metabolic cost and subjective assessment during operation of a rotary tiller with and without an...

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International Journal of Industrial Ergonomics 35 (2005) 361–369

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Metabolic cost and subjective assessment during operation of arotary tiller with and without an operator’s seat

P.S. Tiwari, C.R. Mehta�, A.C. Varshney

Central Institute of Agricultural Engineering, Nabibagh, Berasia Road, Bhopal-462 038, MP, India

Received 1 April 2002; received in revised form 8 October 2004; accepted 10 October 2004

Available online 10 December 2004

Abstract

The acceptance of the power tiller among the Indian farmers has not reached the level expected at the time of its

introduction. This is primarily because of the drudgery involved in its operation. An operator has to walk behind the

machine for a distance of 15–20km to rotatill a hectare of land once. To overcome the problem, an operator’s seat was

developed as an attachment to popular Indian brands of power tiller. This study was conducted to quantify the reduction in

drudgery due to provision of an operator’s seat on a rotary power tiller by measuring metabolic cost and psychophysical

assessment. Heart rate and oxygen consumption rate of the subjects were measured at three levels of forward speed under

actual field conditions using an ambulatory metabolic measurement system. Overall discomfort rating was assessed

subjectively on a 10-point visual analogue discomfort scale. Mean heart rate and oxygen consumption rate varied from 81.7

to 87.6 beats/min and 0.45 to 0.50 l/min, respectively, with increase in forward speed from 0.28 to 0.62m/s with the

operator’s seat. Without the operator’s seat, the heart rate and oxygen consumption rate varied from 94.9 to 108.0 beats/

min and 0.54 to 0.70 l/min, respectively, with increase in forward speed from 0.29 to 0.63m/s. Mean overall discomfort

rating varied from 0.5 to 1.5 with the operator’s seat and from 1.5 to 3.0 without the operator’s seat. Thus, human energy

expenditure during operation of the power tiller varied from 9.40 to 10.44kJ/min with the operator’s seat and from 11.28 to

14.62kJ/min without the operator’s seat. It was observed that the attachment of an operator’s seat to power tiller reduced

human energy expenditure by 16.7–28.6%. On the basis of physiological responses, the rotatilling operation by power tiller

with and without operator’s seat was classified as light and moderate work, respectively. It was concluded that the

provision of an operator’s seat on the power tiller reduces the drudgery of operator.

Relevance to industry

This study emphasizes the need to attach an operator’s seat to power tillers to reduce drudgery of operators during

the rotatilling operation. This may increase work output on a per day basis, which may ultimately increase the adoption

of power tillers by more farmers.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Power tiller; Physiological responses; Psychophysical assessment; Operator’s seat

e front matter r 2004 Elsevier B.V. All rights reserved.

gon.2004.10.002

ing author. Tel.: +91755 2734016; fax: +91 755 2734016.

ess: [email protected] (C.R. Mehta).

www.elsevier.com/locate/ergon

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1. Introduction

Power tillers were introduced in Indian agricul-ture in the early 1960s. However, the pace ofadoption has gained momentum since 1985 due toawareness of their usefulness, development in thenetwork of sales and service facilities, extensivedemonstrations and development of a variety ofmatching equipment for upland and low landcultivation. In view of the scarcity of labourers fortimely farm operations, the demand of powertillers is increasing year after year for carrying outvarious agricultural operations in food crops,orchards and forestry in other parts of thecountry. However, the popularity of power tillersamong the Indian farmers has not reached thelevel expected at the time of their introduction intoIndian agriculture. One of the main reasons forlow adoption of power tiller is the drudgeryinvolved in its operation. An operator has to walkbehind the machine for a distance of about15–20 km, merely to till/puddle a hectare of landonce, with a power tiller of 0.6m working width.This causes a lot of fatigue to the operator. Theproblem is aggravated while walking behind themachine in puddled soil during rotapuddlingoperation in rice fields (Tiwari and Gite, 2002).

