1. health and fitness
2. exercise

Biomechanical differences between
expert and novice workers in a manual
material handling task
André Plamondon, Denys Denis, Christian Larivière, Erik Salazar
Alain Delisle
This article was published in
Ergonomics, vol. 53, no 10, 2010, p. 1239-1253.
1
Biomechanical differences between expert and novice workers in a manual
material handling task
André Plamondona*, Denys Denisa, Alain Delisleb, Christian Larivièrea, Erik
Salazara and the IRSST MMH research group1
Authors’ affiliation:
a
Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST), 505 De
Maisonneuve Blvd West, Montréal, Québec, Canada, H3A 3C2;
b
Faculté d’éducation physique et sportive, Sherbrooke University, Sherbrooke, Québec,
Canada.
*Corresponding author. Email: [email protected]
1
Other members of the IRSST MMH research group: Marie St-Vincent, Iuliana Nastasia, Sophie
Bellefeuille, Maud Gonella; Denis Gagnon (Sherbrooke University).
The objective was to verify whether the methods were safer and more efficient when used by expert
handlers than by novice handlers. Fifteen expert and fifteen novice handlers were recruited. Their task
was to transfer four boxes from a conveyor to a hand trolley. Different characteristics of the load and
lifting heights were modified to achieve a larger variety of methods by the participants. The results
show that the net moments at the L5/S1 joint were not significantly different (P > .05) for the two
groups. However, compared to the novices, the experts bent their lumbar region less ( experts = 54°
(SD = 11°) ; novices = 66° (15°) but bent their knees more (experts ≈ 72°( SD ≈ 30°) ; novices ≈ 53°(
SD ≈ 33°), which brought them closer to the box. The handler’s posture therefore seems to be a major
aspect that should be paid specific attention, mainly when there is maximum back loading.
Keywords: Manual material handling, lifting, expert, low back load, ergonomic intervention.
Statement of Relevance: The findings of this research will be useful for improving manual material
handling training programs. Most biomechanical research is based on novice workers, and adding
information about the approach used by expert handlers in performing their tasks will help provide
new avenues for reducing the risk of injury caused by this demanding physical task.
Acknowledgements: The present research project was funded by the Robert-Sauvé Occupational Health and Safety
Research Institute (IRSST) of Quebec. Special thanks go to the expert and novice subjects and the industries that
participated in this project.
2
1. Introduction
According to the National Research Council (2001), there is a clear relationship between back
injuries and the mechanical load imposed during handling work. The level of the load on the
spinal structures necessarily depends on the type of task performed. A very large number of
studies have evaluated the back loading of subjects during a box lifting task by comparing
different handling techniques such as squatting, stooping and freestyle (de Looze et al., 1993;
Kingma et al., 2004; Kingma et al., 2006; Leskinen et al., 1983; Potvin et al., 1991; Schipplein et
al., 1990; van Dieen et al., 1994). For instance, Kingma et al. (2006) showed that no single lifting
technique can be recommended for all task conditions, and the most important advice is to avoid
lifting wide objects from the floor. Faber et al. (2009) demonstrated that lifting a 20 kg split-load
instead of a 20 kg single load generally resulted in a reduction of the peak L5/S1 compression
force. The effect of load-splitting could be explained mainly by the change in horizontal distance
between the load and L5/S1. Another way for understanding the requirements of a task and for
finding means of prevention or effective means of training involves studying the methods of
expert and novice workers. Ergonomic studies (Authier et al., 1995; Authier et al., 1996) have
shown that experienced handlers, recognized as experts by their colleagues, have developed
methods different from those of novices and that could be both safe and advantageous in terms of
productivity. These methods of experts have not been investigated often in the biomechanical
literature.
Lortie (2002) published a summary of the studies conducted in recent years on the
analysis of the handling activity. The subject of several of these studies was the observation of
methods of experienced and/or expert handlers. In this summary, the author presented several
handling principles favored by handlers, such as: the use of a diagonal grip; the use of
momentum effects to reduce the duration of efforts; moderate knee flexion to avoid unbalancing
the body; the use of the free rear leg as a counterbalance; continuity and regularity of efforts; the
particular position of the feet during lifting and deposit. These methods are interesting because
they could be used as inspiration in developing training programs that are better adapted to the
work. The mitigated success of training programs on back injury prevention in handling can
possibly be explained by our limited practical knowledge about what experts actually do when
performing their work, and the consequences of these methods on biomechanical loading.
A few rare biomechanical studies have been interested in comparing the work methods of
experienced and novice handlers (Gagnon et al., 1996; Granata et al., 1999; Marras et al., 2006;
Hodder et al., 2010). In one of them (Gagnon et al., 1996), the most marked distinction was with
the knees, which were less bent with the experts, who (during lifting) experienced a knee flexion
rather than extension moment as did the beginners. This knee flexion moment would be
generated by the quadriceps which pulls the hamstrings (biarticular muscle group) in order to
promote hip extension in load lifting (de Looze et al., 1993; Burgess-Limerick et al., 1995;
Toussaint et al., 1992). This action of the biarticular muscles facilitates the transfer of work from
the quadriceps to the hip joint (Toussaint et al., 1992). In another study, Granata et al. (1999)
showed that experienced handlers underwent surprisingly greater and more variable lumbar
loading than novice subjects. These handlers generated a greater effort of the flexor muscles of
the rachis (abdominals), which however would ensure better stabilization of the lumbar rachis.
Recently Marras et al. (2006) showed that loading was greater for inexperienced subjects than
experienced lifters over the course of an 8-hour workday. They suggested that the increased
spinal loading levels exhibited by the novice workers were due to an underdeveloped motor
3
control program and that biomechanical risk is greatly reduced with experience. Finally, Hodder
et al.(2010) analyzed the difference between 12 novices nurses (before and after training) and 10
trained experienced nurses during three patient handling tasks. The experienced nurses tended to
have a smaller lumbar spine range of motion than the novices but the results were not
significantly different in all directions and all tasks. Other studies have focused on the validation
of the experts’ methods when simulated by novices, such as foot movements, knee bending, and
the width of the base of support, the lifting dynamics, and the box lifting and tilting strategies.
