Revised NIOSH Lifting Equation May generate spine loads exceeding recommended limits

International Journal of Industrial Ergonomics 47 (2015) 1e8

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Revised NIOSH Lifting Equation May generate spine loads exceeding recommended limits
Plamondon c, 3, Navid Arjmand a, *, Mohammad Amini a, 1, Aboulfazl Shirazi-Adl b, 2, Andre Mohammad Parnianpour a, 4 a b c

Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran
Division of Applied Mechanics, Department of Mechanical Engineering, Ecole Polytechnique, Montr
eal, Qu
ebec, Canada Institut de recherche Robert Sauv
e en sant
e et en s
ecurit
e du travail, Montr
eal, Qu
ebec, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2014 Received in revised form 5 September 2014 Accepted 21 September 2014 Available online

The 1991 NIOSH Lifting Equation (NLE) is widely used to assess the risk of injury to spine by providing estimates of the recommended weight limit (RWL) in hands. The present study uses the predictive equations developed based on a detailed trunk musculoskeletal biomechanical model to verify whether the RWL generates L5-S1 loads within the limits (e.g., 3400 N for compression recommended by NIOSH and 1000 N for shear recommended in some studies). Fifty lifting activities are simulated here to evaluate the RWL by the NLE and the L5-S1 loads by the predictive equations. In lifting activities involving moderate to large forward trunk flexion, the estimated RWL generates L5-S1 spine loads exceeding the recommended limits. The NIOSH vertical multiplier is the likely cause of this inadequacy; a revised multiplier accounting for the trunk flexion angle is hence needed. The use of a fixed 3400 N compression limit is also questioned. Relevance to industry: Ergonomist and occupational health practitioners are advised to use the NLE along with the predictive equations of the spine loads proposed in this study when evaluating the risk of injury to the spine in lifting activities particularly those involving moderate to large trunk flexion. © 2015 Elsevier B.V. All rights reserved.

Keywords: NIOSH lifting equation Biomechanical model Spine loads Multiplier Recommended weight limit

1. Introduction Epidemiological studies have identified manual material handling (MMH) and lifting activities as risk factors for low back pain (LBP) (Garg and Moore, 1992; Hoogendoorn et al., 2000; Manchikanti, 2000; Van Nieuwenhuyse et al., 2004). Both modeling studies, by predicting increased compression, shear and moment loads on the intervertebral discs (Arjmand and ShiraziAdl, 2006), and in vivo investigations, by measuring higher intradiscal pressures (Nachemson, 1981; Wilke et al., 2001), corroborate this association between LBP and MMH. In order to

* Corresponding author. Sharif University of Technology, Tehran, 11155-9567, Iran. Tel.: þ98 21 6616 5684; fax: þ98 21 6600 0021. E-mail addresses: [email protected] (N. Arjmand), mohammad.amini89@ gmail.com (M. Amini), [email protected] (A. Shirazi-Adl), plamondon. [email protected] (A. Plamondon), [email protected] (M. Parnianpour). 1 Tel.: þ98 912 299 7824. 2 Tel.: þ1 514 340 4711x4129; fax: þ1 514 340 4176. 3 Tel.: þ1 514 288 1551x279; fax: þ1 514 288 6097. 4 Tel.: þ98 21 6616 5525; fax: þ98 21 6600 0021. http://dx.doi.org/10.1016/j.ergon.2014.09.010 0169-8141/© 2015 Elsevier B.V. All rights reserved.

manage the increasing rate of work-related LBP, the National Institute for Occupational Safety and Health (NIOSH) has proposed the NIOSH Lifting Equation (NLE) (Waters et al., 1993). For a given manual lifting activity, the NLE estimates the recommended weight limit (RWL) that almost all healthy workers (90% males and 75% females) may handle without an increased risk of LBP. The lifting index (LI) is subsequently defined as the ratio of the load lifted by the worker in the workplace to the RWL computed by the NLE. In the NLE, the RWL (in kg) is estimated by modulating a weight constant (23 kg) by six task-related multipliers varying between 0 and 1; RWL ¼ 23 kg  HM  VM  AM  DM  CM  FM where HM, VM, AM, DM, CM, and FM are respectively, horizontal, vertical, asymmetry, distance, coupling, and frequency multipliers that, in turn, are respectively computed based on the horizontal (H) and vertical (V) positions of the handled load with respect to the interfeet midpoint, handled load asymmetry to the body's midesagittal plane (A), vertical travel distance of the lift (D), hand to load coupling (C), and frequency of lift (F). Three distinct criteria are considered when estimating the NLE multipliers (e.g., relationship

