International Journal of Industrial Ergonomics, 7 (1991) 31-39 31 Elsevier

Biomechanical analysis of raking and comparison of two rakes

Shrawan K u m a r and Chengkung Cheng Department of Physical Therapy, University of Alberta, Edmonton, Alberta, Canada, T6G 2G4

(Received September 5, 1989; accepted in revised form August 3, 1990)

Abstract

Fifteen young adult subjects (4 males - mean age 24.6 years, mean body weight 68.7 kg, mean height 173.5 cra; 11 females - mean age 20.8 yrs, mean body weight 56.4 kg, mean height 165.8 cm) raked dirt with smooth pulling and pushing actions using a conventional and a modified rake. The raking was done in a 75 cm × 75 c m x 30 cm plywood box containing an even spread of 20 cm deep dirt. The raking action consisted of pulling and pushing through a stroke distance of 60 cm, carrying dirt with the rakes in both directions. The activity was recorded on a VCR and analyzed for postural changes in different phases with both rakes. The modified rake allowed an upright stance in the pulling phase of the raking action. However, during pushing considerable spinal flexion was necessary using this rake. Using an assumed force of 100 N a biomechanical analysis was carried out to compare the effect of rakes on spinal stresses. A sensitivity analysis was also done on postural angles, raking angle and the raking force. A biomechanical analysis showed that the modified rake generated a much lower spinal compression than the conventional rake (0.18-0.44 times) during rake pulling. In rake pushing, on the other hand, the modified rake generated a spinal compression of 3 to 8 times the conventional rake. The spinal compression was most sensitive to raking angle followed by raking force, hip angle and shoulder angle during raking activity.

Relevance to industry

In small agricultural industries, including nurseries and green houses and landscaping, raking is a common activity. In order to reduce spinal stress, and thereby reduce the risk of injury and increase productivity, a design and use of an optimal rake is desirable.

Keywords

Rake design, biomechanical analysis, spinal stresses, back pain.

I. Introduction

Stress on the spine is caused by all activities involving the exertion of force through upper ex- tremities and movement of the trunk with planted feet. Raking is one such activity and is performed in a variety of settings such as farming, landscap- ing, gardening, and yard work. Even though it may not constitute a major occupational activity, its superimposition on top of other stressful tasks can be significant. Kumar (1989, 1990) has dem- onstrated that life time cumulative load is an

important risk factor for low-back pain. Further, people with poor fitness and flexibility who are engaging in recreational gardening, may be unsus- pecting targets of raking-generated onset of back pain. Since these accidents typically occur outside working hours and away from work sites, they are noncompensable and no injury statistics are read- ily available. Regardless of the circumstances of injury and compensation status, an injured person is unlikely to be productive or happy at work after having sustained the injury at home. In addition to the direct cost of compensable injuries which

0169-1936/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

32 S. Kumar, C Cheng / Biomechanical analysis of raking

have been widely reported (Holbrook et al., 1984: Kelsey and White, 1980), this reduced productiv- ity will have an indirect cost to society.

Raking in landscaping, gardening and many farming activities consists of alternate pushing and pulling dirt with rakes. The force exertion required in this task will depend on the density, texture and homogeneity of the medium being raked. Unlike with any other manual tool, the force exertion required for raking may be signifi- cantly different from the anticipated force. Un- even lumps may suddenly stop the stroke part way, creating a need for sudden force generation. This factor of unpredictability of force exertion and excursion creates a potentially hazardous situation due to the unexpected modification of a pre-programmed smooth back and forth excursion of predetermined force.

In order to perform a raking task, the operator assumes a staggered posture on firmly planted feet facing the rake site sideways. This posture permits movement of the rake by means of a combined action of arm and back. During this activity there is an alternate flexion and extension of the trunk. During the pushing phase, shoulder flexion and elbow extension are coupled with trunk flexion. In the pulling phase, the trunk and shoulder extend and the elbow flexes. If the trunk motion is re- duced or eliminated, and raking is performed en- tirely by arm movement, a significant portion of the spinal stress will be alleviated.

