. Gait cycle duration, single support phase duration, swing phase duration, and double support phase duration were also calculated from the video record to represent gait temporal characteristics. Gait cycle duration was defined as the time period between two consecutive foot takeoffs. Single support phase duration was defined as the time period from the foot takeoff of the swing leg to the foot strike of swing leg. Swing phase duration was defined as the time period between the takeoff and landing of the swing leg. Double support phase was defined as the time period from the foot strike of the swing leg to the foot takeoff of the supporting leg. Further, seven characteristic magnitudes of the ground forces were identified (Figure 2). These seven ground reaction force variables were the first peak of the vertical force curve (F1); the local minimum of the vertical force after the first peak (F2); the second peak of the vertical force curve (F3); the first peak of the medial-lateral force curve (F4) with a positive value representing the medial direction; the second peak of the medial-lateral force curve (F5); the first peak of the antero-posterior force curve with a positive value representing the anterior direction (F6); and the second peak of the antero-posterior force curve (F7).
FIGURE 2 Three components of the GRF for a typical stair descent trial; the seven dependent variables chosen for analysis (F1 to F7) are shown.
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Data Analysis
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A two-way MANOVA with mixed design was performed to determine the effects of backpack weight and obesity condition. The back weight was treated as a repeated measure while the obesity condition was treated as an independent measure. If the main effect of backpack weight was significant, post-hoc paired t-tests were performed to locate the differences. A Type I error rate of 0.05 was selected as an indication of statistical significance. Bonferroni adjustment was made for a post-hoc paired t-test to guarantee the overall Type I error rate was not greater than 0.05.
RESULTS Compared to children with normal weight, children with obesity showed increased trunk and head forward inclination angles (p = 0.003, p = 0.001), and gait cycle, single support phase, and double support phase durations (p = 0.030, p = 0.000, p = 0.023), and decreased swing phase duration (p = 0.001) (Table 1). The trunk forward inclination angle significantly increased as the backpack weight increased to 10% of body weight (p = 0.012 for children with normal body weight), 15% of body weight (p = 0.011 for children with normal body weight, p = 0.002 for children with obesity), and 20% of body weight (p = 0.000 for children with normal body weight, p = 0.000 for children with obesity). There were no significant differences observed in the head forward inclination angles among different backpack weights. Gait cycle duration significantly increased as the backpack weight increased to 20% of the body weight (p = 0.014 for children with normal body weight, p = 0.003 for children with obesity). Stance duration significantly increased as the backpack weight increased to 15% of body weight (p = 0.012 for children with normal weight, p = 0.002 for children with obesity) and to 20% of body weight (p = 0.001 for children with normal weight, p = 0.000 for children with obesity). Swing duration decreased as backpack weight increased to 15% of body weight (p = 0.031 for children with normal weight, p = 0.022 for children with obesity) and 20% body weight (p = 0.000 for children with normal body weight, p = 0.000 for children with obesity). Double support duration increased as backpack weight increased to 20% of the body weight (p = 0.015 for children with normal weight, p = 0.041 for children with obesity). Children with obesity demonstrated significantly increased F2 (p = 0.005), F3 (p = 0.017), F5 (p = 0.004) and F7 (p = 0.007), but decreased F1 (p = 0.000) in comparison to children with normal body weight. F2 was significantly increased as backpack weight increased to 15% (p = 0.003) and 20% (p = 0.001) of the body weight for children with normal body weight, and as the backpack weight increased to 10% (p = 0.008), 15% (p = 0.000), and 20%
Obese (mean and standard deviation)
34.81 (1.34)a a
0.99 (0.12)b 61.89 (2.56)b 38.11 (2.10)b 29.02 (2.83)b
0.93 (0.11) 61.68 (2.04)b 38.32 (1.57)b 27.69 (3.09)
0.96 (0.11) 61.33 (1.74) 38.67 (1.85) 27.75 (3.15)
29.32 (2.97)
33.93 (2.10)
66.07 (2.10)
1.01 (0.15)
23.82 (8.06)
10.46 (3.79)
a: p < 0.05 between the average backpack loads of normal weight and obese subjects (shown in 0% backpack loads). b: p < 0.05 versus 0% in normal weight subjects. c: p < 0.05 versus 0% in obese subjects.
28.33 (2.51)
65.19 (1.34)a
1.00 (0.09)a
17.91 (12.26) 22.68 (10.03)
17.36 (8.88)
8.55 (2.61)a
16.29 (10.56)
7.57 (3.00)b
7.31 (2.39)b
6.78 (2.22)b
28.86 (2.58)
34.65 (2.07)
65.69 (2.61)
1.07 (0.17)
23.77 (9.95)
11.87 (4.02)c
34.03 (3.30)c
33.38 (2.37)c
66.62 (2.36)c
1.11 (0.20)c
22.48 (10.05)
11.43 (3.53)c
Backpack 0% Backpack 10% Backpack 15% Backpack 20% Backpack 0% Backpack 10% Backpack 15% Backpack 20%
Trunk inclination 5.54 (2.61)a angle (O) Head inclination 18.3 (11.23)a angle (O) Gait cycle 0.96 (0.13)a duration (s) Stance phase 61.06 (1.01)a duration (%) Swing phase 38.94 (1.11)a duration (%) Double support 27.74 (2.10)a phase duration (%)
Variables
Normal weight (mean and standard deviation)
TABLE 1 Posture and temporal variables during stair descent
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(p = 0.000) of the body weight for children with obesity. F3 was significantly increased as the backpack weight increased (p = 0.015 for children with normal body weight, p = 0.000 for children with obesity at 15% of body weight; p = 0.000 for children with normal body weight, p = 0.000 for children with obesity at 20% of body weight). F4 and F5 were significantly increased as backpack weight increased to 20% of the body weight for children with obesity (p = 0.028, p = 0.002). F7 was significantly decreased as the backpack weight increased to 10% (p = 0.044), 15% (p = 0.000), and 20% (p = 0.000) of the body weight for children with normal body weight. F1 was significantly decreased as the backpack weight increased for children with normal body weight and children with obesity (p = 0.003 for children with normal body weight, p = 0.010 for children with obesity at 10% of body weight; p = 0.003 for children with normal body weight, p = 0.005 for children with obesity at 15% of body weight; p = 0.000 for children with normal body weight, p = 0.018 for children with obesity at 20% of body weight). There were no significant differences in F6 among different backpack weights (Table 2).
