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The efficacy of training for three manual handling strategies based on the observation of expert and novice workers
Clinical Biomechanics 18 (2003) 601–611 www.elsevier.com/locate/clinbiomech
The efficacy of training for three manual handling strategies based on the observation of expert and novice workers M. Gagnon Laboratoire de biom ecanique, D epartement de kin esiologie, Universit e de Montr eal, 2100 boul Edouard-Montpetit, C.P. 6128, Succursale Centre-Ville, Montr eal, Qu e., Canada H3C 3J7 Received 8 August 2002; received in revised form 3 April 2003
Abstract Objectives. To evaluate the efficacy of training for three manual handling strategies, i.e. load tilting/hands positioning, shoulders positioning and feet orientation based on the observation of the contrasted strategies of expert and novice workers and free practice using a search approach. Design. Ten novice male workers were tested at pre-training (one trial) and post-training with homogeneous boxes (three trials) and heterogeneous boxes (two trials) sampled from two sessions. Training took place with homogeneous boxes whereas heterogeneous boxes were new situations. Background. The effectiveness of training programs in safe handling and the repetition of specified techniques are contested; they should rather be based on expert workersÕ strategies. Methods. Pre-training and post-training trials were analyzed with five video cameras and a large force plate. The biomechanical variables included three safety criteria: net 3D resulting moments at L5/S1, asymmetry of posture/efforts at L5/S1 and mechanical work on load; kinematics and ergonomic variables were used as explicative variables. Results. Training produced safer strategies by reducing mechanical work and back extensor moments; this occurred in both load conditions, an indication of the transfer of knowledge. These strategies consisted of changes in load maneuvers (tilting/hand positioning) and feet orientation. Conclusions. Training programs should be based on observations of workers. Relevance These results may guide the specialists involved in training programs. Training based on a search approach by the learner and anchored on observations of contrasted strategies (load tilts/hands positioning and feet orientation) by experts and novices appears promising for safe handling. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Biomechanics; Training; Safety; Expert and novice workers; Handling strategies
1. Introduction Low back pain has been a long-standing health problem in manual workers. The training of workers in safe handling techniques is believed to constitute a preventive measure for reducing risk factors in the working environment. Even though a large consensus of opinions exists for the promotion of sound biomechanical principles, their application in training programs does not appear simple. Training programs have
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met little success in reducing low back injuries and their effectiveness appears contested: the reasons appear to be a lack of control conditions and appropriate measurement techniques (Linton and Kamwendo, 1987; Di Fabio, 1995), inadequate training methods either for their lack of applicability or lack of rationale (St-Vincent et al., 1989) and the lack of consideration for adaptability to suit variations in task, workplace and worker (Sedgwick and Gormley, 1998). Several limitations may be present in the administration and evaluation of training programs: (1) the validity of the techniques taught, (2) the restrictions imposed by testing in a laboratory set-up and (3) the validity of
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safety criteria. First, when referring to the validity of the techniques taught, training is generally focused on the acquisition of specific elements such as leg lift rather than back lift and weight distribution on both legs (Nyg ard et al., 1998), a straighter trunk posture (Nussbaum and Torres, 2001) or the use of a back lordosis (Schenk et al., 1996); these elements have been successfully acquired by learners but there is some questioning of an approach based on uniform instructions on lifting techniques (Nyg ard et al., 1998; Burgess-Limerick et al., 1995), specially in the context of highly variable workplace conditions. There is also a lack of consensus in the scientific community as to the technical elements that should be trained (for instance, leg lift or freestyle; straight back or back lordosis). Others have pinpointed that workers rarely used the handling techniques actually taught in these programs and questioned the appropriateness of these techniques (Nyg ard et al., 1998; BurgessLimerick et al., 1995). On the other hand, when the subjects practice freely, biomechanical changes do not occur (Gagnon et al., 2002). In fact, training protocols should be based on workersÕ knowledge about their jobs. Second, the restrictions imposed by laboratory testing must be addressed: the need to increase the reliability of the data and the constraints imposed by the equipment, such as small force plates, have forced the researchers to control workersÕ performance. The most limiting factors of handling performance have been restrictions of feet mobility (feet parallel and static) and load handling manoeuvres (hands fixed on handles), situations rarely encountered in practice. In fact, experiments should be designed to approach real-life conditions as closely as possible. Finally, the evaluation of safety is very complex: low back problems are multi-factorial and the mechanical causes of occupationally related problems do not meet consensus in the scientific community; however, overexertion, repeated loadings and torsion substantiate the mechanical basis of back problems (Hsiang et al., 1997) and the safety criteria most often used include low back loadings and stresses in the soft tissues, asymmetry of back posture and mechanical work or energy requirements. This study will then address the problem of training some specific handling elements on the basis of observations of workers using an experimental context as close as possible to the realities of the field (no feet motion restrictions by using a large force plate (Desjardins and Gagnon, 2001), no handles on the box, no confinements to sagittal-plane lifts). Safety criteria for evaluation will include posture (asymmetry) and loadings of the back and mechanical work; ergonomic observations will be used as a complement for explaining the biomechanical changes. Training based on the observation of workersÕ strategies appears promising and inspired this study. In fact, very few studies have compared experts and novicesÕ
strategies but it has been found that experts adopted very different strategies when compared to novices (Authier et al., 1995a, 1996). Among these strategies, one should mention the role of load tilts/hand positioning, shoulders positioning and feet orientation, three elements that stress the importance of avoiding the movement restrictions (fixed feet, boxes with handles, sagittal lifts) generally encountered in biomechanical studies of handling. In fact, experts placed their hands more often on the box corners, a way to better control the load, and they tilt the load in various directions using its edges or corners for purposes of raising and approaching the load at take-off and deposit; they also anticipate their displacements, orienting their feet towards deposit, always facing the load. Another element was based on the testing of the hypothesis that the maintenance of shoulders parallel to ground and pelvis would be a more efficient handling posture (Gagnon et al., 2000). These strategies are potentially safer (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999) reducing low back asymmetries of efforts and posture (Gagnon et al., 2000; Delisle et al., 1999), loadings on low back, knees and shoulders (Gagnon et al., 1996; Gagnon, 1997) as well as energy requirements (Gagnon et al., 1996, 2000; Gagnon, 1997), and better positions at take-off and deposit, and reductions of the transfer phase during which the load is entirely supported by the worker (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999). These results suggest the importance of emphasizing workersÕ strategies in training programs rather than imposing prescriptive instructions of ÔbestÕ techniques; in fact, expert workers use several strategies depending on the situation (Authier et al., 1995a, 1996) and teaching specified techniques is not realistic. Motor skill acquisition should then rather be seen as a search strategy where the process of practice is characterized as a search for optimal motor solutions to the problem at hand in opposition to the simple repetition of a particular solution, an approach called ecological (Newell, 1991). It was developed from Bernstein (1967) proposals on coordination and optimisation procedures from biology and physics (Gelfand and Tselin, 1962) and applied to motor skill learning (Newell, 1991; Fowler and Turvey, 1978; Gibson, 1979). For instance, expert workers all tilt loads but differ individually in doing so and the variety of situations (height, depth, load characteristics) also impose some changes in strategy: the question is to know if novice workers can learn to tilt loads in a way to minimize back loadings, asymmetries and energy requirements (the optimisation factors examined here). This approach has apparently not been used in manual handling. It implies that workers should be free to use any motion or posture, which requires extended experimental developments (3D analyses, use of large force plates).
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Therefore, this studyÕs objective was to determine if a training program focusing on contrasted strategies of expert and novice workers, referring to load tilting/ hands positioning, feet orientation and shoulders positioning, would lead to significant biomechanical and ergonomic changes in novices. These strategies were observed from video films contrasting experts and novices and the subjects trained using a search approach. The effect of training was evaluated in two situations: while handling homogeneous boxes (with which training was made) and heterogeneous boxes (a new situation for which no training occurred). The homogeneous boxes were evaluated at pre-training (one trial) and posttraining (three trials selected during two experimental sessions). Heterogeneous boxes were evaluated at pretraining (one trial) and post-training (two trials). More specifically, it was hypothesized that some biomechanical and ergonomic changes would occur immediately after training and that they would progress with time; it was also hypothesized that similar changes would be observed with homogeneous and heterogeneous boxes, indicating a transfer of knowledge to new situations. The main objective was to investigate if the strategies investigated in this study should be included in training programs to safe manual handling. The evaluation included biomechanical variables (safety factors related to back loadings, asymmetry of back posture/efforts, energy requirements) and some ergonomic variables (handling manoeuvres; feet positioning) which could explain the change of strategies.
