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Work 51 (2015) 337–348 DOI 10.3233/WOR-141945 IOS Press

Severe obesity effect on low back biomechanical stress of manual load lifting Devender Singha , Woojin Parkb,∗, Dongwook Hwangb and Martin S. Levyc a

Seton Spine and Scoliosis Center, Austin, TX, USA Department of Industrial Engineering, Seoul National University, Seoul, Korea c Department of Quantitative Analysis and Operations Management, College of Business, University of Cincinnati, Cincinnati, OH, USA b

Received 2 September 2013 Accepted 5 February 2014

Abstract. BACKGROUND: Little research is available on low back biomechanical stresses that obese and overweight workers experience from manual load lifting. OBJECTIVE: The study objective was to quantitatively evaluate low back biomechanical stresses of severely obese (BMI  35 kg/m2 ) workers during manual lifts of moderate load weights. METHOD: Twenty severely obese and 20 normal weight participants performed infrequent lifting in 16 task conditions. In each task condition, NIOSH recommended load weights were computed for the origin and destination of lift and were employed as the load weights. Optical motion capture was performed to collect lifting posture data. For each participant and each lifting condition, L5/S1 disc compression forces were computed at the origin and destination of lift using a static low back biomechanical model. RESULTS: The L5/S1 disc compression forces estimated for the severely obese participants ranged from 3000N to 8500N and many exceeded the 3400N NIOSH action limit by large margins. Group mean disc compression force was significantly larger for the severely obese than the normal weight group. CONCLUSION: In light of previous research on spine, bone and obesity, the study results seem to suggest that severely obese individuals are likely at an increased risk of lifting-related low back pain compared with normal weight individuals. Keywords: Corpulence, overweight, manual materials handling, low back pain, L5/S1 disc compression force

1. Introduction Obesity and overweight are physical conditions characterized with excess body fat. They are defined as body mass index (BMI)  30 kg/m2 and 25 kg/m2  BMI < 30 kg/m2 , respectively – while BMI does not measure body fat directly, and therefore, may result in incorrect classification for certain cases (such as athletes), it is known to correlate with direct measures of ∗ Corresponding author: Woojin Park, Department of Industrial Engineering, Seoul National University, 599 Gwanak-ro, Gwanakgu, Seoul 151-744, Korea. Tel.: +82 2 880 4310; Fax: +82 2 889 8560; E-mail: [email protected].

body fat and remains as a surrogate measure of obesity and overweight. Obesity and overweight have reached epidemic proportions globally. According to the World Health Organization (WHO), 11% of the world’s adult population were estimated to be obese and 35%, overweight in 2008 [1]. In many countries, the obese and overweight (hereafter, “large” for the sake of brevity) represent the majority of the population – at least one in two people was reported to be obese or overweight in over half of the Organisation for Economic Co-operation and Development (OECD) countries [2]. In the US and UK, almost 70% of the population was estimated to be large [3,4]. Leaner countries appear to be catching

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up. In South Korea, the largeness rate was over 30% in 2005 [5]; although relatively low compared with the incidence rates of other countries, this represented a 1.6 times increase from the figure 10 years before. According to Kitzinger and Karle, the current largeness trend is affecting countries of all income levels [6]. Despite the global prevalence, largeness has received relatively little attention in the occupational biomechanics and ergonomics research communities [7,8]. It was mostly in the recent years that the physical condition became recognized as a work design issue and researchers began to investigate its implications [9–17]. While the number of occupational biomechanics studies on largeness seems to be on the rise, its impacts on workers’ physical work capacities, work-related physical stresses and occupational health are largely not well understood at this time, due to lack of accumulation of focused research efforts. One of the knowledge deficiencies related to largeness pertains to low back biomechanical stresses from manual load lifting, which are considered as a risk factor of work-related low back pain (LBP) [18,19]. In ergonomics, numerous studies quantified low back biomechanical stresses of different manual lifting tasks to evaluate associated LBP risks. However, very few of them examined low back stresses experienced by large individuals; consequently, relevant quantitative data is scarce. Largeness is expected to increase low back biomechanical stresses as extra body (fat) mass acts as an additional load during lifting. Nonetheless, the magnitudes of the largeness effect for different levels of largeness and the associated LBP risks are not well understood at this time. One study by Blanton provided estimates of liftingrelated low back biomechanical stresses of obese (BMI  30 kg/m2 ) workers based on static biomechanical analyses [20]. However, this study had a limitation that biomechanical stress estimations were not based on obese individuals’ actual lifting posture data. The posture data utilized in this study were obtained from nonobese persons and obesity was simulated by adjusting the body weight parameter in the biomechanical model. Such simulation of obesity would suffice if it can be assumed that obesity does not affect lifting posture or technique; however, no evidence currently supports such an assumption. The scarcity of data/knowledge regarding liftingrelated low back biomechanical stresses of large workers is problematic as it hampers understanding the impacts of largeness on the risk of lifting-related LBP. It also makes it difficult to gauge the significance of the

