Effects of obesity on trunk forward flexion motion in sitting and standing, and postural adaptations and hip joint moment for a standing work task.
Cross-sectional comparison of obese and normal weight groups.
Ten obese subjects (waist girth 121.2±16.8 cm, body mass index (BMI) 38.9±6.6 kg m−2) and 10 age- and height-matched normal weight subjects (waist girth 79.6±6.4 cm, BMI 21.7±1.5 kg m−2).
Trunk motion during seated and standing forward flexion, and trunk posture, hip joint moment and hip-to-bench distance during a simulated standing work task were recorded.
Forward flexion motion of the thoracic segment and thoracolumbar spine was decreased for the obese group with no change in pelvic segment and hip joint motion. Obese subjects showed a more flexed trunk posture and increased hip joint moment and hip-to-bench distance for a simulated standing work task.
Decreased range of forward flexion motion, differing effects within the trunk, altered posture during a standing work task and concomitant increases in hip joint moment give insight into the aetiology of functional decrements and musculoskeletal pain seen in obesity.
Forward flexion of the trunk and hips is used during many activities of daily living,1 therefore reduced range of motion may have implications for loss of functionality. Obese individuals have reported functional limitations in activities of daily living, particularly for tasks requiring increased flexibility.2 Obesity is also a factor in reduced motion magnitude at the hip joint3 and the lumbar spine,4 possibly owing to a mechanical effect of interposing adipose tissue restricting the joint range of motion.3 Strategies may consequently be used to reduce apposition limitations.5 Mechanical obstruction may also lead to altered posture when using a work station as the excess abdominal tissue restricts access to the work surface.6 The musculoskeletal load on the trunk muscles during standing work tasks hence may be increased as a consequence of altered posture, increased load or a combination.6 The effect of obesity on musculoskeletal load on the trunk, however, is unknown.
Although the overall motion of the trunk is reduced in obese individuals,2, 3, 4 it is also possible that there is differential contribution to forward flexion motion from segments within the trunk.5 Differential effects on individual segment motions may affect the musculoskeletal demands on a segment. Differing prevalence within the trunk of vertebral osteophytosis (an indicator of intervertebral disc degeneration) is thought to be related to mechanical factors particularly body size.7 Work-restricting pain in the back area and hip joint is also more common in obese individuals when compared to the general population,8 however, the underlying mechanical factors have not been identified.
Knowledge of trunk segment forward flexion motion, and standing work task postural adaptations and hip joint moments will contribute to the understanding of the effects of obesity on mechanics of trunk motion, and functional limitations in ergonomics. The aims of the project were to investigate the angular displacement of the thoracic and pelvic segments, and the hip joint and thoracolumbar spine, and the base of support mediolateral width during seated and standing forward flexion in obese subjects and compare to a normal weight control group. Hip joint moment and angular displacement, hip-to-bench distance and posture of the thoracic and pelvic segments, and the thoracolumbar spine during a standing work task were also investigated.
Materials and methods
Ten female obese subjects (44.5±10.3 years, height 164.3±5.7 cm, mass 104.7±16.1 kg, waist girth 121.2±16.8 cm, body mass index (BMI) 38.9±6.6 kg m−2) volunteered and were included in the study. Ten age- and height-matched normal weight female subjects (44.2±10.1 years, height 164.2±4.7 cm, mass 58.4±3.6 kg, waist girth 79.6±6.4 cm, BMI 21.7±1.5 kg m−2) formed a control group. All subjects gave informed consent before participating and the study was approved by Southern Cross University Human Ethics Committee.
Motion was recorded using a single camera (50 Hz) motion analysis system (Peak Motus). Self-selected foot position was outlined on a floor mounted paper sheet. For seated flexion, subjects sat on a height adjustable stool with seat height set to 110% of fibular head to floor distance while standing.5 Subjects wore close fitting sport shirts and leggings, and were barefoot. Light weight wand mounted 2 cm retroreflective markers attached using hypoallergenic tape on T4 and T10, L5 and S3 spinous processes defined the thoracic and pelvic segments, respectively. The right thigh was defined by greater trochanter and the lateral femoral condyle markers. Horizontal was defined by two markers attached to the stool edge. Additional markers for the standing work task included the bench edge, right ulnar head and humeral greater tubercle and lateral epicondyle.
Following practice trials, each subject performed three trials of each seated and standing stylized anatomical trunk forward flexion movements at their preferred speed,5 with their feet flat and still. The subjects reached down towards the floor in front of their feet as far as possible, then brought their straight arms above their head looking upwards. This movement was then repeated. For standing forward flexion, the knees were maintained in an extended position. The starting and finishing position for each movement was an upright posture.
