Spinal compression at peak isometric and isokinetic exertions in simulated lifting in symmetric and asymmetric planes

Clinicui Biomechunics Vol. 11. No. 5. pp. 2X31-28’9. 1906 Copyright @ 1996 Elsevier Science Limited. All rights reserved Punted in Great Britam 0268~0033~96 $ I5.W + 0.00

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Spinal compression at peak isometric and isokinetic exertions in simulated lifting in symmetric and asymmetric planes S Kumar

MSC PhD DSC

Department Canada

of Physical Therapy,

University

of Alberta,

Corbett Hall, Edmonton,

Alberta,

Abstract Objective. To determine if the spinal compression can be used as a criterion for the safety of the back. Design. Various lifting strength activities were analysed using a 3-D biomechanical model to determine spinal compression forces. Background. Despite many standards being set and the use of varied strategies, low-back injuries continue to be common in our society. Since the injuries occur to the tissues in the body, it may be more appropriate to investigate the effects of external loads on the tissues. Therefore it was decided to determine the spinal compression in standardized lifting tasks. Methods. Twenty young adults (12 males and 8 females) performed maximal stoop and squat lifts in sagittal, 30” lateral, and 60” lateral planes at half, three-quarters, and full horizontal reach distances. The stoop lifts were performed in isokinetic and isometric modes; the isometric mode was performed with the hip at 60” and 90”. The squat lifts were also performed in isokinetic and isometric modes; the isometric mode consisted of postures with knee at 90” and 135” of flexion. In addition, the subjects also performed isometric lifts in stoop and squat postures at a self-selected optimum posture. During these activities the strength was measured using a static dynamic strength tester employing a load cell and force monitor. Three-dimensional postural recording was made using a 3-D Peak Performance Technologies imaging system. Using the postural and force data as input to a 3-D biomechanical model, the lumbosacral spinal compression was calculated. The values of strength and spinal compression were analysed and compared. Results. The strength was significantly affected by the gender, the type of lift, plane of lift and reach of lift (P
Low-back

C/in. Biomech.

injury

control,

manner in which it can maximize the safety of injuries are biomechanical perturbation of the be used in a manner where tissue load be @ 1996 Elsevier Science Ltd.

spinal compression,

Vol. 11, No. 5, 281-289,

lifting

strength

1996

Introduction

The low-back morbidity in our industrialized society hardly needs to be emphasized. A lifetime prevalence of 70-80%, costing in US $25 billion’, and claiming Received: 13 June 1994; Accepted: 16 January 1996 Correspondence and reprint requests to: Professor S Kumar PhD DSc, Department of Physical Therapy, University of Alberta, Rm 3-75, Corbett Hall, Edmonton, Alberta, T6G 2G4, Canada

second highest activity limitation days’ are grim but real evidence. Though over one hundred risk factors for low-back pain have been identified”,” these can be broadly categorized under four groups: (a) genetic, (b) morphological (c) mechanical, and (d) psychosocial. Little ifanything can be done about the genetic and morphological risk factors; mechanical factors do allow the possibility of administrative control and management. Further, regardless of the contributing

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factor< the fx~cipitation 01 Jnjq is ;I biomechanicai pcrturbatior~ which ewntuall~ I~ads to nociception. ‘f’hu!.. the nrcchanicaf medium i4 ;I funnel through whJch all iactcsrs pass 10 manifest themselves in pain percqtion 2nd paiJl beha\ iour.

II.:)w ma\ ~:0mpressicin pfav a pivotal role? Spinal or ifihi: ifegcnrration begins hoon after skeletal maturity. f‘h~b Jeg~ncr:~tioJl result\ in the loss of intervertebral dJ\\. height ,I< wail as IOW in the \vater content (turgor) ot rhi- nuc~fc~~~..l’hc degeneration occurs most rapidly at th,: I , I_. ;II~L! I,5/SI lcvcfa. These levels are also sub~c~:cci ;ri greatest mechanical loads. It has been demonstrated rhat when ;I spinal unit is subjected to compre~sicm ~ti t’11’0, dri)pfcts of water ooze out from tht: pcripher:)! disc lay:i-y. Application of mechanical ::tml>ressrve load causes a shrinkage in the height of the inierver.tchr.:tf disc :li i~i~rtr’. In everyday living acii~~trcs. the fumbosacral disc as well as 1_.,1’1,, discs are sublrcted 10 large magnitudes of compressive loads and CO~III~U~~LJS \t;Jtic load due to the weight of the torso. The hydrostatic nature crf the nucleus pufposus is due to the high imhihition pressure of the hydrophilic gfvcosaminclgf~~ans nhout 100 kl’a”‘. As application ~1; compre%ivc’ load srfueczcs some water out of the nucfeus it afsit reduces the oxygen tension in it. With such a reduction in oxygen tension the glycosamino&cans begrn to break down into smaller molecules with lower imhibition prcssurc7. The latter, in turn. causes a long-term lowering of the turgor and thereby reduced oxygen tension (as oxygen is dissolved in water). This may continue the process of change which may lead to degeneration. The degenerated disc predisposes the spine to a variety of injuries through