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between HM and H): physiological (limiting maximum energy expenditure to 2.2e4.7 kcal/min), psychophysical (limiting weight magnitude to a value acceptable to 75% of female workers), and biomechanical (limiting L5-S1 compressive force to 3400 N) (Waters et al., 1993). The NLE is widely used worldwide by occupational health practitioners to assess the risk of LBP (Waters et al., 1998). A webbased survey in Canada indicates that most of the certified ergonomists use the NLE in industry to identify and prevent work-related musculoskeletal disorders (Pascual and Naqvi, 2008). The adequacy of the NLE in controlling biomechanical spine loads during lifting activities, however, remains unknown. That is, the RWL estimated by the NLE for a specific lifting activity may indeed generate L5-S1 compressive force greater than the recommended limit of 3400 N. As the 1991 NIOSH committee used very simplified biomechanical

models for the estimation of the L5-S1 compressive loads (Waters et al., 1993) and since biomechanical models have thus far evolved significantly, it is important to critically re-evaluate the biomechanical accuracy of the NLE in determining the RWL. The NIOSH committee has considered the L5-S1 compressive force as the only critical stress factor when determining the RWL while the posterior-anterior shear forces on the lumbar intervertebral discs during lifting activities likely also act as a risk factor for LBP (Norman et al., 1998). Although biomechanical models generally predict much lower shear forces when compared to compressive forces (Arjmand and Shirazi-Adl, 2006; Arjmand et al., 2006) the spine strength is however also much lower in shear. While yield and peak thresholds of about 1500 N and 3000 N have respectively been measured in vitro in lumbar cadaver motion segments (Skrzypiec et al., 2012) maximum permissible limit of 1000 N is

Table 1 Fifty lifting tasks in upright standing and flexed postures with load held at different horizontal (H) and vertical (V) positions relative to the inter-feet midpoint (tasks 1e2: load holding in upright posture close to and away from the chest (see Fig. 1a), tasks 3e6: load holding in upright posture at different vertical but identical horizontal distances (see Fig. 1b), and tasks 7e50: load holding at different flexed postures and horizontal distances (see Fig. 1c for an example)). Corresponding position of the hand load to the L5-S1 joint (DL5-S1), trunk flexion angle (T), and pelvis flexion angle (P) measured in vivo are also given. Predicted NIOSH horizontal (HM) and vertical (VM) multipliers to estimate the recommended weight limit (RWL) and the L5-S1 compressive and shear forces with the RWL in hands for each tasks are estimated. Task

V (cm)

H (cm)

DL5-S1 (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

130 130 90 120 150 180 79.5

12 40 23

25 55 30

25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50

35.6 40.6 50.6 60.6 37.7 42.7 52.7 62.7 39.8 44.8 54.8 64.8 42.2 47.2 57.2 67.2 45.0 50.0 60.0 70.0 47.1 52.1 62.1 72.1 48.5 53.5 63.5 73.5 49.9 54.9 64.9 74.9 51.2 56.2 66.2 76.2 50.7 55.7 65.7 75.7 49.4 54.4 64.4 74.4

77.9

75.3

71.4

66.1

60.6

53.7

47.1

40.7

32.9

27.7

T (deg)

P (deg)

0

0

0

0

10

2.6

20

6.9

30

9.5

40

12.4

50

14.4

60

16.8

70

22.2

80

28.3

90

35

100

41

107.6

46

HM

VM

RWL (kg)

Compressive load (N)

Shear load (N)

1.000 0.625 1.000 1.000 1.000 1.000 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500 1.000 0.833 0.625 0.500

0.835 0.835 0.955 0.865 0.775 0.685 0.987

19.2 12.0 22.0 19.9 17.8 15.8 22.7 18.9 14.2 11.3 22.8 19.0 14.2 11.4 23.0 19.1 14.4 11.5 22.8 19.0 14.2 11.4 22.4 18.7 14.0 11.2 22.0 18.3 13.8 11.0 21.5 17.9 13.5 10.8 21.1 17.6 13.2 10.5 20.6 17.2 12.9 10.3 20.1 16.7 12.6 10.0 19.7 16.4 12.3 9.9

1196 1858 1446 1401 1346 1282 3144 2985 2835 2794 3535 3379 3233 3195 3801 3651 3509 3468 4046 3896 3750 3704 4265 4106 3947 3890 4430 4264 4091 4022 4565 4389 4203 4125 4677 4487 4282 4192 4752 4545 4317 4212 4671 4458 4220 4106 4549 4335 4094 3976

705 963 920 845 771 699 1197 1142 1092 1081 1278 1222 1171 1159 1329 1280 1238 1230 1368 1324 1285 1279 1384 1342 1306 1299 1379 1338 1301 1294 1396 1352 1311 1301 1414 1366 1320 1306 1421 1369 1316 1298 1356 1303 1249 1230 1284 1233 1180 1160