current largeness epidemic in relation to the efforts for LBP reduction and prevention and further define future directions for research. As a step towards addressing the above problem, the current study examined severely obese (BMI  35 kg/m2 ) workers performing manual lifts of moderate hand load weights. The research objective was as follows: 1) To provide quantitative data describing low back biomechanical stresses that severely obese individuals experience during various manual lifts of moderate weights, 2) To determine the severe obesity effects on low back biomechanical stresses through a comparison of severely obese and normal weight participant groups, and 3) To discuss the potential impacts of severe obesity on the risk of lifting-related LBP on the basis of the biomechanical analyses results and the findings from previous research on spine, bone and obesity.

2. Methods 2.1. Participants Two levels of obesity were considered in recruiting participants: normal weight [18.5 kg/m2 < Body Mass Index (BMI) < 24.9 kg/m2 ] and severely obese (BMI > 35 kg/m2 ). For each level, 10 males and 10 females were recruited. Thus, a total of 40 individuals participated in this study. Only the individuals who were free of obvious musculoskeletal disorders and without a history of severe low back disorders were considered. A series of questions were asked to assess the physical conditions of participant candidates and determine the eligibility. All participants were recruited from the Greater Cincinnati and Northern Kentucky area through banners and local newspaper advertisements. From each participant, basic personal data, such as age, gender and occupation, were collected. Also, the following anthropometric dimensions were measured: stature, weight, waist circumference and hip circumference. An industrial weighing scale (SILTEC Electronic weighing scale) was used to measure participants’ body weights. A standard anthropometry kit was used to measure body dimensions. The waist circumference was measured at the level of the umbilicus and the hip circumference, at the widest point between the

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Fig. 1. Task parameters at the origin and destination of lift: horizontal hand location (H), vertical load location (V) and asymmetry angle (A). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141945) Table 1 Age and physical characteristics of the participants

Age (years) Weight (kg)∗ Height (cm) BMI (kg/m2 )∗ Waist Circumference (cm)∗ Hip Circumference (cm)∗ Waist-to-Hip Ratio∗ ∗ Significant

Non-obese (N = 20) Mean (std) 30.1 (7.68) 68.84 (6.89) 175.4 (5.86) 22.4 (1.66) 77.36 (4.96) 95.44 (3.94) 0.8102 (0.03)

Obese (N = 20) Mean (std) 31.6 (6.56) 142.15 (22.79) 173.4 (6.92) 45.96 (7.85) 132.95 (11.68) 136.38 (13.40) 1.0575 (0.06)

at α = 0.05.

hips and buttocks [21]. Based on these anthropometric data, the BMI and the waist-to-hip ratio were calculated. The age and physical characteristics of the normal weight and severely obese groups are summarized in Table 1. T-tests indicated that the severely obese and nonobese groups exhibited significant mean differences in: body weight, BMI, waist circumference, hip circumference and waist-to-hip ratio (p < 0.0001). However, the two groups were not significantly different in the mean height and age. Prior to the experiment, an introduction/training session was held in which participants familiarized themselves with the experimental task; also, during the ses-

sion, all the questions regarding the nature of the experiment and the experiment protocol were answered. No indications about the expectation of results were given in order to avoid conscious or unconscious bias of the results. An informed consent form was signed prior to the experiment. The study was approved by the Institutional Review Board (IRB), University of Cincinnati Medical Center. 2.2. Experimental task Participants performed infrequent low-lying box lifting with moderate hand load weights in a variety of task conditions. Moderate weights here mean hand load weights generally considered acceptable in the current practice of ergonomics design of lifting tasks. The National Institute for Occupational Safety and Health (NIOSH) recommended weight limits (RWLs) were employed as moderate load weights [22]. RWLs are defined as the load weights that nearly all healthy workers could lift in given task conditions without an increased risk of developing lifting-related LBP (note that the term “healthy” workers in the definition of RWLs may not include large workers). Biomechanically, lifting RWLs or less is known to produce L5/S1

