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The effects of obesity on lifting performance
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Applied Ergonomics 39 (2008) 93–98 www.elsevier.com/locate/apergo
The effects of obesity on lifting performance Xu Xu, Gary A. Mirka, Simon M. Hsiang The Ergonomics Laboratory, Edward P. Fitts Department of Industrial and Systems Engineering, North Carolina State University, Raleigh, NC 27695-7906, USA Received 28 June 2006; accepted 2 February 2007
Abstract Obesity in the workforce is a growing problem worldwide. While the implications of this trend for biomechanical loading of the musculoskeletal system seem fairly straightforward, the evidence of a clear link between low back pain (LBP) and body mass index (BMI) (calculated as whole body mass in kilograms divided by the square of stature in meters) has not been shown in the epidemiology literature addressing this topic. The approach pursued in the current study was to evaluate the lifting kinematics and ground reaction forces of a group of 12 subjects—six with a BMI of less than 25 kg/m2 (normal weight) and six with a BMI of greater than 30 kg/m2 (obese). These subjects performed a series of free dynamic lifting tasks with varied levels of load (10% and 25% of capacity) and symmetry (sagittally symmetric and 451 asymmetric). The results showed that BMI had a significant effect (po0.05) on trunk kinematics with the high BMI group exhibiting higher peak transverse plane (twisting) velocity (59% higher) and acceleration (57% higher), and exhibiting higher peak sagittal plane velocity (30% higher) and acceleration (51% higher). When normalized to body weight, there were no significant differences in the ground reaction forces between the two groups. This study provides quantitative data describing lifting task performance differences between people of differing BMI levels and may help to explain why there is no conclusive epidemiological evidence of a relationship between BMI and LBP. r 2007 Elsevier Ltd. All rights reserved. Keywords: Obesity; Lifting; Low back pain
1. Introduction Obesity in the workforce is a considerable problem that shows no indication of slowing. The World Health Organization (1998) has shown that the prevalence of obesity is increasing at a high rate in both developed and developing countries. Obesity is defined using an index called the body mass index (BMI). BMI is calculated as whole body mass in kilograms divided by the square of stature in meters. A BMI between 18.5 and 25 kg/m2 is considered normal weight, between 25 and 30 kg/m2 is considered overweight, between 30 and 40 kg/m2 is obese and greater than 40 kg/m2 is extremely obese. In the United States, obesity has been called an epidemic for both adults and children (Wellman and Friedberg, 2002), with data from 2001 to 2002 showing that 66% of US adults were either overweight or obese and 5% were extremely obese Corresponding author. Tel.: +1 919 515 6399; fax: +1 919 515 5281.
E-mail address: [email protected] (G.A. Mirka). 0003-6870/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2007.02.001
(Hedley et al., 2004). These authors also showed that, in the US workforce, the obesity rate for men is 26.1% and the obesity rate for women is 33.3%. Relevant to the effects of these statistics on occupational safety and health concerns, are the results of the study by Hertz et al. (2004) that showed that, among both men and women, obese workers are more likely to report work limitations due to physical, mental or emotional problems than their normal weight counterparts. An area of particular concern may be low back pain (LBP)/disorder because of the additional muscular force required to move the additional body mass while performing various occupational tasks. LBP is a common occurrence in many occupations. Lifetime prevalence has been estimated at about 70% (Andersson, 1981) and it is one of the most costly problems of modern society, with an estimated cost of $8.8 billion in the US in 1995 (Murphy and Volinn, 1999). People of all ages can be affected by back pain, but it generally begins between the ages of 20 and 40 years, and the prevalence peak between the ages of 45–60 years, with a slight
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difference between males and females (Jayson, 1996). LBP can result from injury to the muscle tissues (muscle strain) or ligamentous sprains and is generated as these tissues work to provide the necessary extension moment to counter the moments created as gravity acts on the body segments and the load being lifted. Even the most simplified biomechanical models are able to show how excessive body mass can adversely impact the loading of the muscles and the spine during the simplest lifting tasks. However, the current epidemiological research does not conclusively show a link between BMI and LBP. LeboeufYde (2000) did a systematic literature review of 65 studies to investigate the relationship between body weight and LBP. From this review, only 21 (32%) of the 65 studies showed a statistically significant positive weak association between body weight and LBP. In these 65 studies, a total of 111 LBP variables were defined and these authors report that a positive association was found between weight and relative weight for 25 of these LBP variables. The author concluded that there is not enough evidence to support a causal relationship between body weight and LBP. As an example of a study that did find a significant relationship, Kostova and Koleva (2001) conducted a cross-sectional study of 898 workers in a fertilizer plant. Their results showed that prevalence rates of LBP was higher in those with a BMI greater than 25 kg/m2 than those under this level (Odds Ratio (OR): 1.46) and this BMI-dependent response was even higher for men greater than 40 years of age (OR 2.76). In another study, Bayramoglu et al. (2001) compared 25 female LBP patients with 20 age-matched controls without known low back trouble. Their results indicate that the LBP patients had significantly higher BMI than the control group (29.08 vs. 26.21 kg/m2). They do note, however, that it is unclear whether the LBP was caused by the higher BMI or, alternatively, the higher BMI might have been caused by the inactivity due to the LBP. Contrary to these results showing a positive relationship were the results of a large-scale study by Lee et al. (2005) that showed no such association between BMI and LBP. In their study of 10,306 participants in a working population in Switzerland, they found that 48% of the participants suffered mild back pain and 7% suffered severe back pain, but found no relationship between BMI and LBP. They did find, however, that the obese subject group (BMI430 kg/m2) exhibited less flexibility (assessed through maximal hip flexion, i.e. hamstring stretch), lower upper body strength, and abdominal strength (assessed through number of pushups/sit-ups performed in 1 min, respectively) than did their normal weight counterparts. Collectively, the research cited above provides a mixed assessment of the relationship between obesity and LBP, but does provide some interesting differences in the physical characteristics between the two groups that may manifest themselves in differences in lifting technique. An assessment of the differences in the human performance aspect of a lifting task (lifting kinetics and kinematics) between normal weight and obese persons might provide
additional insight into this potential occupational safety and health concern. The specific objectives of the current study were to evaluate the lifting kinematics (trunk motions) and kinetics (ground reaction forces) of individuals of differing levels of BMI. 2. Methods 2.1. Participants There were two subject groups in this experiment. The first group was the normal weight subject group (BMI less than 25 kg/m2). The second group was the obese subject group (BMI greater than 30 kg/m2). Twelve male volunteers from North Carolina State University student body and surrounding community participated in this experiment with six subjects in each group (details of anthropometry in Table 1). The experimental protocol was approved by the Institutional Review Board for the Protection of Human Subjects in Research. 2.2. Apparatus The Lumbar Motion Monitor (LMM, Chattanooga Group Inc., TN) was used to capture trunk kinematics during the lifting tasks. This device is positioned along the length of the spine and is attached by a harness at the thorax and pelvis. The LMM captures trunk angular position in the three cardinal planes of human motion (sagittal, coronal, and transverse) at a rate of 60 Hz. Threedimensional angular velocities and accelerations are then obtained by differentiating these angular position data as a function of time (details of this device and its validation can be found in Marras et al., 1992). The ground reaction forces and moments were collected by two force plates (Bertec Corporation, Model 4060A). Table 1 Subject characteristics and loads lifted in experiment Subject no.
