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Accepted Manuscript
Trunk response and stability in standing under sagittal-symmetric pull-push
forces at different orientations, elevations and magnitudes
Z. El Ouaaid, A. Shirazi-Adl, A. Plamondon
BM 8409
To appear in:
Journal of Biomechanics
Accepted Date:
15 October 2017
Please cite this article as: Z. El Ouaaid, A. Shirazi-Adl, A. Plamondon, Trunk response and stability in standing
under sagittal-symmetric pull-push forces at different orientations, elevations and magnitudes, Journal of
Biomechanics (2017), doi:
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Full length paper submitted to Berlin Workshop on Spine Loading & Deformation, May 2017, BM-D17-00436R1, August 2017
Trunk response and stability in standing under sagittal-symmetric pull-push forces
at different orientations, elevations and magnitudes
Z. El Ouaaid, 1 A. Shirazi-Adl, 2 A. Plamondon
Division of Applied Mechanics, Department of Mechanical Engineering, École Polytechnique,
Montréal, Québec, Canada
Institut de recherche Robert Sauvé en santé et en sécurité du travail,
Montréal, Québec, Canada
Word Counts: Abstract: 247
Text (Intro –Discussion): 3900
Address correspondence to:
A. Shirazi-Adl, Professor, Division of Applied Mechanics, Dept. of Mechanical Engineering, École
Polytechnique, P.O. Box 6079, Station ‘centre-ville’, Montréal, Québec, Canada H3C 3A7
Email: [email protected]
Fax: 514- 340 4176; Tel: 514 – 340 4711 (Ext 4129)
To reduce lifting and associated low back injuries, manual material handling operations often
involve pulling-pushing of carts at different weights, orientations, and heights. The loads on spine and
risk of injury however need to be investigated. The aim of this study was to evaluate muscle forces,
spinal loads and trunk stability in pull-push tasks in sagittal-symmetric, static upright standing posture.
Three hand-held load magnitudes (80, 120 and 160N) at four elevations (0, 20, 40 and 60 cm to the L5S1) and 24 force directions covering all pull/push orientations were considered. For this purpose, a
musculoskeletal finite element model with kinematics measured earlier were used. Results
demonstrated that peak spinal forces occur under inclined pull (lift) at upper elevations but inclined
push at the lowermost one. Minimal spinal loads, on the other hand, occurred at and around vertical
pull directions. Overall, spinal forces closely followed variations in the net external moment of pullpush forces at the L5-S1. Local lumbar muscles were most active in pulls while global extensor
muscles in lifts. The trunk stability margin decreased with load elevation except at and around
horizontal push; it peaked under pulls and reached minimum at vertical lifts. It also increased with
antagonist activity in muscles and intra-abdominal pressure. Results provide insight into the marked
effects of variation in the load orientation and elevation on muscle forces, spinal loads and trunk
stability and hence offer help in rehabilitation, performance enhancement training and design of safer
Keywords: Pushing; Pulling; Muscle Forces; Orientation; Elevation; Spine Forces; Stability
In many occupational workplaces (like in service, transport, manufacturing, agriculture and
construction industries), manual material handling (MMH) operations involve both lifting and pullingpushing of objects of various sizes at different elevations and orientations. Manual push and pull of
carts have mainly been introduced to facilitate tasks by replacing or reducing lifting and hence avoid
associated risk of back injuries. It has been reported that manual pull-push operations are nowadays
common in workplaces representing about 50% of MMH tasks, and because of large spinal loads and
diminished stability margin, could potentially place the spine at high risk of injury and low back pain
(Garg et al., 2014, Hoozemans et al., 1998, 2004; Jager et al., 2007; Knapik and Marras, 2009). In
addition, during many rehabilitation and training exercises, weights are lifted-lowered or resisted via
cables/bars/handles in varying orientations/heights. Despite the widespread presence of manual pushpull task conditions, the National Institute for Occupational Safety and Health (NIOSH) lifting equation
addresses repetitive lifts only (Waters et al., 1993). To evaluate the muscle forces, spinal loads, trunk
stability and hence the risk factors during MMH and performance enhancement exercises, in vivo and
musculoskeletal biomechanical model studies are required.