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Fig. 2. Experimental setup. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141945)

compressive forces less than 3400N, which is known as the NIOSH action limit. According to NIOSH, predicted L5/S1 compression values less than 3400N can be considered safe for most healthy workers [22]. In a lifting trial, before the onset of movement, each participant was standing in a neutral position with the hands directly in front of the body and with minimal twisting at the legs, torso or shoulders. At a signal indicating the onset of movement, the participant reached and grabbed the box located at an initial load position (the origin of lift). Then, the participant lifted the box and placed it at a final load location (the destination of lift). A total of 16 lifting task conditions were considered. These 16 task conditions were chosen to simulate various lifting tasks performed in workplaces; they include both symmetrical and asymmetrical lifting conditions. The 16 task conditions were determined as combinations of four initial and four final load locations (16 = 4 × 4); initial and final load locations were specified in terms of the following task descriptor parameters of the NIOSH lifting equation: horizontal load location (H), vertical load location (V) and angle of asymmetry (A). The three parameters are graphically illustrated in Fig. 1. The four initial load locations were the combinations of two levels of vertical load location (25 cm and 35 cm) and two levels of angle of asymmetry (0◦ and 105◦ ); the horizontal load location was fixed at 30 cm. The four final load locations were the combinations of two levels of horizontal load location (48 cm and 60 cm) and two levels of vertical load location (93 cm and 144 cm); the angle of asymmetry was fixed at 0◦ . The initial load locations were created using wooden slabs. A movable and height-adjustable shelf was utilized to create the final load locations (Fig. 2). The 16

lifting task conditions are described in Table 2. For all the 16 lifting task conditions, the coupling quality parameter (C) of the revised NIOSH lifting equation was ‘good’ and the lifting frequency parameter (F) was ‘infrequent’ (< 0.1 lift per minute) with the work duration of less than 1 hour. For each of the 16 task conditions, the vertical load travel distance parameter (D) was determined as the difference between the initial and final vertical load locations (Table 2). See Waters et al. [19] for detailed descriptions of the task descriptor parameters. For each of the 16 lifting conditions, participants performed two lifting motions: one with the box weighing the RWL computed for the origin of lift (RWLORG ) and the other with the box weighing the RWL computed for the destination of lift (RWLDST ). Thus, each participant performed a total of 32 lifting trials. Participants used self-selected, free-style lifting techniques. The order of the 32 trials was randomized for each participant. The 32 trials were assigned on the same day with a sufficient rest period (typically, a few to several minutes) between each trial. 2.3. Data collection and processing A 12-camera VICON motion capture system was utilized to record the lifting motions performed by the participants. Prior to the motion capture session, optical markers (14 mm diameter) were placed on a set of body anatomical landmarks for each participant. The marker placement scheme was based on the VICON motion capture system’s Plug in Gait Model [23]. The excess fat tissues in the body of the severely obese participants caused some difficulties during marker placement and motion recording: first, due to the excess body fat in the abdominal area, palpat-

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Table 2 The sixteen lifting task conditions considered in the study Lifting task condition

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Horizontal load location (cm) 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30

Origin Vertical Angle of load asymmetry location (degrees) (cm) 25 0 25 0 25 105 25 105 35 0 35 0 35 105 35 105 25 0 25 0 25 105 25 105 35 0 35 0 35 105 35 105

RWL (kg)