Stature (m)
BMI (kg/m2)
MVC (Nm)
10% load (kg)
25% load (kg)
73 83 73 64 63 59
23.7 24.9 22.4 20.7 19.5 20.6
171.6 154.3 177.4 194.7 188.9 142.8
5.5 5.5 5.9 6.4 6.4 5.0
13.6 13.6 14.8 15.9 15.9 12.5
102 115 90 105 116 111
31.5 33.2 30.4 35.4 38.8 30.4
200.4 183.1 194.7 223.5 200.4 148.5
5.9 5.5 6.0 7.7 6.4 4.5
14.8 13.6 15 19.3 15.9 11.4
Weight (kg)
Normal weight group 1 1.75 2 1.83 4 1.80 5 1.75 6 1.79 11 1.69 Obese group 3 1.80 7 1.86 8 1.72 9 1.72 10 1.73 12 1.91
Note: MVC values were not statistically different between the two groups of subjects.
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Each force plate has four piezoelectric transducers for measuring the three-dimensional ground reaction forces and moments. The signals from the force plate were sampled at 120 Hz. While these data were collected on both force platforms, the data presented in this paper are only from the right foot platform, as this is the platform that is in the direction of the asymmetric lifting task. The weighted box for the lifting task was 36 cm 36 cm 30 cm tall. The handles on the box were 19 cm from the ground. There was a vertical dowel bar in the middle of the bottom of the box to secure the cast iron weights in a central location. During the lifting task, the weighted box was placed on a wooden platform that located the box at the same level as the top of the force platform (simulated lifting from ground level) at a constant horizontal moment arm of 51 cm. The starting position of the center of the weighted box was either directly in front of the subjects (mid-sagittal plane, the symmetric position) or 451 off to the right of the mid-sagittal plane (the asymmetric position), and the feet were always pointing parallel to the mid-sagittal plane. A lumbar dynamometer (Marras and Mirka, 1989; Mirka and Marras, 1993) was used to establish the trunk extension torque capacity of the individual subjects. Subjects stood in the apparatus in a sagittally symmetric orientation while flexed forward 451 and exerted their maximum trunk extension force into the transducer of the dynamometer. Two repetitions of this exertion were performed, and if the values were within 10% of one another, the larger of the two values was used. If not, another repetition was performed. Using these maximum extension torque values (Table 1), the required handheld load (box weight) that would provide the 10% and 25% of maximum extension torque was calculated for each subject. This choice of ‘‘relative’’ weight (i.e. relative to subject specific capacity) was made so that the relative stress level was held constant across subjects allowing for an assessment of changes in lifting technique as a function of obesity. 2.3. Experimental design The independent variables in this study were BMI (between-subjects variable), LOAD (within-subjects variable), and ASYMMETRY (within-subjects variable). There were two levels of BMI—BMI430 and BMIo25, two levels of LOAD—10% and 25% of lifting capacity, and two levels of ASYMMETRY—01 and 451. There were two types of dependent variables in this study. The first set was focused on the peak values of the trunk kinematics of the lifting process collected by LMM and the peak ground reaction forces collected by force plates during each concentric lifting motion. The trunk kinematics variables included the peak sagittal flexion angle, the peak sagittal extension velocity, the peak sagittal extension acceleration, the peak rotational (twisting in transverse plane) position, the peak rotational velocity, and
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the peak rotational acceleration. The normalized (to body weight) ground reaction force variables were the peak values of lateral shear, anterior shear, and vertical ground reaction forces of the right foot. Only the right foot was considered here as it contained the most pertinent information since the asymmetric lifts were performed to the right side. In addition to these individual values, the ratio of the resultant shear force to the normal force (the required coefficient of friction) was also evaluated. Motivated by biomechanical research that has emphasized the importance of considering the variability of biomechanical measures to the risk of injury (e.g. Mirka and Marras, 1993; Granata, et al. 1999; van Diee¨n et al., 2001, 2002), the second set of dependent variables evaluated the intra-subject variability of the peak kinematic measures. In this research, this variability was obtained by calculating a measure of within-subject variability (details of this calculation provided in Section 2.5) across trials of the same lifting condition for a subject. Therefore, the intra-subject variability of: (1) the peak sagittal flexion angle, (2) the peak sagittal extension velocity, (3) the peak sagittal extension acceleration, (4) the peak rotational (twisting in transverse plane) position, (5) the peak rotational velocity, and (6) the peak rotational acceleration were also dependent measures. 2.4. Procedure Upon arrival, the researcher explained and demonstrated the lifting task. The subject was asked to sign the Informed Consent Form and several basic anthropometric measurements were gathered. The subjects were given a 5-min warm-up and stretching period that focused on the low back to prepare the subject for the lifting task. The subject then proceeded to the lumbar dynamometer and generated a maximum isometric trunk extension moment while in a 451 trunk flexion posture, and using these data the two lifting loads were calculated. The LMM was then placed on the subject’s back and they moved to the lifting platform. A 5-min break was given before the experimental trials commenced. The experimenter first demonstrated the box lifting tasks to be performed by the subject. Before each trial, the subject was asked to lift the box to get a sense of the weight of load. The subject was instructed to stand with the midpoint between the ankles at a specified location to ensure that the load moment arm of the box weight was the required 51 cm and then the position of the subject’s feet were marked with chalk. Between each trial, the subject was allowed to step off of the force plates to take a rest, and then was asked to put his feet on the marked position before the next trial. During the experiment, the subject lifted the box from the starting position on the lifting platform to the mid-chest height in a sagittally symmetric posture. The subjects were instructed that they could use a free-style lifting technique but their feet were not allowed to move during the lift. There were four conditions
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250
900 BMI < 25
200
800 BMI > 30
700
175
600
150
500
125
400
100
300
75
200
50
100
25
0
Angular Velocity (deg/s)
Analysis of variance (ANOVA) techniques were used to analyze this dataset. Prior to conducting these analyses, the assumptions of ANOVA were evaluated and it was found that the homogeneity of variance assumption was violated for the intra-subject variability measures. To address this non-constant variance, a logarithmic transformation was applied to these data and all subsequent statistical analyses were performed on these transformed data. A balanced, two-stage nested design was employed with ‘‘Subject’’ nested within BMI. A Bonferroni-corrected p-value of 0.05 (criterion value now 0.01645) was used as the criterion level for significance in all statistical analyses.
