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Box tilt and knee motions in manual lifting: two differential factors in expert and novice workers
Clrnrcal Bionwlwnics Vol. 12. No. 718,pp. 419-428, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain (‘268-0033197$17.01)+0.00 ELSEVIER
PII: SO268-0033(97)00060-O
Box tilt and knee motions in manual lifting: two differential factors in expert and novice workers M Gagnon Laboratoire de biomecanique, Montreal, Quebec, Canada
Departement
d’education
physique,
Universite
de Montreal,
Abstract Objective. It was the objective of this study to investigate the kinematic and kinetic effects of two specific handling factors that differentiate expert and novice workers, namely the level of knee flexion and box tilt. Design. Seven inexperienced subjects were required to lift a 12-kg box in the sag&al plane using three different strategies: (1) reduced knee flexion and a backward box tilt (more typical of experts); (2) large knee flexion and a backward box tilt; (3) large knee flexion and no box tilt (more typical of novices). Background. The lifting techniques of highly skilled workers differ substantially from those of novices but only limited information is available to compare their biomechanical differences. Methods. The methods included dynamic segmental analyses to calculate the net moments at all body joints and a planar single-muscle equivalent to estimate compression loadings at L5/Sl; total work and joint work distribution were calculated using the integration of joint power. The 3-D kinematic data were acquired with three video cameras and force data were obtained with one AMTI force platform which were synchronized with the film data. Results. Box tilt reduced the load trajectory and loadings on the lower back and shoulders; a reduced knee flexion affected body posture and reduced mechanical work and loadings on all body joints. The combination of these two factors, box tilt and reduced knee flexion, showed added effects for almost all variables. Conclusions. These two factors proper to experts were, in this context, biomechanically more advantageous. Relevance This paper is based on the investigation, with inexperienced subjects, of specific factors consisting of knee flexion and box tilt that differentiate the performance of expert workers from novice workers. The understanding of the biomechanics of their strategies may help provide a theoretical background in the design of preventive measures in manual lifting. 0 1997 Elsevier Science Ltd. All rights reserved Key words: expertise, lifting, box tilt, knee motions, joint loadings
C/in. Biomech.
Vol 12, No. 718, 419-428,
1997
Introduction The training of workers in safe handling techniques is believed to constitute a preventive measure for reducing risk factors in the working environment, Received: 20 August 1996; Accepted: 26 September 1997 Correspondence and reprint requests to: Dr Micheline
Gagnon, DCpartement d’education physique, UniversitC de MontrCal, C. P. 6128, Succ. Centre-ville, MontrCal (Qutbec) H3C 357, Canada.
particularly musculoskeletal injuries to the back. However, training programs have only shown limited success in the reduction of injuries or accidents’,*. The ways of handling loads vary widely and the workers generallv use methods different from those recommended3-s. Even experienced handlers do not generally agree on the best methods of lifting’. To better understand the choices workers make, we have recently conducted an extended laboratory
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study using the methods of ergonomics for comparing expert and novice workers when lifting freely6T7;the same subjects were further examined in an asymmetrical lifting context to compare their biomechanical differences’“. These subjects were six experts, selected from a large transportation firm, and five male physical education students. The experts had been selected from a pool of workers who had been identified by fellow-workers and managers as those workers having the best manual handling skills; they had an average of 20 yr experience (range between 14 and 36yr) and were free from current back and shoulder musculoskeletal problems; the accident data analyses revealed a lower annual rate of handling accidents for the experts (0.13 vs 0.83 accident/worker/yr for the entire firm). The students were required to have at least 3 months’ handling experience (average: 0.7 yr and range: 0.3-0.9 yr); none of them worked in the same firm as the experts, being part-time workers in different manual handling jobs. Several differential factors were found between the groups. Two of these factors have been selected in the present study for further biomechanical analyses, because of their predominance in the majority of the tasks: the level of knee flexion and the use of box tilts. The first aspect is the level of knee flexion. Contrary to what is generally admitted and analysed in biomechanical studies, workers do not use the stoop (straight knees, bent back) nor the squat (flexed knees, straight back) techniques. The workers, whether experts or novices were seen to adopt techniques where both the back and knees were flexed; their difference is rather in the amount of knee flexion, the experts using much less knee flexion and motions than novices6,7*‘0. The premise for advocating a squat technique in training programs is to the effect that by so doing, the workers could spare their backs by shifting loadings from the lower-back to the knee joints. However, this premise remains questionable as several studies have shown that, when the knees were near extension, lower-back loadings either remained unchanged or even slightly decreased”‘-‘4. On the other hand, the squat technique may have adverse effects as it is related to an increased action of knee extensors’“~‘2~‘3~‘5.We therefore thought important to investigate two different postures mostly encountered in practice: trunk flexion combined with large knee flexion (more typical of novice workers) and trunk flexion combined with a moderate level of knee flexion (more typical of expert workers). The other aspect is box tilt. To our knowledge, this factor has not been biomechanically analysed in the past except for our very recent study on asymmetrical handling; the box is generally assumed to remain flat, with the bottom parallel to the ground and no attention is granted to box tilt. In fact, the absence of box
tilt is a characteristic of novice workers. The experts adopt as many as 40 different combinations of box tilt and handgrips in simple asymmetrical contexts: in fact, box tilts in all directions were observed but the predominance was for box tilts towards the subject and sideways7. Authier et a1.7 have observed, in experts, that at the beginning of transfer, the boxes were generally tilted sideways (24%) or towards the worker (30%) with a tilt generally larger than 45”; the same trend was observed during transfer and, at deposit, the boxes also remained tilted 43% of the times. On the other hand, the novices almost never tilted the boxes at the beginning of transfer with only 13% of the times sideways and also 13% towards themselves, and in most caseswith only very slight tilt (below 20”); during transfer, the absence of box tilt was predominant and, similarly at deposit the box was held flat (96% of the cases). We therefore thought pertinent to investigate two different conditions of box tilts that were found current in practice for the manual handling of boxes without handles: a backward box tilt, towards the subject (more typical of expert workers) and the absence of box tilt (more typical of novice workers). It was the purpose of this study to investigate, with inexperienced subjects, the biomechanical effects of two specific factors differentiating novice workers from expert workers in a simple task of sagittal-plane lift. It was hypothesized that a backward box tilt (towards the subject) and reduced knee flexion, characteristics of expert handlers, would present biomechanical advantages such as the reduction of the path of load transfer as well as the reduction of mechanical work and body joint loadings. It was also hypothesized that the combination of both strategies would show added effects over each single strategy. These hypotheses were based on the premise that reduced knee flexion while flexing the back and tilting the load would act on the reduction of the lever arm of the load in relation to the body joints; reduced knee motions should also directly influence mechanical work. Finally it is reasonable to believe that some strategies of experts may involve the objective of minimizing the path of load transfer and thereby the duration of efforts. Methods Subjects
Seven physical education students were used as subjects. ey were all considered as novice workers as they only had marginal experience (or no experience at all) in handling manoeuvres. Their mean age was 22.7 yr (range: 21-28 yr), their height, 177.6 cm (167.5-190.0 cm) and mass, 71.1 kg (59.9-84.5 kg). These subjects were given instructions to perform three different tasks containing specific factors proper to novices or proper to experts.
Gagnon: Differential
Tasks
Three tasks were performed with a 12-kg box (40 cm long x 26 cm wide x 25 cm high), without handles, that was selected from a large sample of a firm’s products. The box was lifted from the ground to a 67-cm high shelf; the front part of the box was positioned at the level of the shelf. The movements were symmetrical; the subjects were free to select their feet spacing or distance between the toes and the nearest part of the box (range: 16-24 cm) but they had to maintain it for the three tasks and no step was allowed during the task; the standard handgrips were also symmetrical (an 8/S position, according to Drury et al.“) where both hands made contact with the lateral edges of the bottom face at pick up. The tasks were the following:
factors in expert and novice workers
421
1. reduced knee flexion was required; the box was tilted backwards with the hands on the lateral edges of the top face that were then slipped to the standard grips for pick up; the box was kept tilted during transfer till deposit where it was left to drop forwards on the shelf (this task was also referred to as the experts’ strategy) [Figure 1(-4-C)]; 2. large knee flexion and similar box tilt were required [Figure l(D-F)]; 3. large knee fiexion and the box was not tilted, rather remaining parallel to the ground at all times (this task is also referred to as the novices’ strategy) [Figure l(G, H)]. A large box tilt of about 45” was requested from the subject for the box tilt condition whereas in the
Figure 1. Body postures at pick-up and deposit for the strategy of reduced knee flexion and box tilt (A-C), for the strategy knee flexion and box tilt (0-F) and for the strategy of large knee flexion and no box tilt (G, H).