A review of literature revealed only one study on raking or rakes (Kumar and Cheng, 1990) contrary to shovelling and shovels. For the latter, an excellent review of literature has been pub- lished by Freivalds (1986a). The energy expendi- ture of the operator using different shovels (Frei- valds, 1986b), and the efficiency of design char- acteristics of shovels (Partridge, 1973; Freivalds, 1986a, b; Grinten, 1987) have been investigated. Freivalds (1986b) and Grinten (1987) studied the effects of the shovel handle length and the lift angle on the biomechanical stresses of the back. Freivalds (1986b) also looked at the energy ex- penditure and the rating of perceived exertion (RPE) for shoulders/arms and low back in the use of the seven shovels. However, the effects of straight or bent handles for rakes have not been well studied. Furthermore, a technique for bio- mechanical evaluation of such a tool has not been

reported. Therefore, in the present study, (a) a conventional rake (straight handle) and (b) a mod- ified rake (with two curves to reduce trunk flexion while holding the rake in position for raking, and a turned-up handle as coupling for one hand), were studied with respect to their structural attri- butes and their possible biomechanical impact on the operator.

2. Materials and methods

2.1. Subjects

Fifteen subjects (4 male and 11 female) volun- teered for the study (table 1). Those with any history of back pain, scoliosis, other musculoske- letal, cardiovascular or metabolic disorders were excluded. A statement from subjects was taken as the proof of their health. No further medical ex- amination was conducted.

2.2. Task

The task consisted of raking dirt in a wooden box 75 cm x 75 cm. An even thickness of 20 cm layer of dirt was spread in the box. The subjects were required to face the box in a staggered pos- ture toward one end of the box with left hand leading (figure 1). They were asked to alternately push and pull through a stroke length of 60 cm moving the dirt medium back and forth with a self-paced uniform velocity in a smooth motion. The same activity was performed with both rakes. The sequence of these conditions was randomized. since people tend to move around during actual raking, the activity studied here is not entirely

Table 1

Anthropometric data of subjects.

Gender Parameter Age Weight Height years kg cm

Male Mean 24.6 68.7 173.5 n = 4 SD 5.7 3.8 3.3

Range 18-28 63-71 171-178 Female Mean 20.8 56.4 165.8 n = 11 SD 2.5 6.1 7.2

Range 18-24 51-66 155-175

n = number of subjects, SD ~ standard deviation

S. Kumar. C Cheng /Biomechanical analysis of raking 33

Fig. l. Raking posture with conventional rake.

blade was attached to the handle at the 72 ° angle. The modified rake had a blade which was at- tached to the handle at an angle of 85 ° and had longer teeth (8.5 cm). Its metal handle had a uniform 3.5 cm diameter with an effective length of 150 cm and three bends measuring 50 °, 60 °, and 50 °. These curvatures allowed the subjects to maintain an upright posture when standing. The shape and size of the rake blade and their teeth resulted in an area of soil contact of 144 cm-" and 117 cm'- for conventional and modified rake, re- spectively.

2.4. Posture analysis

realistic. However, maximal force application is limited to a smaller range of excursion. In the interest of performing an accurate and reliable analysis this task design was chosen.

2.3. Rakes

The rakes consisted of a blade and a handle (figure 2). The conventional rake consisted of a regular blade and a straight 145 cm long wooden tapered handle with a diameter of 3.0 cm. The

The entire raking activity of each of the sub- jects was recorded in profile using a JVC video recorder (Model No. HR-22004). Following the experiment the video cassette was played back repeatedly to identify the postures of initial, mid- dle, and final phases of rake pulling and pushing. These points were identified when the rake blade reached predetermined landmarks along the box wall indicating the ends and middle of the excur- sion span. At those points, the video frames were frozen and printed using a Mitsubishi Video Copy Processor (Model P60 W). F rom these prints, the

25 ° .__'~ o o . / . . ~ 3 150cm

- - \ ° ~ " "J+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +~--

7.5cm ~ ~ 1 4 T e e t h

35.5 cm 60 ° j17 cm

rl .L-" \ - ~.'/I 50°

i ~ . . . . ;h".F'/~"" . . . . . . . . . . . . . . . . . . . . . . ' : '~/ . . . . . 8.5cm I 14 Tee /,,/ / / / / /

3 8 c m v / , ' / / 5 9 c m

I/,2-./

Fig. 2. Raking posture with modified rake.