These studies have shown the potential of these methods to reduce the risk of injury during the
handling operation (Delisle et al., 1996b; Delisle et al., 1996a; Delisle et al., 1998; Delisle et al.,
1999; Gagnon, 2003). Gagnon (2005) produced a summary of experts’ strategies that were
biomechanically safer. Hence, foot placements/movements are characterized by a reduction in
energy expenditures, meaning a reduction in the load transfer time and its path. Similarly, the
experts’ maneuvers on the boxes (grips and tilting) greatly reduced the mechanical work and
slightly reduced the back loading. Gagnon (2005) therefore recommended pursuing research on
these important factors affecting back loading, asymmetrical postures and efforts, and mechanical
work.
It is hypothesized that comparing experts with novices would help to identify some
essential elements for safe manual handling practices (Gagnon et al., 2005) and also for
efficiency. The first aim of a safe method is to ensure the integrity of the back, while the aim of
an efficient method is to meet production objectives while reducing effort. A safe method is
defined mainly by the lumbar load at L5/S1 (resultant and asymmetrical moments) and postures
(asymmetrical bending and postures). An efficient method is evaluated by the total duration of
the handling operation and by a reduction in box transfer distances.
The general hypothesis of this research is to verify whether the methods of expert
handlers are safer and more efficient than those of novice handlers. More specifically, studying
more varied conditions by modifying the characteristics of the load (weight, fragility of the
container and center of gravity off-center) as well as the initial lifting and deposit heights should
promote greater variety in the participants’ methods. This variety of conditions should allow a
better characterization of the experts’ methods and bring out individual technical differences.
2.Methods
2.1 Subjects
Two groups of male subjects were recruited. The first group consisted of 15 expert handlers who
met the four following criteria: a minimum of 5 years of experience, a low incidence of injuries
(particularly to the back), no injury in the year preceding the trial, and finally, was recommended
either by their peers, union or managers. However, the recruitment was quite difficult and we do
not know whether the last criterion was really applied by the person who was in charge of each
company. What we know is that after the first three criteria were met, the number of workers was
quite small and consequently, all had to be recruited. The second group consisted of 15 novice
handlers meeting the following criteria: 3 to 6 months of handling experience and no incidence of
injury in the year preceding the study. None of the subjects had musculoskeletal problems that
could affect the normal performance of their work. Ten expert subjects and three novices had
followed a training program, while the others had not. Table 1 presents the subjects’ main
anthropometric characteristics. The two groups of subjects, namely the experts and the novices,
were not significantly different (T test; P >.05) with respect to weight and height. These latter
4
variables should therefore have no impact on the different biomechanical parameters. It should be
noted that the two groups were significantly different with respect to age and, incidentally,
experience.
Table 1 Anthropometric characteristics of the subjects participating in the experiment (n = 30).
Variables
Experts
Novices
Prob.2
M1
SD1
M
SD
Age (yrs)
38.1
9.8
25.0 5.9
< 0.01
Weight (kg)
75.9
12.2
74.2 11.4
0.70
Height (m)
1.71
0.07
1.75 0.05
0.09
Experience (yrs)
15.4
9.3
0.5
0.4
< 0.01
Horizontal trunk moment
96
17
95
15
0.87
weight at L5/S1 (Nm)
1
M = Mean; SD = Standard deviation; 2 level of probability with bilateral T-test.
2
Significant effects (p< .05) are indicated by bold values.
2.2 Design of the study
The study was divided into three experimental sessions: the first was a familiarization
session on the different experimental and physical capacity measurement procedures; the second
session specifically studied, based on a continuous transfer of boxes from one pallet to another,
the impact of modifying the work pace as well as the cumulative effect of physical fatigue on the
experts’ and novices’ methods; the third session consisted of studying the effect of expertise
during transfer of a box from a conveyor to a hand trolley, by modifying the characteristics of the
load (mass, distribution and stability). This paper discusses only the results of the third session.
During the familiarization session and at the start of each experimental session, the participants
were instructed about the importance of reproducing the technique that they normally used in
their work. For the experts, we stressed the fact that they were teachers and that we wanted to
learn from their expertise. We insisted that the only way to learn from them was for them to
reproduce the technique they normally used at work. During all three sessions, no lifting
technique was ever prescribed to the participants and no comments were given about the
technique they used.
Optotrak
Hand trolley
Force platform
Figure 1. Illustration of the experimental setup.
5
2.3 Experimental procedures
The third experimental session involved the subject transferring 4 boxes from the
conveyor (height from the ground = 12 cm) towards a two-wheel hand trolley (height from the
ground = 2 cm) at a distance of 1.5 m from the lifting location (Figure 1). In the “going” phase to
the hand trolley, the handler had to pull one by one the four boxes on the conveyor towards
himself and proceed with their transfer towards the destination on the hand trolley. The conveyor
was slightly inclined so that the boxes moved by gravity (on the rollers) towards the handler. The
four boxes were transferred one by one to be stacked in a pile on the hand trolley, with the first
box taken from the conveyor at the bottom of the pile, and the last box (fourth one) at the top.
Once all the four boxes were placed on the hand trolley, the subject resumed the task from the
hand trolley towards the conveyor (the return phase) beginning with the box at the top of the pile.
In the return phase, the conveyor was slightly inclined outwards so that the boxes moved away
from the handler. The boxes had the following characteristics: one 15-kg box, one 23-kg box, one
weakened 15-kg box, and one off-center 15-kg box (center of gravity 27 cm laterally from one
side and 8 cm from the other), and all of the same dimensions (26 cm deep x 35 cm wide x 32 cm
high). The weakened box contained 12 bottles of sand and water and had no cover, so as to be
deformable. The order of the boxes had been balanced using a Latin square so that the 15 subjects
within each group performed the handling in a different order and so that both groups had the
same order. The balanced order resulted in each type of box being at each position on the
conveyor (position 1 = first box to position 4 = last box) and at each height on the hand trolley
(height 1 = ground level to height 4 = top of the pile) twice during the experiment (two trials).
Also, two conveyor positions were studied: one facing the hand trolley at a distance of 1.50 m,
and the other at 90° in relation to the hand trolley at the same distance (1.50 m). There were
therefore 4 boxes going, 4 boxes returning, 4 positions, 2 trials and 2 conveyor positions (total of
128 boxes). The conveyor position had also been chosen so that half of the subjects began with
the condition at 180°, and the other half at 90°.