2. Methods 2.1. Subjects The subjects were 10 male novice workers, college students in physical education, who had at least 3 months experience but a maximum of 12 months in part-time manual handling jobs. They regularly practiced sport activities and were free of musculoskeletal problems. Their mean age was 24 yr (range: 21–28 yr), their height, 1.8 m (1.73–1.9 m) and mass, 79.3 kg (63.4– 100 kg). The Universite de MontrealÕs Ethics Review Board had accepted the experimental protocol. 2.2. Training protocol Each subject was trained individually. He was first instructed about some basic biomechanical principles of lifting (reductions of load lever arm and efforts/posture in torsion). This was followed by the observation of video films contrasting experts and novicesÕ strategies in a variety of workplace conditions, with concentration on one specific technical element at a time (either load tilts/ hands positioning, shoulders positioning or feet orien-
603
tation). The subject then practiced the transfer (unloading then reloading) of 16 homogeneous boxes piled over a height of 1.2 m, using two rows and two columns. The protocol was repeated for each of the three elements. The subject was encouraged to try different strategies (search approach) and he received some feedback, limited to the very first trials, to emphasize the biomechanical principles previously taught; however, no instruction was given on the precise technique to adopt. 2.3. Pre-training and post-training tests The pre-training tests involved two sequences of lifts, the one with the consecutive displacement of two heterogeneous 15-kg boxes and the other one with three homogeneous 15-kg boxes, from a low 22-cm shelf to another 22-cm shelf at 90° to the subjectÕs left and distanced by about 1.6 m (Fig. 1). Only one lift (called control trial) for each of the homogeneous and heterogeneous boxes was analysed; these lifts corresponded to the same conditions of take-off and deposit. The boxes were 30 cm 42 cm 33 cm for height, width and antero-posterior distance; the c.g. was either at the centre (homogeneous boxes) or off-centre, i.e. 30% forwards and 86% upwards (heterogeneous boxes). No instructions were provided about the ways to handle. Following pre-training tests, the training protocol was applied with only homogeneous boxes used (Section 2.2). The post-training tests included 25 sequences of practice, in the same set-up, with three homogeneous boxes followed by one last sequence with two heterogeneous boxes; each sequence of lifts was followed by 3– 4 min rest and a longer 30-min rest at mid-session, for minimizing fatigue effects. For homogeneous boxes, the subject executed a total of 156 lifts in two sessions (26 sequences 3 lifts 2 sessions); however, the analyses were restricted to one lift in the pre-training sequence (trial 1 or control trial), and three lifts in the posttraining sequences, i.e. in the first sequence following training (trial 2), in the last sequence of the first session (trial 26) and the last sequence of the second session (trial 51). For heterogeneous boxes, the subject did not have any training and the analyses included all sequences (last lifts), that is, one lift in the pre-training sequence (trial 1), and two lifts in the post-training sequences, the one at the completion of the first session (trial 2) and the other one, at the completion of the second session (trial 3). These trials were selected to test if training effects would occur and progress with time in the situation for which the subject had trained (homogeneous boxes) and also in a new situation for which no training was done (heterogeneous boxes) to investigate if transfer of knowledge occurred. In all these sequences, either during pre-training or post-training, the subject was free to choose his lifting techniques. The initial and final positions of the boxes
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Fig. 1. Experimental set-up and examples of handling strategies for a pre-training trial at take-off (A) and deposit (B) and for a post-training trial at take-off (C) and deposit (D).
were standardized but box handling manoeuvres as well as feet motions were allowed. The subject was not given any feedback during pre-training or post-training; feedback was limited to the very first trials during training. The subject was not informed that some heterogeneous boxes would be used. 2.4. Techniques and methods The 3D kinematic data were obtained using five video cameras and electronically synchronized with the force data obtained from a large 2.4 2.4 m2 force plate that was designed in our laboratory for large displacements (Desjardins and Gagnon, 2001); the positions of markers for the subject plus load were sampled at 30 Hz for five frames around the specific events of take-off (first moment when the load left the shelf) and deposit (last mo-
ment preceding load contact on the other shelf) and only these two events were analysed. Mechanical work on the load was also analysed for the entire duration of transfer and the positions of markers were sampled at 10 Hz. The 3D reconstruction of markers were obtained by DLT procedures (Marzan, 1975) and filtered with quintic splines. The L5/S1 net reaction moments and orientation were obtained from a 3D biomechanical model (Gagnon and Gagnon, 1992; Plamondon et al., 1995). It included 14 segments: feet, shanks, thighs, pelvis, lower-trunk (from L5/S1 to T8), upper-trunk (from T8 to C7/T1), head-neck, arms, and forearms plus hands. It has been validated by comparing an upper-body model with a lower-body model (Plamondon et al., 1996); a sensitivity analysis has also been performed (Desjardins et al., 1998). Net moments and joint orientation were ex-
M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
pressed in the same trunk/pelvis system, a joint referential system of axes formed by the longitudinal axis of the trunk (torsion), the transverse axis of the pelvis (flexion/extension) and a floating axis (lateral bending) (Gagnon et al., 2002). The calculations included L5/S1 net resulting moments and moments about each axis at take-off and deposit. It also included an index characterizing the global L5/S1 asymmetrical moment defined as the square root of the summation of squares of torsion and lateral bending moments. The mechanical work done on load was estimated during transfer from integration of power as a function of time, power being the dot product of the force exerted on the load (inertial force from the second derivative of c.g. position and gravitational force) and c.g. velocity. Three variables were chosen for safety criteria, that is, back efforts represented by L5/S1 net resulting moment at take-off and deposit (events more likely to be associated with higher back loadings), asymmetry using the index of L5/S1 asymmetrical moments, also at take-off and deposit (higher challenge for trunk posture/efforts while handling loads from/to low heights) and finally, the task global difficulty represented by the mechanical work done on load. General kinematics included duration of load transfer as well as the path length of its c.g. during transfer; at take-off and deposit, the variables included height of c.g. for load and for subject and their horizontal distances from the back (L5/S1), lumbar flexion, torsion and lateral bending angles and knee flexion (angle between the longitudinal axes of thigh and shank). Ergonomic observations (load tilts/hands positions and orientation), were made at take-off, deposit and during transfer. Internal validity was verified for one trial per subject analysed twice by the same observer: the average correspondence was 80%; the main discrepancies were for hands placement (near or at the corner of the box) and for tilting (edge and/or corner). Analyses of variance with repeated measures on learning trials were used to determine if the four trials with homogeneous boxes differed significantly and, if the three trials with heterogeneous boxes would also differ; probability values were Greenhouse–Geisser corrected (P 6 0:05). The hypotheses were that the pre-training trial would differ from the post-training trials and that the changes would progress across trials during the session and between sessions.
3. Results The results showed that substantial biomechanical and ergonomic changes occurred immediately, at the first post-training trial and when compared to the pretraining trial, that these changes were maintained
605
throughout the session and between sessions; these changes occurred for two out of the three biomechanical safety criteria selected (mechanical work and back loadings). Similar patterns of changes were observed for both conditions of homogeneous boxes (with which training was made) and heterogeneous boxes (new situation) (Tables 1 and 2). 3.1. Homogeneous boxes Significant differences were found between the pretraining trial and the post-training trials whereas no differences were found between the post-training trials (Table 1). At take-off and deposit, the learning responses corresponded to smaller knee flexion (by about 35°) and higher positions of the box c.g. (by about 6 cm) due to box tilting strategies (Table 3). At take-off, with learning, the resulting horizontal distance of the load from the back (L5/S1) was reduced (by about 10 cm) and the feet were more oriented towards the deposit area (larger angle, between 17° and 33°), whereas, at deposit, smaller net resulting L5/S1 moments (about 40 N m or 25%) were observed; finally, these effects were carried over the transfer period with smaller load transfer time (by about 10%) and smaller load path (by about 50 cm or 27%) and the consequent large reductions of mechanical work done on load (about 50 J or 40%). The absence of statistical differences for the net resulting L5/S1 moment at take-off was explained by the atypical last trial (# 51) by one subject (considerable increases in asymmetrical moments affecting the resulting moment and due to lack of load orientation towards deposit). Learning effects were not detected for asymmetry: back posture was symmetrical with only slight asymmetrical moments (between 20 and 36 N m for torsion or lateral bending) observed in all trials; these moments are low in comparison to literature on asymmetry (Gagnon et al., 1996, 2000, 2002; Plamondon et al., 1995; J€ager and Luttman, 1992; Kingma et al., 1998; Gagnon et al., 1993) (Table 4; discussion). Ergonomic data (Table 3, Fig. 1) showed that, at take-off, the pre-training trial was associated with the absence of box tilts (80% of the cases) and the posttraining trials, with higher frequencies of box tilts on edges and/or corners (83% vs. 20%), with both hands on box corners (23% vs. 0%) suggesting more manoeuvrability; the box was also more oriented towards deposit (more than 30°: 73% vs. 50%); at deposit, similar observations prevailed with higher frequencies of the posttraining trials using box tilts on edges and/or corner (93% vs. 30%), the absence of tilts predominating in the pre-training trial (70% of the cases); finally, during transfer, there were no load rotations in the pre-training trial whereas the post-training trials included load rotation manoeuvres (27% of the cases), probably to better distribute the load on both hands.