figamentous slackening. malalignment of spinal structures. stress concentration. and other factors. However. it must be rccognizcd that for a healthy disc. intervertebral motion and some compression is esscntiaf. ~f’he focus on compression in this paper refers to a level significantly in excess of this normal requirement. In order to curb this common affliction of low-back pain, mechanical load and spinal compression beyond healthy limit needs to be controlled. Many countries around the world have attempted to tackle the problem of incidcnces of low-back pain by setting maximal weight limits for lifting by men and women. For adult male it varies from 25 to 100 kg, and for adult females it ranges from 10 to 41.25 kg”. Ilowever, load in isolation does not provide an accurate measure of low-back compression. The moment arm at which this load is applied will determine the spinal compression. A systematic and predictable reduction in strength production capability with increasing mechanical disadvantage has heen demonstrated”.“‘. Therefore the same load (set weight limit) when applied at greater mechanical disadvantage not only increases the compression by a factor of the ratio of lever arms, but also represents a much greater proportion of the subject’s strength-generating capacity. The higher the level of effort. the lower would be the margin of safety”. Several studies have looked into the mechanical behaviour of spinal units under various compressive loading conditions’.” I’. The spinal units can fail under a large, single compressive load or under repeated moderate loads. Epidemiological studies have reported low-back injury under single violent exertion or impact, and in occupations where moderate loads are handled for longer periods of time. The cumulative load as life-time exposure to compression has been reported to be an important risk factor for precipitation of low-back pain problems’“. Therefore, in order to control the low-back injury and pain, it is important to control exposure to compression. While relationship between load and spinal compression in sagittal plane is better reported. a systematic study investigating the fcve1 of effort and compression in sagittal as well as asymmetric planes has not been reported. Therefore the objective of the study was to measure the peak isometric and isokinetic strengths in sagittal and asymmetric planes and estimate their corresponding compression. The latter may reveal problems in using a simplified approach of establishing a weight limit without regards to task requirements. Methods

Twenty subjects (12 males and 8 females) with no history of low-back pain or any other muscufoskeletal problem within the last year volunteered for the project. Metabolic or cardiovascular disorder, spinal or abdominal surgery, and pregnancy were used as exclusion criteria. Such selected, informed, and con-

Kumar: Table 1. Demographic Gender

data of the experimental

Spinal

compression

at peak

exertions

283

sample

stats

(cm)

Knuckle height (cm)

Knee height (ml

Hip height (cm)

Shoulder height lcml

Full reach (cm)

Weight

Height

(kg) Male ” = 12

Mean SO

28.2 3.7

72.6 6.8

177.4 7.2

79.4 3.1

51.5 3.0

97.2 5.9

145.2 6.4

64.3 2.9

Female n=8

Mean SD

25.6 3.2

54.4 5.8

160.7 3.5

71.6 2.2

45.9 0.8

85.2 1.3

129.7 2.9

55.8 1.4

senting subjects were trained for the tasks at submaximal intensity one or two days before the experiment. The subjects were told that their maxima1 effort was essential for the project; however, they will not be encouraged during the trial. Prior to every trial they will be reminded to give their maxima1 effort. These subjects were measured for their weight, height, horizontal reach (acromion process to the grip centre), and knuckle height (Table 1). The subjects were attired in loose and comfortable clothes and placed on the set-up platform. Tasks