Since the power tiller is a manually operatedwalking type machine, its work performancedepends not only on the machine but also on the

Table 1

Mean values of HR and oxygen consumption rate of Indian power ti

rotapuddling operations

Type of

operation

Forward speed (m/s) HR (beats/min) Ox

ra

Rotatilling 0.18–0.52 105.4–114.6 —

— — 0.6

0.30–0.63 97.1–110.1 0.4

Rotapuddling 0.27–0.42 119.9–132.6 —

— — 0.8

0.30–0.63 101.3–119.2 0.5

operator. If ergonomic aspects are not given dueconsideration, the performance of the man–ma-chine system will be poor and effective workingtime will be reduced because of frequent or longrest periods, resulting in lower work output. Onthe other hand, the machine operation may causeclinical/anatomical disorders due to the heavydemand on the operator’s biological systems andin the long run will affect the operator’s health.Some research has been reported on the devel-

opment of different designs of power tillers andmatching equipment. Research has been reportedon human energy requirements for walking typemachines such as animal drawn mould boardplough (Gite, 1991), animal drawn blade harrow(Gite, 1992), wheeled type manual weeders (Giteet al., 1993), etc. But the ergonomics of power tilleroperation has received little attention in the past.Some researchers have discussed the risk factorsassociated with the operation of power tillers andmetabolic cost of power tiller operation under afew soil and operating conditions (Pawar andPathak, 1980; Kathirvel et al., 1991; Tiwari andGite, 2002). Table 1 presents the mean values ofheart rate (HR) and oxygen consumption rate ofIndian power tiller operators reported by someresearchers at different forward speeds duringrotatilling and rotapuddling operations.In view of the drudgery involved during the

operation of a power tiller, an operator’s seat

ller operators at different forward speeds during rotatilling and

ygen consumption

te (l/min)

Work load

classification

Study conducted

by

Moderately heavy Pawar and

Pathak, 1980

2–1.20 — Kathirvel et al.,

1991

8–0.63 — Tiwari and Gite,

2002

Moderately heavy to

heavy

Pawar and

Pathak, 1980

1–1.05 Heavy Kathirvel et al.,

1991

6–0.86 — Tiwari and Gite,

2002

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P.S. Tiwari et al. / International Journal of Industrial Ergonomics 35 (2005) 361–369 363

suitable for mounting with popular Indian brandsof power tillers was designed and developed(Tiwari et al., 1995) at the Central Institute ofAgricultural Engineering, Bhopal, India. Fieldevaluation of a power tiller with operator’s seatindicated that the uniform operating depth isachieved with more work output on per day basisbecause of less frequent rest pauses of shorterduration as compared to that without operator’sseat (Tiwari et al., 1997). Subjective assessment bythe operators indicated more comfort with opera-tor’s seat. Ride vibration studies conducted on thispower tiller at different terrains and forwardspeeds indicated that power tiller with operator’sseat could be operated for a duration of 2.5 hduring rotatilling and 4.0 h during rotapuddling(Mehta et al., 1997) without exceeding theexposure limits specified by ISO 2631/1. Thepresent study was undertaken to assess thereduction in drudgery in quantitative terms bymeasuring the physiological responses and overalldiscomfort rating (ODR) during operation of arotary type power tiller with and without opera-tor’s seat.

2. Materials and methods

2.1. Subjects

Six randomly selected male subjects participatedin the study. Subjects were screened for posturalabnormalities or movement restrictions. Eachsubject was checked for cardiovascular, neuro-muscular and musculoskeletal disorders. Basicanthropometric data and physiological parametersof the selected subjects are presented in Table 2.

Table 2

Anthropometric and metabolic data for subjects (n ¼ 6)

Statistics Anthropometric variables

Age (Years) Stature (cm) Weight (kg)

Mean 32.5 161.67 57.8

SD 7.3 6.34 7.6

Minimum 23.0 151.0 49.9

Maximum 40.0 169.0 69.0

The subjects selected for the study were wellfamiliar with the controls of power tiller and hadsufficient experience of operating the power tillerunder upland and low land conditions. Thesubjects were made acquainted with experimentalprotocol to enlist their full cooperation. Identicalclothing were provided to all the subjects duringeach trial.