The subjects were free to choose the handling technique and handling velocity. The
subjects had been instructed to always remain on the force platform and to pile the boxes on the
hand trolley. To avoid fatigue building up in the subjects, each series of goings and returns (8
boxes) was followed by 2 minutes of rest. After 64 boxes were transferred, the subject had to rest
for at least 5 minutes in a sitting position. After each “going” transfer with 4 boxes, there was
also a short mandatory 30-second pause to allow the system to be set at zero. An additional rest
time was also planned for the case in which a subject required it or seemed tired (based on the
Borg scale results), which did not occur.
2.4 Measuring system
Two photogrammetric measuring systems were used to record the tridimensional (3D)
coordinates of markers attached to the primary body segments. The first system consisted of
infrared LED diodes whose signals were collected by four “Optotrak” columns (Northern Digital
Inc., Waterloo, Ontario). The Optotrak system’s sampling frequency was set at 30 Hz, and the
markers’ 3D reconstruction error is generally less than 1 mm. Since this system did not generate
video images, a second system consisting of three video cameras allowed verification of the
Optotrak system’s data (both systems were aligned on the same global reference system),
correction of some missing data, and the ergonomic analysis of the handling tasks.
The external forces on the feet during the handling tasks were obtained by using an inhouse-designed force platform (1.90 m x 1.30 m) mounted on 6 AMTI mini platforms (model
6
MC3A-6-1000, Watertown Massachusetts). This type of platform has been validated (Desjardins
and Gagnon, 2001). A synchronization system (Horita, FP-50, Mission Viejo, CA) was used to
synchronize all of the measuring instruments (Optotrak, video and force platform).
The participants were instrumented in such a way that a dynamic 3D linked segment
model was used to estimate the net moments at L5/S1. This model calculates the net moments on
the basis of external forces, the kinematics of body segments and anthropometric data. It was
developed over a period of several years of research (from Gagnon and Gagnon, 1992) and was
the subject of exhaustive validation (Desjardins et al., 1998; Gagnon and Gagnon, 1992;
Plamondon et al., 1996). It requires the attachment of 12 rigid clusters of markers to each of the
following segments: head (1), back at C7 (1), T12 (1) and S1 (1), both arms (2), both forearms
(2), both thighs (2) and both feet. A cluster of markers consisted of 4 LED diodes (except for 7
for the feet) fixed either to an aluminum plate or to a Styrofoam block, which in turn was glued to
the subjects’ skin. The rigid clusters read by the four Optotrak columns were used to locate 48
anatomical landmarks in relation to their respective marker cluster, in order to be able to estimate
the segmental joint centers. The corresponding kinematic data were then filtered using a quintic
spline, and the linear and angular velocities and accelerations were derived. The segmental
parameters were estimated using the elliptical method of Jensen (1978). External forces on the
feet were collected using the dynamometric platform. All of these input data were then integrated
into the segment model to calculate the net moments at L5/S1 (flexion-extension, lateral bending
and torsion moments) using the equations of Hof (1992) (Kingma et al., 2006; Plamondon et al.,
1996). Net moments were expressed in the pelvic system (L5/S1 location). With the subject in
the anatomical position, the longitudinal axis is upward, the sagittal axis is forward, and the
transverse axis is to the left. The Grood and Suntay (1983) method was used to estimate 3D
angular motion: the first rotation about the transverse axis (flexion-extension), the second about
the sagittal axis (lateral bending), and the last about the longitudinal axis. To ensure the validity
of the joint moments at L5/S1, those calculated using a bottom-up approach (approach used in
this study) were compared to the moments determined by a top-down approach for the 30
subjects in the standing anatomical position. The RMS errors between the two approaches were
generally less than 7 Nm, which corresponds to the errors already reported in Plamondon et al.
(1996).
2.5 Data analysis
The total number of box transfers for each subject was 128 [4 boxes x 4 heights x 2
orientations x 2 trips (going-return) x 2 repetitions]. Only the “going” experimental condition,
meaning from the conveyor to the hand trolley, was the subject of analyses in this paper, namely
1920 samples for the 30 subjects. Each handling was broken down into different successive
execution phases. There was a lifting phase and a deposit phase. The lifting phase included a prelift (gripping) where the box is brought close to the subject without being lifted, and lifting of the
box (take-off). The deposit phase begins after the lifting phase, and continues to the deposit of the
box until its final position (placement) on the hand trolley. It should be emphasized that the
duration of the flight phase (meaning the time during which the weight of the box was completely
supported by the subjects) was divided into two equal sections (time/2) such that the first section
was an integral part of the lift, and the second section, of the deposit.
Peak values were calculated in the lifting and deposit phases for the extension, lateral
bending and torsion components of the net moments at L5/S1 as well as for the resultant moment
(i.e., the vector sum of the 3 moment components) and the asymmetrical moment (i.e., the vector
7
sum of the lateral bending and torsion moment components). Moreover, at the instant of the peak
resultant moment, the following were calculated: occurrence of this event (interpretation: 0% =
beginning of flight time; 50% = mid-flight, start of deposit phase; 100% = end of flight time),
lumbar flexion, lumbar lateral bending, lumbar torsion, lumbar flexion angular velocity, upper
trunk flexion angle from vertical (calculated from the local coordinate system at C7), horizontal
distance of the hands (center of a line joining the joint center of both wrists) from the L5/S1
joints, and right and left knee flexions. In addition, several kinematic variables were also
calculated during the lifting and deposit phases: duration of transfer, path length of a box,
maximum upper trunk flexion angle (at C7) from the vertical, upper trunk flexion range (range =
max flexion angle – min flexion angle), maximum horizontal hand distance to L5/S1, maximum
vertical hand distance to L5/S1, minimum (shortest) distance of subject’s CG from the floor, right
and left maximum knee flexion and knee flexion range. The path length of a box is the distance
along which the box moves and was estimated using the midpoint of a line joining the joint
center of both wrists. The path length was calculated only during the flight phase.
Analyses of variance (ANOVA) with repeated measurements were performed. Since all
the experimental trials were repeated twice, the statistical analyses were performed by combining
the two trials. Since the quantity of data was significant, we decided to present only the data
relating to the differences between experts and novices. The dual interactions between expertise
and the other variables (box format “B”, box height “H”, and conveyor orientation “O”), namely
E x B, E x H and E x O were examined; however, the triple interactions were excluded from the
analyses due to the complexity of the interpretation involved.