Even the simplest biomechanical models indicate that obese individuals experience higher joint loading during lifting tasks than their normal weight counterparts. As the mass of the torso increases, the moment created by this additional mass creates the need for greater muscle force and spinal loading (compression and anterior/posterior forces). It is interesting to note, however, that the epidemiology literature is ambiguous as to the relationship between obesity and LBP/disorders (Leboeuf-Yde, 2000). What are the intervening factors that may obscure this
0 Sagittal Acceleration
2.6. Statistical analysis
4. Discussion
Sagittal Velocity
To process the data obtained from LMM, custom software was written to extract the time-dependent kinematic parameters from the dataset. The program identified each lifting cycle and identified the peak values for each of the four dependent variables in the LMM data collected during concentric portion of each lift. Another program was developed that captured the peak ground reaction forces during the concentric range of motion. Because the magnitude of the lateral, anterior, and vertical ground reaction forces are highly correlated to the subject’s body weight, the ground reaction force data were normalized to the subject’s weight. To normalize the force, all lateral, anterior, and vertical ground reaction forces were divided by the subject’s body weight. Finally to calculate the peak required coefficient of friction value, the resultant shear force of the right foot was divided by the vertical ground reaction force of the right foot. The peak of this value for each concentric lifting motion was identified. To assess the intra-subject variability of the trunk kinematics variables, the modified Levene test was used. This required some additional data processing. In the modified Levene test, the absolute deviation of each observation from the treatment median is calculated (Montgomery, 2001). In this case, the median of the 12 observations (four lifts per trial three trials per condition) was identified and the absolute deviation of each observation from this median value is computed. A simple F-test can then be used to evaluate these deviation data to assess differences in variability of the dependent measures as a function of the independent variables.
As expected, LOAD and ASYMMETRY had significant effects on the trunk kinematics and ground reaction force values as has been shown in previous studies (e.g. Ferguson et al., 1992; Allread et al., 1996; Mirka and Baker, 1996), but the main focus of this work was on the effect of BMI on these measures and, interestingly, these effects were much more limited. Univariate ANOVA revealed that four trunk kinematics variables were influenced by BMI level: transverse velocity (F ¼ 9.1, p ¼ 0.013), transverse acceleration (F ¼ 10.57, p ¼ 0.0087), sagittal velocity (F ¼ 12.75, p ¼ 0.0051), and sagittal acceleration (F ¼ 18.36, p ¼ 0.0016) (Fig. 1). Neither of the peak angle variables (peak transverse position, peak sagittal flexion angle) was significantly affected by BMI. In addition, none of the two- or three-way interactions involving BMI were found to be significant, nor were any of the measures of intra-subject variability. Finally, none of the dependent variables associated with the ground reaction forces were significantly affected by BMI or any interactions involving BMI.
Twisting Acceleration
2.5. Data processing
3. Results
Twisting Velocity
(two starting positions two loads) and each of the four conditions was performed three times (4 3 ¼ 12 trials for total). Six consecutive lifts were performed in each trial and finished in 1 min (one lift every 10 s) and the data from the last four lifts of each set were captured by the LMM and the force plates. A 1-min break (standing without load) was given between each trial. Trial order was fully randomized and thus the experiment was a randomized complete block design.
Angular Acceleration (deg /s /s)
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Fig. 1. Main effect of BMI on peak trunk kinematics measures. All effects significant (po0.05).