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Table 1 Biomechanical changes (means and standard deviations (SD)) between the pre-training test and post-training test for the homogeneous boxes (P 6 0:05; n ¼ 10) Variables
Take-off Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Feet orientation towards deposit (deg) Horizontal distance of load from L5/S1 (cm) Height of load c.g. (cm) Deposit Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Horizontal distance of load from L5/S1 (cm) Height of load c.g. (cm) Transfer Mechanical work on loadc (J) Duration (%) Path length of load c.g. (m) a; b; c
Pre-training
Post-training
(1)
(2)
P (26)
Specific comparisons
(51)
Mean
(SD)
Mean
(SD)
Mean
(SD)
Mean
(SD)
1 vs. 2
1 vs. 26
1 vs. 51
192
(38)
179
(49)
182
(51)
197
(58)
0.52
46
(25)
46
(24)
44
(34)
44
(30)
0.99
35 22
(29) (15)
32 29
(23) (20)
23 34
(21) (31)
27 28
(33) (18)
0.70 0.49
6 3
(3) (2)
5 3
(4) (3)
5 3
(2) (3)
5 4
(3) (3)
0.77 0.51
45 65 21
(12) (36) (23)
46 28 38
(12) (20) (24)
44 28 54
(34) (14) (19)
44 25 53
(30) (12) (20)
0.99 0.00 0.01
0.00 0.1
0.00 0.00
0.00 0.00
56
(6)
47
(4)
47
(4)
47
(4)
0.00
0.00
0.00
0.00
39
(2)
44
(3)
44
(4)
44
(3)
0.00
0.00
0.00
0.00
141
(36)
107
(26)
109
(24)
96
(32)
0.01
0.00
0.00
0.00
47
(16)
36
(12)
39
(19)
40
(19)
0.53
36 24
(15) (19)
26 20
(13) (14)
29 21
(20) (13)
29 24
(18) (15)
0.57 0.91
3 3
(1) (2)
3 3
(1) (3)
5 3
(3) (2)
4 4
(4) (2)
0.27 0.70
44 56 55
(15) (38) (9)
42 22 52
(13) (11) (6)
43 25 53
(15) (15) (8)
47 20 54
(15) (12) (10)
0.27 0.01 0.69
0.00
0.00
0.00
41
(2)
47
(4)
47
(4)
47
(3)
0.00
0.00
0.00
0.00
(32) 0.03 (13) 0.04 (0.39) 0.04
0.01 0.00 0.01
0.02 0.04 0.02
0.00 0.09 0.01
106 (70) 59 (44) 66 37 (17) 25 (12) 29 1.87 (0.77) 1.37 (0.50) 1.44
(43) 53 (13) 30 (0.53) 1.35
Variables identified as criteria for safety (see text), respectively for back loadings, asymmetrical efforts and mechanical work or energy.
3.2. Heterogeneous boxes Significant differences were found between the pretraining trial and the post training trials whereas no differences were found between post-training trials (Table 2). The learning responses were similar to those observed for homogeneous loads, even though the lifting of heterogeneous loads were new tasks for which the learner had not had previous training: kinematics and ergonomic observations suggested that transfer of knowledge occurred with similar strategies of box tilts and similar effects of reduced mechanical work and back efforts. However, there was one major difference during transfer: after training, the heterogeneous boxes were
more often rotated compared to the homogeneous loads (45% vs. 27%), suggesting increased manoeuvrability. Symmetry of posture/efforts was also observed and not dependent on learning.
4. Discussion The training of workers is often based on the widespread convention that the Ôleg-liftÕ is the correct technique for lifting or on biomechanical principles without proper anchorage to the physical realities of manual handling (workplace: load placement in height, depth, obstacles; load format and heterogeneity); for
Table 2 Biomechanical changes (means and standard deviations (SD)) between the pre-training test and post-training test for the heterogeneous boxes (P 6 0:05; n ¼ 9d ) Variables
Post-training
Post-training
(1)
Specific comparisons
(2)
(3)
(SD)
Mean
(SD)
Mean
(SD)
P
1 vs. 2
1 vs. 3
Take-off Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Feet orientation towards deposit (deg) Horizontal distance of load from L5/S1 (cm)
210 23 9 19 4 2 43 65 16 57
(38) (18) (6) (20) (2) (2) (13) (44) (15) (9)
197 49 34 31 3 2 47 25 39 47
(40) (31) (24) (27) (2) (2) (14) (13) (31) (8)
188 47 33 23 5 3 47 24 52 48
(35) (29) (35) (15) (4) (1) (14) (18) (21) (9)
0.05 0.10 0.06 0.48 0.19 0.41 0.22 0.03 0.01 0.04
0.13
0.01
0.01 0.01 0.02
0.00 0.00 0.03
Deposit Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Horizontal distance of load from L5/S1 (cm)
148 37 15 30 2 5 41 61 56
(38) (16) (8) (22) (3) (3) (13) (42) (7)
124 36 21 23 4 4 47 24 56
(25) (15) (14) (20) (3) (3) (16) (14) (12)
119 34 19 25 5 4 48 26 56
(32) (16) (12) (18) (3) (2) (15) (17) (11)
0.10 0.85 0.58 0.63 0.25 0.42 0.11 0.03 0.99
0.01
0.01
Transfer Mechanical work on loadc (J) Duration (%) Path length of load (m)
129 39 2.13
(57) (10) (0.52)
(38) (13) (0.44)
0.00 0.05 0.00
0.00 0.04 0.01
0.00 0.01 0.00
73 30 1.67
(49) (15) (0.50)
67 28 1.44
M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
Mean
a; b; c
d
Variables identified as criteria for safety (see text), respectively back loadings, asymmetrical efforts and mechanical work or energy. The data from one trial in one subject had to be discarded; this subject was removed from these analyses.
607
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M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
Table 3 Ergonomic observations of handling elements; frequency of occurrence for the pre-test trial (n ¼ 10) compared to all post-training trials (n ¼ 30 for homogeneous loads; n ¼ 20 for heterogeneous loads) Variables
Homogeneous loads Pre-training
Take-off Hands position
Heterogeneous loads Post-training
Pre-training
Post-training
Edge/edge Edge/corner Corner/corner
20 80 0
20 57 23
10 90 0
25 50 25
Load tilt
None (flat) On edge On corner
80 20 0
17 60a 23a
60 40 0
10 65a 25a
Load orientation relative to deposit
Slight (<30°) Medium (30–60°) Large (>60°)
50 50 0
27 70 3
50 50 0
35 65 0
None (flat) On edge On corner
70 30 0
7 60a 33a
50 50 0
20 60b 20b
100 0
73 27
90 10
55 45
Deposit Load tilt
Transfer Load rotation a b
No Yes
The majority of tilts were either to the right or towards the subject or both. The majority of tilts were either to the right or towards the subject or away from the subject.
instance, one may understand the principle of reducing the lever arm of the load but its application (load manoeuvres) may not be easily discovered. An extensive data collection of opinions of more than 900 health professionals in Australia showed a wide consensus that the essence of lifting skill is adaptability (a way to vary the strategy as a function of the wide variety of situations) and that the basic principles of skill learning must be applied with adaptability as a goal (Sedgwick and Gormley, 1998). It is clear that some drawbacks exist in the biomechanics research of lifting: too many restrictions are imposed on the subject (fixed feet, boxes with handles, sagittal plane lifts) and the focus is often on the back without consideration for other joints or other dimensions such as motor control, fatigue effects, balance. On the other hand, the scientific foundations for training programs, specially for safety, still appear unclear; inadequate training methods may explain the failure of training to improve manual handling (St-Vincent et al., 1989). Our results have demonstrated that the content of training programs should be complemented by observations of workersÕ strategies, contrasting experts and novices, and using a search approach, instead of limiting the programs to theoretical concepts and the teaching of pre-determined techniques. A previous ergonomic evaluation of experts and novices while performing a variety of similar tasks have evidenced that experts adopted very different strategies (Authier et al., 1995a, 1996) and that the strategies
varied as a function of workplace (height of load, weight) (Authier et al., 1995a). These strategies are potentially safer (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999), reducing energy requirements or mechanical work (Gagnon et al., 1996, 2000; Gagnon, 1997), low back asymmetry of efforts and posture (Gagnon et al., 2000; Delisle et al., 1999) as well as loadings on the back, knees and shoulders (Gagnon et al., 1996; Gagnon, 1997). In this study, it was therefore hypothesized that if novices were trained for those most important strategies contrasting experts from novices (namely, box tilting/hands positioning, shoulders positioning and feet orientation), they would improve in those same safety criteria previously evaluated in experts and novices (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999); the objective was to demonstrate that these strategies contrasting experts and novices should be a part of training programs. The experts from whom strategies were observed had been selected from a large transportation firm and identified by fellow workers and managers as those workers having the best manual handling skills; they had at least 10 years of experience and a much lower rate of handling accidents (0.13 vs. 0.83 accident/worker/year) (Gagnon et al., 1996); finally, they possessed an extended knowledge about their jobs (Authier et al., 1995b). The approach of contrasting strategies of experts and novices was a means to encourage the subjects in seeking new ways of handling. Our results partially demonstrated that the strategies investigated in this study should be part of
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Table 4 L5/S1 moments (N m) and angles (deg) as related to trunk posture, feet positioning and box handling manoeuvres from previous publications Investigators
Tasks
Moments (N m) Extensors
Twist
Angles Lateral bending
Flexion
Twist
Lateral bending
–
–
–
(J€ ager and Luttman, 1992)a
Simulation of diagonal lift 20-kg load Fixed feet; no box manoeuvers
200
50
20
(Gagnon et al., 1993)a
16.4-kg box; fixed hand positions Fixed feet, symmetrical; load: 90° Fixed feet, asymmetrical; load: 45° Feet pivoting, symmetrical; load: 90°
202 217 229
78 53 28
60 55 32
47 45 29
21 17 3
16 3 3
11.6-kg box; Fixed hand positions; Shoulders parallel to ground. Fixed feet, symmetrical, load: 90° Fixed feet, symmetrical, load: 0°
217 228
33 20
35 25
65 72
19 1
4 6
200
30
30
50
10
20
11.0-kg box with handles Fixed feet, symmetrical Load at 0° (facing load) Load at 90°
221 191
21 57
24 101
35° 35°
0° 25°
4° 8°
12.4-kg box, fixed feet; asymmetrical (no step but slight feet mobility) Load tilted; shoulders parallel to ground No load tilt; shoulders parallel to ground Load tilt; shoulders not parallel to ground
172 188 165
19 22 14
33 38 113
56 57 58
1 2 3
4 6 19
15.0-kg box; free feet positioning; free box manoeuvres Best trial for back symmetry Worst trial for back symmetry
207 222
9 8
10 71
47 47
2 7
3 6
15.0-kg box; free feet positioning; free box manoeuvres Last trial on homogeneous box Last trial on heterogeneous box
190 178
27 33
28 23
49 47
5 5
4 3
(Plamondon et al., 1995)a
(Gagnon et al., 1996)a
(Kingma et al., 1998)a
(Gagnon et al., 2000)a
(Gagnon et al., 2002)b
This studyb
17-kg load; free box manoeuvers Free feet positions (but constrained on force plates)
The bold figures emphasize asymmetry of posture and/or efforts (moments): twist and lateral bending angles > 15° and moments > 50 N m. a Peak moments. b Data at take-off.