of large
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condition of no box tilt, the angle was not to be larger than about 10” (by visual inspection). In the case of ‘large knee flexion’ the subjects were requested to flex their knees freely whereas for the ‘reduced knee flexion’ the subjects were asked to limit their knee flexion while avoiding straight knees. The subjects were also instructed to lift normally without jerking the load but they were free to choose their lifting patterns even though the initial and final load positions limited the degrees of freedom for motions. The subjects did not receive any instructions concerning trunk posture. Measurement techniques
The tridimensional (3-D) kinematic data were obtained using three video cameras (Panasonic WVD5100) and analyzed with a motion measurement system (Peak Motion Analysis, Palmsprings, CO, USA). Force data were acquired with one AMTI (Advanced Mechanical Technology Inc., Newton, MA, USA) force platform; it was used to record the horizontal and vertical components of the external forces and their point of application. These data were electronically synchronized with film data and sampled at 30 Hz. The 3-D locations of 10 anatomical markers on the subject and of three markers on the box were obtained by Direct Linear Transformation (DLT) procedures i6. Only the x-y co-ordinates of the markers’ positions in the sagittal plane (right side of the body) were retained for analyses; the DLT reconstruction error (RMS) averaged 3.5 and 2.7 mm for the x and y axes respectively. The use of a 3-D approach to analyse a planar motion is justified to eliminate the perspective errors associated with bidimensional analyses. Dynamic segmental model
The body was divided into 13 rigid segments: feet, legs, thighs, pelvis (from hip to L5/Sl joint), torso (from L5/Sl to acromion), head plus neck, upper arms and forearms plus hands. The motions were symmetrical and both lower limbs and upper limbs were considered as single units. The L5 and Sl spinous processes were located by palpation and the L5/Sl joint lied laterally at 34% of the anteroposterior diameter of the abdomen from the backi7. The body segment parameters including masses, centers of mass and moments of inertia were obtained from Dempster” and from Zatsiorsky and Seluyanov”. The coordinates of the markers were filtered by a second-order, zero-phase and low-pass Butterworth filter and the phase shift was eliminated by passing the filtered signals in reverse order. The cut-off frequencies were selected automatically from residual analyses. Finite-difference techniques were applied
for the obtention of linear velocities and accelerations. For angular velocities and accelerations, the principles of relative motion of a rigid body were applied. For calculations of net reaction forces and net moments at each joint, a dynamic analysis was performed on each body segment. The segmental model included the net reaction forces and moments at each joint, the external forces measured by the force platform, the segmental weight, with the inertial forces and moments being determined from the kinematic analysis. The analysis covered each segment, starting at the feet, and working up adjacent segment successively until the hand level was reached. These procedures were explained in detail in Gagnon et a1.20. Model@
of loads at L5fSl
The model was based on net moment equivalence. From the net extensor moment, a single musculoligamentous force was represented to act with a representative lever arm about the L5/Sl joint center. The solutions for the estimation of compression forces and the sources for the input data are presented in Gagnon et a1.20,21.The input data included: an extension force combining the effects of the erectores spinae muscles and interspinous ligaments assumed to act normal to the vertebral plateau with a lever arm corresponding to 26% of the antero-posterior diameter of the trunk; the centroid of the vertebra was an at estimated position 34% from the posterior edge of the antero-posterior diameter of the trunk; the compression force set normal to the plane of the vertebral plateau; and the angle of the sacrum (Sl) with respect to horizontal estimated as a function of trunk flexion and knee flexion.
Mechanical work
Total mechanical work and joint work distribution were determined from a kinetic approach based on the time integration of joint power22. Joint power was calculated from the product of joint net moment and joint angular velocity, The total mechanical work was obtained by adjusting for differences between positive and negative works with the assumption that negative work was 1.5 times more efficient than negative work. The processes of muscular activity including generation (positive work), absorption (negative work), and transfers can be examined thoroughly in this model. The first applications of all these principles to
Gagnon: Differential
Results
practical situations in occupational biomechanics can be found in Gagnon and Smyth2”, and more details on the procedures and sources for input data can be found in that paper.
Box tilt factor (task 2 vs task 3) affkcted the characteristics of load path and body posture (Table l), as well as loadings on the back and shoulders (Table 2). With box tilting, the total duration of the task increased significantly but, at the same time, important advantages were observed in the reduction of both the duration and length of the path during which the load was entirely supported by the subject. Box tilting also affected body posture: the knees did not flex as much (max.: 107 vs 125”) and their range of motions was smaller (90 vs 108”) with this strategy; however, trunk flexion was larger (max. 97 vs 88’) although the relative motions between trunk and pelvis, at L5/Sl, did not follow the same patterns as the trunk motions relative to horizontal, the ranges being similar at about 40”. Tilting the box has approached the center of gravity towards the subject as seen in the significant reduction of the maximal back-to-load distance by 0.04 m; a consequence was the reductions of average compression loadings at L5/Sl by 20% and maximal shoulder flexor moments by 16%. The knee-flexion factor (task 1 vs task 2) had considerable effects on body posture (Table l), loadings on all body joints and mechanical work by lower limbs and the back (Table 2). In both strategies, knee flexion was relatively important but smaller, as expected, with the reduced knee-flexion strategy (54 vs lo?), as was the range of knee motions (40 vs 90”); however, trunk flexion was slightly larger with this strategy (118 vs 97” ) as was
Kinematics of load trajectory The characteristics of the box trajectory included the maximal distance from the center of gravity of the load to the L5/Sl joint, the total duration of load transfer as well as the integration of the path during transfer. This particular event of load transfer was important as it corresponded to the period during which the load was entirely supported, that is from pick-up until deposit. The path of the load was the one associated with its center of gravity. It was calculated from geometrical relations obtained from the three markers on the load. Statistics Analyses of variance involving repeated measurements were conducted on the three experimental tasks to test if there were significant differences between tasks executed with reduced knee flexion or with large knee flexion (task 1 vs task 2), between tasks executed with or without box tilt (task 2 vs task 3) and between the experts’ task and the novices’ task (task 1 vs task 3) where both factors of knee flexion and box tilt were combined. Significant differences were located using linear contrasts with a significance level of 0.05. Table 1. Means (standard
deviations)
Variables
Total lift duration (s) Load supported: duration (s) Load supported: path (m) Max. velocity (ms--') Max. acceleration (msm2) Max. distance load-to-L5/Sla (m) Load-to-L5/Sl dist. at pick-up (m) Trunk flexion: pick-up (“) Max. trunk flexionb,d (“) Min. trunk flexiond(“) Range of trunk motions (“) Max. L5/Sl flexionbsd (“) Min. L5/Sl flexiond (“) Range of L5/Sl motions (“) Knee flexion: pick-up” Max. knee flexionbGe (“) Min. knee flexiona,e (“) Range of knee motions (“)
and statistical
significance
levels for the kinematics
of the three lifting techniques
Reduced knee flexion; box tilted (1)
Large knee flexion; box tilted (2)
Large knee flexion; not tilt (3)
3.13(0.48) 1.34(0.13) 0.96(0.08) l.O(O.1) 2.6(0.4) 0.91(0.04) 0.69(0.03) 105(4) 118(5)
3.29(0.59) 1.39(0.09) 0.94(0.10) l.O(O.1) 2.7(0.4) 0.94(0.05) 0.72(0.02) 84(13) 97(13) 61(5) 36(12) 80(10) 119(7)
2.27(0.42) 1.66(0.26) 1.10(0.10) 0.9(0.1) 2.5(0.7) 0.98(0.05) 0.72(0.02) 81(10) 88(10)
W3) 5W) 82(9) 119(8) 37(5) 51(14) 54(16) 14(7) 40(17)
3W3 98(12) 107(15)
lw3) 90(20)
N.A. Specific comparisons not calculated as the global test showed aOccurrence at or near deposit. bOccurrence at nor near pick-up. co” is for the trunk at the vertical. d180” is for pelvis and trunk segments aligned. “0” is for complete extension. *Statistically significant for PI 0.05.