12.5 12.1 8.3 8.3 13.1 12.6 8.7 8.4 12.5 12.1 8.3 8.6 13.1 12.6 8.7 8.4

ing the correct Right Anterior Superior Iliac (RASI) and Left Anterior Superior Iliac (LASI) locations in normal standing stance was difficult for most of the severely obese participants. This was a problem because estimating the hip joint positions requires the RASI and LASI locations. Second, for some (3 out of 20) severely obese participants, the excess abdominal fat tissues hid the RASI and LASI markers during motion recording. The other body landmarks near the elbow, shoulder, knee and ankle joints did not present problems during marker placement and motion recording. To overcome the first problem, the following measure was taken: before placing markers, the experimenter asked the severely obese participants to lie on their stomach on a flat surface. This procedure enabled us to correctly palpate the RASI and LASI locations by flattening the abdominal fat tissues and revealing the anterior sacral bony landmarks. To solve the second problem, for the three severely obese participants who presented the hidden marker problem, the RASI and LASI markers were moved laterally from the corresponding landmark locations by an equal amount along the anterior sacral axis (while maintaining the coronal orientation of the pelvis) until they became visible by the cameras. The RASI and LASI markers pertain to the estimation of the hip joint center positions and the lateral shift of these markers did not affect the hip joint center position estimations because this change does not affect the construction of the pelvic plane (a planed defined by the anterior and posterior sacral markers), which is part of the hip joint center position estimation procedure [23].

Horizontal load location (cm) 48 48 48 48 48 48 48 48 60 60 60 60 60 60 60 60

Destination Vertical Angle of load asymmetry location (degrees) (cm) 93 0 144 0 93 0 144 0 93 0 144 0 93 0 144 0 93 0 144 0 93 0 144 0 93 0 144 0 93 0 144 0

RWL (kg)

8.8 7.1 8.8 7.1 8.9 7.2 8.9 7.2 7.2 5.7 7.2 5.7 7.1 5.7 7.1 5.7

Vertical load travel distance (cm) 68 119 68 119 58 109 58 109 68 119 68 119 58 109 58 109

For each lifting motion, the position-time trajectories of the optical markers were recorded at a sampling frequency of 120 Hz. These trajectories were then filtered using Woltering’s cross validatory quintic spline routine to interpolate gaps and provide smoothing. The marker position-time data were used to estimate the position-time trajectories of the ankle, knee, hip, shoulder and elbow joint centers and the hand center based on the VICON motion capture system’s Plug in Gait Model [23]. For each participant and each lifting condition, a three-dimensional static low back biomechanical model developed by Chaffin [24–26] was used to estimate the L5/S1 disc compression forces at the origin and destination of lift; the body segment parameters recommended by Chaffin et al. [26] were utilized to estimate the body segment masses and center of mass locations. Initially, more complex surface electromyography (EMG)-driven low back biomechanical models were considered but were not employed as surface EMG data may not be reliably measured from severely obese individuals. Kuiken et al. [27] demonstrated that subcutaneous fat significantly reduces surface EMG signal amplitude and increases EMG crosstalk at nearby surface recording sites. The static low back biomechanical model has been validated using electromyography based estimates of muscle reactions during controlled torso exertions [28]. For the origin, the RWLORG and the posture at the origin of the lifting motion with the RWLORG were used as the input data to the model. For the destination, the RWLDST

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(a)

(b)

Fig. 3. Box-whisker plot summaries of the L5/S1 disc compressive forces of the severely obese participants. (a) At the origin of lift; (b) At the destination of lift.

and the posture at the destination of the lifting motion with the RWLDST were used. For both computations, the height and weight of the performer were used as the input data to the model. 2.4. Statistical analysis For each task condition × load location (origin or destination) combination, the L5/S1 compression forces of lifting trials of the severely obese participant group were summarized using a box-whisker plot. The

individual L5/S1 compression forces were examined to determine the percentage of lifting trials that resulted in violating the 3400N NIOSH action limit (L5/S1 disc compression > 3400N) for each participant group. For each task condition × load location (origin or destination) combination, the mean L5/S1 disc compression forces of the two participant groups were computed. They were visually summarized in multiple bar graphs; also, a t-test (α = 0.05) was conducted to determine if the severely obese and normal weight groups significantly differed in group mean L5/S1 disc

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(a)

(b)

Fig. 4. The group mean L5/S1 disc compression forces of the normal weight and severely obese groups. (a) At the origin of lift; (b) At the destination of lift.

compression force. The t-tests were to determine the statistical significance of the severe obesity effect on lifting-related low back biomechanical stress. For each t-test, t-test assumptions were validated for normality, homoscedasticity and independence. The SAS software program was used to perform the t-tests.