Subjects then performed three trials of a simulated static work task while standing at a 90 cm bench. The subjects first stood comfortably with their body weight evenly distributed over both feet, with arms by their side and looking straight ahead. When instructed, the subjects, while watching their hands, simulated grasping (however not lifting) a light object on the bench located such that their thumb tips were 18 cm from the bench edge. Body contact was made only with the hands and feet.
The two-dimensional trajectories of each marker were optimally filtered (Peak Motus Version 7). For the thoracic and pelvic segments, angular motion in space was calculated relative to the horizontal. The thigh and pelvic segments defined the hip joint, whereas thoracolumbar spine motion was defined as relative motion between the thoracic and pelvic segments. The mediolateral width of the base of support was represented by distance between heel centres.
Hip joint moment during the standing work task was estimated using a two-dimensional static model as described by Paul et al.6 Inputs to the model for the trunk–head–neck, upper arm and forearm–hand segments included segment mass,9 centre of mass,9 segment length and angular position of the segments. Segment lengths were measured by flexible tape. Right and left upper limbs were assumed to be symmetrical. Calculated hip joint moments were expressed relative to body weight and height (N m body weight−1 height−1).10 Hip-to-bench distance was the horizontal difference between the greater trochanter marker and the bench edge maker during the initial quiet standing posture.
Shapiro–Wilks tests showed that data for some variables were not normally distributed. Therefore, Kruskal–Wallis tests were used to investigate differences between the groups, using the mean of three trials for each subject. Spearman's rank order correlations were used to investigate relationships over all trials between BMI and range of motion, and base of support mediolateral width during seated and standing flexion, joint posture during the standing work task, hip joint extension moment and hip-to-bench distance. The total alpha level was set at 0.05 and SPSS (Version 11) was used for statistical analysis.
During seated forward trunk flexion, the thoracic segment angular displacement (P=0.005) and the thoracolumbar spine range of motion (P=0.002) were significantly decreased in the obese group (Table 1). There was no significant effect of obesity on pelvic segment displacement, hip joint range of motion or the base of support mediolateral width (Table 1). Across all subjects, a significant moderate negative relationship was seen for thoracic segment angular displacement (rho=−0.61, P<0.001) and thoracolumbar spine range of motion (rho=−0.57, P<0.001), indicating as BMI increased motion was decreased. A significant low positive relationship was seen for the base of support mediolateral width (rho=0.36, P=0.005), indicating that as BMI increased, the base of support width increased. No significant correlations with BMI were seen for pelvic segment angular displacement and hip joint range of motion (Table 2).
Forward flexion of the trunk in standing demonstrated a significantly smaller thoracic segment angular displacement (P=0.005) and thoracolumbar spine range of motion (P=0.002) in the obese group (Table 1). There was no significant effect of obesity on pelvic segment displacement, hip joint range of motion or the base of support mediolateral width (Table 1). A significant low negative relationship for thoracic segment angular displacement (rho=−0.45, P<0.001), and a moderate negative relationship for thoracolumbar spine range of motion (rho=−0.60, P<0.001) were seen, indicating that as BMI increased, the motion magnitude decreased. A significant low positive relationship was seen for pelvic segment angular displacement (rho=0.48, P<0.001) and the base of support mediolateral width (rho=0.39, P=0.002), indicating that as BMI increased, the base of support width and pelvic segment angular displacement increased. No significant correlations were seen between BMI and hip joint range of motion (Table 2).
For the standing work task (Table 3), the obese group showed a significantly more flexed posture for the thoracic (P=0.002) and pelvic (P=0.010) segments and the hip joint (P=0.023) and thoracolumbar spines (P=0.028). The hip joint net moment was significantly larger in the obese group (P=0.001). During the initial quiet standing posture, the obese group stood further back from the bench with the hip-to-bench distance significantly larger (P=0.001) compared to normal weight subjects (Table 3). BMI showed a high positive significant correlation with the posture of the thoracic segment (rho=0.78, P=<0.001), hip joint moment (rho=0.83, P=<0.001) and hip-to-bench distance (rho=0.73, P=<0.001). BMI also showed a moderate positive significant correlation with the posture of the pelvic segment (rho=0.54, P=<0.001), hip joint (rho=0.57, P=<0.001) and thoracolumbar spine (rho=0.64, P=<0.001), indicating that as BMI increased, the posture was more flexed, hip joint moment increased and the hip-to-bench distance was increased.