423
factors in expert and novice workers
no significance
5W) 30(11) 7w4 119(8) 40(5) 118(11) 125(10) 166’) 108(14) at P
Statistical
(N = 7)
significance
levels
1 vs 2
1 vs 3
2vs 3
0.233 0.254 0.629 N.A. N.A. 0.028' 0.025" 0.005" 0.006" N.A. 0.014x N.A. N.A. N.A. 0.001* 0.001* N.A. 0.002*
o.ooo* 0.009* 0.009* N.A. N.A. 0.001* 0.023" 0.001* o.ooo* N.A. 0.001” Y.A. Y.A. \.A. 0.000” 0.000* t1.A. @.OOO”
0.002" 0.028* 0.001" N.A. N.A. 0.002" 0.466 0.358 0.001" N.A. 0.070 N.A. N.A. N.A. o.ooo* 0.005* N.A. 0.002x
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Table 2. Means (standard
deviations)
Variables
Moments and loadings: Max. knees’ extensorsa (Nm) Max. knees’ flexorsb (Nm) Max. shoulders’ flex&SC (Nm) L5/Sl extensors at pick-up (Nm) L5/Sl extensors at deposit (Nm) Max. compression at L5/Sld (N) Average compression at L5/Sl (N) Work-energy: Total work (J) Upper limbs (J) L5/Sl (J) Lower limbs (J) Knees (J) Ankles (J) N.A. Specific aOccurrence bOccurrence COccurrence dOccurrence *Statistically
and statistical
significance
levels for the kinetics of the three lifting techniques
Reduced knee flexion; box tilted (1)
Large knee flexion; box tilted (2)
Large knee flexion; no tilt (3)
16(20) 136(17) 56(17) 209(22) 204(31) 4819(467) 2961(269)
82(29) 131(13) 64(14) 241(23) 205(31) 5369(365) 3345(208)
73(49) 139(18) 76 (20) 245(44) 224(36) 5587(566) 4171(388)
N.A. 0.048* 0.007* 0.816 0.014” 0.008*
520(82) 105(49) 142(20) 273(36)
615(80) 119(39) 165(33) 331(39) 56(15) 65(14)
645(114) 130(49) 166(32) 349(61) 66(28) 78(19)
0.002* N.A. 0.014* 0.006* 0.028* 0.003”
W8) 33(13)
comparisons not calculated as the global test showed at the beginning of the task, before pick-up. at or near deposit. at mid-transfer. either at pick-up or at deposit. significant for P10.05.
the range of trunk motions (56 vs 36”) but the relative motions between trunk and pelvis, at L5/Sl, were not affected, at about 40” for both tasks. The posture associated with reduced knee-flexion strategy produced a significant reduction in the maximal distance between the load and the back. These postural characteristics might explain the important reduction in total work and joint works including the lower limbs and the back (between 14 and 18%) as well as the important reductions in peak moments for knee extensors (by 80%) shoulder flexors and L5/Sl extensors (by 13%) and for the peak compression force at LSD1 (by 10%). The combined factors (task 1 vs task 3) represented by reduced knee flexion and the use of box tilt (experts’ strategy) and by large knee flexion and no box tilt (novices’ strategy) were compared; the results showed that the effects were still more important than for the single factors separately. The experts’ strategy included the combined advantages of reducing both the duration and length of the path during which the load was entirely supported while reducing peak joint moments, maximal compression force at L5/Sl and joint works. These changes were mainly attributed to the large changes in body posture. Discussion This study was undertaken with inexperienced subjects to investigate the biomechanical effects of two specific handling factors that discriminate expert from novice handlers in a simple task of sagittalplane lifting. It was found that a backward box tilt, a
no significance
Statistical
(N = 7) significance
levels
1 vs 2
1 vs3
2 vs 3
0.001*
0.038” N.A. 0.001* 0.060 0.054 0.004* 0.000”
0.554 N.A. 0.009” 0.821 0.094 0.218 0.001*
0.019*
0.277 N.A. 0.883 0.227 0.182 0.178
N.A. 0.039* 0.013* 0.047* 0.003*
at P50.05.
factor typical of experts, largely influenced the characteristics of load trajectory, minimizing both the duration and length of path during which the load was entirely supported by the handler with the consequences of reducing loadings on the lower back and shoulders. It was also found that a reduced knee flexion, another factor typical of experts, presented advantages over the larger knee flexion that characterizes the performance of novices, affecting body posture with the consequences of reducing total mechanical work and joint works for the back and lower limbs as well as loadings on all body joints. Finally, the combination of both factors showed added effects for almost all variables, indicating that these two factors proper to experts were, in this context, biomechanically more advantageous. It is important to discuss the limitations present in this laboratory context for executing the tasks. Experimental constraints have forced the analysis of a sagittal-plane lift with symmetrical handgrips and symmetrical feet positions. It is in fact restrictive as it is known from several ergonomic studies that lifting is seldom made symmetrically in the sagittal-plane4-“. We have chosen to study two specific technical elements, box tilt and knee flexion, that were clearly generalized in the field and differentiated expert and novice workers”v7. The subjects did not receive any instruction on the way to use their back. They also chose the feet position in relation to the load for which they felt more comfortable: the distance (range: 16-24 cm) is compatible with the observation we made in a previous study of expert and novice workers who were found similar for this variable, 60% of the subjects having adopted distances
Gagnon: Differential
factors in expert and novice workers
without handles affects the comparisons. In spite of this, our results indicated that the strategy of experts led to much smaller knee extensor moments; they are comparable, to this effect, to Gagnon et al.rO previous observation of experts and to Schipplein et al.“” data for a freestyle lift and comparable knee flexion angles (trunk posture not reported); on the other hand, our results on novices’ strategy approach the results of previous investigations on leg liftr2,‘“. It is safer to reduce the action of knee extensors as much as possible as they are responsible for the stresses on the femoro-patellar joint, an important factor in the pathogenesis of osteoarthriti9”. Box tilting has seldom been the subject of biomechanics literature on manual materials handling. However, a few studies in ergonomics have emphasized the importance of this factor when combined with different handgrips3,“. The research emphasis has rather been focused on optimum handle positions and angles that could compensate for the need to tilt the box”‘,.12, however, one cannot find the optimum design appropriate for the numerous conditions present in the working context. The industrial survey conducted by Drury et al.” in nine factories has effectively shown that very few boxes had handles. Recent obsemations of expert handlers from a large transport industry have shown that they were able to develop a large number of handling strategies combining various box tilts with different handgrips”,7. Box tilt, specially when coupled with the appropriate handgrips may provide a large variety of manoeuvres; this handling strategy
between 16 and 32 cm at pick-up and at deposit7. It is important to recognize that other task variables not investigated in the present study, could also influence the lifting performance: the lifting height2”,24, the weight of the loadZ3, its format with the restriction that the load may or may not be brought between the knees”, its horizontal location relative to the back2”, and also the lifting technique with reference to inertial factors2”,27 and to lifting rhythm between lower limbs and trunkz8. The tasks investigated in the present study were controlled for some factors (symmetry, load distance, height, weight, velocity, type of handgrips) but uncontrolled for others (lifting rhythm, trunk posture) in order to approach realistic conditions as much as possible. The scope of the results should then be circumscribed to this context. The interactions of knee flexion and box tilt with other task variables were not addressed in this paper. However, from the present results it is believed that the knee flexion and box tilt/handgrips are important factors that merit further investigation. The present data were compared with previous results on low back and knee moments and trunk/ knee flexion obtained by other investigators (Table 3). Peak extensor moments at L5/Sl were similar to those reported in symmetrical and asymmetrical lifting literature. However, the results on knee moments reflect the large variation attributed to the lifting style, the type of load and the level of expertise of the workers. Trunk flexion was seldom reported which makes the direct comparison difficult; furthermore the use of a load with or
Table 3. Net peak moments (Nm) at the low back and knees and trunk/knees from some previous publications
flexion
angles (“) for asymmetrical
Flexion angle Trunk Investigators
Task/load
Ekholm et al. (1984)‘*
Symmetrical; 0 Leg lift l Back lift Symmetrical; Freestylea Symmetrical; 0 Leg lift l Back lift Symmetrical; 0 Leg lift l Back lift Asymmetrical; Freestyle: l Novices l Experts Symmetrical; 0 Strategy of 0 Strategy of Symmetrical; Freestyle
Schipplein
et al. (1995) 25
Hagen et al. (1995) 2g Toussaint
et al. (1992) l3
Gagnon et al. (1996) lo
This study Deeb et a/.(1985) 31
425
anNd/or symmetrical
Peak moments Knees
lifts
(Nm)
Low back (L5/Sl)
Knees
Max.
Range
Max.
Range
N.R. N.R.
N.R. N.R.
-100 - 20
- 90 -15
N.R. N.R.
50(E)/60(F) -60(F)
N.R.
N.R.
- 50
N.R.
- 245
- IO (E)
N.R. N.R.
40 90
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
-80 - IO
- 70 -5
- 225 - 225
40(E) 180(F)
N.R N.R.
N.R. N.R.
53 31
38 20
- 200 - 200
33(E)/58(F) lO(E)/72(F)
88 118
30 56
125 54
108 40
264 238
73(E)/139(F) 16(E)/136(F)
N.R.
N.R.
-
N.R.
N.R
N.R.