34 s. Kumar. C Cheng / Biomechanical analysis of raking

postural angles described below were measured using a protractor for input to the biomechanical model.

2.5. Biomechanical analysis

The biomechanical analysis was carried out using a simple two dimensional 5-1inks static bio- mechanical model (figure 3). Joint moments [Mz(i)] and reaction forces [Fx(i ), Fy(i)] (as shown in figure 4) were calculated. These calculations were done through each of the links: hand, forearm, upper arm, trunk, thigh and legs. The direction of any force or moment vector was determined by the reactions. The magnitude of the raking force was arbitrarily assumed to be 100 N for biomech- anical comparison of the two rakes. The following equations were used in estimating the joint mo- ments and the reaction forces:

Fx ( i ) = F,x cos a R

Fy(i) = F,x sin a R - (Wi - 1) where W ( i - 1)

= M ( i - 1 ) XG

M:( i ) = M z ( i - 1) + [ F x ( i - 1)-s in a ( i )

- F y ( i - 1) . cos a ( i ) . r( i )

- W ( i - 1) -cos ct(i) x CG(i) x r ( i )

where Fx(i) and Fy(i) are joint horizontal and vertical reaction forces at the ith joint, M~(i) is the moment of force at the ith joint, F~ is the raking force, u R is the angle between the horizon- tal and the first segment of the rake held by the subject, W(i) is the weight of the ith link, M(i) is

~ )

Fig. 3. The design and dimensions of conventional and mod- ified rakes.

F~ (i+1)

a (i.1) f _ ~ ~ a ( i . 1 ) "

W(i-1 )

Fig. 4. The plan of biomechanical analysis.

the mass of the ith link, G is the specific gravity of the links, r(i) is the length of the ith segment, a(i) is the angle contained between the ith seg- ment of the rake and the horizontal, and CG(i) is the percentage of the center of the gravity of the ith segment to the ith joint.

Compression and shear forces of the joints were calculated by the following equations:

F~comp)(i ) = E~( i) " cos a( i) + Fy( i) . sin a( i)

F~shca,~(i ) = ~ . ( i ) cos a( i ) -- Fx(i ) sin a( i )

At the thoraco-lumbar and lumbo-sacral levels, the compressive forces included back muscle forces and the relief offered by the intra-abdominal pres- sure acting at the center of the diaphragm. The foregoing was formulated after Chaffin and Andersson (1984). The thoracolumbar and lumbosacral compressions were determined as fol- lows:

F(:omp) ( i ) = F(comp)(i ) + f m - F a

where F(:omp)(i ) is the compressive force at the level, F(comp)(i ) is the vertical load of the segment above the level, F,. is the back muscle force, and F, is the force due to intra-abdominal pressure.

2.6. Sensitivity analysis

The sensitivity analysis was done to construct an approximation of a design problem that can be analyzed to determine the effect of the change in an individual variable (Haug and Arora, 1979). The model input of the five postural angles (ankle, knee, hip, shoulder, and elbow angles) and two tasks variables (rake angle and raking force) were perturbed by 0. 1%, one variable at the time, and

s. Kumar, C. Cheng / Biomechanical analysis of raking 35

the compressive and shear forces at the thoraco- lumbar and lumbosacral joints were calculated. When the percentage change in these calculated forces equalled or exceeded the percentage change of the input variable, the perturbed input variable was considered sensitive. The sensitivity analysis was formulated as follows:

F % = [ F ( x I . . . . . x I + d % . . . . . x , )

- r ( x , . . . . . x, . . . . . x . ) ]

x [ F ( x , . . . . . . . . . . x . ) ] - '

F% K = d%

where F% is the percentage change in the model calculated force before and after the perturbation, F ( x t . . . . . x~ . . . . . x , ) is the force value obtained before the perturbation of the input variable, f ( x I . . . . . x~+ d% . . . . . ( x , ) is the force value calculated after the variable x,) was perturbed, d% is magnitude of perturbation of the variable in question, K is the sensitivity index (ratio between the percentage change in the calculated force and the percentage value of the perturbation of the selected input variable).

If the sensitivity index was equal to or greater than 1, the input variable was considered sensitive. The frequency of the sensitive cases and the mean magnitude of the sensitivity indices were calcu- lated and tabulated.