DISCUSSION The kinematic data obtained from this study revealed that both normal and obese, male children increased their trunk forward inclination angle as the backpack weight increased to 10% and 15% of the body weight, respectively. These results are similar to those of previous works (Hong & Brueggemann, 2000; Hong & Cheung, 2003; Chow et al., 2006). No previous research investigated the trunk inclination angle during stair descent because the above-mentioned studies only focused on level walking. As the weight of the backpack increases, the trunk forward inclination angle increases progressively. The additional stress placed on the structure of the vertebral column due to the combination of load and trunk forward bend results in increased intradiscal pressure on the spine. Studies have found that intradiscal pressure increased as the forward bending degree of the trunk increases (Rohlmann et al., 2001), therefore, the carriage of heavy backpacks definitely exerts strain on the spine. Chaffin and Andersson (1991) stated that carrying a backpack increases the stress on the lower back. The prolonged postural strain caused by the trunk, which is greatly displaced from its normal position, may lead to postural discomfort, muscular pain in the shoulders, or lower back injury. The results of the current work demonstrate that obesity and backpack weight may increase the trunk forward inclination angle, resulting in higher muscular strain on the chin and on the body posture. Some related studies have examined the head forward inclination angle during walking. Using a weight of 15% of their subjects’ body weight, Chow
91.13 (10.29)b 11.16 (1.77)
80.63 (13.91)a 84.73 (13.45)
10.64 (1.87)
7.33 (1.36)
–10.65 (2.20)
15.74 (7.72)b
9.96 (1.60)
7.23 (2.14)a
–9.82 (1.51)
13.89 (2.66)a
16.74 (2.79)b
–11.07 (2.72)
8.36 (2.36)
10.22 (2.05)
94.47 (11.91)b
72.60 (15.73)b
203.76 (27.68)
b
Backpack 20% a
13.81 (2.64)a
–10.01 (1.40)
8.65 (2.09)a
10.66 (2.19)
83.10 (8.98)a
66.35 (8.02)a
13.80 (1.47)
–10.28 (2.55)
9.28 (2.37)
11.13 (2.34)
87.81 (9.62)
73.61 (4.99)c
c
Backpack 10%
161.87 (16.86) 177.49 (12.27)
Backpack 0%
14.56 (1.99)
–10.75 (2.95)
8.91 (2.37)
9.29 (2.47)
93.92 (7.39)c
80.86 (6.06)c
180.99 (19.00)
c
Backpack 15%
14.04 (2.82)
–10.44 (2.75)
9.93 (1.82)c
11.56 (1.85)c
97.34 (9.86)c
82.63 (5.71)c
181.39 (23.56)c
Backpack 20%
Obese (mean and standard deviation)
a: p < 0.05 between the average backpack loads of normal weight and obese subjects (showed in 0% backpack loads). b: p < 0.05 versus 0% in normal weight subjects. c: p < 0.05 versus 0% in obese subjects.