training programs to safe handling; the novices training in this manner effectively adopted less risky strategies in both conditions of homogeneous and heterogeneous loads: mechanical work was reduced by about 50% and back efforts by 10–30%. The criterion of asymmetry of effort/posture was not affected by the training approach but it is important to mention that, in all trials, asymmetries of posture and efforts were not important (only about 5° of torsion and lateral bending and 30 N m of torsion and lateral bending moments); it is highly probable that the absence of restrictions imposed on the subject for feet positioning and box manoeuvres may have influenced trunk posture and could explain these low levels of asymmetry. It was then of interest to investigate the literature on asymmetry of lifting, on the basis of the three strategies
for which the subjects had trained: posture with reference to shoulders positioning; feet mobility; load tilting (Table 4). Posture, with the maintenance of the shoulders parallel to ground in asymmetrical handling, has been shown to have a high impact on low-back, reducing asymmetrical moments (with shoulders parallel to ground, lateral bending moments and angles were 38 N m and 6°, as compared to 113 N m and 19° when not parallel (Gagnon et al., 2000); even with the load at 90°, twist and lateral bending moments were only about 30 N m when the shoulders were maintained parallel (Plamondon et al., 1995)); with regards to feet mobility, the studies involving asymmetrical lifts with fixed feet have shown higher lateral bending and/or torsion moments (50 N m in torsion (J€ager and Luttman, 1992); 57 and 101 N m in torsion and lateral bending (Kingma
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et al., 1998); about 75 N m in torsion and 60 N m in lateral bending (Gagnon et al., 1993)) whereas asymmetrical lifts executed with free feet positioning usually presented smaller values for these moments (28 N m in torsion and 32 N m in lateral bending with feet pivoting (Gagnon et al., 1993); about 30 N m for these moments with free feet positioning (Gagnon et al., 1996)); finally, load tilting does not appear to affect asymmetry but it is related to reductions of back extensor efforts at take-off (172 N m as compared to 188 N m (Gagnon et al., 2000)) as well as mechanical work done on the load (by 20%: data not presented in Table 4 (Gagnon et al., 2000)). This suggests that posture (shoulders positioning) and feet mobility play a role in asymmetry but the role of load tilting has not been demonstrated. The effect of feet mobility as compared to fixed feet positioning, in particular, merits further investigation; in this study, it was not possible to investigate this aspect as the subjects were free to move their feet at will which is the possible cause of the low levels of asymmetry observed for posture and efforts. The most significant technical change in this study was for load tilting and the accompanying biomechanical changes were similar to those in the literature, i.e., reductions of mechanical work on load and low back extensor moments (Gagnon, 1997; Gagnon et al., 2000) with no effects on asymmetry (Gagnon et al., 2000). Studies on repeated lifting usually involve protocols inducing fatigue effects (Parnianpour et al., 1988; Novak et al., 1993; Hagen et al., 1995; Fogleman and Smith, 1995; Sparto et al., 1997; Marras and Granata, 1997; van Die€en et al., 1998; Dolan and Adams, 1998). Fatigue induced practice leads to changes from a squat to a stoop, that is, reduction of knee flexion (between 5° and 10°) often with increased trunk or lumbar flexion (a few degrees only) (Hagen et al., 1995; Sparto et al., 1997; van Die€en et al., 1998; Dolan and Adams, 1998), decreased back extensor moments (between 5 and 30 N m) (Novak et al., 1993; Sparto et al., 1997; Marras and Granata, 1997; Dolan and Adams, 1998) but increased trunk asymmetry with more twist and/or lateral flexion (about 5° of twist and twisting range of motion) (Parnianpour et al., 1988; van Die€en et al., 1998) and decreased time of lift (Novak et al., 1993; Fogleman and Smith, 1995). In the few cases where fatigue may not have been involved, learning processes with practice were reported to be either very slow (Chaffin et al., 1999) or absent (Gagnon et al., 2002). It is worth noticing that excepted for this last study, the subjects were constrained, lifting in the sagittal plane, either with fixed feet and/or the use of boxes with handles. It is also worth mentioning the wide variety of responses observed not only between subjects but also within subject in the process of repeated lifts (Gagnon et al., 2002; van Die€en et al., 1998, 2001). In the present study, practice based on the observation of workers using a search strategy showed that some bio-
mechanical changes were similar to those recorded in repeated trials associated with fatigue (reduced knee flexion, but to a much larger extent in this study, and reduced back extensor moments of a similar magnitude), whereas other changes were not observed with fatigue and included reductions of mechanical work on load, associated with reduced load trajectory and time of transfer and attributed to load tilting and feet positioning strategies. It is dubious that the results of this study may have been affected by fatigue: the protocol was designed to reduce these effects, specially by including long rest pauses between trials and at mid-session: in fact, throughout the 4-h session, the subjects lifted an average of 15 kg/min.; these conditions appear mild and differ considerably from the protocols reported with studies on fatigue (Parnianpour et al., 1988; Novak et al., 1993; Hagen et al., 1995; Fogleman and Smith, 1995; Sparto et al., 1997; Marras and Granata, 1997; van Die€en et al., 1998; Dolan and Adams, 1998); moreover, our previous control study using a similar protocol of trials for free practice without any training (Gagnon et al., 2002) had not shown any effects. In conclusion, a training program based on observations of contrasting strategies of expert and novice workers, present a potential for improving handling manoeuvres towards more safety, especially for reducing mechanical work done on the load and back efforts. The focus should be mainly on handling load manoeuvres, specially load tilts and hands positioning, and feet displacement strategies. More information is needed to help determine the factors affecting asymmetry, feet positioning appearing an important contributing factor to symmetry/asymmetry of trunk posture.
Acknowledgement The financial assistance of the Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged.
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repetitive lifting employing stoop and squat techniques. Clin. Biomech. 3, 122–127. Hsiang, S.M., Brogmus, G.E., Courtney, T.K., 1997. Low back pain and lifting techniques. A review. Int. J. Indust. Ergonom. 19, 59– 74. J€ager, M., Luttman, A., 1992. The load on the lumbar spine during asymmetrical bi-manual materials handling. Ergonomics 35, 783– 805. Kingma, I., van Die€en, J.H., de Looze, M., Toussaint, H.M., Dolan, P., Baten, C.T.M., 1998. Asymmetrical low back loading in asymmetric lifting movements is not prevented by pelvic twist. J. Biomech. 31, 527–534. Linton, S.J., Kamwendo, K., 1987. Low back schools. A critical review. Phys. Ther. 9, 1375–1383. Marras, W.S., Granata, K.P., 1997. Changes in trunk dynamics and spine loading during repeated trunk exertions. Spine 21, 2564– 2570. Marzan, G.T., 1975. Optimum configuration of data acquisition in close-range photogrammetry. In: Symposium on Close-Range Photogrammetric Systems. Falls Church, Illinois, pp. 558–573. Newell, K.M., 1991. Motor skill acquisition. Annu. Rev. Psychol. 42, 213–237. Novak, G.J., Schipplein, O.D., Trafimow, J.H., Andersson, G.B.J., 1993. Influence of erector spinae muscle fatigue on lumbo-sacral moment during lifting. Eur. J. Exp. Musculoskel. Res. 2, 39–44. Nussbaum, M.A., Torres, N., 2001. Effects of training in modifying working methods during common patient-handling activities. Int. J. Indust. Ergonom. 27, 33–41. Nyg ard, C.-H., Merisalo, T., Arola, H., Manka, M.-L., Huhtala, H., 1998. Effect of work changes and training in lifting technique on physical strain: A pilot study among female workers of different ages. Int. J. Indust. Ergonon. 21, 91–98. Parnianpour, M., Nordin, M., Kahanovitz, N., Frankel, V., 1988. The triaxial coupling of torque generation of trunk muscles during isometric exertions and the effect of fatiguing isoinertial movements on the motor output and movement patterns. Spine 13, 982–992. Plamondon, A., Gagnon, M., Gravel, D., 1995. Moments of the L5/S1 joint during asymmetrical lifting: effects of different load trajectories and initial load positions. Clin. Biomech. 10, 128–136. Plamondon, A., Gagnon, M., Desjardins, P., 1996. Validation of two 3-D segment models to calculate the net reaction forces and moments at the L5/S1 joint in lifting. Clin. Biomech. 11, 101–110. Schenk, R.J., Doran, R.L., Stachura, J.J., 1996. Learning effects of a back education program. Spine 19, 2183–2189. Sedgwick, A.W., Gormley, J.T., 1998. Training for lifting: an unresolved ergonomic issue? Appl. Ergonom. 5, 395–398. Sparto, P.J., Parnianpour, M., Reinsel, T.E., Simon, S., 1997. The effect of fatigue on multijoint kinematics and load sharing during a repetitive lifting test. Spine 22, 2647–2654. St-Vincent, M., Tellier, C., Lortie, M., 1989. Training in handling: an evaluative study. Ergonomics 2, 191–210. van Die€en, J.H., van der Burg, P., Raaijmakers, T.A.J., Toussaint, H.M., 1998. Effects of repetitive lifting on kinematics: inadequate anticipatory control or adaptive changes? J. Motor Behav. 1, 20–32. van Die€en, J.H., Dekkers, J.J.M., Groen, V., Toussaint, H.M., Meijer, O.G., 2001. Within-subject variability in low back load in a repetitively performed, mildly constrained lifting task. Spine 26, 1799–1804.