3. Results

3.1. S p i n a l s t resses

The mean compression forces calculated for thoracolumbar and lumbosacral discs, along with their s tandard deviations for initial, middle and final phases of rake pulling and pushing are given in tables 2 and 3. The shear forces obtained in most conditions were under 150 N and never exceeded 200 N. Therefore, a description of shear forces has been omitted.

With conventional and modified rakes, the compressive forces in the initial, middle, and final phases of rake pushing and pulling, calculated at the lumbosacral level was invariably greater than those at the thoracolumbar level. The compressive forces generated at both levels during the initial and final phases of rake pulling and the initial a n d

middle phases of rake pushing were greater among males than females. In the middle phase of rake pulling and the final phase of rake pushing, the compression generated at both thoracolumbar and lumbosacral levels was higher in females with the conventional rake; whereas, in males using the modified rake, the compression generated at these levels was higher. In the pulling operation, the conventional rake invariably generated higher compression; the reverse was true in the pushing operation at both levels in both sexes.

Table 2

Back compressive forces in Newtons (N) during pulling activities assuming a 100 N rake pulfing force.

Gender Level Rake Initial phase Middle phase Final phase

Mean S D M e ~ S D M e ~ S D

Male TI2/L 1 Cony. 803 634 558 559 708 800 Modi. 224 75 250 71 230 38

Ls/S 1 Cony. 1042 598 913 487 930 844 Modi. 313 83 388 13 284 11

Female T12/LI Cony. 578 395 941 611 463 457 Modi. 155 68 160 68 194 94

Ls/S t Conv. 779 354 1346 691 827 627 Modi. 219 64 247 66 259 75

SD ~ standard deviation; T 1 2 / L 5 ~ Thoraco-lumbar level; L5/S 1 ~ Lumbosacral level; Conv. E conventional rake: Modi. - modified rake.

36 S. Kumar, C. Cheng / Biomechanical analysis of raking

3.2. Sensitivity analysis

The results of the sensitivity analysis revealed that among the seven input variables tested (five postural angles - ankle angle, knee angle, hip angle, shoulder angle and elbow angle; and two rake variables - raking force and raking angle) only four were significantly sensitive. The raking angle was found most sensitive followed by raking force, hip angle, and shoulder angle. The magni- tude of sensitivity has been variable between the subjects. The compression generated at the thoracolumbar and lumbosacral levels by the use of a conventional rake in rake pulling was more sensitive to all four input variables than the com- pression generated at these levels by the use of a modified rake. This was evidenced by the greater number of cases showing sensitivity and the higher levels of percentage change obtained with conven- tional rake as compared to the modified rake. The same pattern was evident for rake pushing for the magnitude of the change. However, the frequency of occurrence was similar. The shoulder angle did seem to influence the compressive force among a few cases with conventional rake, whereas this was not the case with the modified rake. The shear forces were not sensitive to changes in the hip and shoulder angles with either of the two rakes in either pulling or pushing. The external load and the raking force had significant effect of shear forces in about a third of the cases; however, the magnitude was always small. The raking angle had a significant effect in all of the cases except one case with the modified rake. The magnitudes of

the changes obtained were also generally quite high.

4. Discussion

The compressive load on thoracolumbar as well as lumbosacral discs will be less in an upright posture as compared to that in a forward inclined posture due to an increased moment arm in the latter case. In order to overcome this moment arm, a rake with three bends in its handle has appeared on the market. Such a tool allows an upright stance, thereby minimizing the moment arm by changing the posture. However, due to the incorporated design, these two tools will have significant difference in their drag forces. After placing the two rakes in identical raking positions, a comparative drag force analysis revealed very different forces at the point of contact of the rakes with the medium. The moment about the same point was also significantly different. For this calculation, assumptions were made that (i) the muscle forces generated to push and pull were same, (ii) raking involved rigid body motions, and (iii) the trail angles and shapes of these two drags were same. At the point of contact with the raking medium and the conventional rake, O', (figure 5) the forces generated will be given by:

F X = F" cos 0(

Fy = F ' cos sin 01'

Table 3

Back compressive forces in Newtons (N) during pushing activities assuming a 100 N rake pushing force.