2.2. Tasks

The subjects were required to operate a 6.7 kWrotary type power tiller with and without opera-tor’s seat and perform rotatilling operation for atrial duration of 15min each. Tests were con-ducted at three levels of forward speed obtained inthree low-speed gears (L1, L2 and L3) at threequarters of rated engine speed (1500 rpm). Averageforward speeds of power tiller with operator’s seatwere 0.28, 0.44 and 0.62m/s in L1, L2 and L3 gears,respectively. The corresponding values withoutoperator’s seat were 0.29, 0.46 and 0.63m/s.

2.3. Experimental design

There were a total of six conditions (three speedswith and without operator’s seat) under which therotatilling operation was to be performed. Theexperiment was conducted in split-plot designwhere power tiller with and without operator’sseat was taken as main-plots and different levels offorward speed as the sub-plots. The subjects weretaken as replications. Three trials were conductedwith each subject for each of the above conditionsand the mean value of these trials was taken as therepresentative value for that replication. Eachcondition was maintained for 15min of duration

Metabolic variables

HRmax (beats/min) VO2max (l/min) EERmax (kJ/min)

187.5 2.22 46.32

7.29 0.22 4.72

180.0 2.01 41.97

197.0 2.52 52.62

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Fig. 1. Operator’s seat mounted on the 6.7 kW power tiller.


to ensure steady-state oxygen uptake. The treat-ments were given in randomized order to minimizethe effects of variation in environmental and soilconditions. A rest of about 90min was givenbetween the two trials on the same day with thesame subject to enable the operators to start everytrial with overall discomfort at resting level.

2.4. Equipment and procedure

2.4.1. Power tiller

A 6.7 kW rotary type power tiller manufacturedin India and commonly used by the farmers wasselected for the study. The power tiller wasmounted with a single cylinder, four-stroke, watercooled, horizontal diesel engine of 145 kg weightand the size of its rotary was 0.6m having 18 tillingblades. Total weight of power tiller with full fueltank and radiator, and lubricating oil was 510 kg.The power tiller, with full fuel tank and radiatorand without optional front weights and tire ballastwas put in proper test condition before conductingthe tests. The recommended tire pressure of147 kPa was maintained in pneumatic wheels ofthe power tiller.

2.4.2. Operator’s seat

The operator’s seat consisted of a trough typemetallic seat mounted over the power tillerrotavator via two angle iron braces. A telescopicshaft for depth adjustment, a hand wheel mountedat the top of the telescopic shaft for assistancewhile turning the power tiller and two separatefoot rests mounted with hangers welded over thebraces completed the seat system (Fig. 1).

2.4.3. Experimental plot

The study was conducted under heavy soilcondition (55% clay, 30% silt and 15% sand) atthe Central Institute of Agricultural Engineering,Bhopal, India. Preliminary testing of power tillerwith operator’s seat under submerged soil condi-tions indicated that it tended to sink in the field,especially at turns. This was due to its own heavyweight and the additional weight of the operatorand operator’s seat. The power tiller with opera-tor’s seat showed encouraging results duringrotatilling operation. Therefore, tests were con-

ducted during rotatilling operation only. Averagesoil moisture content, bulk density and weedintensity in the experimental plots were 14.6%(db), 1.63 g/cm3 and 18 g/m2 (db), respectively.The length of the experimental plot along whichthe rotatilling operation was conducted was 100m.

2.4.4. Physiological measurement system

Physiological responses were measured underfield conditions using an ambulatory metabolicmeasurement system, KB1-C (Aerosport, USA).Since the dimensions and the weight of theinstrument were 220� 110� 50mm and 1.0 kg,respectively, it could be conveniently carried as arucksack. The instrument performed an auto-zeroat the start of each trial and had a provision ofauto-calibration after 2min warm up. It hadfacility to store data for a trial, which weredownloaded to a computer for analysis at theend of each trial. A HR transmitter (Polar Electro,Finland) was used for transmitting the heart beatsignals to the metabolic measurement system.

2.4.5. Psychophysical measurement system

The psychophysical measurement system used inthe study was a 10-point visual analogue discom-fort (VAD) rating scale. It consisted of a 70 cmlong scale made up of an aluminum box section of20� 20mm size. The scale was divided into 10equal divisions. The left end was marked as ‘0’representing no discomfort and right end wasmarked as ‘10’ representing extreme discomfort.After each trial the subjects were asked to

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subjectively assess the overall discomfort due tothe task completed by them by sliding a pointerover the scale. In a preliminary session, thesubjects were anchored to the VAD scale. Eachsubject walked on a treadmill at different forwardspeeds and slopes ranging from 0.28 to 0.70m/sand 0 to 151, respectively till the extremediscomfort level was experienced. After 5minintervals the subject’s response for ODR wasasked on the VAD scale presented in front of him.