Pull and push forces in static and sudden loading/unloading conditions have been investigated
both in vivo (Andersen et al., 2004; Cholewicki et al., 2000; Brown and McGill, 2008; McGill et al.,
1996; Shahvarpour et al., 2014) and in model studies (Bazrgari et al., 2009, 2011; McGill et al, 1996;).
Under identical moments held constant at the L5-S1 (El Ouaaid et al., 2014a,b) or L3-L4 (Kingma et
al., 2007), muscle EMG activity and spinal loads increased when larger (horizontal) pull forces
(directed anteriorly) were applied at lower elevations. Forces at the lowermost L5-S1 disc and in global
extensor muscles decreased as the external load turned upward from the downward lifts (El Ouaaid et
al, 2014a, b). Available regression equations estimate spinal loads only when applied in the gravity
(lifting) direction (Fathallah et al., 1999, Arjmand et al., 2012). The regression equation proposed by
McGill et al. (1996), although based also on EMG measurements of three subjects in few pull/push
tasks, estimated L4-L5 compression are based only on moments of external forces with no
consideration of pull-push orientations.
Maximum pushing forces of simulated trolleys via instrumented handles at different heights
have been measured with the objective to determine safe push forces in real MMH tasks (Ciriello et al.,
2007; Schaub et al., 2007). Strength of lumbar spine in compression has also been accounted for when
determining associated risk of injury (Schaefer et al., 2007). Net external moments and spinal forces at
lower lumbar levels under simulated pull-push tasks at different conditions (resistance force or cart
weight-design, handle-force elevation-location, one- or two-handed, surface slope-roughness, fixed or
variable orientations, standing or walking, trunk upright posture) have been estimated using
biomechanical models with different degrees of complexity and accuracy (de Looze et al., 2000;
Granata and Bennett, 2005; Hoozemans et al., 2004 and 2007; Jager et al., 2007; Knapik and Marras,
2009; Lett and McGill, 2006; Nimbarte et al., 2013; Sandfeld et al., 2014; Schibye et al., 2001).
Overall, pull forces resulted in greater net moments and spinal internal forces than push forces though
reverse findings have also been reported (Knapik and Marras, 2009; Lett and McGill, 2006). Lower
load handle (at the hip rather than shoulder level) and higher cart weights were found to increase spinal
loads (Hoozemans et al., 2004). More recently in a kinematics-driven musculoskeletal model of trunk
in upright standing, El Ouaaid et al. (2016) varied the magnitude, elevation and pull orientation from
the gravity (lift) all the way to the downward pull. They reported larger spinal forces under lifts but
minimal spinal forces and maximal trunk stability under downward pulls.
Our previous studies in upright standing (El Ouaaid et al., 2014a,b and 2016) demonstrated
substantial changes in recorded EMGs as well as in estimated muscle forces, spinal loads and trunk
stability as external pull orientation altered from downward gravity to upward pull. Push forces were
however totally neglected in these studies. In continuation and using also earlier results, we aim here to
complete estimation of, we aim here to estimate muscle forces, L5-S1 spinal loads and trunk stability
margin for static symmetric tasks in the same upright standing posture but under varying hand-held
load magnitudes (3 levels), elevations (4 levels) and orientations (24 levels) covering all possible pullpush orientations in the sagittal plane. The effects on results of the consideration of antagonist
coactivity and intra-abdominal pressure are also investigated.