Gender Level Rake Initial phase Middle phase Final phase

Mean SD Mean SD Mean SD

Male Txa /L I Cony. 545 341 1096 643 985 554 Modi. 4256 763 4263 543 4590 969

L s / S 1 Conv, 782 343 1376 784 1254 550 Modi. 4565 623 4673 527 5175 775

Female TI2/L~ Cony. 536 403 472 472 1106 713 Modi. 4478 628 3959 760 3757 689

L~ /S I Conv. 837 394 791 477 1409 801 Modi. 4756 515 4353 773 4208 630

SD ~ standard deviation; T12/L s ~ Thoraco-lumbar level; L s / S 1 = Lumbosacral level; C o n v . = conventional rake; Modi. = modified rake.

S. Kumar, C. Cheng / Biomechanical analysis of raking

Y

x t

~-~'~ ec-c--11onventional Rake

12

e Modified Rake

Fig. 5. Joint moments and reaction forces.

37

axial force = F '

shear force = O

(where F~ and Fy are the horizontal and vertical componen ts of the force applied F ' on the handle, M. is the momen t about the point O ' , and O~ is the angle of the rake segment).

For the modified rake at the point of contact with the raking medium, O, the forces generated will be given by:

F~ = F cos 03

= r s in 03

M. -- F c o s 03(12 sin 02 + 11 sin 01)

- F sin 03(12 cos 0~ + 1 t cos 01)

Axial force = F cos 03 cos 01 + F sin 03 sin 01

Shear force = F cos/)3 sin 0 t - F sin 03 cos 01

(where Fx and Fy are the horizontal and vertical componen ts of the force applied F on the handle, M z is the moment about the point O, l t, 12, and ! 3 are lengths of rake segments, and 0 l, 02, and 03 are angles of the rake segments.

Assuming a force of 100 N for a biomechanical compar ison and using the measured values of 01', 0,, 02, 03, and 1 t, 12, and 13; the horizontal and vertical forces at, and the momen t about the points O ' and O, are as summarized below:

Convent ional Modified rake rake

Horizontal force 70.7 N 95.1 N Vertical force 70.7 N 30.9 N Moment 0 N m 54.64 N m

Thus, due to this moment , the modified rake significantly increases the mechanical difficulty

Table 4

The compression generated by the modified rake expressed as a proportion of the conventional rake.

Gender Level Pulling Pushing

Initial Middle Final Initial Middle Final

Males TL 0.27 0.44 0.32 7.8 3.8 4.6 LS 0.30 0.42 0.30 5.8 3.3 4.1

Females TL 0.26 0.17 0.42 8.3 8.3 3.4 LS 0.28 0.18 0.31 5.6 5.5 2.9

TL = Thoraco-lumbar level; LS = Lumbosacral level.

38 S. Kumar, C. Cheng /Biomechanicalanalysis of raking

for pushing activity though it eases the postural stress during rake pulling. This design permits a greater portion of applied force to be resolved in horizontal direction, appropriate for raking, but it is more than offset due to the external moment. If the rakes were to be employed only for pulling, the design change of the modified rake will reduce the stress. However, raking involving alternate pulling and pushing will be faced with significant mechanical disadvantage in pushing phase of ev- ery cycle. During pulling, the modified rake gener- ated compressions 0.17 to 0.42 times the conven- tional rake in females and 0.27 to 0.44 times the conventional rake in males (table 4). However, during pushing, it generated compressions from 2.98 to 8.3 times the conventional rake in females and from 3.3 to 7.8 times the conventional rake in males (table 4). Since many white collar workers with mid back disorders indulge in recreational gardening and raking, the compressive load on thoracolumbar disc was calculated in addition to lumbosacral discs. The compressions generated at the thoracolumbar and lumbosacral joints, using the conventional rake reached a maximum of 1346 N in middle phase for females with an assumed load of 100 N. Since 100 N represents approxi- mately 10 kg of dirt, it was considered a realistic assumption. However, with the use of the mod- ified rake in the pushing phase, the mean com- pressive force ranged between 4256 N to 5175 N for males and 3757 N to 4756 N for females.