12.8 kg IO kg;at 20 cm 17 kg 15.3 kg
17.0 kg
12.0 kg at 16-24 cm novices experts 9 and 13 kg
N.R.Not reported. (E):Extensors; (F): flexors; - symbol for approximation aValues reported at peak L5/Sl moment. bConversion for a reference of 0” in full knee extension.
140b
of readings from graphs,
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appears much more flexible than the use of handles. Depending on the task constraints, the coupling of handgrips and box tilts may fulfil several objectives such as vertical and horizontal stability of the load, proper exertion of momentum, increased stability of the worker and reduced stresses on body joints. A recent biomechanical study by our group has shown that the biomechanical factors are highly sensitive to changes in tilts and handgrips”“. In the present study, the choice was made for a symmetrical 8/8 handgrip in combination with either a backward box tilt or without tilt. The position 8/8 symmetrical has been identified to be within the five most frequent hand positions, accounting for approximately 10% of movements in an industrial survey”; it is recommended for lifting heavy weights from the floor as it is reported to be associated with minimizing forces on the hands and reducing the stress on the upper limbs3*. The objective for tilting the load backward was to raise its center of gravity and to approach it from the worker before lifting. This was accomplished successfully as the time of exertion associated with load transfer was reduced with subsequent reductions of loadings on the upper arms and lower back. The tilting manoeuvres before pick up are, however, responsible for the lengthening of task duration, a necessary compromise to reduce joint loadings. As these results applied to a single tilt/ handgrips strategy, the inferences about its applicability remain limited. Other strategies should be identified in relation to the objectives of the tasks. Our results indicated that these strategies merit further experimental verification to better understand the principles associated with safe handling. The use of flexed knees technique has long been advocated as a strategy to shift the load from the lower-back to the knees34-35,but it has been shown that when the knees were near extension, lower-back loadings either remained unchanged or even slightly decreased’0~“~‘3-‘5~36while there was a shift from knee extensors to knee flexors10-‘2,36 and a reduction in the metabolic cost11,29,37,38. Unfortunately, trunk posture relative to the horizontal was rarely reported and large variations in trunk flexion, from near extension to extreme flexion, may have led to different conclusions on the pertinence of using or not straight knees or flexed knees techniques. In the present study we chose not to control the amount of trunk flexion, rather leaving the subjects free to adopt the most comfortable posture. All subjects chose to flex their trunks near the horizontal. In this respect, their trunk postures are in agreement with several ergonomic observations or with laboratory observations of freestyle lifting in asymmetrical conditions by expert and novice workers lo. Trunk flexion appeared to be related to knee flexion: the smaller the knee flexion, the larger the trunk flexion (comparisons between the means for the three tasks). On the other
hand, the trunk/pelvis angles (max. min. and range of motion) were not related to knee flexion, being similar for all tasks, suggesting that lumbar flexion and the passive bending torque due to discs and ligaments were not influenced differently by the different tasks. One may question if the adjustment of trunk posture may have been influenced by the need to restrict this trunk/pelvis angle. This question should be clarified in future studies. The importance of the interplay between the extensor moment and the bending torque at L5/Sl has already been demonstrated for lifting tasks39. To our knowledge, no study has reported the complete analysis of loadings as well as mechanical works on all body joints; this approach is particularly important at the light of potential shifts of loadings between joints. The use of a restricted knee flexion technique showed reductions of loadings on all body joints with no apparent shifts from one joint to another; the most important reductions occurred for knee extensors thought to be responsible for overuse injuries to the patella. Similarly, mechanical work was considerably reduced with the restricted knee-flexion technique; this was mostly due to reduced work in the lower limbs and the back (L5/Sl). It is apparent that, in this restricted context of sagittal-plane lifting, the reduced knee flexion (about 50” from full extension) combined with large trunk flexion (about 30” below horizontal was more advantageous than a technique combining a large knee flexion (about 120” from full extension) with large trunk flexion (near horizontal). The advantage of minimizing knee flexion was demonstrated in a context associating large trunk flexion and box tilt. This specific situation of sagittal and symmetrical lifting limits the inference that can be made about the applicability of the results to generalized conditions. In conclusion, box tilt and reduced knee flexion, two factors proper to expert workers, produced clear biomechanical advantages of reduced path for load transfer, reduced joint loadings and mechanical works over the novices’ strategies consisting of large knee flexion and no tilt of the box. Future studies should focus on the multiple facets of handgrips/box tilts as applicable to different working contexts. Similarly, the focus should be on the relations between knee flexion versus trunk/pelvis rotation and trunk flexion. One should abandon the idea that the leg lift is the unique solution to handling problems; this is not supported by criteria of minimizing joint loadings and mechanical work. One should probably focus on the compromises between the several objectives that can be fulfilled in the working context. Acknowledgements
The financial assistance of the Institut de recherche en Sante et en securite du travail du Quebec (IRSST)
Gagnon: Differential
and the Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged. References 1. Chaffin, D. B., Gallay, L. S., Woolley, C. B. and
Kuciemba, S. R. An evaluation of the effect of a training program on worker lifting postures. International Journal of Industrial Ergonomics, 1986, 1, 127- 136. 2. Yu, T., Roht, L. H., Wise, R. A., Kilian, D. J. and Weir, F. W. Low-back pain in industry: an old problem revisited. Journal of Occupational Medicine, 1984, 26, 517-525. 3. Drury, C. G., Law, C. H. and Pawenski, C. S. A survey of industrial box handling. Human Factors, 1982, 24, 553-565. 4. Baril-Gingras, G. and Lortie, M. The handling of objects other than boxes: univariate analysis of handling techniques in a large transport company. Ergonomics,
1995,38,905-925.
5. Kuorinka, I., Lortie, M. and Gautreau, M. Manual handling in warehouses: the illusion of correct working postures. Ergonomics, 1994, 37, 65.5-661. 6. Authier, M., Gagnon, M. and Lortie, M. Handling techniques: the influence of weight and height for experts and novices. International Journal of Occupational Safety and Ergonomics, 1995,1, 262-275. 7. Authier, M., Lortie, M. and Gagnon, M. Manual handling techniques: comparing novices and experts. International
Journal of Industrial Ergonomics, 1996, 17,
419-429. 8. St-Vincent, M., Tellier, C. and Lortie, M. Training in handling: an evaluative study. Ergonomics, 1989, 32, 191-210. 9. Authier, M. and Lortie, M. Assessment of factors considered to be important in handling tasks by expert handlers. International Journal of Industrial Ergonomics, 1993, 11, 331-340. 10. Gagnon, M., Plamondon, A., Gravel, D. and Lortie, M. Knee movement strategies differentiate expert from novice workers in asymmetrical manual materials handling. Journal of Biomechanics, 1996, 29, 1445-1453. 11. Garg, A. and Herrin, G. D. Stoop or squat: a biomechanical and metabolic evaluation. AIIE Transactions, 1979, 11, 293-302. 12. Ekholm, J., Nisell, R., Arborelius, V. P., Hammerberg, C. and Nemeth, G. Load on knee joint structures and muscular activity during lifting. Scandinavian Journal of Rehabilitation Medicine, 1984, 16, l-9. 13. Toussaint, H. M., van Baar, C. E., van Langen, P. P., de Looze, M. P. and van Dieen, J. H. Coordination of the leg muscles in backlift and leglift. Journal of Biomechanics, 1992,25, 1279-1289. 14. Dolan, P., Mannion, A. F. and Adams, M. A. Passive tissues help the back muscles to generate extensor moments during lifting. Journal of Biomechanics, 1994, 27, 1077- 1085. 15. van Die&n, J. H., Creemers, M., Draisma, I., Toussaint, H. M. and Kingma, I. Repetitive lifting and spinal shrinkage, effects of age and lifting techniques. Clinical Biomechanics, 1994, 9, 367-374. 16. Marzan, G. T. Optimum configuration of data acquisition in close-range photogrammetry. Symposium on Close-Range Photogrammetric Systems, Falls Church, IL, 1975, pp. 558-573.