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relationship that appears to be so clear? The focus of the current work was to explore the human performance aspect of lifting and how the technique of these two groups may differ and what additional information this might provide to the discussion. The results of this experiment showed that the BMI had a strong effect on the peak value of all dynamic trunk variables kinematics variables captured from the LMM: rotational velocity, rotational acceleration, sagittal velocity, and sagittal acceleration. It would seem that one potential technique that heavier lifters might adopt to maintain lower loading values would be to lift more slowly and thereby minimize the dynamic component of the total loading, and this was the expectation at the start of the study. However, all four kinematics parameters in the group of BMI430 were found to be greater than those in the group of BMIo25 (Fig. 1). This implies that not only is there a greater mass to be lifted (gravity acting on a greater body mass), but these individuals are further increasing the loading by generating greater inertial forces (F ¼ ma). It would seem that a logical explanation for this would be that the high BMI group had a corresponding greater muscle strength that compensates for (and exceeds) the additional forces from the additional body mass, but this explanation is not supported by the data from the current study (Table 1.) More generally, a high BMI value may be found in both a person with a high percent body fat (very unfit) or a very muscular person (very fit) (Gallagher et al., 1996). This observation brings into question the utility of BMI in categorizing individuals along any dimension related to fitness, a result supporting previous studies (Harp and Hecht, 2005a, b; Lesser, 2005; Zhu et al., 2005), and indicates that the incidence of LBP may be related more to functional fitness level (e.g. body composition, trunk extension strength, etc.) than the overly simplistic BMI metric. This perspective can provide an explanation for the poor correlation between BMI and LBP in the epidemiology studies. The effect of obesity on the variability of lifting technique was also explored in the current study. This direction was pursued based on the literature that highlighted obesity as a factor influencing the postural control/ stability (Corbeil et al., 2001; Simoneau and Corbeil, 2005). These authors used simulation to show that when a perturbation force was applied to obese and normal weight humanoid models, the obese model needed non-linear increase of torque at the ankle joint for stabilization—a result that was confirmed in their empirical research. This led to the hypothesis that the normal variability in the lifting kinematics employed may be amplified by the excess weight carried by the group with the higher BMI. This hypothesis was not supported by the results of the current experiment indicating that BMI by itself may not be a good indicator of a person’s capability to maintain postural control/stability. As has been noted in the obesity literature (e.g. Wearing et al., 2006), there is a real need to better understand the
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impact of obesity on the functionality of the human being. These authors have emphasized issues related to range of motion and functional capacity and note ‘‘surprisingly little empirical investigation pertaining to the biomechanics of activities of daily living or into the mechanical and neuromuscular factors that may predispose the obese to injury’’. The goal of this research was to explore one particular work-related aspect of this criticism—how people of different BMI levels perform lifting tasks. The interpretation of the results of this study is made more complicated by the use of the BMI as the index of obesity. Future research (both biomechanical and epidemiological) should use more functional measures of obesity (e.g. strength-to-body mass measures) to characterize the outof-shape individual. It is believed that these indices will be more sensitive in their ability to predict the incidence of LBP/disorder. Another experimental design decision that makes the interpretation of these results more challenging from an application perspective is the use of the subject specific loads. As noted in Section 2, this choice of ‘‘relative’’ weight (i.e. relative to subject specific capacity) was made so that the relative stress level was held constant across subjects allowing for an assessment of changes in lifting technique as a function of obesity. However, in the occupational setting the worker is asked to lift an item that has a certain mass regardless of their personal strength capability. Further research in this area should consider the interplay between the more static anthropometric variables describing fitness and more functional variables of strength and how these influence performance of a lifting task with a designated load.
5. Conclusions The objective of this study was to investigate the differences in lifting patterns between obese people (BMI430) and normal weight (BMIo25) persons. It was hypothesized that the larger individuals would exhibit slower lifting motions to reduce the loading on the muscles and passive structures of the low back region. In contrast to this hypothesis, the results revealed the opposite— measures describing the dynamics of the lifting motion were greater for the obese group than the normal weight group. The result showed that the twisting velocity was 59.2% higher, the twisting acceleration was 57.6% higher, the sagittal velocity was 30.4% higher, and the sagittal acceleration was 50.5% higher in the BMI430 group as compared to the normal weight group. These results bring into question the utility of the BMI as a measure of obesity and may explain why there has been only limited evidence of a positive relationship between BMI and the incidence of LBP in the epidemiological literature. More appropriate measures of obesity are needed that can distinguish between an unfit worker of a given BMI and an individual with the same BMI but with a more healthy body composition and greater occupational functionality.
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References Allread, W., Marras, W., Parnianpour, M., 1996. Trunk kinematics of one-handed lifting, and the effects of asymmetry and load weight. Ergonomics 39, 322–334. Andersson, G., 1981. Epidemiologic aspects on low-back-pain in industry. Spine 6, 53–60. Bayramoglu, M., Akman, M.N., Kilinc, S., Cetin, N., Yavuz, N., Ozker, R., 2001. Isokinetic measurement of trunk muscle strength in women with chronic low-back pain. Am. J. Phys. Med. Rehabil. 80, 650–655. Corbeil, P., Simoneau, M., Rancourt, D., Tremblay, A., Teasdale, N., 2001. Increased risk for falling associated with obesity: mathematical modeling of postural control. IEEE Trans. Neural Syst. Rehabil. Eng. 9, 126–136. van Diee¨n, J., Dekkers, J., Groen, V., Toussaint, H., Meijer, O., 2001. Within-subject variability in low back load in a repetitively performed, mildly constrained lifting task. Spine 26, 1799–1804. van Diee¨n, J., Hoozemans, M., van der Beek, A., Mullender, M., 2002. Precision of estimates of mean and peak spinal loads in lifting. J. Biomech. 35, 979–982. Ferguson, S., Marras, W., Waters, T., 1992. Quantification of back motion during asymmetric lifting. Ergonomics 35, 845–859. Gallagher, D., Visser, M., Sepulveda, D., Pierson, R., Harris, T., Heymsfield, S., 1996. How useful is body mass index for comparison of body fatness across age, sex, and ethnic groups? Am. J. Epidemiol. 143, 228–239. Granata, K., Marras, W., Davis, K., 1999. Variation in spinal load and trunk dynamics during repeated lifting exertions. Clin. Biomech. 14, 367–375. Harp, J., Hecht, L., 2005a. Obesity in the National Football League. J. Am. Med. Assoc. 293, 1061–1062. Harp, J., Hecht, L., 2005b. Obesity in the NFL—reply. J. Am. Med. Assoc. 293, 2999. Hedley, A., Ogden, C., Johnson, C., Carroll, M., Curtin, L., Flegal, K., 2004. Prevalence of overweight and obesity among US children, adolescents, and adults, 1999–2002. J. Am. Med. Assoc. 291, 2847–2850. Hertz, R., Unger, A., McDonald, M., Lustik, M., Biddulph-Krentar, J., 2004. The impact of obesity on work limitations and cardiovascular risk factors in the US workforce. J. Occup. Environ. Med. 46, 1196–1203.