0.991

0.999

0.989

0.973

0.957

0.936

0.916

0.897

0.874

0.858

N. Arjmand et al. / International Journal of Industrial Ergonomics 47 (2015) 1e8

recommended in some studies for spine shear forces in single exertions (Gallagher and Marras, 2012; McGill, 1997; McGill et al., 1998). Therefore, for a given lifting activity even if the RWL proposed by the NLE adequately controls the in vivo L5-S1 compressive force, it is not clear whether the shear forces remain bounded within equally safe limits. This study, hence, aims to investigate the adequacy of the NLE in controlling in vivo biomechanical loads of the spine during infrequent (FM ¼ 1) sagittally-symmetric (AM ¼ 1) manual handling activities with good hand to load coupling (CM ¼ 1) and small vertical travel distance of the lift (DM ¼ 1). For this purpose, a number of in vivo symmetric lifting activities in upright and flexed postures are used to evaluate the horizontal (H) and vertical (V) position of the handled load needed both to compute HM and VM multipliers for the calculation of RWL and to determine body posture required for biomechanical analysis of spine loads. This RWL is subsequently considered as input into our predictive equations of the spine loads (Arjmand et al., 2011) under identical lifting posture measured in vivo. The predictive equations used to estimate compressive and shear loads on the L5-S1 intervertebral disc during lifting activities are accurate easy-to-use tools developed based on a validated finite element musculoskeletal biomechanical model of the spine (Arjmand and Shirazi-Adl, 2006; Arjmand et al., 2007, 2009, 2010). It is hypothesized that under some RWLs the spinal loads likely exceed the recommended safe compression and shear force levels.

2. Materials and methods

3

trunk flexion with a stoop lift, i.e., knee straight and lumbar flexed) at different horizontal distances were studied (Table 1 and Fig. 1). Kinematics data from our previous in vivo studies (Arjmand et al., 2009, 2010) collected on a healthy male subject (52 years, 174.5 cm, and 68.4 kg) under these lifting tasks were used to determine the horizontal (H) and vertical (V) position of the handled load with respect to the inter-feet midpoint needed in the NLE for the estimation of the RWL. For these tasks, corresponding body posture including sagittal trunk (T) and pelvis (P) rotations with respect to the upright posture as well as horizontal position (DL5-S1) of the handled load with respect to the L5-S1 needed for the estimation of spinal loads using the predictive equations were also determined. Kinematics was measured using 12 clusters; one on each foot, thigh, upper arm and forearm in addition to one on the pelvis, T12, C7, head, and handled load. Four light-emitting diodes (LEDs) markers were glued on each cluster (except for the feet with seven LEDs) for a total of 54 LEDs. Positions of the markers were measured in three dimensions at a sampling rate of 30 Hz using a four-camera Optotrak system (Northern Digital, Waterloo ON, Canada). The study was limited to the simulation of infrequent lifting tasks performed symmetrically in the sagittal plane, at relatively slow movement speed. Lifting tasks in the upright posture with load held at two different horizontal distances (tasks 1 and 2 in Table 1), in the upright posture with load held at four different vertical positions (tasks 3 through 6 in Table 1) and in flexed postures (Tasks 7 to 50 in Table 1) were performed with respectively 19.8, 10.4, and 0 kg in hands in our previous in vivo studies (Arjmand et al., 2009, 2010). Weight in hands were comfortably grasped via handles.

2.1. In vivo study 2.2. Recommended weight limit (RWL) Fifty sagittally-symmetric manual lifting activities while holding a weight in upright posture at different horizontal and vertical distances as well as in different flexed postures (from 10 to full

For each lifting task, based on the measured horizontal (H) and vertical (V) distance of the hand load with respect to the midpoint

Fig. 1. A schematic presentation of the tasks performed by the subject in the in vivo study to determine position of load and body posture: (a) holding symmetrically a box of 19.8 kg close to and far away from the chest (b) holding symmetrically a bar of 10.4 kg at four different heights (90, 120, 150, and 180 cm with respect to the ground) while preserving its horizontal moment arm at 30 cm with respect to the S1, and (c) an example of tasks performed with forward trunk flexion (T ¼ 40 ).

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N. Arjmand et al. / International Journal of Industrial Ergonomics 47 (2015) 1e8

between the feet (Table 1), the horizontal (HM) and vertical (VM) multipliers were calculated using the NIOSH proposed relationships between H (in cm) and HM, i.e. HM ¼ 25/H (except for H < 25 cm where HM ¼ 1), as well as V (in cm) and VM, i.e. VM ¼ 1(0.003  jV75j) (Waters et al., 1993). Other NIOSH multipliers including asymmetry (AM), distance (DM), coupling (CM), and frequency (FM) multipliers were taken as their reference values (AM ¼ DM ¼ CM ¼ FM ¼ 1).