16.91 (2.85)b
–10.56 (2.86)
8.29 (1.63)
67.92 (14.98)b
197.80 (33.58)
b
Backpack 15%
58.06 (12.69)
55.53 (8.50)a
171.52 (30.72) 195.70 (33.92)
b
Backpack 10%
F1 (Body weight%) F2 (Body weight%) F3 (Body weight%) F4 (Body weight%) F5 (Body weight%) F6 (Body weight%) F7 (Body weight%)
a
Backpack 0%
Variables
Normal weight (mean and standard deviation)
TABLE 2 GRF variables during stair descent
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et al. (2006) found no significant differences in the range of head motions in the antero-posterior direction among schoolgirls with adolescent idiopathic scoliosis. The current research also showed that schoolbag weight did not increase the head forward inclination angle for both normal body weight children and children with obesity. However, children with obesity tended to lean their heads further than children with normal body weight. During stair descent, normal body weight subjects had to identify the location of the next staircase step, while children with obesity had to lean their head to avoid the blocking of the sightline resulting from their relatively raised abdomen; in turn, the increased head forward inclination angle leads to greater muscular strain on the cervical vertebra. The results of the current work also showed that both obesity and backpack weight altered gait patterns in children, in accordance with the results of previous works. For example, Hong and Brueggemann (2000) found that carrying a backpack equal to 20% of the body weight induced a significant increase in double support duration and a decrease in swing duration; they also reported that the higher weight on the back raised the subject’s center of gravity, resulting in further instability. In our work, obese subjects were forced to compensate by reducing their swing phase, which may be an attempt to minimize the duration of the unsteady single-limb stance. Hong and Li (2005) found that a load equal to 20% of the body weight induces a significant increase in gait cycle duration and a load of 15% of the body weight increases double support duration during stair descent. Singh and Koh (2009) found that a load equal to 20% of the body weight significantly increases double support time. Increasing gait cycle duration and decreasing swing phase duration can serve as compensatory mechanisms to allow children to minimize either the induced gait instability or the strain on the musculoskeletal system. Compared with children with normal weight, children with obesity conducted a significant increase in stance phase duration and a decrease in swing duration during level walking (McGraw, McClenaghan, Willams, & Dickerson, 2000). These changes may have been caused by the altered body size and shape, which influenced postural stability by altering the location of the center of gravity (Fregly, Oberman, Graybiel, & Mitchell, 1968). In our research, obesity and a load of 15% of the body weight altered gait patterns, which reflect decreased body stability in these children. The vertical GRF profiles were characterized by two unequal peaks, where the first peak was consistently greater than the second one. The antero-posterior component exhibited breaking (negative) and propulsive (positive) peaks, whereas the medio-lateral GRFs were always directed medially. F1 showed the greatest value among all direction forces, with the F1 peak showing the greatest shock wave encountered during stair descent. When the F1 peak occurred, eccentric contractions in the antigravity muscles were observed, i.e., the muscles generated force as they lengthened (Hong & Li,
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2005). This can be related to the potential risk of muscle fiber damage when the speed or load becomes higher. We also found that backpack weight equal to 10% (or higher) of the body weight may be deleterious to the musculoskeletal system in children both with normal body weight and with obesity. To avoid the possible hazards caused by stair descent, caution should be exercised in relation to the load being carried, the frequency of the movement performed, and the use of shoes with a proper cushioning ability (Hong & Li, 2005). Although some studies have concluded that F1 tends to decrease in children with obesity, no study has yet to examine GRF during stair descent for obese subjects. Dowling et al. (2004) compared GRFs during level walking and found that children with obesity exerted greater vertical force. The difference between Dowling et al.’s (2004) result and that of the current work may be due to the nature of stair descent examined in either work. Riener, Rabuffetti, and Frigo (2002) found that during stair descent, the potential (gravitational) energy of the body is first transferred into kinetic energy and then absorbed by the muscles; thus, the first peak of the vertical GRF reflects the nature of energy absorption. For children with obesity, the relatively larger plantar contact area (Mickle, Steele, & Munro, 2006) and thicker plantar skin may cushion energy absorption that, in turn, can help children with obesity avoid the first vertical shock. F5 and F7 values showed that children with obesity exerted more mediallateral and anterior-posterior forces. Different postural control strategies generally indicate that medial-lateral versus anterior-posterior stability may be influenced in different ways by the addition of noncontributory mass to the body system. For example, Mizrahi and Susak (1989) concluded that anteriorposterior stability is applied to the support surface using the feet and ankle concurrently, whereas medial-lateral adjustments result from foot forces applied in the opposite direction. Ankle rotation versus lateral weight shift is typically used to maintain anterior-posterior versus medial-lateral stability. Therefore, the addition of noncontributory mass to the body system would have greater influence on one’s ability to shift weight laterally, thus leading to difficulties in maintaining medial-lateral stability. The additional backpack weight can certainly be problematic for children with obesity attempting to maintain anterior-posterior and medial-lateral stability. Daily activities (e.g., running, jumping, and stair walking) place a considerable demand on the postural control system to maintain stability in the anterior-posterior and medial-lateral directions. The addition of excess, noncontributory weight in children with obesity decreases stability, possibly affecting their willingness to participate in strenuous physical activities that can further challenge their postural control system. These children with obesity may feel less confident and be less motivated to engage in physical activity, resulting in lower daily caloric expenditure.
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CONCLUSIONS
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The results of this study warranted the following conclusions: 1. Children with obesity increased their trunk forward inclination angle, head forward inclination angle, and gait cycle durations in comparison to children with normal body weight when carrying a backpack weight between 0 to 20% of their body weight during stairs descent. 2. Children with obesity exerted greater GRF in the medial-lateral and anterior-posterior directions and less GRF in the vertical direction in comparison to children with normal body weight. 3. A backpack weight of more than 15% of body weight led to further instability and damage on the already strained bodies of children with obesity.