NCSS software (NCSS 2007, Version: 07.1.14) was used to process the statistical data. In
order to be able to do the parametric analyses, the data were transformed using a transformation
(Van Albada and Robinson, 2007) allowing normal distributions to be obtained according to the
Wilk-Shapiro test. Also, to offset a violation of sphericity in the repeated measurement
ANOVAs, the probability threshold was adjusted using the Geisser Greenhouse Epsilon
correction factor.
3. Results
3.1 Duration of transfer and path
There was no significant difference between the two groups in the duration of the different
phases, despite the tendency of the experts to be slower in performing the task (Table 2). The
flight time was slightly longer for the experts (experts = 2.2 s vs novices = 1.9 s), but once again,
this difference was not significant. However, there was a significant interaction (Expertise ×
Height) in the duration of the flight phase. This interaction is explained by the fact that the flight
time decreases with the box deposit height, for experts as well as novices, except that the
difference between the two groups decreases with box height. Furthermore, the path of the box
was not significantly different for the experts (mean = 1.81 m) and novices (mean = 1.71 m).
8
Table 2 Duration of the different phases (n =1920 trials)
Variables
Task duration (s)
Pre-flight (s)
Flight time (s)
Post flight (s)
Experts
M1
SD1
Novices
M
SD
4.7
1.5
2.2
1.0
4.5
1.5
1.9
1.1
1.5
0.8
0.7
0.7
1.5
0.7
0.7
0.7
P Main
Effect
E1
0.61
0.99
0.35
0.82
Interaction (P)1
EB
EH
EO
0.63
0.91
0.20
0.25
0.50
0.11
0.59
0.92
0.94
0.92
0.042
0.77
1
M = Mean; SD = Standard deviation; E = Expertise; EB = Expertise × Box interaction; EH =
Expertise × Height interaction; EO = Expertise × Orientation interaction
2
Significant effects (p< .05) are indicated by bold value.
3.2 Peak resultant moment
Lifting phase: The peak resultant moment at L5/S1 towards the hand trolley was similar
for the two groups (Table 3: experts = 226 Nm vs novices = 228 Nm). However, at the time of
the peak resultant moment, several variables showed significant differences between experts and
novices, mainly posture-related variables (Table 3). Thus, the lumbar flexion angle (experts = 54°
vs novices = 66°; Figure 2) and the upper trunk flexion angle with respect to the vertical (experts
= 62° vs novices = 76°) were significantly smaller for the experts than for the novices. The
experts therefore bent forward less than the novices at the time of the peak resultant moment.
However, the experts bent their knees significantly more (experts ≈ 73° vs novices ≈ 53°). Three
other variables also caught our attention. The peak resultant moment occurred slightly earlier
with the experts, meaning just after the start of the flight phase (experts = 3% vs novices = 6%).
Despite a slight postural asymmetry (lateral bending and torsion angles smaller than 4°), the
lumbar torsion angle was significantly different (experts = 4° vs novices = -1°). The lumbar
angular velocity in flexion was significantly greater for the experts (experts = 25°/s vs novices =
20°/s). It is also interesting to note that the hand distance to L5/S1 was similar in both groups
(experts = 0.40 m vs novices = 0.42 m). Finally, there was no significant interaction between the
Expertise variable and the other independent variables.
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Table 3 L5/S1 peak resultant moment for expert and novice handlers and corresponding
kinematic variables during the lifting (1) and deposit (2) phases (n = 1920 trials)
Variables
Peak L5/S1 resultant moment
(Nm)
Occurrence (%)
Lumbar flexion angle (°)
Lumbar flexibility index (%)
Lumbar lateral bending angle
(°)
Lumbar torsion angle (°)
Upper trunk flexion angle
from the vertical (°)
Horizontal hand distance to
L5/S1 (m)
Right knee flexion (°)
Left knee flexion (°)
Lumbar flexion angular
velocity (°/s)
Lifting
=1
Deposit
=2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
Experts
M1
SD1
226
152
3
88
54
31
83
48
0
-2
4
2
62
39
0.40
0.37
71
46
74
46
25
6
37
57
8
21
11
20
18
31
6
6
5
6
15
23
0.05
0.10
31
30
29
31
12
23
Novices
M
SD
228
164
6
86
66
45
96
65
3
-1
-1
1
76
51
0.42
0.41
51
41
55
35
20
3
43
60
9
21
15
25
21
37
6
6
8
7
17
27
0.06
0.12
36
24
30
24
12
23
P Main
Effect
E1
0.92
0.23
0.043
0.59
0.01
0.00
0.06
0.01
0.08
0.47
0.03
0.59
0.02
0.01
0.15
0.10
0.03
0.50
0.05
0.01
0.04
0.34
Interactions (P)1
EB
EH2
EO
0.43
0.12
0.56
0.22
0.82
0.22
0.70
0.25
0.25
0.56
0.40
0.67
0.98
0.12
0.38
0.54
0.48
0.65
0.95
0.59
0.98
0.06
0.93
0.40
0.53
0.63
0.75
0.27
0.67
0.43
0.64
0.32
0.29
0.20
0.14
0.06
0.69
0.02
0.09
0.13
0.89
0.10
0.47
0.25
0.86
0.66
0.73
0.58
0.48
0.48
0.46
0.40
0.66
0.50
0.74
0.87
0.76
0.85
0.64
0.77
0.36
0.69
0.17
0.35
0.34
0.98
1
M = Mean; SD = Standard deviation; E = Expertise; EB = Expertise × Box; EH = Expertise ×
Height; EO = Expertise × Orientation
2
Height during the lifting phase refers to the position of the boxes on the conveyor; height during
the deposit refers to the height of the boxes on the hand trolley.
32
Significant effects (p< .05) are indicated by bold values.
Lumbar flexion(°)
90
80
70
Experts
60
Novices
50
40
30
1
2
3
4
Box position
Figure 2. Lumbar flexion angle for expert and novice handlers at the instant of the peak resultant
moment during the lifting phase. Errorbars indicate 1 SD.
10
Lumbar flexion(°)
Deposit phase: The results during the deposit phase are essentially the same as those
obtained during the lifting phase. At the instant of the peak resultant moment, the experts were
less bent than the novices for the upper trunk (Table 3) and for lumbar flexion (Table 3, Figure
3), but bent their left knee more. Note that there is an interaction between expertise and box
height for the hand distance from L5/S1, which is explained by the fact that this distance
decreased with the height of the boxes in all cases, but the distance was shorter for the experts
than for the novices (44 cm vs 49 cm) during deposit of the 1st box on the hand trolley, and
practically identical (≈30 cm) for the last box (4th) on the hand trolley (Figure 4).