(Table 1). In practical application, one needs to only measure trunk rotation while the LP ratio is estimated based on the available database in the literature (Arjmand et al., 2011; Tafazzol et al., 2014). To determine load distance to shoulder joint (DS) the following equations developed based on our model posture prediction were used for the tasks in upright and flexed postures, respectively (Arjmand et al., 2011):

2.3. Predictive equations for biomechanical analysis of spine loads

DS ðFlexedÞ ¼ DL5S1 þ 6:0682  0:8205  T þ 1:301  LP

Predictive equations are simple yet accurate user-friendly tools that relate outputs (spine compressive and shear loads) of a complex detailed trunk finite element biomechanical model of the spine to its input variables during static lifting activities (Arjmand et al., 2011). Sagittal trunk flexion angle (T), lumbopelvic ratio (LP), mass (M) of the handled load and its horizontal distance (DS) with respect to the shoulder joint are the model inputs. These equations are developed to serve ergonomists and occupational health practitioners in management of low back disorders in estimation of spinal loads and design of workplace. Validity of the predictive equations has been verified (Arjmand et al., 2011) by comparing their predictions for spine compressive loads under a number of lifting activities with the measured intradiscal pressure values under identical tasks (R2 ¼ 0.99 and RMSE ¼ 0.12 MPa). As for the estimation of the L5-S1 compressive and shear forces during lifting tasks in upright and flexed postures four equations were used as follows (Arjmand et al., 2011):

Cupright ¼ 407:143 þ 9:088  M  1:457  DS  1:133  M2 þ 0:106  Ds 2 þ 1:657  MDS (1) Supright ¼ 93:797  6:621  M þ 2:093  DS þ 0:097  M2 þ 0:009  Ds 2 þ 1:067  MDS (2) Cflexed ¼ 56:435 þ 56:584  T þ 21:6  M  214:667  LP þ 10:247  DS  0:332  T2 þ 0:238  M2 þ 50:532  ðLPÞ2 þ 0:027  Ds 2 þ 0:764  TM  0:745  TðLPÞ  0:053  TDS  3:823  MðLPÞ þ 2:412  MDS  0:923  ðLPÞDS

DS ðUprightÞ ¼ DL5S1 þ 5:271 þ 0:0229  M

þ 0:0035  T2  0:2551  LP2 þ 0:007  T  LP where DL5-S1 is load distance to the L5-S1 joint centre in cm as given in Table 1. For each of fifty lifting activities the magnitude of T, M (equal to the estimated RWL by the NLE), LP, and DL5-S1 as given in Table 1 were replaced in the appropriate predictive equations and the L5-S1 spine loads were estimated. Subsequently, the maximum weight in hands (M3400) that results in the L5-S1 compression force of 3400 N (calculated by trial-and-error using the predictive equations) was compared to the RWL for each task. 3. Results The NLE adequately controlled the L5-S1 spinal loads during lifting activities in upright standing posture by yielding RWLs that correspond to compressive loads far smaller than the NIOSH action limit of 3400 N and shear loads smaller than 1000 N (Table 1: tasks 1 through 6). The NLE is therefore very conservative (Cupright < 2000 N) for these activities as far as the L5-S1 compressive load is concerned (Table 1). Also, the NLE adequately controlled the L5-S1 compressive loads to remain lower than 3400 N during almost all lifting tasks involving small trunk flexion angles (T < 30 ) (tasks 7 through 14 with the exception of task 11 in Table 1). For all lifting tasks with trunk flexion angles larger than 30 (tasks 15 through 50), however, the proposed RWL of the NLE generated L5-S1 compressive loads up to 40% beyond the NIOSH limit of 3400 N. Also, for all lifting tasks in flexed postures the predicted L5-S1 shear load with the RWL in hands exceeded 1000 N (Table 1). Comparison of the maximum weight in hands (M3400) that results in the L5-S1 compression limit of 3400 N to the RWL for lifting tasks indicated greater RWL in lifting activities involving trunk flexion angles larger than 30 (i.e., RWL/M3400 > 1 and in some cases close or even larger than 2) (Table 2). 4. Discussion