3. Results Box plot summaries of the L5/S1 disc compression forces of the severely obese participants are provided in Figs 3a and 3b. Figure 3a is for the origin of lift and Fig. 3b, for the destination. Most of the L5/S1 disc compression forces computed for the severely obese participants were greater than 3400N at both the origin and destination – only three out of the 640 (640 = 20 severely obese participants × 16 lifting conditions × 2 load locations) estimated compression forces were smaller than 3400N. The compression forces ranged from 3078N to 7787N at the origin of lift (Fig. 3a) and from 3408N to 8417N at the destination (Fig. 3b). For all the 16 lifting task conditions and both at the origin and the destination, all of the individual L5/S1 disc compression forces computed for the nor-

mal weight participants were found to be less than 3400N. The group mean L5/S1 disc compression forces of the two participant groups are presented for each of the 16 lifting task conditions in Figs 4a and 4b. Figure 4a is for the origin of lift and Fig. 4b, for the destination. Across all of the 16 lifting task conditions and at both the origin and the destination, the mean L5/S1 disc compression force of the severely obese group was significantly greater than that of the nonobese (p < 0.0001). The group mean differences (severely obese – nonobese) at the origin of lift ranged from 1183N to 1858N (Fig. 4a); at the destination, from 1584N to 2473N (Fig. 4b). The overall means of L5/S1 disc compression forces of the severely obese and normal weight groups were 4394.7N and 2866.1N, respectively, at the origin of lift; at the destination, they were 4128.6N and 2181.6 N, respectively.

4. Discussion It was found that the L5/S1 disc compression forces estimated for the severely obese participants ranged roughly from 3000N to 8500N; most of the estimates

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(99.5%) exceeded the 3400N action limit, and many, by a large margin as shown in Figs 3a and 3b. In contrast, for the normal weight group, none of the individual L5/S1 disc compression forces exceeded the 3400N NIOSH action limit. The above results are not very surprising when considering the intention behind the revised NIOSH lifting equation. The NIOSH lifting equation was designed to determine RWLs that produce low back compression forces less than 3400N for healthy workers [22]. Healthy workers, according to the NIOSH, are workers without any adverse health conditions. The nonobese participants were within the normal or ‘healthy’ range of BMI (18.5 kg/m2 < BMI < 24.9 kg/m2 ). Severe obesity is generally regarded as a serious health concern. Thus, the above results seem to indicate that: 1) the NIOSH lifting equation accomplishes what it intends to accomplish for workers within the healthy weight range and 2) the severely obese population is not within the coverage of the equation. The effect of severe obesity on low back biomechanical stress was found to be practically significant. Across the 16 lifting task conditions, the group mean differences (severely obese – normal weight) at the origin of lift ranged from 1183N to 1858N (Fig. 4a); at the destination, from 1584N to 2473N (Fig. 4b); the differences were all statistically significant (p < 0.0001). The severe obesity-associated increases in low back spine compression are thought to be primarily due to extra body (fat) masses that act as additional loads during lifting. It is possible that severely obese individuals adopted unique lifting postures or techniques; however, determining the existence of such unique lifting postures/techniques and quantifying their effects were beyond the scope of the current research study. The results of the current study may be construed as an indication that severely obese individuals are at an increased risk of lifting-related LBP compared with normal weight counterparts even when lifting moderate load weights, such as NIOSH RWLs. However, some care is required in interpreting the results. The 3400N criterion was based on previous cadaver spine studies [29] and very few of the spine specimens used in these studies were obtained from severely obese individuals. While the previous studies found that spinal compressive strengths ranged roughly from 2000N to 8000N for the specimens examined, it is not certain whether or not this range adequately represents the strength distribution for severely obese individuals. Some questions arise:

– Does the severely obese population differ from the general population in the distribution of low back spinal compression strength? If yes, what is the difference? – Is the 3400N criterion appropriate for effectively protecting severely obese workers? If not, what is the optimal cut-off point? Addressing the above questions obviously requires a large set of low back spine strength data empirically collected from severely obese individuals. Unfortunately, to the authors’ knowledge, such data is currently scant. The above questions, however, may be considered in light of the knowledge and findings from previous research on spine, bone and obesity. Some inferences may be drawn from them. Spinal compression strength is known to be affected by spine geometry and material property. Multiple studies [30–35] showed that the compression strength of human thoracolumbar vertebrae increases in proportion to endplate size as well as bone mineral density (BMD)/bone mineral content (BMC). In terms of end-plate area, severe obesity or largeness in general does not seem to provide advantages. Seidl et al. [36] examined computed tomography (CT) scans of lumbar spinal units of 53 male donors and determined the sizes of the cross-sectional areas of endplates L3-L5. The study further investigated possible relationships between endplate areas and selected anthropometric dimensions, such as body mass, body height, shoulder height, elbow height and diameters of ankle, knee, elbow and wrist. The result was that the endplate area cannot be accurately predicted by anthropometric parameters. In particular, body mass was found to have no relationship with endplate area, implying that severe obesity does not guarantee large endplate areas – in fact, in the dataset of Seidl et al. [36], the lightest male (52.3 kg) had even a somewhat larger inferior endplate of L4 than the heaviest one (135 kg). Many previous studies on bone health issues, including osteopenia and osteoporosis, have examined the relationship between largeness (high BMI) and BMD/BMC [37–43]. Nonetheless, however, the relationship has not been clearly elucidated. While some studies indicated that high BMI individuals have high BMD/BMC, and therefore, are protected from osteoporosis [37,38], other studies, especially more recent ones, suggested that obesity (BMI>30) is associated with low BMD/BMC and actually interferes with bone health. Pollock et al. [39] reported that excess fat mass may potentially compromise the health of adolescent bone. Greco et al. [40] examined vertebral BMD in

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398 overweight and obese patients and found that overweight was neutral or protective for BMD whereas obesity was associated with low bone mass compatible with a diagnosis of osteoporosis. Bhupathiraju et al. [41] reported that higher abdominal body fat is associated with lower BMD and poorer bone health in a Puerto Rican sample. Katzmarzyk et al. [42] found that BMD was inversely related to abdominal visceral adipose tissue and subcutaneous adipose tissue in white and African American adults. Kim et al. [43] showed that fat mass was inversely related to BMC in Korean men and women. One may think that severe obesity should increase BMC and BMD on the basis of the well-known Wolff’s law [44]. The Wolff’s law states that bone in a healthy person will adapt to the loads it is placed under through remodeling; if loading on a particular bone increases, the bone will remodel itself over time, increase bone density and strength in both the cortical and cancellous (trabecular) portions in order to resist the type of loading. The Wolff’s law predicts that severe obesity would increase BMC/BMD of the spine bones as the vertebral bodies are constantly under the additional loadings from extra body mass. According to McGill [45], upon spinal compression, the cortical portion of a vertebral body remains rigid but the nucleus of the disc pressurizes and causes the endplates of the vertebrae to bulge inward to compress the cancellous bone. The cancellous bone normally fails first and usually is the determinant of failure tolerance of the spine. Thus, an increase in the density and strength of the cancellous bone through mechanical loading and bone remodeling is expected to increase the overall low back spinal compression strength. However, a caution must be exercised in linking severe obesity to possible benefits in bone density and strength. Severely obese individuals may be on average physically less active than the normal weight counterparts. Therefore, it is not clear if severely obese individuals are indeed subjected to higher levels of mechanical loadings than normal weight persons during daily and occupational activities. In addition to endplates size and vertebral BMC/ BMD, the health of intervertebral discs also influences the spinal compression strength. Of note is that obesity may adversely affect intervertebral discs. Spinal mobility generally decreases with increasing body weight [46]. Decreased spinal mobility is known to interfere with disc nutrition. Moreover, obese individuals are likely to have dyslipidemia, which plays a major role in the development of atherosclerosis [47].