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17. McNeil], T., Warwick, D., Andersson, G. B. J. and Schultz, A. Trunk strengths in attempted flexion, extension, and lateral bending in healthy subjects and patients with low-back disorders. Spine, 1980, 5, 529-538. 18. Dempster, W. T. Space requirements of the seated operator. WADC Technical Report, 55-159, WrightPatterson Air Force Base, OH, 1955. 19. Zatsiorsky, V. and Seluyanov, H. The mass and inertia characteristics of the main segments of the human body. In Biomechanics VIII-B, eds H. Matsui, K. Kobayashi. Human Kinetics Publishers, Champaign, IL, 1983, pp. 1152-l 159. 20. Gagnon, M., Chehade, A., Kemp, F. and Lortie, M. Lumbo-sacral loads and selected muscle activity while turning patients in bed. Ergonomics.. 1987, 30, 1013-1032. 21. Gagnon, M., Sicard, C. and Drouin. G. Evaluation of forces on the lumbo-sacral joint and assessmentof work and energy transfers in nursing aides lifting patients. Ergonomics, 1986, 29, 407-421. 22. Aleshinsky, S. Y. An energy ‘sources’ and ‘fractions’ approach to the mechanical energy expenditure problem - II. Movement of the multi-link chain model. Journal qf Biomechanics. 1986, 19,295-300. 23. Gagnon, M. and Smyth, G. Muscular mechanical energy expenditure as a process for detecting potential risks in manual materials handling. Journal of Biomechanics, 1991, 24, 191-203. 24. Leskinen, T. P.J., Stalhammar, H. R., Kuorinka, 1. A. A. and Troup, J. D. G. Hip torque, lumbosacral compression, and intraabdominal pressure in lifting and lowering tasks. In Biomechanics IX-B, eds D. A. Winter, R. W. Norman, R. P. Wells, K. C. Hayes and A. E. Patla. Human Kinetics Publishers, Champaign, IL, 1985, pp. 55-59. 25. Schipplein, 0. D., Reinsel, T. E., Andersson, G. B. J. and Lavender, S. A. The influence of initial horizontal weight placement on the loads at the lumbar spine while lifting. Spine, 1995, 20, 1895-1898. 26. Gagnon, M. and Smyth, G. Biomechanical exploration on dynamic modes of lifting. Ergonomics, 1992, 35, 329-345. 27. Leskinen, T. I’. J., Stalhammar, H. Ii. and Kuorinka, I. A. A. The effect of inertial factors on spinal stress when lifting. Engineering in Medicine, 1983, 12, 87-89. 28. Troup, J. D. G., Leskinen, T. P. J., Stalhammar, H. R. and Kuorinka, I. A. A. A comparison of intraabdominal pressure increases, hip torque, and lumbar vertebral compression in different lifting techniques. Human Factors, 1983, 25, 517-525. 29. Hagen, K. B., Sorhagen, 0. and Harms-Ringdahl, K. Influence of weight and frequency on thigh and lowertrunk motion during repetitive lifting employing stoop and squat techniques. Clinical Biomechanics, 1995, 10, 122-127. 30. Goodfellow, J., Hungerford, D. S. and Woods, C. Patello-femoral joint mechanics and pathology. 2. Chondromalacia patellae. Journal of Bone and Joint Surgery, 1976,5?3B, 291-299.
31. Deeb, J. M., Drury, C. G. and Begbie, K. L. Handle positions in a holding task as a function of task height. Ergonomics, 1985,28, 747-763.
32. Drury, C. G. and Deeb, J. M. Handle positions and angles in a dynamic lifting task. Part 1. Biomechanical considerations. Ergonomics, 1986, 29, 743-768. 33. Delisle, A., Gagnon, M. and Desjardins, P. Handgrip and box tilting strategies in handling: effects on
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stability and trunk and knee efforts. International Journal of Occupational
Safety and Ergonomics,
1996, 2,
109-118. 34. Miller, R. L. When you lift, bend your knees. Occupational
Health and Safety, 1980, 45, 46-47.
35. Bendix, T. and Eid, S. E. The distance between the load and the body with three bi-manual lifting techniques. Applied Ergonomics, 1983, 14, 18.5-192. 36. Delisle, A., Gagnon, M. and Desjardins, P. Effects of the base of support and knee Rexion on trunk and knee efforts and stability while handling low lying loads. Book of Abstracts. 2nd International Scientific Conference on Prevention of Work-related
Musculoskeletal Disorders, Montreal, 1995, pp. 301-302. 37. Welbergen, E., Kemper, H. C. G., Knibbe, J. J., Toussaint, H. M. and Clysen, L. Efficiency and effectiveness of stoop and squat lifting at different frequencies. Ergonomics, 1991, 34, 613-624. 38. Trafimow, J. H., Schipplein, 0. D., Novak, J. and Andersson, G. B. J. The effects of quadriceps fatigue on the technique of lifting. Spine, 1993, 18, 364-367. 39. Dolan, P., Early, M. and Adams, M. A. Bending and compressive stresses acting on the lumbar spine during lifting activities. Journal of Biomechanics, 1994, 27, 1237-l 248.
PII: SO268-0033(97)00060-O
Box tilt and knee motions in manual lifting: two differential factors in expert and novice workers M Gagnon Laboratoire de biomecanique, Montreal, Quebec, Canada
Departement
d’education
physique,
Universite
de Montreal,
Abstract Objective. It was the objective of this study to investigate the kinematic and kinetic effects of two specific handling factors that differentiate expert and novice workers, namely the level of knee flexion and box tilt. Design. Seven inexperienced subjects were required to lift a 12-kg box in the sag&al plane using three different strategies: (1) reduced knee flexion and a backward box tilt (more typical of experts); (2) large knee flexion and a backward box tilt; (3) large knee flexion and no box tilt (more typical of novices). Background. The lifting techniques of highly skilled workers differ substantially from those of novices but only limited information is available to compare their biomechanical differences. Methods. The methods included dynamic segmental analyses to calculate the net moments at all body joints and a planar single-muscle equivalent to estimate compression loadings at L5/Sl; total work and joint work distribution were calculated using the integration of joint power. The 3-D kinematic data were acquired with three video cameras and force data were obtained with one AMTI force platform which were synchronized with the film data. Results. Box tilt reduced the load trajectory and loadings on the lower back and shoulders; a reduced knee flexion affected body posture and reduced mechanical work and loadings on all body joints. The combination of these two factors, box tilt and reduced knee flexion, showed added effects for almost all variables. Conclusions. These two factors proper to experts were, in this context, biomechanically more advantageous. Relevance This paper is based on the investigation, with inexperienced subjects, of specific factors consisting of knee flexion and box tilt that differentiate the performance of expert workers from novice workers. The understanding of the biomechanics of their strategies may help provide a theoretical background in the design of preventive measures in manual lifting. 0 1997 Elsevier Science Ltd. All rights reserved Key words: expertise, lifting, box tilt, knee motions, joint loadings
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1997
Introduction The training of workers in safe handling techniques is believed to constitute a preventive measure for reducing risk factors in the working environment, Received: 20 August 1996; Accepted: 26 September 1997 Correspondence and reprint requests to: Dr Micheline
Gagnon, DCpartement d’education physique, UniversitC de MontrCal, C. P. 6128, Succ. Centre-ville, MontrCal (Qutbec) H3C 357, Canada.