(3) Sflexed ¼ 14:908 þ 23:810  T þ 12:077  M  146:166  LP þ 3:385  DS  0:146  T2 þ 0:091  M2 þ 49:924  ðLPÞ2 þ 0:013  Ds 2 þ 0:195  TM  2:113  TðLPÞ  0:015  TDS  3:034  MðLPÞ þ 0:936  MDS  0:273  ðLPÞDS (4) where Cupright, Supright, Cflexed, Sflexed are the L5-S1 compressive and shear loads (N) in upright and flexed postures, respectively. T, M, and DS are in degree, kg, and cm, respectively and the spine loads are calculated in N. The lumbopelvic ratio is calculated as the lumbar rotation divided by the pelvis rotation where the former is equal to the algebraic difference between measured trunk (T) and pelvis (P) rotations for the lifting activity under consideration

Accurate yet easy-to-use predictive equations of the spine loads were used to verify whether or not the recommended weight limit (RWL) estimated by the NIOSH Lifting Equation (NLE) for a wide range of lifting activities would result in L5-S1 compressive loads in agreement with its recommended action limit of 3400 N. Moreover, the L5-S1 shear forces under these lifting activities were also estimated by the predictive equations and compared to the threshold of 1000 N recommended in some studies in the literature. Results indicated that while the RWL estimated by the NLE was conservative (L5-S1 compressive loads < 2000 N) for lifting tasks performed in the upright standing posture and those involving small trunk flexion angles (T < 30 where the L5-S1 compressive loads were generally < 3400 N) it was too large for tasks involving moderate to large trunk flexion (T  30 ). Furthermore, while the RWL estimated by the NLE was adequate (L5-S1 shear loads < 1000 N) for lifting tasks performed in the upright standing posture it was not so in flexed postures. Therefore, the NLE may not

N. Arjmand et al. / International Journal of Industrial Ergonomics 47 (2015) 1e8 Table 2 Recommended weight limit (RWL) of the NIOSH Lifting Equations for lifting activities in flexed postures compared to the maximum weight in hands (M3400) that results in the NIOSH L5-S1 compression force limit of 3400 N. Task

V (cm)

H (cm)

DL5-S1 (cm)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

79.5

25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50 25 30 40 50

35.6 40.6 50.6 60.6 37.7 42.7 52.7 62.7 39.8 44.8 54.8 64.8 42.2 47.2 57.2 67.2 45.0 50.0 60.0 70.0 47.1 52.1 62.1 72.1 48.5 53.5 63.5 73.5 49.9 54.9 64.9 74.9 51.2 56.2 66.2 76.2 50.7 55.7 65.7 75.7 49.4 54.4 64.4 74.4

77.9

75.3

71.4

66.1

60.6

53.7

47.1

40.7

32.9

27.7

T (deg)

P (deg)

RWL (kg)

M3400 (kg)

RWL/M3400

10

2.6

20

6.9

30

9.5

40

12.4

50

14.4

60

16.8

70

22.2

80

28.3

90

35

100

41

107.6

46

22.7 18.9 14.2 11.3 22.8 19.0 14.2 11.4 23.0 19.1 14.4 11.5 22.8 19.0 14.2 11.4 22.4 18.7 14.0 11.2 22.0 18.3 13.8 11.0 21.5 17.9 13.5 10.8 21.1 17.6 13.2 10.5 20.6 17.2 12.9 10.3 20.1 16.7 12.6 10.0 19.7 16.4 12.3 9.9

24.8 22.1 17.9 14.8 21.6 19.2 15.4 12.6 19.3 17.0 13.6 11.1 16.8 14.8 11.8 9.5 14.5 12.8 10.2 8.2 12.8 11.3 9.0 7.3 11.4 10.1 8.0 6.5 10.4 9.3 7.4 6.0 9.9 8.8 7.1 5.9 10.2 9.2 7.5 6.2 10.9 9.8 8.1 6.8

0.9 0.9 0.8 0.8 1.1 1.0 0.9 0.9 1.2 1.1 1.1 1.0 1.4 1.3 1.2 1.2 1.5 1.5 1.4 1.4 1.7 1.6 1.5 1.5 1.9 1.8 1.7 1.7 2.0 1.9 1.8 1.8 2.1 2.0 1.8 1.7 2.0 1.8 1.7 1.6 1.8 1.7 1.5 1.5

adequately control spinal loads as intended in protecting the spinal column during lifting activities in flexed postures.

4.1. Methodological issues Both our in vivo and biomechanical modeling (predictive equations) studies have some limitations. In our in vivo study to measure trunk kinematics, for a given trunk flexion angle, the pelvis rotations needed for the estimation of spine loads in predictive equations were not measured with loads in hands equal to the calculated RWL. It was therefore assumed that for a given lifting activity under a specific flexed trunk angle (T), the pelvis rotation (P) and thus the LP ratio would not have altered had the RWL been handled in these lifting tasks. It has been shown that holding 10 kg in hands can increase the LP ratio during forward bending activities by about 15% at most (Granata and Sanford, 2000). A sensitivity analysis on the effect of LP ratio on spinal loads estimated by our predictive equations showed that increasing the LP ratio at any trunk angle by 15% influences the L5-S1 compressive and shear loads by less than 1 and 5%, respectively.