90
80
70
60
50
40
30
20
10
0
Experts
Novices
1
2
3
4
Box height
Figure 3. Lumbar flexion angle for expert and novice handlers at the instant of the peak resultant
moment during the deposit phase. Errorbars indicate 1 SD.
3.3 Other moments at L5/S1
Lifting phase: As with the peak resultant moment, there is no significant difference
between the experts and novices for the moments around the transverse and sagittal axes (Table
4). However, the torsion moment towards the right was significantly smaller for experts than for
novices (-26 Nm vs -34 Nm), while the moment towards the left was greater for the experts
(experts = 23 Nm; novices = 17 Nm). It must also be emphasized that the lumbar posture
observed at the time of the maximum asymmetrical resultant moment was only slightly
asymmetric (less than 3° for lateral bending or torsion). Also, the experts’ lumbar flexion was
significantly less than that of the novices (expert =34° vs novice = 50°) at the time of the
maximum asymmetrical resultant moment.
11
Table 4 Moments at the L5/S1 joint for expert and novice handlers during the lifting (1) and
deposit (2) phases (n = 1920 trials)
Variables
Max extension moment
(Nm)
Peak L5/S1 asymmetrical
moment (Nm)
Max lateral bending
moment (Nm)2
Min lateral bending
moment (Nm)2
Max torsion moment
(Nm)3
Min torsion moment
(Nm)3
Lifting =1
Deposit =2
1
2
1
2
1
2
1
2
1
2
1
2
Experts
M1
SD1
223
146
59
64
36
34
-52
-49
23
14
-26
-24
37
59
20
19
18
24
18
29
12
12
14
12
Novices
M
SD
224
157
60
64
30
38
-49
-41
17
9
-34
-30
41
61
21
23
20
25
22
35
11
12
16
13
P Main
Effect
E1
0.94
0.32
0.95
0.94
0.23
0.60
0.49
0.32
0.054
0.04
0.02
0.02
Interactions (P)1
EB
EH
EO
0.42
0.23
0.73
0.08
0.39
0.41
0.99
0.64
0.45
0.80
0.05
0.14
0.94
0.28
0.99
0.40
0.99
0.68
0.78
0.02
0.68
0.02
0.56
0.08
0.75
0.84
0.50
0.91
0.64
0.91
0.46
0.24
0.71
0.78
0.47
0.59
1
M = Mean; SD = Standard deviation; E = Expertise; EB = Expertise × Box interaction; EH =
Expertise × Height interaction; EO = Expertise × Orientation
interaction
2
Positive lateral bending moment = towards the right of the subject
3
Positive torsion moment = towards the left of the subject
42
Significant effects (p< .05) are indicated by bold values.
Deposit phase: The torsion moments were significantly different for experts and novices
(Table 4). The interactions between expertise and the height of the boxes must also be
emphasized. Thus, for the experts, the maximum lateral bending moment towards the left was
greater than for the novices, but the difference between the two groups tended to decrease at the
highest deposit height. For the second interaction, the experts were characterized by a greater
torsion moment towards the left than the novices for deposit, but the difference between the two
groups decreased with the height of the boxes.
3.4 Kinematic variables
Lifting phase: The maximum upper trunk angle with respect to the vertical showed that
experts were less bent than novices (Table 5). However, the upper trunk flexion range was similar
for the two groups (around 52°). One also notes that the maximum hand distances from L5/S1,
horizontally (experts = 0.42 m vs novices = 0.45 m) as well as vertically (experts = 0.35 m vs
novices = 0.43 m), were significantly shorter for experts than for novices. As a result, the experts
got closer to the box, horizontally and vertically. To achieve this, the experts seemed to
compensate for less flexion of the upper trunk by more knee flexion (experts ≈ 85° vs novices ≈
60°-70°) and a greater knee flexion range (experts ≈ 69° vs novices ≈ 48°), which brought them
closer to the ground than the novices (experts = 0.63 m vs novices = 0.70 m). There were also a
few significant interactions for the knee flexion angle (Expert x Height). They are explained by a
decrease and an increase in knee flexion between the lifting of the 1st and 4th boxes with the
novices, whereas for the experts, the knee angle did not vary as much with the order of the boxes.
The upper trunk flexion range was seen to remain practically identical with the experts,
regardless of the box format (≈ 51°), whereas it varied by a few degrees with the novices.
12
Table 5 Important kinematic variables for expert and novice handlers during the lifting (1) and
deposit (2) phases (n = 1920 trials)
Variables
Lifting =1
Deposit =2
Max upper trunk flexion angle
from vertical (°)
Upper trunk flexion range (°)
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
Max horizontal hand distance
to L5/S1 (m)2
Max vertical hand distance to
L5/S1 (m)a
Subject’s min CG height (dist
from floor) (m)
Max right knee flexion (°)
Right knee flexion range (°)
Max left knee flexion (°)
Left knee flexion range(°)
Experts
M1
SD1
74
45
51
27
0.42
0.42
0.35
0.17
0.63
0.80
84
69
68
54
89
65
70
55
Novices
M
SD
15
24
17
17
0.05
0.09
0.1
0.16
0.08
0.13
34
26
34
26
29
27
31
27
86
58
52
31
0.45
0.46
0.43
0.21
0.70
0.83
62
62
48
43
71
56
47
46
15
27
22
20
0.06
0.12
0.13
0.20
0.09
0.12
36
23
31
23
30
24
29
23
P Main
Effect
E1
0.023
0.00
0.82
0.12
0.01
0.15
0.02
0.05
0.02
0.08
0.03
0.20
0.07
0.08
0.05
0.17
0.01
0.16
Interactions (P)1
EB
EH
EO
0.91
0.46
0.00
0.62
0.42
0.24
0.89
0.99
0.45
0.42
0.07
0.18
0.19
0.34
0.38
0.19
0.05
0.05
0.80
0.06
0.07
0.61
0.17
0.00
0.36
0.01
0.06
0.39
0.02
0.29
0.88
0.16
0.05
0.06
0.01
0.06
0.49
0.88
0.97
0.55
0.75
0.91
0.70
0.80
0.11
0.07
0.12
0.98
0.25
0.64
0.40
0.46
0.35
0.50
1
M = Mean; SD = Standard deviation; E = Expertise; EB = Expertise × Box interaction; EH =
Expertise × Height interaction; EO = Expertise × Orientation interaction
2
This distance was estimated only during flight
3
Significant effects (p< .05) are indicated by bold values.