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Hong, Y., & Brueggemann. G. (2000). Changes in gait patterns in 10-year-old boys with increasing loads when walking on a treadmill. Gait and Posture, 11(3), 254–259. Hong, Y., & Cheung, C. (2003). Gait and posture responses to backpack load during level walking in children. Gait and Posture, 17, 28–33. Hong, Y., & Li, J. (2005). Influence of load and carrying methods on gait phase and ground reactions in children’s stair walking. Gait and Posture, 22, 63–68. Hong. Y., Li. J., & Fong, D. T. (2008). Effect of prolonged walking with backpack loads on trunk muscle activity and fatigue in children. Journal of Electromyography and Kinesiology, 8(6), 990–996. Johnson, R. F., Knapik. J. J., & Merullo, D. J. (1995). Symptoms during load carrying effects of mass and load distribution during a 20-km road march. Perceptual and Motor Skills, 81(1), 331–338. Li, J. X., Hong, Y., & Robinson, P. D. (2003). The effect of load carriage on movement kinematics and respiratory parameters in children during walking. Eur J Appl Physiol, 90(1-2), 35–43. Malhotra, M. S., & Gupta, J. S. (2007). Carrying of backpacks by children. Ergonomics, 8, 55–60. McGraw, B., McClenaghan, B. A., Willams, H. G., & Dickerson, J. (2000) Gait and postural stability in obese and nonobese prepubertal boys. Arch Phys Med Rehabil, 81, 484–489. Mickle, K. J., Steele, J. R., & Munro, B. J. (2006) Does excess mass affect plantar pressure in young children? International Journal of Pediatric Obesity, 1(3), 183–188 Mizrahi, J., & Susak, Z. (1989) Bi-lateral reactive force patterns in postural sway activity of normal subjects. Biol Cybern, 60, 297–305. Pascoe, D. D., Pascoe, D. E., Wang, Y. T., Shin, D. M., & Kim, C. K. (1997). Kinematics analysis of book bag weight on gait cycle and posture of yourth. Ergonomics, 41, 251–253. Riener, R., Rabuffetti, M., & Frigo, C. (2002) Stair ascending and descending at different inclinations. Gait and Posture, 15, 32–44. Rohlmann, A., Claes, L. E., Bergmann, G., Graichen, F., Neef, P., & Wilke, H. J. (2001) Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises. Ergonomics, 44(8), 781–794. Seven, Y. B., Akalan, N. K, & Yucesoy, C. A. (2008). Effects of back loading on the biomechanics of sit-to-stand motion in healthy children. Human Movement Science, 27, 65–79. Singh. T., & Koh, M. (2009). Effects of backpack load position on spatiotemporal parameters and trunk forward lean. Gait and Posture, 29, 49–53. Stacoff, A., Diezi, C., Luder, G., Stussi, E., & Quervain, I. K. (2005). Ground reaction forces on stairs: effects of stair inclination and age. Gait and Posture, 21, 24–38. Whittfield, J., Legg, S., & Hedderley, D. I. (2005). Schoolbag weight and musculoskeletal symptoms in New Zealand secondary schools. Applied Ergonomics, 36, 193–198.
Research in Sports Medicine: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gspm20
Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent ab
c
ab
ab
Qipeng Song , Bing Yu , Cui Zhang , Wei Sun
& Dewei Mao
ab
a
Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China b
Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
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c
University of North Carolina at Chapel Hill, Chapel Hill, USA Published online: 21 Mar 2014.
To cite this article: Qipeng Song, Bing Yu, Cui Zhang, Wei Sun & Dewei Mao (2014) Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent, Research in Sports Medicine: An International Journal, 22:2, 172-184, DOI: 10.1080/15438627.2014.881823 To link to this article: http://dx.doi.org/10.1080/15438627.2014.881823
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Research in Sports Medicine, 22:172–184, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 1543-8627 print/1543-8635 online DOI: 10.1080/15438627.2014.881823
Effects of Backpack Weight on Posture, Gait Patterns and Ground Reaction Forces of Male Children with Obesity during Stair Descent
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QIPENG SONG Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China; Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
BING YU University of North Carolina at Chapel Hill, Chapel Hill, USA
CUI ZHANG, WEI SUN, and DEWEI MAO Shandong Sports Science Research Center, Laboratory of Sports Biomechanics, 3008, Fengming Road, Jinan, China; Shandong Sports University, 3008, Fengming Road, Jinan, Shandong, China
This study investigates the effects of backpack weight on posture, gait pattern, and ground reaction forces for children with obesity in an attempt to define a safe backpack weight limit for them. A total of 16 obese (11.19 ± 0.66 years of age) and 21 normal body weight (11.13 ± 0.69 years of age) schoolboys were recruited. Two force plates and two video cameras were used. Multivariate analysis of variance with repeated measures was employed. Obese children showed increased trunk and head forward inclination angle, gait cycle duration and stance phase, decreased swing phase, and increased ground reaction force in the medial-lateral and anteriorposterior directions when compared with male children with a normal body weight. The changes were observed even with an empty backpack in comparison with normal body weight children
Received 31 July 2013; accepted 8 December 2013. Project of Shandong Education Department, grant ID: J08LE13. Natural Science Foundation of Shandong Province, grant ID: ZR2010CM064. Address correspondence to Dewei Mao, Shandong Sports Science Research Center, Jinan, China. Email: [email protected] 172
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and a 15% increase in backpack weight led to further instability and damage on their already strained bodies. KEYWORDS children with obesity, gait and posture, stair descent