5

In this study, the spinal loads were estimated through the predictive equations that are developed based on a complex detailed trunk finite element biomechanical model while considering a constant body weight (68.4 kg) and height (174.5 cm). This body weight is below that of the 50th percentile of American females (McDowell et al., 2008). Our recent detailed modeling studies (Arjmand et al., 2014a; Hajihosseinali et al., 2015) as well as those of Han et al. (2013) have indicated the crucial role of body weight on predicted spinal loads. As body weight almost doubles from 56 to 110 kg, our simulations while properly scaling musculature anatomy based on in vivo data (Anderson et al., 2011) yield increases of 20e120% in spinal loads depending on the lifting activity (Arjmand et al., 2014a; Hajihosseinali et al., 2015). Therefore, had a body weight of 84 kg and the same height of ~175 cm (corresponding to 50th percentile of North American males (McDowell et al., 2008)) for instance been considered, substantially larger spinal loads compared to those reported (Table 1) would have been predicted. The inadequacy of the NLE in controlling spine compressive load at 3400 N would in this case extend to additional lifting tasks considered in this study. On the other hand, had a smaller body weight of ~56 kg (corresponding to 25th percentile of North American females (Waters et al., 1993)) for instance been considered, lower (by 15% at most) L5-S1 compressive loads (Arjmand et al., 2014a; Hajihosseinali et al., 2015) compared to those listed in Table 1 would have been predicted. In this case, the L5-S1 compression in all tasks involving trunk flexion angle of 50 and larger (except in task 26) would exceed 3400 N. Finally, it is to be noted that the NLE is applied to all workers equally regardless of their individual body weight. Therefore, based on the representative tasks considered in this study, our conclusion that the RWL in the NLE may generate spine loads beyond recommended limits remains valid irrespective of the body weight considered. Also, in our in vivo study the subject was instructed to stretch his arms in the gravity direction while V (vertical position of the hand load with respect to the inter-feet midpoint) was measured for different trunk rotations (T ¼ 10, 20, 30, …, 100, and 107.6 ). It is important to note that the measured V and the corresponding T represent one plausible lifting posture among many others as the arms flex or knees bend. The tasks investigated in the present study represent some typical ones performed in an occupational environment. It is important to note that for a given task (i.e., for an identical trunk flexion angle, hand load magnitude, and horizontal load distance to the L5-S1) individuals may adapt alternative postures with different lumbopelvis (LP) ratios (or different pelvis rotations than those given in Table 1). As the LP ratio is an input into our predictive equations in flexed postures, its variation can affect the predicted spinal loads. Our sensitivity analyses on the effect of variations in LP ratio on the predicted L5-S1 compressive force, however, indicate that the predicted spinal loads are only slightly altered with the LP ratio. For instance, in tasks (47e50) in which the pelvic rotation is taken as 46 (Table 1), a substantial reduction by 20 (from 56 or 36 ) equivalent to a marked 115% increase in LP ratio (from 0.92 to 1.99) decreases the L5-S1 compression only by 5% at most. The predictive equations are developed based on a detailed musculoskeletal model of the spine whose predictions for the L4-L5 intradiscal pressure and back muscle forces for a wide range of lifting activities are found in agreement with in vivo measured intradiscal pressure values and qualitatively with electrographic (EMG) data (Arjmand and Shirazi-Adl, 2006; Arjmand et al., 2007, 2009, 2010; 2011). This model similar to its counterparts, however, has its own assumptions that are discussed in details elsewhere (Arjmand and Shirazi-Adl, 2006). For example, abdominal antagonistic coactivities are neglected in the model and thus in predictive equations. Also, the model is not sensitive to the vertical position of