particularly musculoskeletal injuries to the back. However, training programs have only shown limited success in the reduction of injuries or accidents’,*. The ways of handling loads vary widely and the workers generallv use methods different from those recommended3-s. Even experienced handlers do not generally agree on the best methods of lifting’. To better understand the choices workers make, we have recently conducted an extended laboratory
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study using the methods of ergonomics for comparing expert and novice workers when lifting freely6T7;the same subjects were further examined in an asymmetrical lifting context to compare their biomechanical differences’“. These subjects were six experts, selected from a large transportation firm, and five male physical education students. The experts had been selected from a pool of workers who had been identified by fellow-workers and managers as those workers having the best manual handling skills; they had an average of 20 yr experience (range between 14 and 36yr) and were free from current back and shoulder musculoskeletal problems; the accident data analyses revealed a lower annual rate of handling accidents for the experts (0.13 vs 0.83 accident/worker/yr for the entire firm). The students were required to have at least 3 months’ handling experience (average: 0.7 yr and range: 0.3-0.9 yr); none of them worked in the same firm as the experts, being part-time workers in different manual handling jobs. Several differential factors were found between the groups. Two of these factors have been selected in the present study for further biomechanical analyses, because of their predominance in the majority of the tasks: the level of knee flexion and the use of box tilts. The first aspect is the level of knee flexion. Contrary to what is generally admitted and analysed in biomechanical studies, workers do not use the stoop (straight knees, bent back) nor the squat (flexed knees, straight back) techniques. The workers, whether experts or novices were seen to adopt techniques where both the back and knees were flexed; their difference is rather in the amount of knee flexion, the experts using much less knee flexion and motions than novices6,7*‘0. The premise for advocating a squat technique in training programs is to the effect that by so doing, the workers could spare their backs by shifting loadings from the lower-back to the knee joints. However, this premise remains questionable as several studies have shown that, when the knees were near extension, lower-back loadings either remained unchanged or even slightly decreased”‘-‘4. On the other hand, the squat technique may have adverse effects as it is related to an increased action of knee extensors’“~‘2~‘3~‘5.We therefore thought important to investigate two different postures mostly encountered in practice: trunk flexion combined with large knee flexion (more typical of novice workers) and trunk flexion combined with a moderate level of knee flexion (more typical of expert workers). The other aspect is box tilt. To our knowledge, this factor has not been biomechanically analysed in the past except for our very recent study on asymmetrical handling; the box is generally assumed to remain flat, with the bottom parallel to the ground and no attention is granted to box tilt. In fact, the absence of box
tilt is a characteristic of novice workers. The experts adopt as many as 40 different combinations of box tilt and handgrips in simple asymmetrical contexts: in fact, box tilts in all directions were observed but the predominance was for box tilts towards the subject and sideways7. Authier et a1.7 have observed, in experts, that at the beginning of transfer, the boxes were generally tilted sideways (24%) or towards the worker (30%) with a tilt generally larger than 45”; the same trend was observed during transfer and, at deposit, the boxes also remained tilted 43% of the times. On the other hand, the novices almost never tilted the boxes at the beginning of transfer with only 13% of the times sideways and also 13% towards themselves, and in most caseswith only very slight tilt (below 20”); during transfer, the absence of box tilt was predominant and, similarly at deposit the box was held flat (96% of the cases). We therefore thought pertinent to investigate two different conditions of box tilts that were found current in practice for the manual handling of boxes without handles: a backward box tilt, towards the subject (more typical of expert workers) and the absence of box tilt (more typical of novice workers). It was the purpose of this study to investigate, with inexperienced subjects, the biomechanical effects of two specific factors differentiating novice workers from expert workers in a simple task of sagittal-plane lift. It was hypothesized that a backward box tilt (towards the subject) and reduced knee flexion, characteristics of expert handlers, would present biomechanical advantages such as the reduction of the path of load transfer as well as the reduction of mechanical work and body joint loadings. It was also hypothesized that the combination of both strategies would show added effects over each single strategy. These hypotheses were based on the premise that reduced knee flexion while flexing the back and tilting the load would act on the reduction of the lever arm of the load in relation to the body joints; reduced knee motions should also directly influence mechanical work. Finally it is reasonable to believe that some strategies of experts may involve the objective of minimizing the path of load transfer and thereby the duration of efforts. Methods Subjects
Seven physical education students were used as subjects. ey were all considered as novice workers as they only had marginal experience (or no experience at all) in handling manoeuvres. Their mean age was 22.7 yr (range: 21-28 yr), their height, 177.6 cm (167.5-190.0 cm) and mass, 71.1 kg (59.9-84.5 kg). These subjects were given instructions to perform three different tasks containing specific factors proper to novices or proper to experts.
Gagnon: Differential
Tasks
Three tasks were performed with a 12-kg box (40 cm long x 26 cm wide x 25 cm high), without handles, that was selected from a large sample of a firm’s products. The box was lifted from the ground to a 67-cm high shelf; the front part of the box was positioned at the level of the shelf. The movements were symmetrical; the subjects were free to select their feet spacing or distance between the toes and the nearest part of the box (range: 16-24 cm) but they had to maintain it for the three tasks and no step was allowed during the task; the standard handgrips were also symmetrical (an 8/S position, according to Drury et al.“) where both hands made contact with the lateral edges of the bottom face at pick up. The tasks were the following:
factors in expert and novice workers
421
1. reduced knee flexion was required; the box was tilted backwards with the hands on the lateral edges of the top face that were then slipped to the standard grips for pick up; the box was kept tilted during transfer till deposit where it was left to drop forwards on the shelf (this task was also referred to as the experts’ strategy) [Figure 1(-4-C)]; 2. large knee flexion and similar box tilt were required [Figure l(D-F)]; 3. large knee fiexion and the box was not tilted, rather remaining parallel to the ground at all times (this task is also referred to as the novices’ strategy) [Figure l(G, H)]. A large box tilt of about 45” was requested from the subject for the box tilt condition whereas in the
Figure 1. Body postures at pick-up and deposit for the strategy of reduced knee flexion and box tilt (A-C), for the strategy knee flexion and box tilt (0-F) and for the strategy of large knee flexion and no box tilt (G, H).
of large
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condition of no box tilt, the angle was not to be larger than about 10” (by visual inspection). In the case of ‘large knee flexion’ the subjects were requested to flex their knees freely whereas for the ‘reduced knee flexion’ the subjects were asked to limit their knee flexion while avoiding straight knees. The subjects were also instructed to lift normally without jerking the load but they were free to choose their lifting patterns even though the initial and final load positions limited the degrees of freedom for motions. The subjects did not receive any instructions concerning trunk posture. Measurement techniques
The tridimensional (3-D) kinematic data were obtained using three video cameras (Panasonic WVD5100) and analyzed with a motion measurement system (Peak Motion Analysis, Palmsprings, CO, USA). Force data were acquired with one AMTI (Advanced Mechanical Technology Inc., Newton, MA, USA) force platform; it was used to record the horizontal and vertical components of the external forces and their point of application. These data were electronically synchronized with film data and sampled at 30 Hz. The 3-D locations of 10 anatomical markers on the subject and of three markers on the box were obtained by Direct Linear Transformation (DLT) procedures i6. Only the x-y co-ordinates of the markers’ positions in the sagittal plane (right side of the body) were retained for analyses; the DLT reconstruction error (RMS) averaged 3.5 and 2.7 mm for the x and y axes respectively. The use of a 3-D approach to analyse a planar motion is justified to eliminate the perspective errors associated with bidimensional analyses. Dynamic segmental model
The body was divided into 13 rigid segments: feet, legs, thighs, pelvis (from hip to L5/Sl joint), torso (from L5/Sl to acromion), head plus neck, upper arms and forearms plus hands. The motions were symmetrical and both lower limbs and upper limbs were considered as single units. The L5 and Sl spinous processes were located by palpation and the L5/Sl joint lied laterally at 34% of the anteroposterior diameter of the abdomen from the backi7. The body segment parameters including masses, centers of mass and moments of inertia were obtained from Dempster” and from Zatsiorsky and Seluyanov”. The coordinates of the markers were filtered by a second-order, zero-phase and low-pass Butterworth filter and the phase shift was eliminated by passing the filtered signals in reverse order. The cut-off frequencies were selected automatically from residual analyses. Finite-difference techniques were applied
for the obtention of linear velocities and accelerations. For angular velocities and accelerations, the principles of relative motion of a rigid body were applied. For calculations of net reaction forces and net moments at each joint, a dynamic analysis was performed on each body segment. The segmental model included the net reaction forces and moments at each joint, the external forces measured by the force platform, the segmental weight, with the inertial forces and moments being determined from the kinematic analysis. The analysis covered each segment, starting at the feet, and working up adjacent segment successively until the hand level was reached. These procedures were explained in detail in Gagnon et a1.20. Model@
of loads at L5fSl
The model was based on net moment equivalence. From the net extensor moment, a single musculoligamentous force was represented to act with a representative lever arm about the L5/Sl joint center. The solutions for the estimation of compression forces and the sources for the input data are presented in Gagnon et a1.20,21.The input data included: an extension force combining the effects of the erectores spinae muscles and interspinous ligaments assumed to act normal to the vertebral plateau with a lever arm corresponding to 26% of the antero-posterior diameter of the trunk; the centroid of the vertebra was an at estimated position 34% from the posterior edge of the antero-posterior diameter of the trunk; the compression force set normal to the plane of the vertebral plateau; and the angle of the sacrum (Sl) with respect to horizontal estimated as a function of trunk flexion and knee flexion.
Mechanical work
Total mechanical work and joint work distribution were determined from a kinetic approach based on the time integration of joint power22. Joint power was calculated from the product of joint net moment and joint angular velocity, The total mechanical work was obtained by adjusting for differences between positive and negative works with the assumption that negative work was 1.5 times more efficient than negative work. The processes of muscular activity including generation (positive work), absorption (negative work), and transfers can be examined thoroughly in this model. The first applications of all these principles to
Gagnon: Differential
Results
practical situations in occupational biomechanics can be found in Gagnon and Smyth2”, and more details on the procedures and sources for input data can be found in that paper.