The efficacy of training for three manual handling strategies based on the observation of expert and novice workers M. Gagnon Laboratoire de biom ecanique, D epartement de kin esiologie, Universit e de Montr eal, 2100 boul Edouard-Montpetit, C.P. 6128, Succursale Centre-Ville, Montr eal, Qu e., Canada H3C 3J7 Received 8 August 2002; received in revised form 3 April 2003
Abstract Objectives. To evaluate the efficacy of training for three manual handling strategies, i.e. load tilting/hands positioning, shoulders positioning and feet orientation based on the observation of the contrasted strategies of expert and novice workers and free practice using a search approach. Design. Ten novice male workers were tested at pre-training (one trial) and post-training with homogeneous boxes (three trials) and heterogeneous boxes (two trials) sampled from two sessions. Training took place with homogeneous boxes whereas heterogeneous boxes were new situations. Background. The effectiveness of training programs in safe handling and the repetition of specified techniques are contested; they should rather be based on expert workersÕ strategies. Methods. Pre-training and post-training trials were analyzed with five video cameras and a large force plate. The biomechanical variables included three safety criteria: net 3D resulting moments at L5/S1, asymmetry of posture/efforts at L5/S1 and mechanical work on load; kinematics and ergonomic variables were used as explicative variables. Results. Training produced safer strategies by reducing mechanical work and back extensor moments; this occurred in both load conditions, an indication of the transfer of knowledge. These strategies consisted of changes in load maneuvers (tilting/hand positioning) and feet orientation. Conclusions. Training programs should be based on observations of workers. Relevance These results may guide the specialists involved in training programs. Training based on a search approach by the learner and anchored on observations of contrasted strategies (load tilts/hands positioning and feet orientation) by experts and novices appears promising for safe handling. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Biomechanics; Training; Safety; Expert and novice workers; Handling strategies
1. Introduction Low back pain has been a long-standing health problem in manual workers. The training of workers in safe handling techniques is believed to constitute a preventive measure for reducing risk factors in the working environment. Even though a large consensus of opinions exists for the promotion of sound biomechanical principles, their application in training programs does not appear simple. Training programs have
E-mail address: [email protected] (M. Gagnon). 0268-0033/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-0033(03)00076-7
met little success in reducing low back injuries and their effectiveness appears contested: the reasons appear to be a lack of control conditions and appropriate measurement techniques (Linton and Kamwendo, 1987; Di Fabio, 1995), inadequate training methods either for their lack of applicability or lack of rationale (St-Vincent et al., 1989) and the lack of consideration for adaptability to suit variations in task, workplace and worker (Sedgwick and Gormley, 1998). Several limitations may be present in the administration and evaluation of training programs: (1) the validity of the techniques taught, (2) the restrictions imposed by testing in a laboratory set-up and (3) the validity of
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safety criteria. First, when referring to the validity of the techniques taught, training is generally focused on the acquisition of specific elements such as leg lift rather than back lift and weight distribution on both legs (Nyg ard et al., 1998), a straighter trunk posture (Nussbaum and Torres, 2001) or the use of a back lordosis (Schenk et al., 1996); these elements have been successfully acquired by learners but there is some questioning of an approach based on uniform instructions on lifting techniques (Nyg ard et al., 1998; Burgess-Limerick et al., 1995), specially in the context of highly variable workplace conditions. There is also a lack of consensus in the scientific community as to the technical elements that should be trained (for instance, leg lift or freestyle; straight back or back lordosis). Others have pinpointed that workers rarely used the handling techniques actually taught in these programs and questioned the appropriateness of these techniques (Nyg ard et al., 1998; BurgessLimerick et al., 1995). On the other hand, when the subjects practice freely, biomechanical changes do not occur (Gagnon et al., 2002). In fact, training protocols should be based on workersÕ knowledge about their jobs. Second, the restrictions imposed by laboratory testing must be addressed: the need to increase the reliability of the data and the constraints imposed by the equipment, such as small force plates, have forced the researchers to control workersÕ performance. The most limiting factors of handling performance have been restrictions of feet mobility (feet parallel and static) and load handling manoeuvres (hands fixed on handles), situations rarely encountered in practice. In fact, experiments should be designed to approach real-life conditions as closely as possible. Finally, the evaluation of safety is very complex: low back problems are multi-factorial and the mechanical causes of occupationally related problems do not meet consensus in the scientific community; however, overexertion, repeated loadings and torsion substantiate the mechanical basis of back problems (Hsiang et al., 1997) and the safety criteria most often used include low back loadings and stresses in the soft tissues, asymmetry of back posture and mechanical work or energy requirements. This study will then address the problem of training some specific handling elements on the basis of observations of workers using an experimental context as close as possible to the realities of the field (no feet motion restrictions by using a large force plate (Desjardins and Gagnon, 2001), no handles on the box, no confinements to sagittal-plane lifts). Safety criteria for evaluation will include posture (asymmetry) and loadings of the back and mechanical work; ergonomic observations will be used as a complement for explaining the biomechanical changes. Training based on the observation of workersÕ strategies appears promising and inspired this study. In fact, very few studies have compared experts and novicesÕ
strategies but it has been found that experts adopted very different strategies when compared to novices (Authier et al., 1995a, 1996). Among these strategies, one should mention the role of load tilts/hand positioning, shoulders positioning and feet orientation, three elements that stress the importance of avoiding the movement restrictions (fixed feet, boxes with handles, sagittal lifts) generally encountered in biomechanical studies of handling. In fact, experts placed their hands more often on the box corners, a way to better control the load, and they tilt the load in various directions using its edges or corners for purposes of raising and approaching the load at take-off and deposit; they also anticipate their displacements, orienting their feet towards deposit, always facing the load. Another element was based on the testing of the hypothesis that the maintenance of shoulders parallel to ground and pelvis would be a more efficient handling posture (Gagnon et al., 2000). These strategies are potentially safer (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999) reducing low back asymmetries of efforts and posture (Gagnon et al., 2000; Delisle et al., 1999), loadings on low back, knees and shoulders (Gagnon et al., 1996; Gagnon, 1997) as well as energy requirements (Gagnon et al., 1996, 2000; Gagnon, 1997), and better positions at take-off and deposit, and reductions of the transfer phase during which the load is entirely supported by the worker (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999). These results suggest the importance of emphasizing workersÕ strategies in training programs rather than imposing prescriptive instructions of ÔbestÕ techniques; in fact, expert workers use several strategies depending on the situation (Authier et al., 1995a, 1996) and teaching specified techniques is not realistic. Motor skill acquisition should then rather be seen as a search strategy where the process of practice is characterized as a search for optimal motor solutions to the problem at hand in opposition to the simple repetition of a particular solution, an approach called ecological (Newell, 1991). It was developed from Bernstein (1967) proposals on coordination and optimisation procedures from biology and physics (Gelfand and Tselin, 1962) and applied to motor skill learning (Newell, 1991; Fowler and Turvey, 1978; Gibson, 1979). For instance, expert workers all tilt loads but differ individually in doing so and the variety of situations (height, depth, load characteristics) also impose some changes in strategy: the question is to know if novice workers can learn to tilt loads in a way to minimize back loadings, asymmetries and energy requirements (the optimisation factors examined here). This approach has apparently not been used in manual handling. It implies that workers should be free to use any motion or posture, which requires extended experimental developments (3D analyses, use of large force plates).