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weights in hands in upright postures, i.e., the same spinal loads are predicted by the model as load height increases (tasks 3 through 6 in Table 1) at a constant horizontal distance to the body. This is in disagreement with in vivo studies (Arjmand et al., 2009; Granata and Orishimo, 2001) indicating that back and abdominal muscle EMG activities increase as load height increases (with a constant lever arm) which is due to the stability requirements. The predicted spine loads during these lifting activities with the corresponding RWL in hands are however well below the recommended permissible limits (Table 1). 4.2. Analysis of results Analysis of results indicated that the vertical multiplier (VM) in NLE was a primary reason for the inadequate estimation of RWL in lifting tasks with flexed postures while the horizontal multiplier (HM) properly modulated RWL. According to our predictive equations during lifting activities in flexed postures as horizontal distance of weights in hands increased from H ¼ 25 cm to H ¼ 50 cm the maximum hand load (M3400) should have decreased by ~40% in order to maintain the L5-S1 compressive load at 3400 N (Fig. 2). The horizontal multiplier (HM ¼ 25/H) also reduced from 1 to 0.5 as load moved away from H ¼ 25e50 cm thus decreasing the weight limit by about 50%. The NIOSH horizontal multiplier, hence, appropriately controlled the L5-S1 compressive load to meet the recommended limit. On the other hand, our predictive equations showed that at any given horizontal load distance, H, as the vertical distance of the weight in hands reduced from V ¼ 79.5 cm (at T ¼ 10 ) to V ¼ 47.1 cm (at T ¼ 80 ) the maximum weight (M3400) should have decreased by ~60% in order to maintain the L5-S1 compressive load at 3400 N (Fig. 3). The NIOSH vertical multiplier (VM ¼ 1(0.003  jV75j)), however, slightly reduced from ~0.987 to ~0.916 (Table 1) thereby decreasing the weight by only ~7% as load vertical distance dropped from 79.5 to 47.1 cm (or equivalently trunk flexion angle increased from 10 to 80 ). The difference between RWL and M3400 increased (i.e., inadequacy of the NLE in controlling spine loads grew) as the trunk flexion angle increased; reaching its peak at the trunk flexion angle of 90 (e.g., RWL/ M3400 ¼ 2.1) (Table 2). The 1991 NIOSH committee has acknowledged that for lifting activities near the floor the biomechanical studies suggest an increased lumbar load and physiological studies indicate an increased energy expenditure (Waters et al., 1993). However, due to the lack of direct empirical data to provide a specific reduction in RWL for lifts at floor level (V ¼ 0), the

Fig. 2. Variation of the maximum handled weight (M3400) that preserves the L5-S1 compressive load at 3400 N with the horizontal position of the weight (H) at different vertical positions of load (V).

committee diminished the RWL by 22.5% versus lifts at the waist level (V ¼ 75 cm) ((i.e., VM ¼ 1(0.003  jV75j)). Revision of the NLE by the introduction of a trunk flexion angle multiplier can hence markedly improve the estimations of RWL. The NLE does not adequately control the L5-S1 compressive load in lifting activities involving moderate to large trunk flexion as it underestimates the importance of trunk flexion angle on the spine compressive load. In agreement with our biomechanical modeling simulations (Arjmand and Shirazi-Adl, 2006; Arjmand et al., 2010), in vivo studies also point to the great impact of trunk flexion angle on spine loads by measuring ~230% increase (from 1 to 2.3 MPa) in the L4-L5 intradiscal pressure (as an indicator of the spine compressive load) as trunk flexes forward from upright to ~70 flexion with 20 kg in hands or by ~255% (from 0.45 to 1.6 MPa) as trunk flexes forward from upright to full flexion without load in hands (Wilke et al., 2001). This illustrates the importance of increasing the height of the lift (or decreasing trunk flexion) in an ergonomic intervention in order to reduce external back loading as demonstrated in a recent manual material handling study (Plamondon et al., 2012). This can also be achieved by adapting a squat lifting technique that involves relatively smaller trunk angles when compared to a stoop one. Our previous in vivo-modeling investigations have indicated that spinal compression/shear forces are smaller in squat lifts than in stoop ones (Bazrgari et al., 2008). The inadequacy in the vertical NLE could partly be due to the fact that the 1991 NIOSH committee used very simplistic biomechanical models with limited number of trunk muscles (e.g., one equivalent muscle for synergetic muscles with linear line of action and equilibrium at a single level) for the estimation of the L5-S1 compressive loads (Waters et al., 1993). The predictive equations, however, are developed based on a detailed multi-level biomechanical model in which much more accurate musculature architecture (52 trunk muscle fascicles each with its own line of action), their curved-shape (wrapping) geometry during flexion postures, and nonlinear passive properties of the ligamentous spine are incorporated (Arjmand and Shirazi-Adl, 2006; Arjmand et al., 2006, 2011). This is the most detailed biomechanical model ever used to investigate whether or not the NLE maintains spinal loads below the recommended limits. It is evident that a simplistic model such as the L5-S1 model of 3DSSPP software (University of Michigan Center for Ergonomics, 2014) likely yields spinal loads different from those listed in Table 1. In a recent study, Potvin (2014) states that RWLs from NLE yield L5-S1 compression forces close to the limit of 3400 N for lifts below knuckle and much lower compression forces for lifts at higher height. These findings are, however, based on a very simplistic biomechanical model of the spine assuming a constant moment arm of 6 cm for a

Fig. 3. Variation of the maximum handled weight (M3400) that preserves the L5-S1 compressive load at 3400 N with the vertical position of the weight (V) at different horizontal positions of weight (H).