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INTRODUCTION Carrying a backpack is an activity in daily life for students, especially for elementary, middle school, and high school students. Many studies have been conducted to investigate the effects of backpack weight on children. These studies reported that excessive backpack weight result in back pain, muscle soreness, numbness, shoulder pain, and muscle fatigue (Johnson, Knapik, & Merullo, 1995; Pascoe, Pascoe, Wang, Shin, & Kim, 1997). Many studies also investigated the effects of carrying weight on ground reaction force (GRF) (Stacoff, Diezi, Luder, Stussi, & Quervain, 2005), gait pattern (Hong & Brueggemann, 2000; Hong & Cheung, 2003), body posture (Grimmer, Dansie, Milanese, Pirunsan, & Trott, 2002; Hong & Brueggemann, 2000; Hong & Chueng, 2003; Li, Hong, & Robinson, 2003; Singh & Koh, 2009; Stacoff et al., 2005), and electromyography (Hong, Li, & Fong, 2008) during level walking. However, only few of these studies revealed the biomechanical averages of carrying a backpack during stair walking. Stair walking requires an impulsive force equivalent to 155% of the body weight while level walking requires 120% of the body weight (Dowling, Steele, & Baur, 2004). Hong and Li (2005) investigated the influence of different schoolbag weight on GRF during stair walking. The results of their experiment showed that a load equal to 15% of the body weight induces significant increases in maximum peak force, stance, and double support duration for the backpack condition. However, only vertical reaction forces have been measured, and only normal body weight subjects are recruited in their respective works. Previous studies on the effects of backpack weight on gait were mainly focused on individuals with normal body weight. Our literature review did not show any study on the effects of backpack weight on posture, gait patterns, and ground reaction forces of children with obesity. Obesity results in a significant disadvantage in movement and discomfort in simple daily activities such as walking and stair-climbing (Hills & Parker, 1991) because large proportions of their body weight do not contribute to their movement performance. A good understanding of the effects of backpack weight on these children’s posture, gait, and ground reaction forces will provide significant information for preventing musculoskeletal system injuries and disorders among these children. Stair descent results in increased GRF in comparison to level walking and stair ascent. Andriacchi, Andersson, Fermier, Stern, and Galante (1982) found that the maximum external knee flexion moment in stair descent was 2.7 times that in stair ascent. Hong and Li (2005) reported that the maximum peak force in stair descent with a backpack of 15% body weight was1.89 times greater than that in stair ascent, while the ground reaction force was three times of
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that in stair ascent. Tripping is also more common during stair descent than in stair ascent, because people usually descend a flight of stairs with their center of pressure near the edge of the stairs (Beaulieu, Pelland, & Robertson, 2008). Many studies have attempted to establish safe weight carrying limits for children, and the guidelines vary among countries. Load carrying limits were set as 10% of the body weight in New Zealand (Whittfield, Legg, & Hedderley, 2005); 10% to 12% in India (Malhotra & Gupta, 2007); 15% in Hong Kong (Hong & Li, 2008), Australia (Chansirinukor, Wilson, Grimmer, & Dansie, 2001), and Singapore (Singh & Koh, 2009); and 20% in Turkey (Seven, Akalan, & Yucesoy, 2008). These load carrying limits were set for subjects with normal body weight in standing or level walking, and thus may not be appropriate for children with obesity in stair descent. The purpose of this study was to determine the effects of backpack weight on posture, gait pattern, and ground reaction forces of male obese children in stair descent. We hypothesized that increasing backpack weight would increase trunk and head forward inclination angle, gait cycle duration, and ground reaction force during stair descent. We also hypothesized that male children with obesity would increase their trunk and head forward inclination angle, gait cycle durations and would exert greater GRF in the mediallateral and anterior-posterior directions and less GRF in the vertical direction in comparison to children with normal body weight.
METHODS Subjects A total of 16 school boys with obesity and 21 school boys with normal body weight between 10 to 12 years of age from Shimuyuan Primary School in Jinan, China were recruited as subjects. The mean age, standing height, body weight, and body mass index (BMI) were 11.19 ± 0.66 years, 148.85 ± 14.07 cm, 75.26 ± 15.99 kg and 29.78 ± 4.10 kg/m2, respectively, for children with obesity, and 11.13 ± 0.69 years, 152.22 ± 6.95 cm, 48.13 ± 9.52 kg, and 20.03 ± 3.08 kg/m2, respectively, for children with normal body weight. A BMI of 26 kg/m2 or higher was defined as obesity for 12-year-old male children (Cole, Bellizzi, Flegal, & Dietz, 2000). The use of human subjects in this study was approved by the Internal Review Board of Shandong Sport University. Written consent was obtained from each subject’s parents before data collections.
Testing Protocol Each subject was asked to complete one testing session per day for a total of four testing sessions. In each session, the subject was asked to carry a twostrap backpack with both shoulders in a given backpack weight (0, 10, 15, or 20% of body weight). The order of backpack weight conditions was
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randomized using a Latin Square method. In each session, the subject was allowed to have a warm up to get used to ascending and descending the stairs. They were then asked to walk with the backpack for 400 m at a selfselected speed as a level walking test before the stair descent test. In the stair descent test, the subject was asked to perform five successful stair descent trials. A successful stair descent trial was defined as a trial in which the subject continuously descended from the top of the staircase and then continuously walked for 10 m with all data successfully collected. The subject had a 1 minute break between trials. After each trial, subjects had 1 minute to rest. The subject used the same pair of shoes for all testing sessions.
Data Collection A staircase with six steps (Figure 1) was built for the data collection in this study. The step dimensions were 17.0 cm (riser) × 29 cm (tread). A hand rail was set on the right side of the descent. Two force plates (Kistler, 9287BA and 9281CA, Switzerland) were embedded in the third and fourth steps of the descent to collect GRF data at a sample rate of 1000 Hz. Two high-speed video cameras were placed 12 m from the walking track with an angle of 90° to record movements of level walking and stair descent for each subject at a sample rate of 50 frames/second. Six landmarks were manually digitized from video records (Left and right shoulder, left and right hip, forehead and lower jaw). Three-dimensional (3-D) coordinates of the six landmarks were reduced from digitized two-dimensional coordinates. The 3-D coordinates were filtered through a low-pass recursive Butterworth digital filter with a cut-off frequency of 6 Hz.