Jayson, M., 1996. ABC of work related disorders—back pain. Br. Med. J. 313, 355–358. Kostova, V., Koleva, M., 2001. Back disorders (low back pain, cervicobrachial and lumbosacral radicular syndromes) and some related risk factors. J. Neurol. Sci. 192, 17–25. Leboeuf-Yde, C., 2000. Body weight and low back pain—a systematic literature review of 56 journal articles reporting on 65 epidemiologic studies. Spine 25, 226–237. Lee, C., Kratter, R., Duvoisin, N., Taskin, A., Schilling, J., 2005. Crosssectional view of factors associated with back pain. Int. Arch. Occup. Environ. Health 78, 319–324. Lesser, G., 2005. Obesity in the NFL. J. Am. Med. Assoc. 293, 2999. Marras, W., Mirka, G., 1989. Trunk strength during asymmetric trunk motion. Hum. Factors 31, 667–677. Marras, W., Fathallah, F., Miller, R., Davis, S., Mirka, G., 1992. Accuracy of a three-dimensional lumbar motion monitor for recording dynamic trunk motion characteristics. Int. J. Ind. Ergon. 9, 75–87. Mirka, G., Baker, A., 1996. An investigation of the variability in human performance during sagittally symmetric lifting tasks. IIE Trans. 28, 745–752. Mirka, G., Marras, W., 1993. A stochastic-model of trunk muscle coactivation during trunk bending. Spine 18, 1396–1409. Montgomery, D., 2001. Design and Analysis of Experiments, fifth ed. Wiley, New York. Murphy, P., Volinn, E., 1999. Is occupational low back pain on the rise? Spine 24, 691–697. Simoneau, M., Corbeil, P., 2005. The effect of time to peak ankle torque on balance stability boundary: experimental validation of a biomechanical model. Exp. Brain Res. 165, 217–228. Wearing, S., Hennig, E., Byrne, N., Steele, J., Hills, A., 2006. The biomechanics of restricted movement in adult obesity. Obes. Rev. 7, 13–24. Wellman, N., Friedberg, B., 2002. Causes and consequences of adult obesity: health, social and economic impacts in the United States. Asia Pacific J. Clin. Nutr. 11, S705–S709. World Health Organization, 1998. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. World Health Organization, Geneva. Zhu, S., Heymsfield, S., Toyoshima, H., Wang, Z., Pietrobelli, A., Heshka, S., 2005. Race-ethnicity-specific waist circumference cutoffs for identifying cardiovascular disease risk factors. Am. J. Clin. Nutr. 81, 409–415.
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The effects of obesity on lifting performance Xu Xu, Gary A. Mirka, Simon M. Hsiang The Ergonomics Laboratory, Edward P. Fitts Department of Industrial and Systems Engineering, North Carolina State University, Raleigh, NC 27695-7906, USA Received 28 June 2006; accepted 2 February 2007
Abstract Obesity in the workforce is a growing problem worldwide. While the implications of this trend for biomechanical loading of the musculoskeletal system seem fairly straightforward, the evidence of a clear link between low back pain (LBP) and body mass index (BMI) (calculated as whole body mass in kilograms divided by the square of stature in meters) has not been shown in the epidemiology literature addressing this topic. The approach pursued in the current study was to evaluate the lifting kinematics and ground reaction forces of a group of 12 subjects—six with a BMI of less than 25 kg/m2 (normal weight) and six with a BMI of greater than 30 kg/m2 (obese). These subjects performed a series of free dynamic lifting tasks with varied levels of load (10% and 25% of capacity) and symmetry (sagittally symmetric and 451 asymmetric). The results showed that BMI had a significant effect (po0.05) on trunk kinematics with the high BMI group exhibiting higher peak transverse plane (twisting) velocity (59% higher) and acceleration (57% higher), and exhibiting higher peak sagittal plane velocity (30% higher) and acceleration (51% higher). When normalized to body weight, there were no significant differences in the ground reaction forces between the two groups. This study provides quantitative data describing lifting task performance differences between people of differing BMI levels and may help to explain why there is no conclusive epidemiological evidence of a relationship between BMI and LBP. r 2007 Elsevier Ltd. All rights reserved. Keywords: Obesity; Lifting; Low back pain
1. Introduction Obesity in the workforce is a considerable problem that shows no indication of slowing. The World Health Organization (1998) has shown that the prevalence of obesity is increasing at a high rate in both developed and developing countries. Obesity is defined using an index called the body mass index (BMI). BMI is calculated as whole body mass in kilograms divided by the square of stature in meters. A BMI between 18.5 and 25 kg/m2 is considered normal weight, between 25 and 30 kg/m2 is considered overweight, between 30 and 40 kg/m2 is obese and greater than 40 kg/m2 is extremely obese. In the United States, obesity has been called an epidemic for both adults and children (Wellman and Friedberg, 2002), with data from 2001 to 2002 showing that 66% of US adults were either overweight or obese and 5% were extremely obese Corresponding author. Tel.: +1 919 515 6399; fax: +1 919 515 5281.