In continuation of our previous studies (El Ouaaid et al., 2014a,b and 2016), measured
kinematics of one healthy subject (181.5 cm stature and 68.3 kg body weight) from our previous study
(El Ouaaid et al., 2014a) was used in an iterative kinematics-driven nonlinear finite element (FE)
model (El Ouaaid et al., 2013a,b) to estimate muscle forces, spinal loads and stability margin under
push forces at different orientations and elevations. An external force at 3 different magnitudes (80,
120 and 160 N resisted in hands at a constant horizontal anterior moment arm of 25 cm to the L5-S1
disc center) and orientations in the sagittal plane covering all push directions (13 in total) (Fig. 1) were
considered each at 4 different elevations (0, 20, 40 and 60 cm to the L5-S1 disc center). To introduce
muscle coactivity in this study, flexor or extensor moment of 10 Nm (El Ouaaid et al., 2013a, b and
2016) was added as antagonistic moment at different cases. Small to moderate intra-abdominal
pressures (≤8 kPa) (Arjmand and Shirazi-Adl, 2006) were also taken into account only when
abdominal muscles were found active while counterbalancing the trunk gravity and external push
forces. Furthermore, to complete the data, earlier reported muscle forces, spinal loads and trunk
stability margin under pull forces at 3 elevations (20, 40 and 60 cm to the L5-S1) (El Ouaaid et al.,
2016) along with new results under pull forces at lower elevation (0 cm to the L5-S1) are also
Kinematics and Loads: Measured changes in subject’s trunk kinematics relative to the unloaded
upright posture were considered by applying T12 (trunk) and S1 (pelvis) rotations that also verified to
match measured positions at the C7 vertebra relative to the S1 (EL Ouaaid et al., 2014a,b). The lumbar
rotation, as the difference between T12 and S1 rotations, was subsequently partitioned between its
segments (8% at the T12–L1, 14% at the L1–L2, 16% at the L2–L3, 22% at the L3–L4, 25% at the L4–
L5 and 15% at the L5–S1) (El Ouaaid et al., 2013b). External pull-push forces (at different
orientations-heights) were applied onto the model. The trunk weight distributed at each spinal level T1S1 was evaluated based on subject’s body weight (Pearsall, 1994). Head, hands, forearms and upper
arms gravity loads were applied as additional gravity forces at their measured mass centers (de Leva
Kinematics-driven model: Muscle forces, spinal loads (at the L5-S1 level) as well as stability margin
and critical muscle stuffiness (see below) were calculated employing an iterative kinematics-driven FE
model combined with an optimization algorithm (EL Ouaaid et al., 2013a, b and 2016). The multisegment FE model simulated the T12-L1 to the L5-S1 levels using six shear deformable beams with
nonlinear proprieties representing entire motion segments (i.e. discs, facets and ligaments) (see El
Ouaaid et al., 2016). Deformable beams were inserted at endplates into rigid vertebrae. The thoracic
spine (T1 to T12) was simulated as a single rigid body. The sagittal-symmetric muscle architecture of
the FE model (Fig. 1A) consisted of 56 muscles; 10 global muscles inserted into the thoracic spine
(rigidly attached to the T12) (ICPT: iliocostalis lumborum pars thoracic, LGPT: longissimus thoracis
pars thoracic as global extensor muscles while RA : rectus abdominus, EO: external oblique and IO:
internal oblique as global abdominal muscles) and 46 local lumbar muscles inserted into lumbar
vertebrae (ICPL: iliocostalis lumborum pars lumborum; LGPL: longissimus thoracis pars lumborum;
MF: multifidus; QL: quadratus lumborum and IP: iliopsoas).
Optimization and antagonist moments: To solve redundant equilibrium equations at the T12 to L5
spinal levels, we used optimisation algorithm with the sum of cubed agonist muscle stresses subtracted
by the sum of cubed antagonist muscle stresses as the cost function while the equilibrium equations and
antagonist moments were enforced as equality constraints (El Ouaaid et al., 2013a). Unknown muscle
forces also remained positive and smaller than the maximum active forces (i.e. 0.6 MPa×PCSA mm2
where PCSA is the physiological cross- sectional area) overlooking the negligible change in the muscle
passive resistance in the upright standing posture. The estimated muscle forces were re-applied
iteratively as additional external forces onto the corresponding vertebral level and the analysis was
repeated until convergence. For the estimation of muscle forces, spinal loads and stability margin, an
antagonist moment (MC) of 10 Nm (and in limited cases for stability analyses 5 Nm as well) was
introduced to generate some coactivity (El Ouaaid et al, 2013a, 2014b and 2016) in either flexor
abdominal (EL Ouaaid et al., 2014b) or extensor back muscles depending on the moment of applied
pull-push forces. This added moment generated modest antagonist muscle forces in better agreement
with our recorded EMG. Nonlinear analyses were carried out by ABAQUS (Simulia Inc., Providence,
RI) and the optimization was analytically solved by the Lagrange multipliers method. For more details,
see earlier works (El Ouaaid et al., 2013a,b).