FIGURE 1 Laboratory staircase, force plates 1 and 2 were embedded in the third and fourth steps of the staircase, respectively.
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Data Reduction Trunk and head inclination angles were reduced from filtered 3-D coordinate data. Trunk inclination angle was defined as the angle between trunk longitudinal axis and horizontal plane projected to the sagittal plane. The longitudinal axis of the trunk is defined as the vector from the mid-point of the line connecting hip joint centers to the mid-point of the line connecting shoulder joint centers. A positive trunk inclination angle means the trunk is leaning forward. The head inclination angle was defined as the angle between the line connecting digitized forehead and lower jaw landmarks and the horizontal plane projected to the sagittal plane. Gait cycle duration, single support phase duration, swing phase duration, and double support phase duration were also calculated from the video record to represent gait temporal characteristics. Gait cycle duration was defined as the time period between two consecutive foot takeoffs. Single support phase duration was defined as the time period from the foot takeoff of the swing leg to the foot strike of swing leg. Swing phase duration was defined as the time period between the takeoff and landing of the swing leg. Double support phase was defined as the time period from the foot strike of the swing leg to the foot takeoff of the supporting leg. Further, seven characteristic magnitudes of the ground forces were identified (Figure 2). These seven ground reaction force variables were the first peak of the vertical force curve (F1); the local minimum of the vertical force after the first peak (F2); the second peak of the vertical force curve (F3); the first peak of the medial-lateral force curve (F4) with a positive value representing the medial direction; the second peak of the medial-lateral force curve (F5); the first peak of the antero-posterior force curve with a positive value representing the anterior direction (F6); and the second peak of the antero-posterior force curve (F7).
FIGURE 2 Three components of the GRF for a typical stair descent trial; the seven dependent variables chosen for analysis (F1 to F7) are shown.
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Data Analysis
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A two-way MANOVA with mixed design was performed to determine the effects of backpack weight and obesity condition. The back weight was treated as a repeated measure while the obesity condition was treated as an independent measure. If the main effect of backpack weight was significant, post-hoc paired t-tests were performed to locate the differences. A Type I error rate of 0.05 was selected as an indication of statistical significance. Bonferroni adjustment was made for a post-hoc paired t-test to guarantee the overall Type I error rate was not greater than 0.05.
RESULTS Compared to children with normal weight, children with obesity showed increased trunk and head forward inclination angles (p = 0.003, p = 0.001), and gait cycle, single support phase, and double support phase durations (p = 0.030, p = 0.000, p = 0.023), and decreased swing phase duration (p = 0.001) (Table 1). The trunk forward inclination angle significantly increased as the backpack weight increased to 10% of body weight (p = 0.012 for children with normal body weight), 15% of body weight (p = 0.011 for children with normal body weight, p = 0.002 for children with obesity), and 20% of body weight (p = 0.000 for children with normal body weight, p = 0.000 for children with obesity). There were no significant differences observed in the head forward inclination angles among different backpack weights. Gait cycle duration significantly increased as the backpack weight increased to 20% of the body weight (p = 0.014 for children with normal body weight, p = 0.003 for children with obesity). Stance duration significantly increased as the backpack weight increased to 15% of body weight (p = 0.012 for children with normal weight, p = 0.002 for children with obesity) and to 20% of body weight (p = 0.001 for children with normal weight, p = 0.000 for children with obesity). Swing duration decreased as backpack weight increased to 15% of body weight (p = 0.031 for children with normal weight, p = 0.022 for children with obesity) and 20% body weight (p = 0.000 for children with normal body weight, p = 0.000 for children with obesity). Double support duration increased as backpack weight increased to 20% of the body weight (p = 0.015 for children with normal weight, p = 0.041 for children with obesity). Children with obesity demonstrated significantly increased F2 (p = 0.005), F3 (p = 0.017), F5 (p = 0.004) and F7 (p = 0.007), but decreased F1 (p = 0.000) in comparison to children with normal body weight. F2 was significantly increased as backpack weight increased to 15% (p = 0.003) and 20% (p = 0.001) of the body weight for children with normal body weight, and as the backpack weight increased to 10% (p = 0.008), 15% (p = 0.000), and 20%
Obese (mean and standard deviation)
34.81 (1.34)a a
0.99 (0.12)b 61.89 (2.56)b 38.11 (2.10)b 29.02 (2.83)b
0.93 (0.11) 61.68 (2.04)b 38.32 (1.57)b 27.69 (3.09)
0.96 (0.11) 61.33 (1.74) 38.67 (1.85) 27.75 (3.15)
29.32 (2.97)
33.93 (2.10)
66.07 (2.10)
1.01 (0.15)
23.82 (8.06)
10.46 (3.79)
a: p < 0.05 between the average backpack loads of normal weight and obese subjects (shown in 0% backpack loads). b: p < 0.05 versus 0% in normal weight subjects. c: p < 0.05 versus 0% in obese subjects.