All subjects were required to perform 56 standardized maximal effort lifting task against an instrumented industrial size handle. The tasks consisted of stoop and squat lifts. Within each of these two lifting techniques 28 different conditions were studied separately. Both

lifting techniques were studied under isokinetic condition and two isometric postures. The isokinetic lifts for both stoop and squat technique started at the ground level and terminated past the knuckle height. The isokinetic velocity was chosen at 60 cm SC’. Thus isokinetic lifts lasted under 2 s. The isometric stoop lifts were performed in two postures : (a) isometric 1 was done with 60” of hip flexion, and (b) isometric 2 was done with 90” of hip flexion (Figure 1). The isometric squat was also performed in two postures: (a) isometric 1 was performed with 90” of knee flexion, and (b) isometric 2 was executed with knee at 135” of flexion (Figure 2). For isometric tests the subjects were asked to lift without jerking, reaching their full strength within the first 2 s and maintaining the effort for another 3, for a total of 5 s. All isokinetic and isometric stoop and squat lifts were performed in sagittal, 30” lateral and 60” lateral planes at half, three-quarters, and full horizontal reach distances (Figures 1 and 2). In

60’

9

Standard Posture stoop Half Reach

Figure 1. The postures

Three-quatiers Reach

of stoop lift.

Full Reach

Half Reach

Three-quarters Reach

Full Reach

Half Reach

Three-quarters Reach

Full Reach

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60’

tiall Reach

Three-quailers Reach

Full Reach

tia’!

Reach

FU Reach

i

;t~.ld~tio~. <;jch ~bjcct performed a maximal isometric lift in elf--sciixied ‘optimal stoop and squat posture f01 maxima! ilutput. ‘Fhcsi- wcr~’ termed standard stoop. and standard squat postures as described in literature’.‘. ‘l‘hc suhjccls \vc”rc allowed at least a ?-min rest period bc=tween an\ two test c~onditions.

I lie Statri, :rrrd Dynamic Slrcngth ‘I’cster (SIXI’) ~vas used for r- recording the only \rrtical force. The <)trtput trl‘ the load ccl1 wan t’ed to the Force Monitor [ I’rototypc I>esipn (‘ompany. Ann Arbor. Ml) for signal conditioning and feeding the AiD hoard (Data Translation 2X01 A) which sampled the signals at SO Hz. I‘hc activities of’ the suhiccts were recorded using two mutually orthogonal videocameras of the Peak I’erformance Sy>tcm with an event synchronizing. SMPTI: time eodc generator. and an autodigitizer module. ‘The two cameras wet-c placed 5 m away from the experimental site rn a mutually perpendicular direction

cuntreing upon the same point. Once the camera position> were selected. the positions of the tripod stands were marked and kept unaltered throughout the experiment. ‘I‘hc zoom lense\- of the vidcocamcras were used to fill the field with the subject’s image, allouing room for any anticipated movement. The camera and VCR were started at lcast I min before the activities began. ‘I’htz VC’Rs were allowed to run continuously. capturing each activity in its entirety for all subjects. The experimental site was illuminated with two SOOW spotlights. also placed mutually perpendicular to each other. The posture measuring system was calibrated for three-dimensional coordinate data using Peak Calibration System. The Peak 5 Motion Measuring System used direct linear transformation method from man) two-dimensional \icws. This method established a direct linear relationship between digitized coordinates f’rom the views of both cameras and the threedimensional space coordinate by using intersections of lines from each camera view to determine point in space. The direct linear transformation method used known coordinates in object space of’ the calibration frame. ‘Though for a two-camera. three-dimensional system, six non-coplanar points on the calibration frame were required; all eight rods with three markers on each were used. IJsing the least-square method to determine the best fit. an equation (containing co-

Kumar:

efficients that translated, rotated, and scaled data according to the coordinates of points from the calibration frame) was created. This set-up and procedure calibrated the object space and stored the camera parameters. Using the above-described calibrated system, the angles at wrist, elbow, shoulder, hip, knee, and ankle were measured. The Peak 5.0.0 software was used along with the hardware to convert the coordinates into angles at the joints and fed to a 486 computer through the Peak Frame Grabber Board. The Peak Performance System sampled at a frequency of 60 frames per second. The centroids of the spherical markers were considered as the respective joint centres. These angles were used as kinematic input to the biomechanical nlodel’x. Experimentul

desigrl

A combination of two lifting techniques (stoop and squat), three lifting modes (isokinetic, isometric 1, and isometric 2), three planes (sagittal, 30” lateral, and 60 lateral), three reaches (half, three-quarters, and full horizontal reach distances), and two standard postures resulted in a total of 56 conditions. The sequence of these 56 conditions were randomized. Analysis

The input variables of force generated and coordinates of the ankle, knee, hip, shoulder, elbow, wrist, lumbosacral joints, and vertex were fed to the threedimensional biomechanical model” for calculation of external moments and back compressive forces. The model used a linear optimization technique (Simplex) which minimized the compressive force at one disc joint with three moment equation constraints and muscle Table 2. The mean peak force (N) generated and 60" lateral planes at half, three-quarters.