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Atherosclerosis could cause malnutrition of the disc cells [48,49], which may lead to disc degeneration. Obesity has been shown to be associated with disc degeneration [50]. People with severe disc degeneration are more likely to have low back pain [51]. Overall, the previous research on spine, bone and obesity suggests that severe obesity may not necessarily increase low back spinal compression strength; in fact, some of the previous research studies suggest that severe obesity may compromise spinal strength by adversely affecting the health of bones and discs. While new empirical data is needed to confirm the possible adverse effects of severe obesity on low back spinal strength, it is clear that no sufficient evidence currently exists that supports using a cut-off point greater than 3400N for the severely obese population. In light of this, the biomechanical analyses results of the current study (Figs 3a, 3b, 5a and 5b) are thought to suggest that severely obese individuals are likely at an increased risk of lifting-related LBP even when lifting moderate load weights, such as NIOSH RWLs. The current global prevalence of largeness, henceforth, may be hindering our efforts for controlling liftingrelated LBP. Future LBP prevention research and practices may need to consider this possibility. The current research results have some practical and research implications: first, they indicate that many severely obese individuals are likely to be at an increased risk of lifting-related LBP, even when performing infrequent lifts of moderate load weights, such as NIOSH RWLs. Since severely obese individuals need to perform lifting tasks during both occupational and daily life activities, methods and/or interventions for protecting them are needed. Some possibilities include: developing new lifting task design tools or guidelines similar to the current NIOSH lifting equation, identifying safer lifting techniques and creating new mechanical lifting aids specifically for severely obese workers. Second, lifting of heavy loads could be detrimental to severely obese individuals as even moderate load weights, such as NIOSH RWLs, were found to produce unsafe low back compression forces. Third, repetitive load lifting may be very hazardous to severely obese individuals even with light loads. Repeated loading lowers tissue failure tolerance [45] and fatigue failure occurs with fewer repetitions when the applied load is closer to the tissue strength [52]. Severely obese individuals seem to experience high loadings from the body fat mass alone, and thus, repetitive load lifting even with light load weights may easily cause fatigue failure. Similarly, sustained load holding

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may be hazardous to severely obese workers. Fourth, weight loss may help reduce lifting-related LBP risks for severely obese workers. Of course, how different ways of weight loss affect the mechanical properties of the spinal structure must be understood and optimal strategies for reducing fat mass while preserving bone mass need to be determined. Certain weight loss methods may compromise spinal strength. For example, upper gastrointestinal surgery is known to adversely affect bone health [53–55]. Limitations of the current study and some future research directions are provided here: first, this study examined low back biomechanical stresses of severely obese individuals. Future studies are needed to investigate low back biomechanical stresses associated with the other largeness levels, that is, overweight and moderate obesity, so as to provide a complete understanding as to the largeness impacts on lifting-related LBP risks. Second, the current study used a static low back biomechanical model to quantify low back biomechanical stresses of severely obese workers. In general, static biomechanical analyses are known to underestimate biomechanical stresses compared with dynamic analyses – Leskinen [56] reported that dynamic peak compressions were 33 ∼ 60% higher than static depending on lifting technique; de Looze et al. [57] showed that during fast lifting, the peak dynamic force was 42% higher than the static one. Dynamic analyses are expected to show the severe obesity impacts and associated risks more vividly. Third, in estimating body segment masses and segmental center of mass locations for computing L5/S1 disc compression forces, this study utilized the body segment parameters recommended by Chaffin et al. [26]. The body segment parameters used pertain to the general population rather than severely obese individuals. Therefore, their usage inevitably results in some errors – disc compression forces may be underestimated for apple-shaped severely obese individuals and be overestimated for pear-shaped ones. Currently, very few studies seem to provide the body segment parameters values for severely obese individuals with different somatotypes – the authors are not aware of any. Future studies are needed to provide such data. Fourth, in vitro studies that examine the spinal strengths of large individuals are needed to identify the spinal strength distribution of the population. Finally, this study employed only the biomechanical approach in considering the lifting-related LBP risks of severely obese individuals. Studies based on the epidemiological, physiological and psychophysical approaches are needed

to fully understand the severe obesity impacts. Also, more medically oriented studies may be needed along with those based on the traditional ergonomics research methods. Karppinen [58] suggested that obesity may cause LBP through systemic chronic inflammation; it is associated with increased production of cytokines and acute-phase reactants and with activation of pro-inflammatory path ways, which are known to cause pain.

5. Conclusion This study quantitatively evaluated low back biomechanical stresses of severely obese (BMI  35 kg/m2 ) workers during manual lifts of moderate load weights (NIOSH RWLs). The L5/S1 disc compression forces estimated for the severely obese participants ranged from 3000N to 8500N and many exceeded the 3400N NIOSH action limit by large margins. In light of previous research on spine, bone and obesity, the study results seem to suggest that severely obese individuals are likely at an increased risk of lifting-related low back pain compared with normal weight individuals.

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