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Therefore, this studyÕs objective was to determine if a training program focusing on contrasted strategies of expert and novice workers, referring to load tilting/ hands positioning, feet orientation and shoulders positioning, would lead to significant biomechanical and ergonomic changes in novices. These strategies were observed from video films contrasting experts and novices and the subjects trained using a search approach. The effect of training was evaluated in two situations: while handling homogeneous boxes (with which training was made) and heterogeneous boxes (a new situation for which no training occurred). The homogeneous boxes were evaluated at pre-training (one trial) and posttraining (three trials selected during two experimental sessions). Heterogeneous boxes were evaluated at pretraining (one trial) and post-training (two trials). More specifically, it was hypothesized that some biomechanical and ergonomic changes would occur immediately after training and that they would progress with time; it was also hypothesized that similar changes would be observed with homogeneous and heterogeneous boxes, indicating a transfer of knowledge to new situations. The main objective was to investigate if the strategies investigated in this study should be included in training programs to safe manual handling. The evaluation included biomechanical variables (safety factors related to back loadings, asymmetry of back posture/efforts, energy requirements) and some ergonomic variables (handling manoeuvres; feet positioning) which could explain the change of strategies.
2. Methods 2.1. Subjects The subjects were 10 male novice workers, college students in physical education, who had at least 3 months experience but a maximum of 12 months in part-time manual handling jobs. They regularly practiced sport activities and were free of musculoskeletal problems. Their mean age was 24 yr (range: 21–28 yr), their height, 1.8 m (1.73–1.9 m) and mass, 79.3 kg (63.4– 100 kg). The Universite de MontrealÕs Ethics Review Board had accepted the experimental protocol. 2.2. Training protocol Each subject was trained individually. He was first instructed about some basic biomechanical principles of lifting (reductions of load lever arm and efforts/posture in torsion). This was followed by the observation of video films contrasting experts and novicesÕ strategies in a variety of workplace conditions, with concentration on one specific technical element at a time (either load tilts/ hands positioning, shoulders positioning or feet orien-
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tation). The subject then practiced the transfer (unloading then reloading) of 16 homogeneous boxes piled over a height of 1.2 m, using two rows and two columns. The protocol was repeated for each of the three elements. The subject was encouraged to try different strategies (search approach) and he received some feedback, limited to the very first trials, to emphasize the biomechanical principles previously taught; however, no instruction was given on the precise technique to adopt. 2.3. Pre-training and post-training tests The pre-training tests involved two sequences of lifts, the one with the consecutive displacement of two heterogeneous 15-kg boxes and the other one with three homogeneous 15-kg boxes, from a low 22-cm shelf to another 22-cm shelf at 90° to the subjectÕs left and distanced by about 1.6 m (Fig. 1). Only one lift (called control trial) for each of the homogeneous and heterogeneous boxes was analysed; these lifts corresponded to the same conditions of take-off and deposit. The boxes were 30 cm 42 cm 33 cm for height, width and antero-posterior distance; the c.g. was either at the centre (homogeneous boxes) or off-centre, i.e. 30% forwards and 86% upwards (heterogeneous boxes). No instructions were provided about the ways to handle. Following pre-training tests, the training protocol was applied with only homogeneous boxes used (Section 2.2). The post-training tests included 25 sequences of practice, in the same set-up, with three homogeneous boxes followed by one last sequence with two heterogeneous boxes; each sequence of lifts was followed by 3– 4 min rest and a longer 30-min rest at mid-session, for minimizing fatigue effects. For homogeneous boxes, the subject executed a total of 156 lifts in two sessions (26 sequences 3 lifts 2 sessions); however, the analyses were restricted to one lift in the pre-training sequence (trial 1 or control trial), and three lifts in the posttraining sequences, i.e. in the first sequence following training (trial 2), in the last sequence of the first session (trial 26) and the last sequence of the second session (trial 51). For heterogeneous boxes, the subject did not have any training and the analyses included all sequences (last lifts), that is, one lift in the pre-training sequence (trial 1), and two lifts in the post-training sequences, the one at the completion of the first session (trial 2) and the other one, at the completion of the second session (trial 3). These trials were selected to test if training effects would occur and progress with time in the situation for which the subject had trained (homogeneous boxes) and also in a new situation for which no training was done (heterogeneous boxes) to investigate if transfer of knowledge occurred. In all these sequences, either during pre-training or post-training, the subject was free to choose his lifting techniques. The initial and final positions of the boxes
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Fig. 1. Experimental set-up and examples of handling strategies for a pre-training trial at take-off (A) and deposit (B) and for a post-training trial at take-off (C) and deposit (D).
were standardized but box handling manoeuvres as well as feet motions were allowed. The subject was not given any feedback during pre-training or post-training; feedback was limited to the very first trials during training. The subject was not informed that some heterogeneous boxes would be used. 2.4. Techniques and methods The 3D kinematic data were obtained using five video cameras and electronically synchronized with the force data obtained from a large 2.4 2.4 m2 force plate that was designed in our laboratory for large displacements (Desjardins and Gagnon, 2001); the positions of markers for the subject plus load were sampled at 30 Hz for five frames around the specific events of take-off (first moment when the load left the shelf) and deposit (last mo-
ment preceding load contact on the other shelf) and only these two events were analysed. Mechanical work on the load was also analysed for the entire duration of transfer and the positions of markers were sampled at 10 Hz. The 3D reconstruction of markers were obtained by DLT procedures (Marzan, 1975) and filtered with quintic splines. The L5/S1 net reaction moments and orientation were obtained from a 3D biomechanical model (Gagnon and Gagnon, 1992; Plamondon et al., 1995). It included 14 segments: feet, shanks, thighs, pelvis, lower-trunk (from L5/S1 to T8), upper-trunk (from T8 to C7/T1), head-neck, arms, and forearms plus hands. It has been validated by comparing an upper-body model with a lower-body model (Plamondon et al., 1996); a sensitivity analysis has also been performed (Desjardins et al., 1998). Net moments and joint orientation were ex-
M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
pressed in the same trunk/pelvis system, a joint referential system of axes formed by the longitudinal axis of the trunk (torsion), the transverse axis of the pelvis (flexion/extension) and a floating axis (lateral bending) (Gagnon et al., 2002). The calculations included L5/S1 net resulting moments and moments about each axis at take-off and deposit. It also included an index characterizing the global L5/S1 asymmetrical moment defined as the square root of the summation of squares of torsion and lateral bending moments. The mechanical work done on load was estimated during transfer from integration of power as a function of time, power being the dot product of the force exerted on the load (inertial force from the second derivative of c.g. position and gravitational force) and c.g. velocity. Three variables were chosen for safety criteria, that is, back efforts represented by L5/S1 net resulting moment at take-off and deposit (events more likely to be associated with higher back loadings), asymmetry using the index of L5/S1 asymmetrical moments, also at take-off and deposit (higher challenge for trunk posture/efforts while handling loads from/to low heights) and finally, the task global difficulty represented by the mechanical work done on load. General kinematics included duration of load transfer as well as the path length of its c.g. during transfer; at take-off and deposit, the variables included height of c.g. for load and for subject and their horizontal distances from the back (L5/S1), lumbar flexion, torsion and lateral bending angles and knee flexion (angle between the longitudinal axes of thigh and shank). Ergonomic observations (load tilts/hands positions and orientation), were made at take-off, deposit and during transfer. Internal validity was verified for one trial per subject analysed twice by the same observer: the average correspondence was 80%; the main discrepancies were for hands placement (near or at the corner of the box) and for tilting (edge and/or corner). Analyses of variance with repeated measures on learning trials were used to determine if the four trials with homogeneous boxes differed significantly and, if the three trials with heterogeneous boxes would also differ; probability values were Greenhouse–Geisser corrected (P 6 0:05). The hypotheses were that the pre-training trial would differ from the post-training trials and that the changes would progress across trials during the session and between sessions.
3. Results The results showed that substantial biomechanical and ergonomic changes occurred immediately, at the first post-training trial and when compared to the pretraining trial, that these changes were maintained
605
throughout the session and between sessions; these changes occurred for two out of the three biomechanical safety criteria selected (mechanical work and back loadings). Similar patterns of changes were observed for both conditions of homogeneous boxes (with which training was made) and heterogeneous boxes (new situation) (Tables 1 and 2). 3.1. Homogeneous boxes Significant differences were found between the pretraining trial and the post-training trials whereas no differences were found between the post-training trials (Table 1). At take-off and deposit, the learning responses corresponded to smaller knee flexion (by about 35°) and higher positions of the box c.g. (by about 6 cm) due to box tilting strategies (Table 3). At take-off, with learning, the resulting horizontal distance of the load from the back (L5/S1) was reduced (by about 10 cm) and the feet were more oriented towards the deposit area (larger angle, between 17° and 33°), whereas, at deposit, smaller net resulting L5/S1 moments (about 40 N m or 25%) were observed; finally, these effects were carried over the transfer period with smaller load transfer time (by about 10%) and smaller load path (by about 50 cm or 27%) and the consequent large reductions of mechanical work done on load (about 50 J or 40%). The absence of statistical differences for the net resulting L5/S1 moment at take-off was explained by the atypical last trial (# 51) by one subject (considerable increases in asymmetrical moments affecting the resulting moment and due to lack of load orientation towards deposit). Learning effects were not detected for asymmetry: back posture was symmetrical with only slight asymmetrical moments (between 20 and 36 N m for torsion or lateral bending) observed in all trials; these moments are low in comparison to literature on asymmetry (Gagnon et al., 1996, 2000, 2002; Plamondon et al., 1995; J€ager and Luttman, 1992; Kingma et al., 1998; Gagnon et al., 1993) (Table 4; discussion). Ergonomic data (Table 3, Fig. 1) showed that, at take-off, the pre-training trial was associated with the absence of box tilts (80% of the cases) and the posttraining trials, with higher frequencies of box tilts on edges and/or corners (83% vs. 20%), with both hands on box corners (23% vs. 0%) suggesting more manoeuvrability; the box was also more oriented towards deposit (more than 30°: 73% vs. 50%); at deposit, similar observations prevailed with higher frequencies of the posttraining trials using box tilts on edges and/or corner (93% vs. 30%), the absence of tilts predominating in the pre-training trial (70% of the cases); finally, during transfer, there were no load rotations in the pre-training trial whereas the post-training trials included load rotation manoeuvres (27% of the cases), probably to better distribute the load on both hands.