Spinal

compression

at peak

285

exertions

stress upper limits to solve muscle force distributions. These solved muscle forces were then substituted into three force equilibrium equations to solve the disc compressive and shear forces. For muscle force calculations a lever arm length of 6.5 cm was assumed for erector spinae as reported before’“. The peak exertion forces and their corresponding external moments and the back compressive forces were qualitatively and numerically analysed. Subsequently they were subjected to the analyses of variances with post hoc analyses. Results

The mean peak force generated in the maximal lifting effort in isokinetic, isometric-l, and isometric-2 modes in sagittal, 30” lateral, and 60” lateral planes at half, three-quarters, and full reach and their corresponding external moments and the back compressive forces are presented in Tables 2 and 3 for males and females respectively. The maximum force was generated by males in isometric stoop lift at half reach with 90” of hip flexion, whereas in females it was produced in isometric stoop lift with hip at 60” and half horizontal reach distance. This is somewhat contrary to the expectation that the maximal forces would be generated in standard (subjective optimum) posture squat lift. Maximal strength was always produced at half reach in every plane followed by three-quarters reach and subsequently full reach (Tables 2 and 3). These differences are generally found between the symmetrical and asymmetrical planes. The strengths recorded in sagittal plane were generally higher than those recorded in asymmetrical planes. Though the variation in strength production in stoop lift among males ranged between 22 and 108% of the normalizing factor (standardsubjective, optimum-posture stoop lift) the correspond-

during stoop and squat lifts among normal males in isokinetic and isometric modes in sagittal, 30" lateral, and full reach, and their corresponding external moments (N m) and back compressive forces (N) Mode

Lift

Plane

lsokinetic

Reach

(degl stoop

Isometric

Isometric

1

2

Force (N/

Ext. mom (N m

BCF (NI

Force (N)

Ext. mom IN ml

BCF (A’)

Force IN)

Ext. mom /N ml

BCF (NI

0

Half T-Q Full

545 300 241

169 148 156

6186 5966 6229

679 353 205

242 169 134

7593 6495 5911

723 380 209

287 195 133

7813 5750 5292

30

Half T-Q Full

505 302 210

158 140 132

6171 6229 5855

523 323 208

171 145 122

6841 6163 5796

603 323 201

181 158 122

6521 5765 5162

60

Half T-Q Full

344 246 173

110 108 111

5331 5499 5453

451 232 153

133 115 93

5647 5579 5136

393 230 148

133 111 91

5512 5298 4746

0

Half T-Q Full

402 339 221

146 171 139

5798 6437 5767

391 287 201

166 157 135

6539 5994 5711

401 279 186

175 139 123

6273 5261 5246

30

Half T-Q Full

425 274 218

136 131 132

5242 5339 5736

358 273 188

148 142 121

5911 5776 5495

414 266 188

166 134 118

5814 5362 4988

60

Half T-Q Full

303 250 188

104 112 124

4605 4889 5508

324 239 162

118 119 114

5223 5312 5457

322 227 168

117 112 105

5018 4833 4919

Squat

0". sagittal plane; T-Q, three-quarters

horizontal

reach;

Ext. mom,

externmal

manent;

BCF, back compressive

force.

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Tabta 3. The mean peak force (N) generated and 60” lsteral planes at half, three-quarters,

i St

during stoop and squat lifts among normal females in isokinetic and isometric modes in sagittal, and full reach, and their corresponding moments (N m) and back compressive forces (N)

_-Force (NI

stoop

sqii;i,

Mode __.-___ isometric 1

lsok/net,c Ext.

mom iN m

BCF I__

IN)

Force ..-.

--.