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single extensor muscle incorporated in the model. In a recent study (Arjmand et al., 2014b; Rajaee et al., 2015), we have shown that different lifting analysis tools such as AnyBody Modeling System (Damsgaard et al., 2006), 3DSSPP (University of Michigan Center for Ergonomics, 2014), our predictive equations, HCBCF equation (Merryweather et al., 2009), and simple polynomial model (McGill et al., 1996) could yield very different spinal loads and hence risk of injury when applied to an identical task. This is due to distinct underlying assumptions and simplifications used in these models. Among the forgoing five tools, the Anybody Modeling System and our predictive equations estimated intradiscal pressures in closer agreement with available in vivo measurements (RMSE z 0.12 MPa). Our biomechanical modeling analysis demonstrated that the L5S1 intervertebral disc is the site of greatest lumbar posterioranterior shear force due to its initial forward slope in the sagittal plane (Arjmand et al., 2006). The L5-S1 shear load predicted based on the RWL exceeded 1000 N suggested as a limit in some earlier studies (Gallagher and Marras, 2012; McGill, 1997; McGill et al., 1998) but remained below the in vitro measured yield shear forces of 1.57 ± 0.11 kN (Skrzypiec et al., 2012) in all lifting activities involving trunk flexion (Table 1). In vitro ultimate shear forces of up to 2.8 kN (Cyron et al., 1976), 3.29 ± 0.16 kN (Skrzypiec et al., 2012), and 2.24 ± 0.57 kN (Frei et al., 2002) are reported using different experimental setups. A recent review study suggests the average ultimate shear forces of 1.9 and 1.7 kN for working age males and females, respectively (Gallagher and Marras, 2012) and recommends the 1000 N limit for occasional exposure to shear which is also in agreement with the National Research Council & Institute of Medicine (NRC/IOM) Report (NRC-IOM 2001) as well as the limit reported elsewhere (McGill, 1997; McGill et al., 1998). It is, however, to be noted that the L5-S1 shear yield and strength forces are likely different from those at the upper lumbar levels due to substantial changes in disc size, facet orientations and compression preloads. It is to be noted that, apart from the segmental level, the spinal strength in shear likely depends also on the compression preload and posture (Howarth and Callaghan, 2013). Some epidemiological studies suggest a relationship between the LI and the prevalence of LBP (statistically significant increase in prevalence as LI exceeds 1.0 or 2.0) (e.g., Waters et al., 2011; Lu et al., 2014). On the other hand, the NLE application to predict the incidence of low back disorders has been questioned; no relations have been detected between the LI and either the incidence of lost time due to LBP or the prevalence of LBP (Pinder and Frost, 2011). Our findings here suggest that a modification in the NIOSH vertical multiplier for tasks performed with moderate to large trunk angles is necessary in order to maintain the L5-S1 compression below 3400 N. In parallel and in light of data on spinal strength in compression and its dependence on the worker's body height, weight, gender, and sex (Jager and Luttmann, 1991; Genaidy et al., 1993), the limit of 3400 N set on the compression force should also be re-visited The biomechanics and epidemiologic bases of this limit has indeed been questioned (Jager and Luttmann, 1999). 4.3. Conclusion The NLE failed to adequately control spine compressive and shear loads in lifting activities involving moderate to large trunk forward flexion. The NIOSH vertical multiplier was in part found responsible for this inadequacy while the horizontal multiplier correctly modulated the hand load. In order to address this shortcoming in the NLE, it is suggested that a new multiplier accounting for the trunk flexion angle be considered. In addition, the recommended compression limit of 3400 N can be revised in accordance with the data on the spine compression strength and its

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dependence on the individual's body weight, body height, gender, and age (Jager and Luttmann, 1991; Genaidy et al., 1993) . Ergonomist and occupational health practitioners are advised to use the NLE along with the predictive equations of the spine loads proposed in this study when evaluating the risk of injury to the spine in lifting activities particularly those involving moderate to large trunk flexion. The NLE may be used for lifting activities in upright postures as well as lifting tasks involving small trunk flexion such as those performed with a squat technique. Acknowledgment This work is supported by grants from Sharif University of
Technology (Tehran, Iran) and Institut de recherche Robert-Sauve
et en se
curite
du travail, IRSST (grant number: 2010-0023) en sante
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