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M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
Table 1 Biomechanical changes (means and standard deviations (SD)) between the pre-training test and post-training test for the homogeneous boxes (P 6 0:05; n ¼ 10) Variables
Take-off Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Feet orientation towards deposit (deg) Horizontal distance of load from L5/S1 (cm) Height of load c.g. (cm) Deposit Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Horizontal distance of load from L5/S1 (cm) Height of load c.g. (cm) Transfer Mechanical work on loadc (J) Duration (%) Path length of load c.g. (m) a; b; c
Pre-training
Post-training
(1)
(2)
P (26)
Specific comparisons
(51)
Mean
(SD)
Mean
(SD)
Mean
(SD)
Mean
(SD)
1 vs. 2
1 vs. 26
1 vs. 51
192
(38)
179
(49)
182
(51)
197
(58)
0.52
46
(25)
46
(24)
44
(34)
44
(30)
0.99
35 22
(29) (15)
32 29
(23) (20)
23 34
(21) (31)
27 28
(33) (18)
0.70 0.49
6 3
(3) (2)
5 3
(4) (3)
5 3
(2) (3)
5 4
(3) (3)
0.77 0.51
45 65 21
(12) (36) (23)
46 28 38
(12) (20) (24)
44 28 54
(34) (14) (19)
44 25 53
(30) (12) (20)
0.99 0.00 0.01
0.00 0.1
0.00 0.00
0.00 0.00
56
(6)
47
(4)
47
(4)
47
(4)
0.00
0.00
0.00
0.00
39
(2)
44
(3)
44
(4)
44
(3)
0.00
0.00
0.00
0.00
141
(36)
107
(26)
109
(24)
96
(32)
0.01
0.00
0.00
0.00
47
(16)
36
(12)
39
(19)
40
(19)
0.53
36 24
(15) (19)
26 20
(13) (14)
29 21
(20) (13)
29 24
(18) (15)
0.57 0.91
3 3
(1) (2)
3 3
(1) (3)
5 3
(3) (2)
4 4
(4) (2)
0.27 0.70
44 56 55
(15) (38) (9)
42 22 52
(13) (11) (6)
43 25 53
(15) (15) (8)
47 20 54
(15) (12) (10)
0.27 0.01 0.69
0.00
0.00
0.00
41
(2)
47
(4)
47
(4)
47
(3)
0.00
0.00
0.00
0.00
(32) 0.03 (13) 0.04 (0.39) 0.04
0.01 0.00 0.01
0.02 0.04 0.02
0.00 0.09 0.01
106 (70) 59 (44) 66 37 (17) 25 (12) 29 1.87 (0.77) 1.37 (0.50) 1.44
(43) 53 (13) 30 (0.53) 1.35
Variables identified as criteria for safety (see text), respectively for back loadings, asymmetrical efforts and mechanical work or energy.
3.2. Heterogeneous boxes Significant differences were found between the pretraining trial and the post training trials whereas no differences were found between post-training trials (Table 2). The learning responses were similar to those observed for homogeneous loads, even though the lifting of heterogeneous loads were new tasks for which the learner had not had previous training: kinematics and ergonomic observations suggested that transfer of knowledge occurred with similar strategies of box tilts and similar effects of reduced mechanical work and back efforts. However, there was one major difference during transfer: after training, the heterogeneous boxes were
more often rotated compared to the homogeneous loads (45% vs. 27%), suggesting increased manoeuvrability. Symmetry of posture/efforts was also observed and not dependent on learning.
4. Discussion The training of workers is often based on the widespread convention that the Ôleg-liftÕ is the correct technique for lifting or on biomechanical principles without proper anchorage to the physical realities of manual handling (workplace: load placement in height, depth, obstacles; load format and heterogeneity); for
Table 2 Biomechanical changes (means and standard deviations (SD)) between the pre-training test and post-training test for the heterogeneous boxes (P 6 0:05; n ¼ 9d ) Variables
Post-training
Post-training
(1)
Specific comparisons
(2)
(3)
(SD)
Mean
(SD)
Mean
(SD)
P
1 vs. 2
1 vs. 3
Take-off Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Feet orientation towards deposit (deg) Horizontal distance of load from L5/S1 (cm)
210 23 9 19 4 2 43 65 16 57
(38) (18) (6) (20) (2) (2) (13) (44) (15) (9)
197 49 34 31 3 2 47 25 39 47
(40) (31) (24) (27) (2) (2) (14) (13) (31) (8)
188 47 33 23 5 3 47 24 52 48
(35) (29) (35) (15) (4) (1) (14) (18) (21) (9)
0.05 0.10 0.06 0.48 0.19 0.41 0.22 0.03 0.01 0.04
0.13
0.01
0.01 0.01 0.02
0.00 0.00 0.03
Deposit Net L5/S1 resulting momenta (N m) L5/S1 asymmetrical momentb (N m) jL5/S1 torsion momentj (N m) jL5/S1 lateral bending momentj (N m) jL5/S1 torsion anglej (deg) jL5/S1 lateral bending anglej (deg) Lumbar flexion angle (deg) Knee flexion (deg) Horizontal distance of load from L5/S1 (cm)
148 37 15 30 2 5 41 61 56
(38) (16) (8) (22) (3) (3) (13) (42) (7)
124 36 21 23 4 4 47 24 56
(25) (15) (14) (20) (3) (3) (16) (14) (12)
119 34 19 25 5 4 48 26 56
(32) (16) (12) (18) (3) (2) (15) (17) (11)
0.10 0.85 0.58 0.63 0.25 0.42 0.11 0.03 0.99
0.01
0.01
Transfer Mechanical work on loadc (J) Duration (%) Path length of load (m)
129 39 2.13
(57) (10) (0.52)
(38) (13) (0.44)
0.00 0.05 0.00
0.00 0.04 0.01
0.00 0.01 0.00
73 30 1.67
(49) (15) (0.50)
67 28 1.44
M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
Mean
a; b; c
d
Variables identified as criteria for safety (see text), respectively back loadings, asymmetrical efforts and mechanical work or energy. The data from one trial in one subject had to be discarded; this subject was removed from these analyses.
607
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M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
Table 3 Ergonomic observations of handling elements; frequency of occurrence for the pre-test trial (n ¼ 10) compared to all post-training trials (n ¼ 30 for homogeneous loads; n ¼ 20 for heterogeneous loads) Variables
Homogeneous loads Pre-training
Take-off Hands position
Heterogeneous loads Post-training
Pre-training
Post-training
Edge/edge Edge/corner Corner/corner
20 80 0
20 57 23
10 90 0
25 50 25
Load tilt
None (flat) On edge On corner
80 20 0
17 60a 23a
60 40 0
10 65a 25a
Load orientation relative to deposit
Slight (<30°) Medium (30–60°) Large (>60°)
50 50 0
27 70 3
50 50 0
35 65 0
None (flat) On edge On corner
70 30 0
7 60a 33a
50 50 0
20 60b 20b
100 0
73 27
90 10
55 45
Deposit Load tilt
Transfer Load rotation a b
No Yes
The majority of tilts were either to the right or towards the subject or both. The majority of tilts were either to the right or towards the subject or away from the subject.