0,7
Distance (m)
0,6
0,5
0,4
Experts
0,3
Novices
0,2
0,1
0
1
2
3
4
Box height
Figure 4. Horizontal distance (m) between the hands and the L5/S1 joint at the instant of the peak
resultant moment during the deposit phase. Errorbars indicate 1 SD.
Deposit phase: The maximum upper trunk flexion angle with respect to the vertical (Table
5) was significantly smaller for the experts than the novices, regardless of the deposit height (4
heights). However, contrary to lifting, the maximum knee flexion angle was not different for the
groups. Experts also differed from novices by being vertically closer to the box. Whether it was
with the maximum horizontal or vertical hand distance from L5/S1 (significant interaction), the
experts were always closer than the novices when the box was deposited near the ground, with
13
this distance decreasing as the deposit was done at a higher height, to be practically identical for
the experts and novices at the 4th height (Figure 5). This trend was also observed during the peak
resultant moment (see Figure 4 above).
0,2
Distance (m)
0
-0,2
Experts
-0,4
Novices
-0,6
-0,8
1
2
3
4
Box height
Figure 5. Maximum vertical distance between the hands and the L5/S1 joint during the deposit
phase. Errorbars indicate 1 SD.
4. Discussion
The results show that the experts bent their lumbar region and upper trunk less and knees more
than the novices. Also, they brought the box closer (or get closer to it), both horizontally and
vertically. Figure 6 illustrates the difference between an expert and a novice worker at the time of
the peak resultant moment during lifting. Despite these differences in posture, back loading was
the same, as revealed by the peak resultant moment at L5/S1. Experts would be expected to be
more efficient by reducing the path of the boxes as well as the flight time, but this was not the
case.
EXPERT
NOVICE
Figure 6. Example of posture adopted by an expert and a novice handler.
14
4.1 The moments
According to Marras (2006), the most important factor associated with the risk of workrelated back injury involves the external moment imposed on the spine. From this standpoint, it is
vital that this external moment be kept as small as possible and that, regardless of the technique
used, it allows the load’s center of mass to be brought as close as possible to the spine. There are
reasons why the most commonly recognized and recommended handling principle is to minimize
the distance between the load and the trunk (Graveling et al., 2003; Marras, 2006; Marras, 2008;
McGill, 2002) and why it is the principle most observed with experts and experienced handlers
(Authier et al., 1996; Baril-Gingras and Lortie, 1995). It is known that the moments vary with the
technique used by the handler, as shown by the studies of Kingma et al. (2004) and Kingma et al.
(2006). Contrary to what we thought, there was no significant difference between experts and
novices for the resultant moment, whether it was in lifting or deposit. This is surprising because
the experts were less bent and closer to the box (not significant at the time of the resultant
moment). However, since the lumbar angular velocity was greater for the experts, it had the
effect of increasing the magnitude of the moment and of eliminating the impact of a reduction in
vertical upper trunk flexion and horizontal distance. This concurs with Faber et al. (2007) who
observed that experienced workers did not decrease the moments, as expected, when the mass of
the load was reduced because they increased their trunk angular acceleration and the horizontal
L5/S1-hand distance at the instant of the peak resultant moment. They indicated that individual
adaptations to lifting behavior are likely to interact with the worker’s experience with the task.
For experts, it may be advantageous to increase the lumbar angular velocity in order to lift the
load, at the expense of an increase in the resultant moment.
The asymmetrical moment and the lateral bending and torsion moments were relatively
large, on average reaching levels of 64 Nm, 52 Nm and 34 Nm respectively. Since the subjects’
postures were generally not asymmetrical, the position of the box in relation to the L5/S1 joint
had to deviate sufficiently from the sagittal plane to cause this loading asymmetry. Although
these load levels can increase the risk of injury, they are still less, for example, than in situations
where the feet are solidly in place on the ground, conditions in which Kingma et al. (1998)
measured moments in the order of 101 Nm for lateral (sagittal) bending and 57 Nm for torsion.
Furthermore, Gagnon (2003) emphasized the importance of encouraging foot mobility, in order
to limit asymmetrical back loading. Despite the fact that foot movement on the platform was not
the subject of any constraint in this study, our subjects, experts and novices nevertheless reached
levels greater than those of Gagnon (2003), with the amplitude of the torsion and lateral bending
(sagittal) moments in the order of 28 Nm (for lifting 15-kg boxes from a low 22 cm shelf to
another 22 cm shelf at 90°).
The lateral bending moments between experts and novices were not significantly
different, in contrast to the torsion moments. The positive torsion moments (towards the left in
the anatomical position) were greater for the experts than the novices, but smaller in amplitude
than the negative moments (towards the right in the anatomical position), with the novices having
higher values (around -30 Nm). The differences between experts and novices, although
significant in a majority of cases, were less than 8 Nm, which is within the margin of error of the
linked segment model and likely to be physiologically negligible. It is consequently very difficult
to explain the reason for these differences and to know whether they are relevant in the incidence
of injuries. Moreover, it is interesting to note that Lavender et al. (2007) have shown that
handlers who limited the torsion moments below the value of 30 Nm were less likely to suffer
from back problems. To minimize the risk of injury, handlers would therefore benefit from a
reduction in lumbar torsion loading.
15
4.2 Posture
Three posture-related variables differentiated experts from novices: lumbar flexion, knee
flexion, and bringing the box closer (to the trunk).
4.2.1 Lumbar flexion
The experts bent their lumbar region less than the novices did. So in all the situations
studied, the difference between the two groups for lumbar flexion (or even upper trunk flexion)
was in the order of 10° or more. First, it could be assumed that expertise was the first factor
responsible for this difference. However, in addition to the experts being significantly different
from the novices in the number of years of experience as handlers, they were significantly older
(experts = 38 years of age vs novices = 25 years of age). This factor may have had an impact on
the results. In fact, lumbar flexibility decreases with age (Troke et al., 2005; Intolo et al., 2009)
and could explain in part why the experts bent their backs less than the novices did.