28.33 (2.51)
65.19 (1.34)a
1.00 (0.09)a
17.91 (12.26) 22.68 (10.03)
17.36 (8.88)
8.55 (2.61)a
16.29 (10.56)
7.57 (3.00)b
7.31 (2.39)b
6.78 (2.22)b
28.86 (2.58)
34.65 (2.07)
65.69 (2.61)
1.07 (0.17)
23.77 (9.95)
11.87 (4.02)c
34.03 (3.30)c
33.38 (2.37)c
66.62 (2.36)c
1.11 (0.20)c
22.48 (10.05)
11.43 (3.53)c
Backpack 0% Backpack 10% Backpack 15% Backpack 20% Backpack 0% Backpack 10% Backpack 15% Backpack 20%
Trunk inclination 5.54 (2.61)a angle (O) Head inclination 18.3 (11.23)a angle (O) Gait cycle 0.96 (0.13)a duration (s) Stance phase 61.06 (1.01)a duration (%) Swing phase 38.94 (1.11)a duration (%) Double support 27.74 (2.10)a phase duration (%)
Variables
Normal weight (mean and standard deviation)
TABLE 1 Posture and temporal variables during stair descent
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(p = 0.000) of the body weight for children with obesity. F3 was significantly increased as the backpack weight increased (p = 0.015 for children with normal body weight, p = 0.000 for children with obesity at 15% of body weight; p = 0.000 for children with normal body weight, p = 0.000 for children with obesity at 20% of body weight). F4 and F5 were significantly increased as backpack weight increased to 20% of the body weight for children with obesity (p = 0.028, p = 0.002). F7 was significantly decreased as the backpack weight increased to 10% (p = 0.044), 15% (p = 0.000), and 20% (p = 0.000) of the body weight for children with normal body weight. F1 was significantly decreased as the backpack weight increased for children with normal body weight and children with obesity (p = 0.003 for children with normal body weight, p = 0.010 for children with obesity at 10% of body weight; p = 0.003 for children with normal body weight, p = 0.005 for children with obesity at 15% of body weight; p = 0.000 for children with normal body weight, p = 0.018 for children with obesity at 20% of body weight). There were no significant differences in F6 among different backpack weights (Table 2).
DISCUSSION The kinematic data obtained from this study revealed that both normal and obese, male children increased their trunk forward inclination angle as the backpack weight increased to 10% and 15% of the body weight, respectively. These results are similar to those of previous works (Hong & Brueggemann, 2000; Hong & Cheung, 2003; Chow et al., 2006). No previous research investigated the trunk inclination angle during stair descent because the above-mentioned studies only focused on level walking. As the weight of the backpack increases, the trunk forward inclination angle increases progressively. The additional stress placed on the structure of the vertebral column due to the combination of load and trunk forward bend results in increased intradiscal pressure on the spine. Studies have found that intradiscal pressure increased as the forward bending degree of the trunk increases (Rohlmann et al., 2001), therefore, the carriage of heavy backpacks definitely exerts strain on the spine. Chaffin and Andersson (1991) stated that carrying a backpack increases the stress on the lower back. The prolonged postural strain caused by the trunk, which is greatly displaced from its normal position, may lead to postural discomfort, muscular pain in the shoulders, or lower back injury. The results of the current work demonstrate that obesity and backpack weight may increase the trunk forward inclination angle, resulting in higher muscular strain on the chin and on the body posture. Some related studies have examined the head forward inclination angle during walking. Using a weight of 15% of their subjects’ body weight, Chow
91.13 (10.29)b 11.16 (1.77)
80.63 (13.91)a 84.73 (13.45)
10.64 (1.87)
7.33 (1.36)
–10.65 (2.20)
15.74 (7.72)b
9.96 (1.60)
7.23 (2.14)a
–9.82 (1.51)
13.89 (2.66)a
16.74 (2.79)b
–11.07 (2.72)
8.36 (2.36)
10.22 (2.05)
94.47 (11.91)b
72.60 (15.73)b
203.76 (27.68)
b
Backpack 20% a
13.81 (2.64)a
–10.01 (1.40)
8.65 (2.09)a
10.66 (2.19)
83.10 (8.98)a
66.35 (8.02)a
13.80 (1.47)
–10.28 (2.55)
9.28 (2.37)
11.13 (2.34)
87.81 (9.62)
73.61 (4.99)c
c
Backpack 10%
161.87 (16.86) 177.49 (12.27)
Backpack 0%
14.56 (1.99)
–10.75 (2.95)
8.91 (2.37)
9.29 (2.47)
93.92 (7.39)c
80.86 (6.06)c
180.99 (19.00)
c
Backpack 15%
14.04 (2.82)
–10.44 (2.75)
9.93 (1.82)c
11.56 (1.85)c
97.34 (9.86)c
82.63 (5.71)c
181.39 (23.56)c
Backpack 20%
Obese (mean and standard deviation)
a: p < 0.05 between the average backpack loads of normal weight and obese subjects (showed in 0% backpack loads). b: p < 0.05 versus 0% in normal weight subjects. c: p < 0.05 versus 0% in obese subjects.