E-mail address: [email protected] (G.A. Mirka). 0003-6870/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2007.02.001
(Hedley et al., 2004). These authors also showed that, in the US workforce, the obesity rate for men is 26.1% and the obesity rate for women is 33.3%. Relevant to the effects of these statistics on occupational safety and health concerns, are the results of the study by Hertz et al. (2004) that showed that, among both men and women, obese workers are more likely to report work limitations due to physical, mental or emotional problems than their normal weight counterparts. An area of particular concern may be low back pain (LBP)/disorder because of the additional muscular force required to move the additional body mass while performing various occupational tasks. LBP is a common occurrence in many occupations. Lifetime prevalence has been estimated at about 70% (Andersson, 1981) and it is one of the most costly problems of modern society, with an estimated cost of $8.8 billion in the US in 1995 (Murphy and Volinn, 1999). People of all ages can be affected by back pain, but it generally begins between the ages of 20 and 40 years, and the prevalence peak between the ages of 45–60 years, with a slight
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difference between males and females (Jayson, 1996). LBP can result from injury to the muscle tissues (muscle strain) or ligamentous sprains and is generated as these tissues work to provide the necessary extension moment to counter the moments created as gravity acts on the body segments and the load being lifted. Even the most simplified biomechanical models are able to show how excessive body mass can adversely impact the loading of the muscles and the spine during the simplest lifting tasks. However, the current epidemiological research does not conclusively show a link between BMI and LBP. LeboeufYde (2000) did a systematic literature review of 65 studies to investigate the relationship between body weight and LBP. From this review, only 21 (32%) of the 65 studies showed a statistically significant positive weak association between body weight and LBP. In these 65 studies, a total of 111 LBP variables were defined and these authors report that a positive association was found between weight and relative weight for 25 of these LBP variables. The author concluded that there is not enough evidence to support a causal relationship between body weight and LBP. As an example of a study that did find a significant relationship, Kostova and Koleva (2001) conducted a cross-sectional study of 898 workers in a fertilizer plant. Their results showed that prevalence rates of LBP was higher in those with a BMI greater than 25 kg/m2 than those under this level (Odds Ratio (OR): 1.46) and this BMI-dependent response was even higher for men greater than 40 years of age (OR 2.76). In another study, Bayramoglu et al. (2001) compared 25 female LBP patients with 20 age-matched controls without known low back trouble. Their results indicate that the LBP patients had significantly higher BMI than the control group (29.08 vs. 26.21 kg/m2). They do note, however, that it is unclear whether the LBP was caused by the higher BMI or, alternatively, the higher BMI might have been caused by the inactivity due to the LBP. Contrary to these results showing a positive relationship were the results of a large-scale study by Lee et al. (2005) that showed no such association between BMI and LBP. In their study of 10,306 participants in a working population in Switzerland, they found that 48% of the participants suffered mild back pain and 7% suffered severe back pain, but found no relationship between BMI and LBP. They did find, however, that the obese subject group (BMI430 kg/m2) exhibited less flexibility (assessed through maximal hip flexion, i.e. hamstring stretch), lower upper body strength, and abdominal strength (assessed through number of pushups/sit-ups performed in 1 min, respectively) than did their normal weight counterparts. Collectively, the research cited above provides a mixed assessment of the relationship between obesity and LBP, but does provide some interesting differences in the physical characteristics between the two groups that may manifest themselves in differences in lifting technique. An assessment of the differences in the human performance aspect of a lifting task (lifting kinetics and kinematics) between normal weight and obese persons might provide
additional insight into this potential occupational safety and health concern. The specific objectives of the current study were to evaluate the lifting kinematics (trunk motions) and kinetics (ground reaction forces) of individuals of differing levels of BMI. 2. Methods 2.1. Participants There were two subject groups in this experiment. The first group was the normal weight subject group (BMI less than 25 kg/m2). The second group was the obese subject group (BMI greater than 30 kg/m2). Twelve male volunteers from North Carolina State University student body and surrounding community participated in this experiment with six subjects in each group (details of anthropometry in Table 1). The experimental protocol was approved by the Institutional Review Board for the Protection of Human Subjects in Research. 2.2. Apparatus The Lumbar Motion Monitor (LMM, Chattanooga Group Inc., TN) was used to capture trunk kinematics during the lifting tasks. This device is positioned along the length of the spine and is attached by a harness at the thorax and pelvis. The LMM captures trunk angular position in the three cardinal planes of human motion (sagittal, coronal, and transverse) at a rate of 60 Hz. Threedimensional angular velocities and accelerations are then obtained by differentiating these angular position data as a function of time (details of this device and its validation can be found in Marras et al., 1992). The ground reaction forces and moments were collected by two force plates (Bertec Corporation, Model 4060A). Table 1 Subject characteristics and loads lifted in experiment Subject no.
Stature (m)
BMI (kg/m2)
MVC (Nm)
10% load (kg)
25% load (kg)
73 83 73 64 63 59
23.7 24.9 22.4 20.7 19.5 20.6
171.6 154.3 177.4 194.7 188.9 142.8
5.5 5.5 5.9 6.4 6.4 5.0
13.6 13.6 14.8 15.9 15.9 12.5
102 115 90 105 116 111
31.5 33.2 30.4 35.4 38.8 30.4
200.4 183.1 194.7 223.5 200.4 148.5
5.9 5.5 6.0 7.7 6.4 4.5
14.8 13.6 15 19.3 15.9 11.4
Weight (kg)
Normal weight group 1 1.75 2 1.83 4 1.80 5 1.75 6 1.79 11 1.69 Obese group 3 1.80 7 1.86 8 1.72 9 1.72 10 1.73 12 1.91
Note: MVC values were not statistically different between the two groups of subjects.