Unfortunately, we did not measure our subjects’ lumbar flexibility. However, based on literature
data, the effect of age on lumbar flexibility can be taken into account. Using the data of Intolo et
al. (2009) and Troke et al. (2005), our subjects’ maximum lumbar flexibility was estimated in
relation to age. Then, each subject’s lumbar flexion angles were divided by his corresponding
predicted maximum flexibility to obtain a lumbar flexibility index 1. The result is presented in
Table 3 and shows that age could in fact explain part of the variance between the two groups.
During lifting, the difference between the two groups virtually remained significant (P = 0.06),
which tends to show that age, but also experience, have an impact in reducing our experts’
lumbar flexibility. Furthermore, during the deposit phase, expertise always seemed to play an
important role because the difference between experts and novices for the flexibility index
remained significant (P= 0.02). One can conclude that age is an important factor to consider, but
also expertise, because it is a key aspect, particularly during the deposit phase.
Schipplein et al. (1990) suggested that quadriceps muscle strength limits the subjects’ ability to
lift with their knee flexed. Some studies have shown that leg weakness affects the lifting
technique (Puniello et al, 2001; Zhang and Buhr, 2002) and recently, Li et al. (2009)
demonstrated that individuals with back strength greater than their total knee strength tended to
use a back-preferred lift technique, and vice versa. This suggests that muscular strength is an
important factor in the lifting technique. In the present study, back strength tests in different
directions (flexion, extension, lateral bending, and torsion) and a lifting strength test (Chaffin et
al., 1978), involving the legs and the back, were not different between experts and novices
(Bellefeuille, 2009). Considering that back strength was not different, it can be concluded from
the lifting strength test that the leg strength was also similar. Therefore, strength cannot be
considered here as explaining the difference in lumbar flexion between the two groups.
Now, how does the lumbar flexion range of motion or even the flexibility index represent
factors to be considered in the prevention of back injuries? There are two schools of thought on
this subject in the literature, as discussed elsewhere (Dolan et al., 1994). The first promotes
lumbar flexion close to the maximum in order to elongate the passive lumbar structures and the
lumbodorsal fascia, which would result in an increase in the contribution of the passive elements
to balancing the external moment and would reduce the internal compressive forces on the disk
(Gracovetsky et al., 1981; Gracovetsky et al., 1989). Dolan et al. (1994) indicated, moreover, that
when a load is lifted from the ground, most of the participants in their study bent their lumbar
1
By multiplying by 100, a percentage of the maximum lumbar flexibility is obtained.
16
region from 80 to 95% of the maximum, and the contribution of passive elements to the external
moment could be as high as 30%. Also, Maduri et al. (2008) hypothesized that elongation of the
passive tissues is a method of energy transfer that promotes energy accumulation during
elongation, which is subsequently restored in the return movement during the lift (stretchingshortening cycle). Although this transfer can be very beneficial in terms of energy savings, it may
also increase the risk of injury. This is the second school of thought, which recommends
maintaining as much as possible a neutral posture of the lumbar spine (or close to lordosis) so as
to limit the elongation of the passive elements of the spine (McGill, 2002; McGill, 2009). Several
reasons support this recommendation. With pronounced lumbar spine flexion: the shear forces are
much greater and would be close to the maximum tolerance of the tissues (around 1000 N); the
ligaments support a large part of the shear forces, which is not the case in a neutral posture where
the muscles play a greater role; at maximum flexion, the disk is 20 to 40% weaker for supporting
the loads than when it is in a neutral posture (McGill, 2002). Also, when the ligaments are
elongated in a cyclical or sustained manner, desensitization of the mechanical receptors would
occur, which would result in a decrease in the reflex activities of the muscles (Solomonow et al.,
1999). Between the two schools, there is the viewpoint of Adams et al. (2002) who emphasize
that moderate lumbar flexion: allows passive structure elongation in order to benefit from a
certain type of elastic energy; reduces the risk of intervertebral disk hernia by avoiding excessive
stretching of the posterior anulus fibrosus; flattens the lumbar curvature, which reduces the
muscle activity required to stabilize the lumbar spine. The posture adopted by our experts
corresponds more to the viewpoint of Adams et al. (2002), where the handler benefits from
certain mechanical advantages of elongation of the passive structures, while leaving a safety
margin for these structures that would be missing in the case of quasi maximum lumbar flexion.
The use of a detailed internal biomechanical model of the spine could help provide a better
estimate of the contribution of the active and passive tissues.
Several authors (Burgess-Limerick, 2006; Adams et al., 2002; McGill, 2007; Marras,
2008) have recommended the avoidance of extreme postures, mainly in flexion, but also in lateral
flexion and axial rotation. Burgess-Limerick (2006) has furthermore indicated that there is no
basis for avoiding moderate lumbar flexion postures. Another study (Authier et al., 1995) also
observed that experts bent their trunk less, particularly when the lift was near the ground. In fact,
these authors observed that 73% of upper trunk flexion was considered severe (>45°) in the case
of experts, whereas these severe flexions occurred in 98% of the trials for the novices. A safety
margin for any handler could be important in the case of fatigue or unanticipated events. For
example, Dolan and Adams (1998) observed in participants an increase in lumbar flexion (from
83.3% to 90.4%) after more than 100 box lifts. Fatigue caused these subjects to work close to
their flexibility limit, which had the advantage of making the passive tissues support a greater
load (energy savings), but had the disadvantage of putting these tissues in a zone that
considerably reduced their safety margin and consequently increased the risk of damage to the
tissues. Experts, despite their age, therefore seem to give themselves a larger safety margin.
4.2.2 Knee flexion
The experts bent their knees significantly more than the novices did, mainly in the lifting
phase and slightly less in the deposit phase. By bending their trunk less, experts had to
compensate by bending their knees more to get closer to the boxes to be lifted. Also observed
was a negative correlation (r ≈-0.65; P<.05) between trunk flexion and knee flexion, mainly in
the lifting phases. This contradicts previous findings by Gagnon et al. (1996) and Authier et al.
(1996), where experts bent their knees significantly less (P≤ .05) on lifting and deposit than
17
novices did. It is difficult to identify the reason for these differences, mainly in relation to the
study of Authier et al. (1996) where the working context was only very slightly different. On the
other hand, in Gagnon et al. (1996), the experts were limited in their movement to two small
force platforms and could not really move their feet.