Box tilt factor (task 2 vs task 3) affkcted the characteristics of load path and body posture (Table l), as well as loadings on the back and shoulders (Table 2). With box tilting, the total duration of the task increased significantly but, at the same time, important advantages were observed in the reduction of both the duration and length of the path during which the load was entirely supported by the subject. Box tilting also affected body posture: the knees did not flex as much (max.: 107 vs 125”) and their range of motions was smaller (90 vs 108”) with this strategy; however, trunk flexion was larger (max. 97 vs 88’) although the relative motions between trunk and pelvis, at L5/Sl, did not follow the same patterns as the trunk motions relative to horizontal, the ranges being similar at about 40”. Tilting the box has approached the center of gravity towards the subject as seen in the significant reduction of the maximal back-to-load distance by 0.04 m; a consequence was the reductions of average compression loadings at L5/Sl by 20% and maximal shoulder flexor moments by 16%. The knee-flexion factor (task 1 vs task 2) had considerable effects on body posture (Table l), loadings on all body joints and mechanical work by lower limbs and the back (Table 2). In both strategies, knee flexion was relatively important but smaller, as expected, with the reduced knee-flexion strategy (54 vs lo?), as was the range of knee motions (40 vs 90”); however, trunk flexion was slightly larger with this strategy (118 vs 97” ) as was
Kinematics of load trajectory The characteristics of the box trajectory included the maximal distance from the center of gravity of the load to the L5/Sl joint, the total duration of load transfer as well as the integration of the path during transfer. This particular event of load transfer was important as it corresponded to the period during which the load was entirely supported, that is from pick-up until deposit. The path of the load was the one associated with its center of gravity. It was calculated from geometrical relations obtained from the three markers on the load. Statistics Analyses of variance involving repeated measurements were conducted on the three experimental tasks to test if there were significant differences between tasks executed with reduced knee flexion or with large knee flexion (task 1 vs task 2), between tasks executed with or without box tilt (task 2 vs task 3) and between the experts’ task and the novices’ task (task 1 vs task 3) where both factors of knee flexion and box tilt were combined. Significant differences were located using linear contrasts with a significance level of 0.05. Table 1. Means (standard
deviations)
Variables
Total lift duration (s) Load supported: duration (s) Load supported: path (m) Max. velocity (ms--') Max. acceleration (msm2) Max. distance load-to-L5/Sla (m) Load-to-L5/Sl dist. at pick-up (m) Trunk flexion: pick-up (“) Max. trunk flexionb,d (“) Min. trunk flexiond(“) Range of trunk motions (“) Max. L5/Sl flexionbsd (“) Min. L5/Sl flexiond (“) Range of L5/Sl motions (“) Knee flexion: pick-up” Max. knee flexionbGe (“) Min. knee flexiona,e (“) Range of knee motions (“)
and statistical
significance
levels for the kinematics
of the three lifting techniques
Reduced knee flexion; box tilted (1)
Large knee flexion; box tilted (2)
Large knee flexion; not tilt (3)
3.13(0.48) 1.34(0.13) 0.96(0.08) l.O(O.1) 2.6(0.4) 0.91(0.04) 0.69(0.03) 105(4) 118(5)
3.29(0.59) 1.39(0.09) 0.94(0.10) l.O(O.1) 2.7(0.4) 0.94(0.05) 0.72(0.02) 84(13) 97(13) 61(5) 36(12) 80(10) 119(7)
2.27(0.42) 1.66(0.26) 1.10(0.10) 0.9(0.1) 2.5(0.7) 0.98(0.05) 0.72(0.02) 81(10) 88(10)
W3) 5W) 82(9) 119(8) 37(5) 51(14) 54(16) 14(7) 40(17)
3W3 98(12) 107(15)
lw3) 90(20)
N.A. Specific comparisons not calculated as the global test showed aOccurrence at or near deposit. bOccurrence at nor near pick-up. co” is for the trunk at the vertical. d180” is for pelvis and trunk segments aligned. “0” is for complete extension. *Statistically significant for PI 0.05.
423
factors in expert and novice workers
no significance
5W) 30(11) 7w4 119(8) 40(5) 118(11) 125(10) 166’) 108(14) at P
Statistical
(N = 7)
significance
levels
1 vs 2
1 vs 3
2vs 3
0.233 0.254 0.629 N.A. N.A. 0.028' 0.025" 0.005" 0.006" N.A. 0.014x N.A. N.A. N.A. 0.001* 0.001* N.A. 0.002*
o.ooo* 0.009* 0.009* N.A. N.A. 0.001* 0.023" 0.001* o.ooo* N.A. 0.001” Y.A. Y.A. \.A. 0.000” 0.000* t1.A. @.OOO”
0.002" 0.028* 0.001" N.A. N.A. 0.002" 0.466 0.358 0.001" N.A. 0.070 N.A. N.A. N.A. o.ooo* 0.005* N.A. 0.002x
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Vol. 12, No. 719, 1997
Table 2. Means (standard
deviations)
Variables
Moments and loadings: Max. knees’ extensorsa (Nm) Max. knees’ flexorsb (Nm) Max. shoulders’ flex&SC (Nm) L5/Sl extensors at pick-up (Nm) L5/Sl extensors at deposit (Nm) Max. compression at L5/Sld (N) Average compression at L5/Sl (N) Work-energy: Total work (J) Upper limbs (J) L5/Sl (J) Lower limbs (J) Knees (J) Ankles (J) N.A. Specific aOccurrence bOccurrence COccurrence dOccurrence *Statistically
and statistical
significance
levels for the kinetics of the three lifting techniques
Reduced knee flexion; box tilted (1)
Large knee flexion; box tilted (2)
Large knee flexion; no tilt (3)
16(20) 136(17) 56(17) 209(22) 204(31) 4819(467) 2961(269)
82(29) 131(13) 64(14) 241(23) 205(31) 5369(365) 3345(208)
73(49) 139(18) 76 (20) 245(44) 224(36) 5587(566) 4171(388)
N.A. 0.048* 0.007* 0.816 0.014” 0.008*
520(82) 105(49) 142(20) 273(36)
615(80) 119(39) 165(33) 331(39) 56(15) 65(14)
645(114) 130(49) 166(32) 349(61) 66(28) 78(19)
0.002* N.A. 0.014* 0.006* 0.028* 0.003”
W8) 33(13)
comparisons not calculated as the global test showed at the beginning of the task, before pick-up. at or near deposit. at mid-transfer. either at pick-up or at deposit. significant for P10.05.
the range of trunk motions (56 vs 36”) but the relative motions between trunk and pelvis, at L5/Sl, were not affected, at about 40” for both tasks. The posture associated with reduced knee-flexion strategy produced a significant reduction in the maximal distance between the load and the back. These postural characteristics might explain the important reduction in total work and joint works including the lower limbs and the back (between 14 and 18%) as well as the important reductions in peak moments for knee extensors (by 80%) shoulder flexors and L5/Sl extensors (by 13%) and for the peak compression force at LSD1 (by 10%). The combined factors (task 1 vs task 3) represented by reduced knee flexion and the use of box tilt (experts’ strategy) and by large knee flexion and no box tilt (novices’ strategy) were compared; the results showed that the effects were still more important than for the single factors separately. The experts’ strategy included the combined advantages of reducing both the duration and length of the path during which the load was entirely supported while reducing peak joint moments, maximal compression force at L5/Sl and joint works. These changes were mainly attributed to the large changes in body posture. Discussion This study was undertaken with inexperienced subjects to investigate the biomechanical effects of two specific handling factors that discriminate expert from novice handlers in a simple task of sagittalplane lifting. It was found that a backward box tilt, a
no significance
Statistical
(N = 7) significance
levels
1 vs 2
1 vs3
2 vs 3
0.001*
0.038” N.A. 0.001* 0.060 0.054 0.004* 0.000”
0.554 N.A. 0.009” 0.821 0.094 0.218 0.001*
0.019*
0.277 N.A. 0.883 0.227 0.182 0.178
N.A. 0.039* 0.013* 0.047* 0.003*
at P50.05.