Intra-abdominal pressure (IAP): IAP likely affects muscle and spinal forces. Since abdominal
muscles generate IAP, small to moderate values (0 – 8kPa) were assumed that varied linearly with the
net flexion moment (portion to be resisted by the abdominal musculature) at the T12 level when and if
caused by pushing forces. The maximum IAP (8 kPa) was considered under the largest abdominal
activity (in between the upward pull and inclined upward push forces). IAP generated a net extension
moment at the T12 when applied as an upward force on the upper diaphragm 4 cm anterior to the T12.
A diaphragm area of 200 cm2 was assumed (Arjmand and Shirazi-Adl, 2006).
Stability analyses: After the calculation of muscle forces in the equilibrium phase of analyses, the trunk
stability was analyzed under all orientations (with 120 N pull-push forces at two heights 20 and 40 cm).
Muscles between their insertion points at the final deformed configurations were replaced by uniaxial
elements with a force-proportional stiffness taken as
computed muscle force,
(Bergmark, 1989), where F denotes the
the instantaneous total muscle length and
a non-dimensional muscle
stiffness coefficient. In stability analyses, two levels of q (15 and 25) were considered (Cholewicki and
McGill, 1995) to evaluate stability margins as the pull-push orientation and elevation changed. Critical
muscle stiffness
(EL Ouaaid et al., 2009) was also calculated as the minimum q value required for
the trunk to remain at a neutral stable condition.
At a constant external force elevation of 40 cm to the L5-S1 (Fig. 1) and under a constant
coactivity moment (MC = 10 Nm), maximum local compression (2315 N) and shear (1365 N) forces
at the L5-S1were reached at 30⁰, between forward horizontal pull (0o
or 360⁰) and lift (90⁰)
orientations. The minimal values were found under inclined push force at 150⁰ (Fig. 2). While the
compression force was influenced by both global and lumbar extensor muscles in horizontal to
downward pulls, the shear force was mainly affected in this range by the former (global extensor
muscles) alone (Fig. 2). The activity in abdominals, however, was most influential on these spinal
forces under push forces. Moments of muscle forces inserted at the T12 (required reaction moment at
the T12) and into lumbar vertebrae L1-L5 (lumbar required moment) as well as the external moment
(Moment of pull-push forces alone at the L5-S1) peaked, along with spinal forces, at ~30⁰ (Fig. 3).
Under constant pull-push forces forces (120 N) and a constant co-activity moment (10 Nm), peak
values of moments (Fig. 3), spinal loads (Fig. 4) and muscle forces (Fig. 5) substantially increased with
force elevation. Spinal forces were however larger under forces at lower elevations (at 0 and 20 cm)
when oriented in upward pushes (105o-165o). As expected, spinal and muscle forces remained
unchanged at different load elevations when the force acted either upward (pull at 270o) or downward
(lift at 90o) (Fig. 4 L5-S1 spinal forces at 90o and 270o orientations reached, respectively, ~1864 N and
~1069 N in compression and ~940 N and ~480N, in shear.
Unlike spinal forces, the trunk stability margin decreased with load elevation and peaked under
inclined upward pulls (285o – 345o) (Fig. 6A). This change was also evident under upward and
downward forces despite computed identical muscle and spinal forces (Figs 4 and 5). The critical
muscle stiffness coefficient qcr also varied with pull-push orientations and reached its minimum
(indicating the most stable trunk configuration) at around 315o under inclined upward pull. The
maximum qcr was found under lifts at 90⁰-135⁰ (Fig. 7). Coactivity in antagonist muscles markedly
increased stability margin and decreased qcr (Fig. 7). Consideration of IAP in oblique upward push
demonstrated a modest decreasing effect on spinal loads and muscle forces whereas an increasing
effect on trunk stability margin (Fig. 6B).