instance, one may understand the principle of reducing the lever arm of the load but its application (load manoeuvres) may not be easily discovered. An extensive data collection of opinions of more than 900 health professionals in Australia showed a wide consensus that the essence of lifting skill is adaptability (a way to vary the strategy as a function of the wide variety of situations) and that the basic principles of skill learning must be applied with adaptability as a goal (Sedgwick and Gormley, 1998). It is clear that some drawbacks exist in the biomechanics research of lifting: too many restrictions are imposed on the subject (fixed feet, boxes with handles, sagittal plane lifts) and the focus is often on the back without consideration for other joints or other dimensions such as motor control, fatigue effects, balance. On the other hand, the scientific foundations for training programs, specially for safety, still appear unclear; inadequate training methods may explain the failure of training to improve manual handling (St-Vincent et al., 1989). Our results have demonstrated that the content of training programs should be complemented by observations of workersÕ strategies, contrasting experts and novices, and using a search approach, instead of limiting the programs to theoretical concepts and the teaching of pre-determined techniques. A previous ergonomic evaluation of experts and novices while performing a variety of similar tasks have evidenced that experts adopted very different strategies (Authier et al., 1995a, 1996) and that the strategies
varied as a function of workplace (height of load, weight) (Authier et al., 1995a). These strategies are potentially safer (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999), reducing energy requirements or mechanical work (Gagnon et al., 1996, 2000; Gagnon, 1997), low back asymmetry of efforts and posture (Gagnon et al., 2000; Delisle et al., 1999) as well as loadings on the back, knees and shoulders (Gagnon et al., 1996; Gagnon, 1997). In this study, it was therefore hypothesized that if novices were trained for those most important strategies contrasting experts from novices (namely, box tilting/hands positioning, shoulders positioning and feet orientation), they would improve in those same safety criteria previously evaluated in experts and novices (Gagnon et al., 1996, 2000; Gagnon, 1997; Delisle et al., 1996, 1999); the objective was to demonstrate that these strategies contrasting experts and novices should be a part of training programs. The experts from whom strategies were observed had been selected from a large transportation firm and identified by fellow workers and managers as those workers having the best manual handling skills; they had at least 10 years of experience and a much lower rate of handling accidents (0.13 vs. 0.83 accident/worker/year) (Gagnon et al., 1996); finally, they possessed an extended knowledge about their jobs (Authier et al., 1995b). The approach of contrasting strategies of experts and novices was a means to encourage the subjects in seeking new ways of handling. Our results partially demonstrated that the strategies investigated in this study should be part of
M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
609
Table 4 L5/S1 moments (N m) and angles (deg) as related to trunk posture, feet positioning and box handling manoeuvres from previous publications Investigators
Tasks
Moments (N m) Extensors
Twist
Angles Lateral bending
Flexion
Twist
Lateral bending
–
–
–
(J€ ager and Luttman, 1992)a
Simulation of diagonal lift 20-kg load Fixed feet; no box manoeuvers
200
50
20
(Gagnon et al., 1993)a
16.4-kg box; fixed hand positions Fixed feet, symmetrical; load: 90° Fixed feet, asymmetrical; load: 45° Feet pivoting, symmetrical; load: 90°
202 217 229
78 53 28
60 55 32
47 45 29
21 17 3
16 3 3
11.6-kg box; Fixed hand positions; Shoulders parallel to ground. Fixed feet, symmetrical, load: 90° Fixed feet, symmetrical, load: 0°
217 228
33 20
35 25
65 72
19 1
4 6
200
30
30
50
10
20
11.0-kg box with handles Fixed feet, symmetrical Load at 0° (facing load) Load at 90°
221 191
21 57
24 101
35° 35°
0° 25°
4° 8°
12.4-kg box, fixed feet; asymmetrical (no step but slight feet mobility) Load tilted; shoulders parallel to ground No load tilt; shoulders parallel to ground Load tilt; shoulders not parallel to ground
172 188 165
19 22 14
33 38 113
56 57 58
1 2 3
4 6 19
15.0-kg box; free feet positioning; free box manoeuvres Best trial for back symmetry Worst trial for back symmetry
207 222
9 8
10 71
47 47
2 7
3 6
15.0-kg box; free feet positioning; free box manoeuvres Last trial on homogeneous box Last trial on heterogeneous box
190 178
27 33
28 23
49 47
5 5
4 3
(Plamondon et al., 1995)a
(Gagnon et al., 1996)a
(Kingma et al., 1998)a
(Gagnon et al., 2000)a
(Gagnon et al., 2002)b
This studyb
17-kg load; free box manoeuvers Free feet positions (but constrained on force plates)
The bold figures emphasize asymmetry of posture and/or efforts (moments): twist and lateral bending angles > 15° and moments > 50 N m. a Peak moments. b Data at take-off.
training programs to safe handling; the novices training in this manner effectively adopted less risky strategies in both conditions of homogeneous and heterogeneous loads: mechanical work was reduced by about 50% and back efforts by 10–30%. The criterion of asymmetry of effort/posture was not affected by the training approach but it is important to mention that, in all trials, asymmetries of posture and efforts were not important (only about 5° of torsion and lateral bending and 30 N m of torsion and lateral bending moments); it is highly probable that the absence of restrictions imposed on the subject for feet positioning and box manoeuvres may have influenced trunk posture and could explain these low levels of asymmetry. It was then of interest to investigate the literature on asymmetry of lifting, on the basis of the three strategies
for which the subjects had trained: posture with reference to shoulders positioning; feet mobility; load tilting (Table 4). Posture, with the maintenance of the shoulders parallel to ground in asymmetrical handling, has been shown to have a high impact on low-back, reducing asymmetrical moments (with shoulders parallel to ground, lateral bending moments and angles were 38 N m and 6°, as compared to 113 N m and 19° when not parallel (Gagnon et al., 2000); even with the load at 90°, twist and lateral bending moments were only about 30 N m when the shoulders were maintained parallel (Plamondon et al., 1995)); with regards to feet mobility, the studies involving asymmetrical lifts with fixed feet have shown higher lateral bending and/or torsion moments (50 N m in torsion (J€ager and Luttman, 1992); 57 and 101 N m in torsion and lateral bending (Kingma
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M. Gagnon / Clinical Biomechanics 18 (2003) 601–611
et al., 1998); about 75 N m in torsion and 60 N m in lateral bending (Gagnon et al., 1993)) whereas asymmetrical lifts executed with free feet positioning usually presented smaller values for these moments (28 N m in torsion and 32 N m in lateral bending with feet pivoting (Gagnon et al., 1993); about 30 N m for these moments with free feet positioning (Gagnon et al., 1996)); finally, load tilting does not appear to affect asymmetry but it is related to reductions of back extensor efforts at take-off (172 N m as compared to 188 N m (Gagnon et al., 2000)) as well as mechanical work done on the load (by 20%: data not presented in Table 4 (Gagnon et al., 2000)). This suggests that posture (shoulders positioning) and feet mobility play a role in asymmetry but the role of load tilting has not been demonstrated. The effect of feet mobility as compared to fixed feet positioning, in particular, merits further investigation; in this study, it was not possible to investigate this aspect as the subjects were free to move their feet at will which is the possible cause of the low levels of asymmetry observed for posture and efforts. The most significant technical change in this study was for load tilting and the accompanying biomechanical changes were similar to those in the literature, i.e., reductions of mechanical work on load and low back extensor moments (Gagnon, 1997; Gagnon et al., 2000) with no effects on asymmetry (Gagnon et al., 2000). Studies on repeated lifting usually involve protocols inducing fatigue effects (Parnianpour et al., 1988; Novak et al., 1993; Hagen et al., 1995; Fogleman and Smith, 1995; Sparto et al., 1997; Marras and Granata, 1997; van Die€en et al., 1998; Dolan and Adams, 1998). Fatigue induced practice leads to changes from a squat to a stoop, that is, reduction of knee flexion (between 5° and 10°) often with increased trunk or lumbar flexion (a few degrees only) (Hagen et al., 1995; Sparto et al., 1997; van Die€en et al., 1998; Dolan and Adams, 1998), decreased back extensor moments (between 5 and 30 N m) (Novak et al., 1993; Sparto et al., 1997; Marras and Granata, 1997; Dolan and Adams, 1998) but increased trunk asymmetry with more twist and/or lateral flexion (about 5° of twist and twisting range of motion) (Parnianpour et al., 1988; van Die€en et al., 1998) and decreased time of lift (Novak et al., 1993; Fogleman and Smith, 1995). In the few cases where fatigue may not have been involved, learning processes with practice were reported to be either very slow (Chaffin et al., 1999) or absent (Gagnon et al., 2002). It is worth noticing that excepted for this last study, the subjects were constrained, lifting in the sagittal plane, either with fixed feet and/or the use of boxes with handles. It is also worth mentioning the wide variety of responses observed not only between subjects but also within subject in the process of repeated lifts (Gagnon et al., 2002; van Die€en et al., 1998, 2001). In the present study, practice based on the observation of workers using a search strategy showed that some bio-
mechanical changes were similar to those recorded in repeated trials associated with fatigue (reduced knee flexion, but to a much larger extent in this study, and reduced back extensor moments of a similar magnitude), whereas other changes were not observed with fatigue and included reductions of mechanical work on load, associated with reduced load trajectory and time of transfer and attributed to load tilting and feet positioning strategies. It is dubious that the results of this study may have been affected by fatigue: the protocol was designed to reduce these effects, specially by including long rest pauses between trials and at mid-session: in fact, throughout the 4-h session, the subjects lifted an average of 15 kg/min.; these conditions appear mild and differ considerably from the protocols reported with studies on fatigue (Parnianpour et al., 1988; Novak et al., 1993; Hagen et al., 1995; Fogleman and Smith, 1995; Sparto et al., 1997; Marras and Granata, 1997; van Die€en et al., 1998; Dolan and Adams, 1998); moreover, our previous control study using a similar protocol of trials for free practice without any training (Gagnon et al., 2002) had not shown any effects. In conclusion, a training program based on observations of contrasting strategies of expert and novice workers, present a potential for improving handling manoeuvres towards more safety, especially for reducing mechanical work done on the load and back efforts. The focus should be mainly on handling load manoeuvres, specially load tilts and hands positioning, and feet displacement strategies. More information is needed to help determine the factors affecting asymmetry, feet positioning appearing an important contributing factor to symmetry/asymmetry of trunk posture.
Acknowledgement The financial assistance of the Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged.
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