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Each force plate has four piezoelectric transducers for measuring the three-dimensional ground reaction forces and moments. The signals from the force plate were sampled at 120 Hz. While these data were collected on both force platforms, the data presented in this paper are only from the right foot platform, as this is the platform that is in the direction of the asymmetric lifting task. The weighted box for the lifting task was 36 cm 36 cm 30 cm tall. The handles on the box were 19 cm from the ground. There was a vertical dowel bar in the middle of the bottom of the box to secure the cast iron weights in a central location. During the lifting task, the weighted box was placed on a wooden platform that located the box at the same level as the top of the force platform (simulated lifting from ground level) at a constant horizontal moment arm of 51 cm. The starting position of the center of the weighted box was either directly in front of the subjects (mid-sagittal plane, the symmetric position) or 451 off to the right of the mid-sagittal plane (the asymmetric position), and the feet were always pointing parallel to the mid-sagittal plane. A lumbar dynamometer (Marras and Mirka, 1989; Mirka and Marras, 1993) was used to establish the trunk extension torque capacity of the individual subjects. Subjects stood in the apparatus in a sagittally symmetric orientation while flexed forward 451 and exerted their maximum trunk extension force into the transducer of the dynamometer. Two repetitions of this exertion were performed, and if the values were within 10% of one another, the larger of the two values was used. If not, another repetition was performed. Using these maximum extension torque values (Table 1), the required handheld load (box weight) that would provide the 10% and 25% of maximum extension torque was calculated for each subject. This choice of ‘‘relative’’ weight (i.e. relative to subject specific capacity) was made so that the relative stress level was held constant across subjects allowing for an assessment of changes in lifting technique as a function of obesity. 2.3. Experimental design The independent variables in this study were BMI (between-subjects variable), LOAD (within-subjects variable), and ASYMMETRY (within-subjects variable). There were two levels of BMI—BMI430 and BMIo25, two levels of LOAD—10% and 25% of lifting capacity, and two levels of ASYMMETRY—01 and 451. There were two types of dependent variables in this study. The first set was focused on the peak values of the trunk kinematics of the lifting process collected by LMM and the peak ground reaction forces collected by force plates during each concentric lifting motion. The trunk kinematics variables included the peak sagittal flexion angle, the peak sagittal extension velocity, the peak sagittal extension acceleration, the peak rotational (twisting in transverse plane) position, the peak rotational velocity, and
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the peak rotational acceleration. The normalized (to body weight) ground reaction force variables were the peak values of lateral shear, anterior shear, and vertical ground reaction forces of the right foot. Only the right foot was considered here as it contained the most pertinent information since the asymmetric lifts were performed to the right side. In addition to these individual values, the ratio of the resultant shear force to the normal force (the required coefficient of friction) was also evaluated. Motivated by biomechanical research that has emphasized the importance of considering the variability of biomechanical measures to the risk of injury (e.g. Mirka and Marras, 1993; Granata, et al. 1999; van Diee¨n et al., 2001, 2002), the second set of dependent variables evaluated the intra-subject variability of the peak kinematic measures. In this research, this variability was obtained by calculating a measure of within-subject variability (details of this calculation provided in Section 2.5) across trials of the same lifting condition for a subject. Therefore, the intra-subject variability of: (1) the peak sagittal flexion angle, (2) the peak sagittal extension velocity, (3) the peak sagittal extension acceleration, (4) the peak rotational (twisting in transverse plane) position, (5) the peak rotational velocity, and (6) the peak rotational acceleration were also dependent measures. 2.4. Procedure Upon arrival, the researcher explained and demonstrated the lifting task. The subject was asked to sign the Informed Consent Form and several basic anthropometric measurements were gathered. The subjects were given a 5-min warm-up and stretching period that focused on the low back to prepare the subject for the lifting task. The subject then proceeded to the lumbar dynamometer and generated a maximum isometric trunk extension moment while in a 451 trunk flexion posture, and using these data the two lifting loads were calculated. The LMM was then placed on the subject’s back and they moved to the lifting platform. A 5-min break was given before the experimental trials commenced. The experimenter first demonstrated the box lifting tasks to be performed by the subject. Before each trial, the subject was asked to lift the box to get a sense of the weight of load. The subject was instructed to stand with the midpoint between the ankles at a specified location to ensure that the load moment arm of the box weight was the required 51 cm and then the position of the subject’s feet were marked with chalk. Between each trial, the subject was allowed to step off of the force plates to take a rest, and then was asked to put his feet on the marked position before the next trial. During the experiment, the subject lifted the box from the starting position on the lifting platform to the mid-chest height in a sagittally symmetric posture. The subjects were instructed that they could use a free-style lifting technique but their feet were not allowed to move during the lift. There were four conditions
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250
900 BMI < 25
200
800 BMI > 30
700
175
600
150
500
125
400
100
300
75
200
50
100
25
0
Angular Velocity (deg/s)
Analysis of variance (ANOVA) techniques were used to analyze this dataset. Prior to conducting these analyses, the assumptions of ANOVA were evaluated and it was found that the homogeneity of variance assumption was violated for the intra-subject variability measures. To address this non-constant variance, a logarithmic transformation was applied to these data and all subsequent statistical analyses were performed on these transformed data. A balanced, two-stage nested design was employed with ‘‘Subject’’ nested within BMI. A Bonferroni-corrected p-value of 0.05 (criterion value now 0.01645) was used as the criterion level for significance in all statistical analyses.
Even the simplest biomechanical models indicate that obese individuals experience higher joint loading during lifting tasks than their normal weight counterparts. As the mass of the torso increases, the moment created by this additional mass creates the need for greater muscle force and spinal loading (compression and anterior/posterior forces). It is interesting to note, however, that the epidemiology literature is ambiguous as to the relationship between obesity and LBP/disorders (Leboeuf-Yde, 2000). What are the intervening factors that may obscure this
0 Sagittal Acceleration
2.6. Statistical analysis
4. Discussion
Sagittal Velocity
To process the data obtained from LMM, custom software was written to extract the time-dependent kinematic parameters from the dataset. The program identified each lifting cycle and identified the peak values for each of the four dependent variables in the LMM data collected during concentric portion of each lift. Another program was developed that captured the peak ground reaction forces during the concentric range of motion. Because the magnitude of the lateral, anterior, and vertical ground reaction forces are highly correlated to the subject’s body weight, the ground reaction force data were normalized to the subject’s weight. To normalize the force, all lateral, anterior, and vertical ground reaction forces were divided by the subject’s body weight. Finally to calculate the peak required coefficient of friction value, the resultant shear force of the right foot was divided by the vertical ground reaction force of the right foot. The peak of this value for each concentric lifting motion was identified. To assess the intra-subject variability of the trunk kinematics variables, the modified Levene test was used. This required some additional data processing. In the modified Levene test, the absolute deviation of each observation from the treatment median is calculated (Montgomery, 2001). In this case, the median of the 12 observations (four lifts per trial three trials per condition) was identified and the absolute deviation of each observation from this median value is computed. A simple F-test can then be used to evaluate these deviation data to assess differences in variability of the dependent measures as a function of the independent variables.