A nonlinear musculoskeletal kinematics-driven FE model (EL Ouaaid et al., 2014b) was used in
the upright standing posture to quantify the effects of changes in the static sagittal-symmetric hand-held
load magnitude, elevation and orientation (simulating all pull-push directions) on muscle forces, spinal
loading (at the L5-S1) and trunk stability. Loads, at 4 elevations, 3 magnitudes and 24 orientations,
were applied in hands (held at a constant anterior moment arm of 25 cm with respect to the L5-S1). To
improve simulation results, effects of small to moderate antagonistic coactivity moment (10 Nm) and
intra-abdominal pressure (≤8 kPa) on results were also investigated. A moment of 10 Nm was
considered as co-contraction in better agreement with our recorded EMG (EL Ouaaid et al., 2014) and
for a better evaluating of trunk stability and spinal loads (Granata and Bennett, 2005). Variations in
load orientation, height and magnitude substantially affected muscle forces, spinal loads and stability
margin. Coactivity moment and IAP both improved the trunk stability.
Under 120 N load at 40 cm elevation and antagonist coactivity (10 Nm), the maximum and
minimum compression and shear forces at the L5-S1 disc were found respectively at ~30⁰ (upward pull
by subject) and ~300⁰ (downward pull by subject) orientations (Fig. 2). ). In accordance with changes
in external moments (at the L5-S1) and as the force orientation altered, the peak compression/shear
forces of 2315/1365 N (at 30o) reduced to 2023/ 930N in the horizontal pull direction (360 o), to
1840/930N in lift (90⁰) and further to 1147/570N in horizontal push (180⁰) with minimum 966/488N at
300o orientation. These results demonstrate that spinal forces follow closely variations in external
moments that could be considered as surrogate measures of spinal forces as pull-push force orientations
alter. This trend was also found true at other force elevations (Fig. 3 and 4). Spine forces, when applied
at higher elevations, are hence much smaller in horizontal push than in vertical lift and horizontal pull
which could also be due both to greater lever arms of abdominal muscles and intra-abdominal pressure
when flexor muscles are contracted (Arjmand and Shirazi-Adl, 2006).
Larger spinal compression and shear forces were estimated in some earlier musculoskeletal
model studies of different workplace tasks involving pull and push of objects. Using a singleequivalent muscle model, de Looze (1995) estimated compression forces of ~5.8 kN when handling
large containers. Hoozemans et al (2004) computed with an EMG-assisted model compressive forces at
the L5-S1 of ~4 kN while pulling objects at the hip elevation. Jaeger et al (2007) computed peak
compression forces of ~3 kN at the L5-S1 in push-pull of flight attendant trolleys. With an EMG8
assisted model of subjects in a simulated push-pull at different elevations and levels, Knapik and
Marras (2009) reported peak compression/shear forces of ~1.5 (L5-S1)/1.0 (L1-L2) kN in both pushing
and pulling. Although direct comparison with other model studies in the literature is not possible due to
numerous variable distinct pull-push conditions, earlier studies also generally found greater net external
moments and lumbar spine forces under pulling forces than pushing ones (de Looze et al., 2000;
Hoozemans et al., 2004; Jager et al., 2007; Sandfeld et al., 2014; Schibye et al., 2001). Mixed or
inverse findings were however reported as well (Knapik and Marras, 2009; Lett and McGill, 2006).
Changes in the load orientation markedly affected the activity levels in different muscle groups
(i.e., abdominal, global extensor and local lumbar muscles) and their contributions to spinal loads. For
example, at 40 cm elevation (Fig. 2), local lumbar extensors inserted into L1-L5 vertebrae increased
their activities at and around horizontal pull (0⁰) generating a large portion of compression (~44.4% of
total compression at the L5-S1). On the other hand, global extensor muscles (i.e., attached to the
thorax) produced more compression in lift (~43.4% of total compression). Abdominals, however, had
the greatest role in pushes producing ~35% of the total compression at horizontal push (180 o). Shear
forces at the L5-S1 were most influenced by global extensors in pulls (0 o-90o) and by abdominals in
pushes (180o-270o) (Fig. 2). Comparisons between conventional lift and pull-push activities
demonstrated that in horizontal pull local lumbar extensor muscles became more active and generated
more compression on spine (~900 N of total compression of 2023 N at 40cm elevation and 120 N force
(Figs 3 and 5)). This effect was even more pronounced at lower force elevations. In lifting, however,
global extensor muscles remained more influential and caused much greater proportion of compression
at the L5-S1 (~800N of total compression (1840N)) than the lumbar muscle group (478.7 N at the same
elevation and force of 120N). In orientations between horizontal push (180⁰) and pull (270⁰),
abdominal muscles primarily counterbalanced net external moments generated by pull-push forces and
gravity; the spine compression and shear forces at the L5-S1 decreased (under 120 N force at 40 cm
elevation) to 1147N and 570N, respectively.