factor typical of experts, largely influenced the characteristics of load trajectory, minimizing both the duration and length of path during which the load was entirely supported by the handler with the consequences of reducing loadings on the lower back and shoulders. It was also found that a reduced knee flexion, another factor typical of experts, presented advantages over the larger knee flexion that characterizes the performance of novices, affecting body posture with the consequences of reducing total mechanical work and joint works for the back and lower limbs as well as loadings on all body joints. Finally, the combination of both factors showed added effects for almost all variables, indicating that these two factors proper to experts were, in this context, biomechanically more advantageous. It is important to discuss the limitations present in this laboratory context for executing the tasks. Experimental constraints have forced the analysis of a sagittal-plane lift with symmetrical handgrips and symmetrical feet positions. It is in fact restrictive as it is known from several ergonomic studies that lifting is seldom made symmetrically in the sagittal-plane4-“. We have chosen to study two specific technical elements, box tilt and knee flexion, that were clearly generalized in the field and differentiated expert and novice workers”v7. The subjects did not receive any instruction on the way to use their back. They also chose the feet position in relation to the load for which they felt more comfortable: the distance (range: 16-24 cm) is compatible with the observation we made in a previous study of expert and novice workers who were found similar for this variable, 60% of the subjects having adopted distances
Gagnon: Differential
factors in expert and novice workers
without handles affects the comparisons. In spite of this, our results indicated that the strategy of experts led to much smaller knee extensor moments; they are comparable, to this effect, to Gagnon et al.rO previous observation of experts and to Schipplein et al.“” data for a freestyle lift and comparable knee flexion angles (trunk posture not reported); on the other hand, our results on novices’ strategy approach the results of previous investigations on leg liftr2,‘“. It is safer to reduce the action of knee extensors as much as possible as they are responsible for the stresses on the femoro-patellar joint, an important factor in the pathogenesis of osteoarthriti9”. Box tilting has seldom been the subject of biomechanics literature on manual materials handling. However, a few studies in ergonomics have emphasized the importance of this factor when combined with different handgrips3,“. The research emphasis has rather been focused on optimum handle positions and angles that could compensate for the need to tilt the box”‘,.12, however, one cannot find the optimum design appropriate for the numerous conditions present in the working context. The industrial survey conducted by Drury et al.” in nine factories has effectively shown that very few boxes had handles. Recent obsemations of expert handlers from a large transport industry have shown that they were able to develop a large number of handling strategies combining various box tilts with different handgrips”,7. Box tilt, specially when coupled with the appropriate handgrips may provide a large variety of manoeuvres; this handling strategy
between 16 and 32 cm at pick-up and at deposit7. It is important to recognize that other task variables not investigated in the present study, could also influence the lifting performance: the lifting height2”,24, the weight of the loadZ3, its format with the restriction that the load may or may not be brought between the knees”, its horizontal location relative to the back2”, and also the lifting technique with reference to inertial factors2”,27 and to lifting rhythm between lower limbs and trunkz8. The tasks investigated in the present study were controlled for some factors (symmetry, load distance, height, weight, velocity, type of handgrips) but uncontrolled for others (lifting rhythm, trunk posture) in order to approach realistic conditions as much as possible. The scope of the results should then be circumscribed to this context. The interactions of knee flexion and box tilt with other task variables were not addressed in this paper. However, from the present results it is believed that the knee flexion and box tilt/handgrips are important factors that merit further investigation. The present data were compared with previous results on low back and knee moments and trunk/ knee flexion obtained by other investigators (Table 3). Peak extensor moments at L5/Sl were similar to those reported in symmetrical and asymmetrical lifting literature. However, the results on knee moments reflect the large variation attributed to the lifting style, the type of load and the level of expertise of the workers. Trunk flexion was seldom reported which makes the direct comparison difficult; furthermore the use of a load with or
Table 3. Net peak moments (Nm) at the low back and knees and trunk/knees from some previous publications
flexion
angles (“) for asymmetrical
Flexion angle Trunk Investigators
Task/load
Ekholm et al. (1984)‘*
Symmetrical; 0 Leg lift l Back lift Symmetrical; Freestylea Symmetrical; 0 Leg lift l Back lift Symmetrical; 0 Leg lift l Back lift Asymmetrical; Freestyle: l Novices l Experts Symmetrical; 0 Strategy of 0 Strategy of Symmetrical; Freestyle
Schipplein
et al. (1995) 25
Hagen et al. (1995) 2g Toussaint
et al. (1992) l3
Gagnon et al. (1996) lo
This study Deeb et a/.(1985) 31
425
anNd/or symmetrical
Peak moments Knees
lifts
(Nm)
Low back (L5/Sl)
Knees
Max.
Range
Max.
Range
N.R. N.R.
N.R. N.R.
-100 - 20
- 90 -15
N.R. N.R.
50(E)/60(F) -60(F)
N.R.
N.R.
- 50
N.R.
- 245
- IO (E)
N.R. N.R.
40 90
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
N.R. N.R.
-80 - IO
- 70 -5
- 225 - 225
40(E) 180(F)
N.R N.R.
N.R. N.R.
53 31
38 20
- 200 - 200
33(E)/58(F) lO(E)/72(F)
88 118
30 56
125 54
108 40
264 238
73(E)/139(F) 16(E)/136(F)
N.R.
N.R.
-
N.R.
N.R
N.R.
12.8 kg IO kg;at 20 cm 17 kg 15.3 kg
17.0 kg
12.0 kg at 16-24 cm novices experts 9 and 13 kg
N.R.Not reported. (E):Extensors; (F): flexors; - symbol for approximation aValues reported at peak L5/Sl moment. bConversion for a reference of 0” in full knee extension.
140b
of readings from graphs,
426
Clin. Biomech.
Vol. 12, No. 7/8, 1997
appears much more flexible than the use of handles. Depending on the task constraints, the coupling of handgrips and box tilts may fulfil several objectives such as vertical and horizontal stability of the load, proper exertion of momentum, increased stability of the worker and reduced stresses on body joints. A recent biomechanical study by our group has shown that the biomechanical factors are highly sensitive to changes in tilts and handgrips”“. In the present study, the choice was made for a symmetrical 8/8 handgrip in combination with either a backward box tilt or without tilt. The position 8/8 symmetrical has been identified to be within the five most frequent hand positions, accounting for approximately 10% of movements in an industrial survey”; it is recommended for lifting heavy weights from the floor as it is reported to be associated with minimizing forces on the hands and reducing the stress on the upper limbs3*. The objective for tilting the load backward was to raise its center of gravity and to approach it from the worker before lifting. This was accomplished successfully as the time of exertion associated with load transfer was reduced with subsequent reductions of loadings on the upper arms and lower back. The tilting manoeuvres before pick up are, however, responsible for the lengthening of task duration, a necessary compromise to reduce joint loadings. As these results applied to a single tilt/ handgrips strategy, the inferences about its applicability remain limited. Other strategies should be identified in relation to the objectives of the tasks. Our results indicated that these strategies merit further experimental verification to better understand the principles associated with safe handling. The use of flexed knees technique has long been advocated as a strategy to shift the load from the lower-back to the knees34-35,but it has been shown that when the knees were near extension, lower-back loadings either remained unchanged or even slightly decreased’0~“~‘3-‘5~36while there was a shift from knee extensors to knee flexors10-‘2,36 and a reduction in the metabolic cost11,29,37,38. Unfortunately, trunk posture relative to the horizontal was rarely reported and large variations in trunk flexion, from near extension to extreme flexion, may have led to different conclusions on the pertinence of using or not straight knees or flexed knees techniques. In the present study we chose not to control the amount of trunk flexion, rather leaving the subjects free to adopt the most comfortable posture. All subjects chose to flex their trunks near the horizontal. In this respect, their trunk postures are in agreement with several ergonomic observations or with laboratory observations of freestyle lifting in asymmetrical conditions by expert and novice workers lo. Trunk flexion appeared to be related to knee flexion: the smaller the knee flexion, the larger the trunk flexion (comparisons between the means for the three tasks). On the other
hand, the trunk/pelvis angles (max. min. and range of motion) were not related to knee flexion, being similar for all tasks, suggesting that lumbar flexion and the passive bending torque due to discs and ligaments were not influenced differently by the different tasks. One may question if the adjustment of trunk posture may have been influenced by the need to restrict this trunk/pelvis angle. This question should be clarified in future studies. The importance of the interplay between the extensor moment and the bending torque at L5/Sl has already been demonstrated for lifting tasks39. To our knowledge, no study has reported the complete analysis of loadings as well as mechanical works on all body joints; this approach is particularly important at the light of potential shifts of loadings between joints. The use of a restricted knee flexion technique showed reductions of loadings on all body joints with no apparent shifts from one joint to another; the most important reductions occurred for knee extensors thought to be responsible for overuse injuries to the patella. Similarly, mechanical work was considerably reduced with the restricted knee-flexion technique; this was mostly due to reduced work in the lower limbs and the back (L5/Sl). It is apparent that, in this restricted context of sagittal-plane lifting, the reduced knee flexion (about 50” from full extension) combined with large trunk flexion (about 30” below horizontal was more advantageous than a technique combining a large knee flexion (about 120” from full extension) with large trunk flexion (near horizontal). The advantage of minimizing knee flexion was demonstrated in a context associating large trunk flexion and box tilt. This specific situation of sagittal and symmetrical lifting limits the inference that can be made about the applicability of the results to generalized conditions. In conclusion, box tilt and reduced knee flexion, two factors proper to expert workers, produced clear biomechanical advantages of reduced path for load transfer, reduced joint loadings and mechanical works over the novices’ strategies consisting of large knee flexion and no tilt of the box. Future studies should focus on the multiple facets of handgrips/box tilts as applicable to different working contexts. Similarly, the focus should be on the relations between knee flexion versus trunk/pelvis rotation and trunk flexion. One should abandon the idea that the leg lift is the unique solution to handling problems; this is not supported by criteria of minimizing joint loadings and mechanical work. One should probably focus on the compromises between the several objectives that can be fulfilled in the working context. Acknowledgements
The financial assistance of the Institut de recherche en Sante et en securite du travail du Quebec (IRSST)
Gagnon: Differential
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