4.2.3 Bringing the box closer
The experts were closer to the box to be lifted or deposited than were the novices,
although this did not occur during the peak resultant moment. It is not surprising to see this
principle with experts, as already observed by Authier et al. (1995) and Authier et al. (1996). This
explains why the experts, in wanting to get vertically closer to the box, bent their knees more
than the novices did. However, one way of avoiding excessive knee bending is to tilt the box onto
its edge in order to raise its center of gravity. Not all the experts tilted the boxes in this way,
either on lifting or depositing, which could have had the consequence that they had to get lower
than the others. Gagnon (2005) stressed that box tilting could lead to a reduction in the
mechanical work carried out on the box, as well as a reduction in the path of the box and in the
path of the handler’s center of gravity.
4.3 The interactions
The main significant statistical interactions occurred during deposit towards the hand
trolley. In fact, box height affected the expert and novice subjects differently, mainly in the
distance of the box to L5/S1 and the knee flexion angle. Generally, the differences between the
two groups were greater when the box height was minimal (box on the ground) and this
difference blurred when the height was the highest. As a result, the closer the handling operation
was to the ground (maximum back loading), the greater the difference between the experts and
novices. The types of boxes had practically no impact on the methods between the two groups,
but nevertheless had major impacts, just like height, on most of the kinematic and kinetic
variables (results not presented in this paper). It can be concluded that box height can amplify the
differences between experts and novices, and that this factor is important to consider in a training
program.
4.4 Generalization of results
The subjects all met the criteria that had been established. Back problems were absent
during their careers for the very great majority of the subjects, and particularly for the experts.
None of the subjects presented musculoskeletal problems that could affect their normal way of
working, and the problems were minor. The experts were older than the novices, but novices in
industries are generally young people and it would not have been representative to select an older
group. One of our major concerns was to allow the handlers as much freedom as possible so as
not to affect their usual methods. The experimental constraints were therefore minimized as much
as possible so as not to affect these methods. Also, before the study, the participants benefited
from a familiarization session. We believe that the work situation that was presented to them,
although very specific, placed the handlers in familiar handling conditions.
All the boxes had the same size (26 cm deep x 35 cm wide x 32 cm high) which is an
important factor to take into account in the generalization of the results to larger boxes. As
Kingma et al. (2006) substantiated, box size is an important factor if the load does not fit between
the worker’s legs. They demonstrated that lumbar flexion and trunk inclination were larger when
the subjects lifted a wide box (width = 600 mm) than when they lifted a narrow box (width =300
mm). However, it was observed that some experts did not necessarily keep their knees straight
ahead or lift the box squared (as novices frequently did) and some of them, during take-off, tilted
18
the box and spread their knees apart to be closer to the box and to avoid excessive forward
bending. Gagnon (2005) indicated that box tilts led to a reduction in knee flexion which was
attributed to a higher position of the load CG and reduced the distance to the subject’s back.
Increasing the size or the format of the load can certainly affect the way handlers can lift the load
but also introduce new ways of doing the job. Therefore, we do not know whether our results
would have been the same with different box sizes but it is important to take this factor into
account in the generalization of the results.
Experimental errors are always possible in this type of study and could have resulted in an
increase or reduction in the differences between experts and novices. Lumbar flexion was
comparable to the results that can be found in Troke et al. (2005) or in Kingma et al. (2006)
which use the same methodology as ours. If we compared our results with the radiographic study
of Pearcy (1985), the lumbar spine motion of their subjects reached on average 52°; this value did
not include the flexion component from the T12/L1 joint. It is important to note that the T12
cluster was placed just above the T12 vertebra in order to be able to relate the anatomical
landmark at T12 with the cluster. Therefore, additional flexion motion of the L1/T12 and
T11/T12 joints was part of the lumbar flexion in our study and should be considered in the
interpretation of the data.
A final point is that the experts in this study favored a handling method taught as safe, meaning
less trunk flexion and more knee flexion. The majority of the experts knew the “safe” technique:
“back straight – knees bent” (squat lifting technique) for lifting a box, since they had followed a
training program. Nevertheless, it was assumed at the start that these handlers would favor a more
direct, more efficient handling method, transferring the loads in a continuous way rather than by
distinct phases, thus reducing the duration of loading. Furthermore, the past work of Authier
(Authier et al., 1995; Authier et al., 1996) and Gagnon (2005) led us to believe that the experts
would use much more efficient methods than those observed in the laboratory, for example,
bending their knees less during lifting. Of course, because someone was responsible for the
experts’ recruitment in each company, one could hypothesize that our recruitment was biased, by
virtue of the fourth inclusion criterion (expert recognized by their peers) toward those experts that
were already using a “safe” handling technique. However, although this possibility cannot be
clearly rejected, we repeat that the most important criteria (first three) were met, which ensures
that these experts had a negligible incidence of injury. Another possibility that is common to all
MMH studies is that the experts modified their usual work methods to obtain the investigator’s
approval (social desirability bias), despite our specific instructions to reproduce the technique
they normally used at work. Nevertheless, we are confident that the subjects followed our
instructions based on two facts: 1) by means of a questionnaire, we asked the participants after
the experimental sessions whether they had used the same technique as at work and all the
responses were affirmative; and 2) the results for the second (results not presented here) and third
experiments are similar regarding the lifting technique. Therefore in a very different work context
where the participants had to transfer from one pallet to another a total of 250 boxes at an
unimposed and imposed speed (9 boxes/min), the subjects reproduced the same lifting technique
as in the present experiment (Bellefeuille, 2009). We believe that in this specific work context it
would have been very difficult for them (more than 30 minutes of transfer) to fake their
technique. These two facts reinforce our beliefs that the subjects followed our instructions, but
again, there is a possibility that the subjects were impressed by the researchers and lab
environment and changed their usual work technique. It is impossible to verify this without
evaluating workers in their real environment, which should be the next step in this type of
research.
19
5. Conclusion
The results clearly show that the experts bend their lumbar region and upper trunk less than
novices, but bend their knees more, which brings them closer to the box vertically. If one
assumes that the experts’ age could partly explain the differences between experts and novices
for lumbar flexion, it is nevertheless true that in the deposit phase, expertise always explains a
large part of the differences. The experts were horizontally closer to the box, but it was expected
that they would be more efficient by reducing the path of the boxes as well as the flight time; this
was not the case. Also, the peak resultant moment was expected to be less with the experts, but
this did not occur. Therefore, the handler’s posture seems to be a major aspect that should be paid
particular attention, mainly when there is maximum back loading.
20
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