Due to its anticipated effects on the lever arm of pull-push forces and hence net external
moments at different levels, alterations in the force elevation interacted with changes in orientations
and markedly affected results as well. Peak spinal forces markedly shifted from pulling at highest
elevation of 60 cm to pushing at the lowest elevation of 0 cm (Fig. 4). In horizontal pull (0o), results
also substantially altered in tandem with changes in the lever arm of external forces applied at different
heights. In lifting (90o) and vertical pull (270 o), when the load height lowered from 60 to 0 cm, required
and external moments as well as total local, global and abdominal muscle forces and consequently
spinal forces remained however almost unchanged (Fig. 4) with minor differences due to slight changes
in the arm positions at different force elevations (El Ouaaid et al, 2016). In these conditions; in vivo
studies have either recorded greater abdominal and extensor muscle activity at higher load elevations
when applied at a constant lever arm (Granata and Orishimo, 2001) or remained inconclusive when
reporting loads on instrumented vertebral replacement in limited patients (Rohlmann et al., 2013). In
our musculoskeletal model study, estimation of greater spinal loads at higher force elevations during
lifts under identical lever arms and moments would occur had the stability constraint been directly or
indirectly considered in the optimization algorithm (Arjmand et al., 2008). As load elevation varied,
our results clearly show maximum spinal forces at highest elevation (60 cm) whereas minimum occurs
at 20 cm and not at the lowest elevation (0 cm). This is due to the lever arm of loads to the L5-S1 and
suggests that the trend likely reverses thereafter at lower elevations. Earlier studies are not conclusive
as one (Hoozemans et al., 2007) reported greater net L5-S1 external moment under push forces at
shoulder compared to hip whereas in contrast another (Hoozemans et al., 2004) reported larger net
moment and spine forces under both pulls and pushes at hip height.
Stability analyses with and without IAP, was carried out under constant external load (120 N) at
two coactivity moments (5 and 10 Nm), two load heights (20 and 40 cm) and two muscle stiffness
coefficients (q=15 and 25). As expected, an increase in the muscle stiffness (q) from 15 to 25 increased
the stability margin in all orientations and heights (Fig. 6). Interestingly and due to the varying activity
in muscle groups and spinal compression, trunk stability margin increased at lower force elevation (20
cm compared to 40 cm) in all orientations except at and around horizontal pushes (Fig. 6). This is
found in agreement with model studies of Granata and Bennett (2005) reporting more stable spine
under pushing at waist height compared to shoulder height but not with their estimation of vertical
lifting as being much more stable than pushing. In the gravity lift (90 o) and vertical pull (270o)
directions where muscle forces and spinal loads remained virtually the same as force elevation
changed, the stability margin increased at lower load elevation; especially so in the latter direction.
These results also demonstrate that the trunk stability margin does depend on both spinal compression
and activity in different muscle groups. The maximum stability margin happens in vertical pull forces
(with larger lumbar muscle activity and smaller compression) (Figs 6 and 7) whereas the minimum
occurs at and around the vertical lift (with large extensor activity and compression). Critical muscle
stiffness coefficient required for the trunk to remain at a neutral stable condition markedly altered with
the external force orientation and coactivity moment (Figs 6 and 7). Pulling tasks were found in general
more stable than pushing and lifting. IAP, considered here only in oblique pushes in presence of
activity in abdominals, slightly increased spinal loads and stability margin.