INI __

Ext. mom (N ml .._-_ __.--

-.-

Isometric

BCF WI

30’ lateral,

-~________

Force

2

lNi

Ext. mom (N m/ _____I_

BCF (N)

321

97

234 191

67 102

3617 3733 4042

362 233 143

107 90 73

4207 3835 3522

343 284 159

119 125 90

416’1 4145 3455

284 246 138

14 88 69

3397 3899 3531

320 230 137

93 88 69

3968 3916 3544

355 247 141

125 106 79

4383 3832 3333

231

67

16 i

1:’

178

?I

3202 3451 3611

751 171 91

70 66 51

3367 3409 3106

237 169 121

85 72 68

3563 3402 3190

t;

waif T.0 F,ili

280 212 : 17

94 85 90

3670 3674 3847

214 157 115

85 72 66

3757 3256 3355

210 134 100

81 66 56

3439 3109 2875

Haif r-l2 iliil

287

;, :

?l!

160

81 81 68

3794 3655 3444

218 153 97

19 69 55

3549 3462 3076

214 131 9:

72 57 50

3035 2826 7756

/.

Half 7 0 Full

214 158 15i

61 70 61

3189 3417 3493

160 143 86

47 60 48

2741 3137 2940

188 134 93

59 58 50

2701 2898 2699

0, sayitrai plane i !> three-quarters horizontal vach, Ext. morri. externmai momsnt; BCF, back com~,‘ess,ve force

ing back csnnpressive force varied between 62 and 107”’ _ ‘l, of the same normaiizing lumbosacral compression (Figures 3 and 3). Among males the standard posttire squat lift strength was higher than standard (subjective optimum) posture stoop. which was not the cast among females. Among males the mean peak squat lift strqgh ranged between 22 and 67% of the normalizing factor (standard posture squat lift strength) but the corresponding back compressive forces werr ftrund to range from 56 tcl 80% of the normalizing iumbosacral compression (Figures 5 and 6). For stoop lift among fcmates the peak strength ranged between 29 ;tnd 11.5”;, of female stoop standard posture *;rrrqth, ‘l‘h~ corresponding back compressive forces. howevz. ranged between a low of 75% to a high of Ifih”, i j!‘ t LL. t‘<>mpressivc load under stoop standard pc~sturc r:i)mprcssion. I;~jr squat lift conditions, the naasrmunr
of force (producad) with corresponding lumbosacral for Males In Isokinetlc, stoop

: dLk :9:;

30oLal Condltlons Figure 3. The peak forces and the back compressive d!ir;ng Isokineticsouat lifts by male subiects.

compression

:.a

Force

STY<

BCF

6OoLat.

forces generated

strength. The lumbosacral compression on the hand ranged between 87 and 124% of squat standard posture hack compressive force. The analyses of variance demonstrated significant main effects due to gender, type of lift. plane of lift and the horizontal reach distance at which lift was performed for the peak strength as well as the external moment (P~0.01). IIowever, for the back compressive forces the reach did not have significant main effect although it accounted for up to 70% decrement in strength production capability due to increase in reach. For the peak strength there were significant two-way interactions between reach and plane, reach and mode, reach and type of lift. and type of lift and plane of lift. ‘The latter implied that the reach significantly affected the strcsngth exertion in different planes. lifts in different modes, and lifts of different types. Further. the generation of strength due to the type of the lift was significantlv affected by the plane. For back comComparison

of force (produced) wlth corresponding lumbosacral for Males in Isometric, stoop at 600

compresslon

ZZ%B Force +TGX3 BCF

300LN. Condltlons

60QLal.

Figure 4. The peak forces and the back compressive forces generated during isometric 1 (knee 90’flexlon) squat lifts by male subjects.

Kumar:

Comparlson

of force (produced) wlth corresponding lumbosacral for Males In Isoklnetlc, squat

I I

sagma

SOoLat. Conditions

Figure 5. The peak forces and the back compressive during isokinetic stoop lifts by female subjects.

compression

BOoLat.

forces generated

pressive forces, however, the interactions of reach with only mode and plane were significant. Therefore any combination of type of lift, mode of lift, plane of lift, or reach of lift did not produce any significant difference in the back compression. They did, however, result in significant differences in the amount of strength which could be exerted. For strength, the three-way interactions between type of lift, plane of lift, mode of lift, and also reach were significant. The latter indicates each of these variables influenced the generation of strength in a significant way when they were factors in a lifting task. However, their variation in combination of three failed to produce any significant impact on the lumbosacral compression. The statistical results are presented in Table 4.