As expected, LOAD and ASYMMETRY had significant effects on the trunk kinematics and ground reaction force values as has been shown in previous studies (e.g. Ferguson et al., 1992; Allread et al., 1996; Mirka and Baker, 1996), but the main focus of this work was on the effect of BMI on these measures and, interestingly, these effects were much more limited. Univariate ANOVA revealed that four trunk kinematics variables were influenced by BMI level: transverse velocity (F ¼ 9.1, p ¼ 0.013), transverse acceleration (F ¼ 10.57, p ¼ 0.0087), sagittal velocity (F ¼ 12.75, p ¼ 0.0051), and sagittal acceleration (F ¼ 18.36, p ¼ 0.0016) (Fig. 1). Neither of the peak angle variables (peak transverse position, peak sagittal flexion angle) was significantly affected by BMI. In addition, none of the two- or three-way interactions involving BMI were found to be significant, nor were any of the measures of intra-subject variability. Finally, none of the dependent variables associated with the ground reaction forces were significantly affected by BMI or any interactions involving BMI.
Twisting Acceleration
2.5. Data processing
3. Results
Twisting Velocity
(two starting positions two loads) and each of the four conditions was performed three times (4 3 ¼ 12 trials for total). Six consecutive lifts were performed in each trial and finished in 1 min (one lift every 10 s) and the data from the last four lifts of each set were captured by the LMM and the force plates. A 1-min break (standing without load) was given between each trial. Trial order was fully randomized and thus the experiment was a randomized complete block design.
Angular Acceleration (deg /s /s)
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Fig. 1. Main effect of BMI on peak trunk kinematics measures. All effects significant (po0.05).
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relationship that appears to be so clear? The focus of the current work was to explore the human performance aspect of lifting and how the technique of these two groups may differ and what additional information this might provide to the discussion. The results of this experiment showed that the BMI had a strong effect on the peak value of all dynamic trunk variables kinematics variables captured from the LMM: rotational velocity, rotational acceleration, sagittal velocity, and sagittal acceleration. It would seem that one potential technique that heavier lifters might adopt to maintain lower loading values would be to lift more slowly and thereby minimize the dynamic component of the total loading, and this was the expectation at the start of the study. However, all four kinematics parameters in the group of BMI430 were found to be greater than those in the group of BMIo25 (Fig. 1). This implies that not only is there a greater mass to be lifted (gravity acting on a greater body mass), but these individuals are further increasing the loading by generating greater inertial forces (F ¼ ma). It would seem that a logical explanation for this would be that the high BMI group had a corresponding greater muscle strength that compensates for (and exceeds) the additional forces from the additional body mass, but this explanation is not supported by the data from the current study (Table 1.) More generally, a high BMI value may be found in both a person with a high percent body fat (very unfit) or a very muscular person (very fit) (Gallagher et al., 1996). This observation brings into question the utility of BMI in categorizing individuals along any dimension related to fitness, a result supporting previous studies (Harp and Hecht, 2005a, b; Lesser, 2005; Zhu et al., 2005), and indicates that the incidence of LBP may be related more to functional fitness level (e.g. body composition, trunk extension strength, etc.) than the overly simplistic BMI metric. This perspective can provide an explanation for the poor correlation between BMI and LBP in the epidemiology studies. The effect of obesity on the variability of lifting technique was also explored in the current study. This direction was pursued based on the literature that highlighted obesity as a factor influencing the postural control/ stability (Corbeil et al., 2001; Simoneau and Corbeil, 2005). These authors used simulation to show that when a perturbation force was applied to obese and normal weight humanoid models, the obese model needed non-linear increase of torque at the ankle joint for stabilization—a result that was confirmed in their empirical research. This led to the hypothesis that the normal variability in the lifting kinematics employed may be amplified by the excess weight carried by the group with the higher BMI. This hypothesis was not supported by the results of the current experiment indicating that BMI by itself may not be a good indicator of a person’s capability to maintain postural control/stability. As has been noted in the obesity literature (e.g. Wearing et al., 2006), there is a real need to better understand the
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impact of obesity on the functionality of the human being. These authors have emphasized issues related to range of motion and functional capacity and note ‘‘surprisingly little empirical investigation pertaining to the biomechanics of activities of daily living or into the mechanical and neuromuscular factors that may predispose the obese to injury’’. The goal of this research was to explore one particular work-related aspect of this criticism—how people of different BMI levels perform lifting tasks. The interpretation of the results of this study is made more complicated by the use of the BMI as the index of obesity. Future research (both biomechanical and epidemiological) should use more functional measures of obesity (e.g. strength-to-body mass measures) to characterize the outof-shape individual. It is believed that these indices will be more sensitive in their ability to predict the incidence of LBP/disorder. Another experimental design decision that makes the interpretation of these results more challenging from an application perspective is the use of the subject specific loads. As noted in Section 2, this choice of ‘‘relative’’ weight (i.e. relative to subject specific capacity) was made so that the relative stress level was held constant across subjects allowing for an assessment of changes in lifting technique as a function of obesity. However, in the occupational setting the worker is asked to lift an item that has a certain mass regardless of their personal strength capability. Further research in this area should consider the interplay between the more static anthropometric variables describing fitness and more functional variables of strength and how these influence performance of a lifting task with a designated load.
5. Conclusions The objective of this study was to investigate the differences in lifting patterns between obese people (BMI430) and normal weight (BMIo25) persons. It was hypothesized that the larger individuals would exhibit slower lifting motions to reduce the loading on the muscles and passive structures of the low back region. In contrast to this hypothesis, the results revealed the opposite— measures describing the dynamics of the lifting motion were greater for the obese group than the normal weight group. The result showed that the twisting velocity was 59.2% higher, the twisting acceleration was 57.6% higher, the sagittal velocity was 30.4% higher, and the sagittal acceleration was 50.5% higher in the BMI430 group as compared to the normal weight group. These results bring into question the utility of the BMI as a measure of obesity and may explain why there has been only limited evidence of a positive relationship between BMI and the incidence of LBP in the epidemiological literature. More appropriate measures of obesity are needed that can distinguish between an unfit worker of a given BMI and an individual with the same BMI but with a more healthy body composition and greater occupational functionality.
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