For more details regarding model validation and limitations during symmetric lifting tasks in
upright standing posture or forward trunk flexion, one may refer to previous publications (El Rich et
al., 2004; Arjmand and Shirazi-Adl, 2005; El Ouaaid et al., 2009). In this study and based on earlier
measurements (El Ouaaid et al., 2014a), identical upright trunk posture was considered in both pull and
push loads at different magnitudes, orientations and elevations. Due to small changes in the posture
prescribed here onto the upright standing under gravity load alone (see Fig. 1B), passive muscle
contributions were neglected. Moreover and for the same reason, we do not expect noticeable effects
on results had we altered the assumed imaging-based distribution of the total lumbar rotation among
different lumbar levels. Abdominal muscles and back muscles were each modelled by a single muscle
on each side. In symmetric lifting activities, consideration of more than one muscle for abdominal
oblique muscles has however little effects on the spinal loads and trunk stability (Davis and Mirka,
2000; Arjmand et al., 2008a).
In summary, Results demonstrate that activity patterns in various muscle groups, spinal forces
and trunk stability are markedly affected by changes in the pull-push orientation, elevation and
magnitude of hand-held forces in the sagittal plane. Local compression and shear forces peaked under
oblique upward pulls at higher elevations and oblique upward pushes under lower elevations. Trunk
was found most stable under vertical pulls and least stable under pushes. This research provides insight
into the effect of the load orientation and elevation on muscle forces, spinal loads and trunk stability
and hence is helpful in rehabilitation, performance enhancement training and design of safer
Acknowledgements: This work was supported by the Institut de recherche Robert-Sauvé en santé et en
sécurité du travail (IRSST 2014-0009).
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Conflict of Interest Declaration: No conflict of interest to declare.
Figure 1: (A) Finite element (FE) model with trunk local and global muscles in the sagittal plane
(XZ). Six inter-vertebral beam elements (shown as discs) represent the stiffness of various motion
segments. Global muscles: ICPT, iliocostalis lumborum pars thoracic; LGPT, longissimus thoracis pars
thoracic; IO, internal oblique; EO, external oblique and RA, rectus abdominus. Local lumbar muscles:
ICPL, iliocostalis lumborum pars lumborum; LGPL, longissimus thoracis pars lumborum; MF,
multifidus; QL, quadratus lumborum and IP, iliopsoas (Bogduk et al. 1992; Stokes and Gardner-Morse
1999). (B) Initial upright standing posture under gravity alone (EL-Rich et al., 2004) and under sagittal
symmetric tasks at 24 different orientations (0 to 360o), 3 external force magnitudes (80, 120 and 160
N) and 4 elevations (0, 20, 40 and 60 cm). At 40 cm elevation, arrows shown different directions of
Figure 2: Calculated total compression (A) and shear (B) forces (N) at the L5-S1. Portions generated
by forces in local lumbar muscles, global extensor (attached at the T12 and above) and abdominal
muscles are also separately shown. The force of 120 N acts at 40 cm elevation with respect to the L5S1 along with a constant coactivity moment that changes from flexion to extension to act opposite to
the moment of the external force (MC = 10 Nm). The force orientation varies from 0⁰ (or 360⁰) that
represents a horizontal pull by intervals of 15o to simulate lifting (90⁰), horizontal pushing (180⁰) and
downward pulling (270o). Arrows show the direction of applied forces at hands. Arrows indicate handheld forces acting on the human body.
Figure 3: External moments at the L5-S1 as well as the required moments (be resisted by muscles) at
the T12 (global muscles) and lumbar levels (local muscles) under 120 N force at (A) 40 cm and (B) 20
cm elevations at different orientations. Arrows indicate hand-held forces acting on the human body.
Figure 4: Effect of load height on total spinal (A) compression and (B) shear forces at the L5-S1 under
constant coactivity moment (MC=10 Nm) and load magnitude (120 N) at different orientations.
Figure 5: Effect changes in force orientation on total forces in local lumbar, global extensor and
abdominal muscles under constant coactivity moment (MC=10 Nm) and external pull-push force of
120 N acting at (A) 40 cm and (B) 20 cm elevations to the L5-S1.
Figure 6: Effect of load height (A) and Intra-abdominal pressure (B) on trunk stability margin under
constant coactivity moment (10 Nm) and external load magnitude (120 N) at two muscle stiffness
coefficients q = 15 and 25 and different pull-push orientations.
Figure 7: Critical muscle stiffness coefficient q (required for a neutral stable trunk configuration) at
different pull-push orientations and coactivity moment levels under 120 N force acting at 40 cm
elevation. A smaller q denotes a more stable trunk.
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