Spinal

compression

The variation in strength capability due to the technique of lifting, the horizontal reach distance at which lift is performed, the plane in which it is executed, and

Comparison

of force (produced) wlth corresponding lumbosacral for Males In Isometric, squat at 900

compresslon

110 100 90 /

~

I

5

80

s

70

% e

60

g c eaI

50

If

30

Degrees of freedom

Peak force Sex Type Plane Reach Mode

9.54 28.53 81.89 105.66 0.09

1 1 2 2 2

0.01 0.01 0.01 0.01 NS

28.67 61.27 4.27 28.23 2.61 1.18

2 2 2 4 4 4

0.01 0.01 0.01 0.01 NS NS

6.27 7.19 1.44 1.73

4 4 4 8

0.01 0.01 NS NS

Type Type Mode Plane Type Mode

x plane x reach x reach x reach x mode x plane x x x x

20 10

2-way and 3-way H

TQ

Sagltta1

F

H

TQ

3OoLat. Condltlons

F

H

6OoLat.

plane reach reach reach

Probability

moment

Back compressive Sex

15.19 10.94 41.34 8.89 interactions

0.01 0.01

2 2

0.01 0.01

1 1 2

0.01 0.01 0.01

not significant

force

Tvw

Plane Figure 6. The peak forces and the back compressive forces generated during isometric 1 (hip 60” flexion) stoop lifts by female subjects,

A summary table for peak force, external force during lifting F ratio

External Sex Type Plane Reach

0

287

Part of model

Type x mode Type x plane Type x mode Mode x plane

40

exertions

the velocity of lifting observed in this study concurs with previous studies”.“‘~““. It clearly reflects that the physical human system is significantly affected by the mechanical variables. While a variation in the reach distance directly affects the mechanical advantage, the impact of the plane of activity is less evident. In many such asymmetric lifting activities, the moment arm at which load is applied in these lifting conditions do not differ significantly from their corresponding symmetrical effort. Nonetheless, a change in plane was found to vary the strength from 30 to 50% and 30 to 35% for stoop lifts at half reach among males and females respectively. The corresponding variation in back compressive forces for males and females were lo-25% and lo-20% respectively. Despite the variation in the strength produced ranged from quite low to very high depending on the task variables, the back compressive force always remained quite high. An identical pattern has been observed for squat lift as well in both genders. The latter becomes obvious by looking at the low and high values of strengths produced and compressions generated (Figures 3-6). Extrapolating from the results reported by Hutton and Adams14 the lumbosacral compression generated in lifting activities tested ranged between 57 and 76% of the ultimate compressive strength (UCS) of the spinal units. The lower values of the compressive loading was recorded in activities which were associated with greater mechanical disadvantage. In a self-selected optimal (standard) posture the maximal compressive force of 74% of the UCS was reached. The latter indicated only 24% margin of safety, which was exceeded in efforts at half reach. Table 4. Analysis of variance. moment and back compressive

Discussion

at peak

2-way and S-way interactions

25.68 8.76 26.24 not significant

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The significance of the above-mentioned numerical vaiucs becomes more clear in the light of results of the ALO\ A for thcsc variables. The variable of reach. due to which up to 50% of variation in strength production wa\ manifested. did not have a significant main effect for fhc back comprcssivc force. Furthermore, though the combinations of type: of lift, plane of lift. type 01 hit, and reach had sigmficani effect on the strength produced the! did not significantly affect the back comprt:ssiv:c Ior~ccs. Similarly. three-way interactions (t)[w ‘X plani! )-. mode. and type x plane x reach) had significant tbftczt cm production of strength but failed to product a signtticant effect on back compressive force. Such a compl;*x interaction between the hehaviour c3t scverai variahlcs which affect the strength generation. but fail to impact significantly on the back comprcssivc tori:: does ni31.indicate to the insensitive nature of the spinal compr,cssion. On the contrary. since the spinal compression 1sgenerated through the strength exertion. it ~ctuld appc;~r that the variables interact in a manner that the compression does not cxcced a lcvcl to violate the margin of safety. Thus the spinal compression may hc naturr’s ultimate optimizing v~ariable or the most howcvcr. this study was important cs3\t function: neither desipnetl to prove. nor does it prove. thih hvpotkeai\ t’hc 1iteratur.t~ with respect to association of the lowback pain with risk factor\ is inconclusive with divergen. -i/. 1 Some stud& have reported a lack of mi,,‘,:‘;,::~~ * . :, ~1: low-back pain with physical factors especially isometric strength” .‘<. 01~ the contrary they report a much stronger association bctwecn low-hack pain and psychosocial factors. 1,ack of job satisfaction and enjovmtnr was highly correlated with the low-back pain rep~~3rtctiin their study. Tow-back pain is a multifacctcd problem. None of the reported studies. cxperimenlai or cpidcmiologicai. have proved a causal rc)ationshil3. C)vcr 100 risk factors have been identified lo:- Low back nain.‘.’ and their association is reasonably ctmvincing in studies in which they arc presented. F-I~~wevc’r hCCitUSC of the multifactorial nature of low-back pain. the extent of contribution in causation of the prc3blem i> not ;tscertainablc. There may be cases where problems may bc largely, psychosomatic. and in others it may bc cntnely physical and mechanical. In cases where work is heavily physical. despite high motivation and work enjoyment. a high degree of low-back injury’ pain has been reported. for example in healthcare workers’!‘ Injury bv dctinition IS a biomechanical perturbation resulting in n(>ctception. Most physical and industrial activities. regardless of the nature of the exact action. arc associated with compression due to anatomical structure and mechanical configuration. Given a strong association between magnitude of the physical tasks performed and precipitation of low-back syndromet6.‘“. rhc role ol‘ physical factors in causation of low-back injuries remains strongly likely. Some of the studies which have focused on the psychosocial factors suffer from limitations in resolution of their methodology for