16.91 (2.85)b
–10.56 (2.86)
8.29 (1.63)
67.92 (14.98)b
197.80 (33.58)
b
Backpack 15%
58.06 (12.69)
55.53 (8.50)a
171.52 (30.72) 195.70 (33.92)
b
Backpack 10%
F1 (Body weight%) F2 (Body weight%) F3 (Body weight%) F4 (Body weight%) F5 (Body weight%) F6 (Body weight%) F7 (Body weight%)
a
Backpack 0%
Variables
Normal weight (mean and standard deviation)
TABLE 2 GRF variables during stair descent
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et al. (2006) found no significant differences in the range of head motions in the antero-posterior direction among schoolgirls with adolescent idiopathic scoliosis. The current research also showed that schoolbag weight did not increase the head forward inclination angle for both normal body weight children and children with obesity. However, children with obesity tended to lean their heads further than children with normal body weight. During stair descent, normal body weight subjects had to identify the location of the next staircase step, while children with obesity had to lean their head to avoid the blocking of the sightline resulting from their relatively raised abdomen; in turn, the increased head forward inclination angle leads to greater muscular strain on the cervical vertebra. The results of the current work also showed that both obesity and backpack weight altered gait patterns in children, in accordance with the results of previous works. For example, Hong and Brueggemann (2000) found that carrying a backpack equal to 20% of the body weight induced a significant increase in double support duration and a decrease in swing duration; they also reported that the higher weight on the back raised the subject’s center of gravity, resulting in further instability. In our work, obese subjects were forced to compensate by reducing their swing phase, which may be an attempt to minimize the duration of the unsteady single-limb stance. Hong and Li (2005) found that a load equal to 20% of the body weight induces a significant increase in gait cycle duration and a load of 15% of the body weight increases double support duration during stair descent. Singh and Koh (2009) found that a load equal to 20% of the body weight significantly increases double support time. Increasing gait cycle duration and decreasing swing phase duration can serve as compensatory mechanisms to allow children to minimize either the induced gait instability or the strain on the musculoskeletal system. Compared with children with normal weight, children with obesity conducted a significant increase in stance phase duration and a decrease in swing duration during level walking (McGraw, McClenaghan, Willams, & Dickerson, 2000). These changes may have been caused by the altered body size and shape, which influenced postural stability by altering the location of the center of gravity (Fregly, Oberman, Graybiel, & Mitchell, 1968). In our research, obesity and a load of 15% of the body weight altered gait patterns, which reflect decreased body stability in these children. The vertical GRF profiles were characterized by two unequal peaks, where the first peak was consistently greater than the second one. The antero-posterior component exhibited breaking (negative) and propulsive (positive) peaks, whereas the medio-lateral GRFs were always directed medially. F1 showed the greatest value among all direction forces, with the F1 peak showing the greatest shock wave encountered during stair descent. When the F1 peak occurred, eccentric contractions in the antigravity muscles were observed, i.e., the muscles generated force as they lengthened (Hong & Li,
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2005). This can be related to the potential risk of muscle fiber damage when the speed or load becomes higher. We also found that backpack weight equal to 10% (or higher) of the body weight may be deleterious to the musculoskeletal system in children both with normal body weight and with obesity. To avoid the possible hazards caused by stair descent, caution should be exercised in relation to the load being carried, the frequency of the movement performed, and the use of shoes with a proper cushioning ability (Hong & Li, 2005). Although some studies have concluded that F1 tends to decrease in children with obesity, no study has yet to examine GRF during stair descent for obese subjects. Dowling et al. (2004) compared GRFs during level walking and found that children with obesity exerted greater vertical force. The difference between Dowling et al.’s (2004) result and that of the current work may be due to the nature of stair descent examined in either work. Riener, Rabuffetti, and Frigo (2002) found that during stair descent, the potential (gravitational) energy of the body is first transferred into kinetic energy and then absorbed by the muscles; thus, the first peak of the vertical GRF reflects the nature of energy absorption. For children with obesity, the relatively larger plantar contact area (Mickle, Steele, & Munro, 2006) and thicker plantar skin may cushion energy absorption that, in turn, can help children with obesity avoid the first vertical shock. F5 and F7 values showed that children with obesity exerted more mediallateral and anterior-posterior forces. Different postural control strategies generally indicate that medial-lateral versus anterior-posterior stability may be influenced in different ways by the addition of noncontributory mass to the body system. For example, Mizrahi and Susak (1989) concluded that anteriorposterior stability is applied to the support surface using the feet and ankle concurrently, whereas medial-lateral adjustments result from foot forces applied in the opposite direction. Ankle rotation versus lateral weight shift is typically used to maintain anterior-posterior versus medial-lateral stability. Therefore, the addition of noncontributory mass to the body system would have greater influence on one’s ability to shift weight laterally, thus leading to difficulties in maintaining medial-lateral stability. The additional backpack weight can certainly be problematic for children with obesity attempting to maintain anterior-posterior and medial-lateral stability. Daily activities (e.g., running, jumping, and stair walking) place a considerable demand on the postural control system to maintain stability in the anterior-posterior and medial-lateral directions. The addition of excess, noncontributory weight in children with obesity decreases stability, possibly affecting their willingness to participate in strenuous physical activities that can further challenge their postural control system. These children with obesity may feel less confident and be less motivated to engage in physical activity, resulting in lower daily caloric expenditure.
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The results of this study warranted the following conclusions: 1. Children with obesity increased their trunk forward inclination angle, head forward inclination angle, and gait cycle durations in comparison to children with normal body weight when carrying a backpack weight between 0 to 20% of their body weight during stairs descent. 2. Children with obesity exerted greater GRF in the medial-lateral and anterior-posterior directions and less GRF in the vertical direction in comparison to children with normal body weight. 3. A backpack weight of more than 15% of body weight led to further instability and damage on the already strained bodies of children with obesity.
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