In the obese group, trunk forward flexion motion was restricted in both sitting and standing, supporting previous studies where obesity was reported to be a factor in reducing forward flexion motion magnitude.3, 4 Obstruction to the seated movement would be expected as the excessive anterior trunk tissue is located adjacent to the thighs when seated on a chair. Apposition between the anterior thigh and the abdominal tissue would restrict the forward flexion motion. Standing forward flexion, however, although not mechanically obstructed by the thighs, may be difficult to perform owing to decreased forward stability. Difficulties with balance2, 11 may have made the obese subjects more cautious in moving forward to the end of range.
The reduced forward flexion motion was not seen across all segments and joints. Pelvic segment displacement and hip joint range of motion for both seated and standing motion was similar between the groups. Therefore, excessive anterior trunk tissue did not alter pelvic segment forward flexion motion. Trunk forward flexion, however, is achieved by a combination of spinal and pelvic segment motion, with part of the motion accomplished by reducing the distance between the thoracic cage and the pelvis. It is possible that the excessive anterior trunk tissue provided a physical obstruction to the motion of the thoracic segment with a consequent reduction in thoracolumbar spinal motion.5 Also the higher the BMI, the more the angular displacement of the thoracic segment and thoracolumbar spine range of motion was decreased, indicating that increasing adiposity will lead to further motion restriction.
Although a wider foot placement was not seen in the obese group in comparison to the control group, the base of support mediolateral width increased as BMI increased for both seated and standing motion. Wider foot placement may be a strategy to minimize obstruction of the pelvis during forward flexion tasks.5
During the standing work task, the obese group positioned themselves further back from the work bench, possibly because their body dimensions precluded them from standing closer to the bench. As a consequence, in order to complete the task, the obese group showed a more flexed posture of both the thoracic and pelvic segments, which was reflected in the hip and thoracolumbar spine postural adaptations. These postural adaptations increased with increasing BMI similar to adaptations seen as pregnancy progressed.6
Increased hip joint moment would be expected with increasing body mass. The obese group, however, showed a significantly increased standing work task hip joint moment, even though the hip joint moment was normalized to body weight and height. The increased moment therefore was likely to have been the result of postural changes rather than increased body mass.6
Postural adaptations and trunk segment motion limitations may have implications for further understanding the aetiology of musculoskeletal pain in obesity. Although there is no clear relationship between BMI and low back pain,12 self-reported work-restricting pain in the neck and back areas and hip joints is more common in obese people.8 As postural loading and discomfort increase, and the time a posture can be held decreases with increased trunk flexion,13 the postural adaptations and increased hip joint moments seen for the obese group during the standing work task are a possible explanation for the reported increase in work-restricting pain in this group. Further evidence of the consequences of postural adaptations concomitant with obesity are seen in the results of weight reversal, where weight reduction reduces the risk of developing work-restricting pain and increases the likelihood of recovering from such pain.8 Differing effects on individual trunk segments may also lead to altered movement patterns within the trunk in obese individuals, which may affect the musculoskeletal demands on the segment. Evidence for differential effects on trunk segment loads in obesity is seen in the stronger relationship, between increased body mass and the frequency of vertebral osteophytes, in the thoracic region in comparison to that in the lumbar spine.7
The postural adaptations and consequent musculoskeletal loading seen in the obese group has ergonomic implications. Although it is expected that the postural adaptations and increased loading seen in the obese group would be revoked with weight loss, similar to improvements in walking and balance11, 14 and changes post-birth,15 the current pandemic of obesity16 is unlikely to be resolved in the short term. Although no comparable investigation was found for obese subjects, a similar investigation of standing work surface height and areas in late pregnancy showed a preference for lower table height and a task position closer to the bench edge.17 Postural differences in late pregnancy were also minimized in a self-selected workplace design.15 Adaptable workspaces for obese employees therefore may need to be provided to minimize the potential for musculoskeletal injury or pain in the workplace. As the prevalence of obesity is increasing, further research is required to investigate the implications of obesity on workstation design.
Decreased range of forward flexion motion, differing effects within the trunk, altered posture during a standing work task and concomitant increases in hip joint moment give insight into the aetiology of functional decrements and musculoskeletal pain seen in obesity. Further research is required to develop design strategies to minimize postural adaptations consequent to increased body dimensions as seen in obesity. In summary, decreased forward flexion motion of the thoracic segment and thoracolumbar spine in both sitting and standing, and a more flexed posture and increased hip joint moment and hip-to-bench distance for a standing work task were seen in an obese group of females in comparison to a normal weight age, gender and height matched group.
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This project was supported by a Southern Cross University Research Grant.
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Gilleard, W., Smith, T. Effect of obesity on posture and hip joint moments during a standing task, and trunk forward flexion motion. Int J Obes 31, 267–271 (2007). https://doi.org/10.1038/sj.ijo.0803430
- work task
- trunk flexion
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