answering the questions conclusively” ‘.‘. The latter statement is supported by the fact that these studies did not relate the strengths to the intensities of the jobs performed. Also. the strengths tested were not job simulated. and thus compared dissimilar data sets. Finally. a question may be raised about YXS subjects with low-back pain screened out”. The latter will significantly skew the sample. possibly affecting the conclusion. As stated above for other studies, the current study has its limitations also. It does not consider many other mechanical factors such as bending. twisting. pulling, pushing and other physical stresses. The selection of compression alone as the factor to be studied is based on the premise that regardless of the nature of the physical task. as its magnitude is increased. it increases the compression force. Not all injuries are discogenic. Nonetheless. compression is an indicator of the magnitude of physical load cndurcd by the spine. Additional limitations of the study relate to the assumptions made in the biomcchanical model, which arc as follows: I. ‘l’hc centroid of the marker represented the joint ccntre. -.7 The human body consisted of a series of only 3h nodal joints with 3.5 rigid body segments. 3. The chosen nodal joints wcrc located in the ccntrc of joints.

of the body segment 4. The percentage distribution weights and segment ccntre of gravity with respect tc3 the proximal joints were taken from 1)empstcr (195.5)” and wcrc not verified individually. .‘i The moment arms of the spinal and paraspinal muscles vvcrc taken from Kumar”’ and were not vcrihud for each subject. h. The errors in the process were assumed to be insignificant. 7. The disc compressive force was assumed to be a rcliablc index of biomcchanical load and an indicator of other physical stresses. it is also emphasized that the spinal compression is not deemed to be undesirable regardless of its nature, magnitude and frequency. Clearly an extent of compression is needed to maintain healthy discs and spine. When the compression exceeds the magnitude. duration, and frequency for health, it becomes a concern. Maximal exertions are considered to exceed those limits, hence the focus of attention. The observation made and the hypothesis stated above gets its credence from epidemiological observations. Maximal efforts frequently associated with industrial injuries have typically been shown to reach approximately 75% of the ultimate compressive strength”. From the observations made in this study. it is obvious that the magnitude of the load or weight is not the primary factor threatening injury; it is rather the entire mechanical configuration that may translate into excessive magnitudes of spinal compression. Thus for single exertions violating safety it is not the

Kumar: Spinal compression at peak exertions magnitude of the load but the magnitude of the compression that needs to be controlled. Industrial operations, of necessity, are repetitious. The mechanical medium through which these industrial activities are performed is biological. The biological materials are viscoelastic in nature and consequently time rate dependent. Even submaximal exertions lead to deformation. If an optimum recovery period is not allowed, there is likely to be a residual effect. Therefore, even if the compressions do not reach the boundary of trauma, they will contribute to a cumulative effect, which in turn is a proven risk factor for precipitation of low-back painlh. Thus, it is proposed here that adoption of a standard, and limit of the magnitude of load to be lifted, may be steps in the right direction, but are unlikely to produce an optimum effect in controlling low-back injuries. On the basis of observations made in this study, it is suggested that a prevention strategy based on spinal compression may be more appropriate for maximizing the desired outcome. Although design and implementation of such a strategy may be a challenge, we need to strive toward it.

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