It must also be emphasized that an assumption of a 100 N external load acted as a ceiling for the calculation of compression. In most cases, though, the force exerted was subjectively assessed to be significantly greater than 100 N. In order to apply such increased force, the subjects had to make postural changes, considerably increasing the hip angle. Such a posture, increased the moment arm of the trunk, as reflected in the higher compressive load. An invariably higher compression load at the lumbosacral disc was to be expected as a result of a larger segmental mass at the level.

The width of the blade of the modified rake was 2.5 cm greater and its teeth 1.0 cm longer. However, due to the narrower teeth of the mod- ified rake, the conventional rake had a greater area of contact. This resulted in increased mecha- nical resistance of the medium to the conventional rake. This difference in the contact area resulted

in 8.9% and 22.6% increase in raking force for sand and clay respectively for the conventional rake due to difference in the cohesive forces of the two media. When translated to the effect it may have on the disc compressive forces the conven- tional rakes as a result of this difference in the rake-medium contact area will generate up to 2.5% more for sand and up to 19.8% more for clay. In spite of this disadvantage to the conventional rake, the modified rake was calculated to generate several times more compressive force. The mod- ified handle was smooth and gloss finished, having a much lower coefficient of friction compared to the wooden handle of the conventional rake. Such a handle may require a higher grip force in order to deliver the same raking force. However, this feature will not alter the force required to do the raking. Higher grip force will necessitate stronger contraction of the hand and forearm muscles. This may increase the chances of localized muscle fa- tigue increasing the physiological cost of the activ- ity.

It is acknowledged that the human body is a series of links joined together, and a change in one joint is likely to affect the entire posture. How- ever, in conducting sensitivity analysis, a magni- tude of 0.1% of perturbation was selected. This was on the premise that such a small change in one of the postural angles will not drastically affect the orientation of linked segments to render the analysis erroneous. Under these circumstances, the raking angle was determined to be the most sensitive variable, both in terms of frequency as well as magnitude. A change in this will determine the direction of the vector force delivered by the upper extremities. In order to place the hands and arms in an optimum position to deliver this force, the operator will have to adjust his entire posture. Such a pivotal role of the angle of rake renders it most sensitive. The magnitude of the external load (raking force) will clearly determine the muscular force required. Thus, the compressive forces will also be affected. Therefore, any feature which increases the force exertion requirement is unde- sirable. For agricultural activities, landscaping and domestic gardening raking involves alternate activ- ities of pulling and pushing, carrying dirt with the rake for spreading or grading. Therefore, due to its moment, the modified rake has to be consid- ered undesirable. However, if the raking is done

s. Kumar, C Cheng / Biomechanical analysis of raking 39

o n l y to co l lec t d e a d leaves o n t he g r o u n d it will

i n v o l v e o n l y p u l l i n g , a n d t h e n t he m o d i f i e d r a k e

wil l b e a b e t t e r cho ice .

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Freivalds. A. 1986a. The ergonomics of shovelling and shovel design - a review of literature. Ergonomics, 29(1): 3-18.

Freivalds, A. 1986b. The ergonomics of shovelling and shovel design - an experimental study. Ergonomics. 29 (I): 19-30,

Grinten. M. van der. 1987. Shovel design and back load in digging trenches. In: P. Buckle (Ed.), Musculoskeletal Dis- orders at Work. London. Taylor & Francis, pp+ 96-101.

Haug, E.J. and Arora, J,S., 1979. Applied optimal design: Mechanical and structural systems. John Wiley, New York,

Holbrook+ T.L.. Grazier. K.. Kelsey. J.L. and Stauffer. R.N.. 1984. The frequency of occurrence, impact and cost of selected musculoskeletal conditions in the United States. American Academy of Orthopaedic Surgeons.

Kelsey, S., J.L. and White. A.A., 1980. Epidemiology and impact of low-back pain. Spine, 5: 133-142.

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Kumar. S.. 1990, Cumulative load as a risk factor for low-back pain. Spine (in press).

Kumar, S. and Cheng. C.K., 1990. Spinal stresses in simulated raking with various rake handles. Ergonomics, 33, 1-11.

NIOSH (National Institute for Occupational Safety and Health). 1981. Work practices guide for manual lifting. U.S. Dept. of Health and Human Services, NIOSH Pub- lication No. 81-122. Cincinnati, OH.

Partridge, M., 1973. Farm Tools Through the Ages: New York Graphic Society. Boston.