2.4.6. Procedure

The subjects were asked to report at the worksite at 09.30 h in post absorptive stage and had arest of 30min before the start of a trial. All thetrials were conducted between 10.00 to 12.30 and15.00 to 16.30 h during the month of April (springseason). A minimum gap of 2 h was given betweenfood intake and start of a trial. The observedvariations in the mean dry bulb temperature, wetbulb temperature and relative humidity during thetests were 30.0–39.01C, 18.0–22.01C and11.0–29.0%, respectively. Each trial was precededby taking 5min data for physiological responses ofthe subject while resting (seated on a stool undershade). After 5min, the subject was asked tooperate the power tiller (already started by anotherperson and engine throttle position set at threequarters of rated engine speed) for the trialduration of 15min. The forward speed of opera-tion was measured during each trial by timing thepower tiller between two mark points at a distanceof 25m, using a digital stop watch.

2.4.7. Data analysis

For calculation of the physiological responses,data of HR and oxygen consumption rate duringwork between 6 and 15min of the trial were used.The values recorded for different replications(subjects) were averaged to get the mean valuesfor a particular treatment. The calorific value ofoxygen was taken as 20.88 kJ/l of oxygen forcalculating the energy expenditure rates as sug-gested by Nag et al. (1980). To have a meaningfulcomparison of physiological responses duringoperation of power tiller with and without theoperator’s seat, the delta values (increase inphysiological responses over resting values) of

heart rate (DHR) and oxygen consumption rate(DVO2) were calculated. The data were analyzed toobtain the effect of various treatments on physio-logical parameters and ODR using an analysis ofvariance (ANOVA) and multiple comparison tests.

3. Results

3.1. Physiological measures

Mean values of physiological parameters suchas maximum oxygen consumption rate and max-imum HR of six subjects participated in the studywere 2.22 l/min and 187.5 beats/min, respectively(Table 2). Data on mean values of HR and oxygenconsumption rate of the subjects during rotatillingoperation by the power tiller with and without theoperator’s seat at different levels of forward speedare presented in Table 3.Fig. 2 presents the variation in HR of the

subjects while operating power tiller with andwithout the operator’s seat at different levels offorward speed. It showed that mean HR increasedlinearly from 94.9 to 108.0 beats/min with increasein forward speed of travel from 0.29 to 0.63m/swithout the operator’s seat on power tiller.However, the mean HR increased from 81.7to 87.6 beats/min with increase in forward speedfrom 0.28 to 0.62m/s with the operator’s seat.It was also observed that the mean HR reducedby 13.91–18.89% (Table 3) with the provisionof operator’s seat on power tiller as the walkingon tilled land was completely eliminated,which reduced the cardiac cost of power tilleroperation.Fig. 3 presents the variation in oxygen con-

sumption rate of the subjects while operatingpower tiller with and without the operator’s seat atdifferent levels of forward speed. It showed alinear relationship of oxygen consumption ratewith forward speed under both the operatingconditions. The mean value of oxygen consump-tion rate without the operator’s seat increasedfrom 0.54 to 0.70 l/min with increase in forwardspeed from 0.29 to 0.63m/s. However, it increasedfrom 0.45 to 0.50 l/min with increase in forwardspeed from 0.28 to 0.62m/s with the operator’s

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Table 3

Mean values of physiological responses and psychophysical ratings of operators (n ¼ 6) during operation of a rotary tiller with and

without operator’s seat

Parameters Value (SD) Reduction in physiological

responses and ODR, net (%)

Without seat With seat

Gear selected L1 L2 L3 L1 L2 L3 — — —

Forward speed (m/s) 0.29 0.46 0.63 0.28 0.44 0.62 — — —

HRwork (beats/min) 94.9 100.5 108.0 81.7 84.2 87.6 13.2

(13.91)

16.3

(16.22)

20.4

(18.89)

DHR (beats/min) 22.6 28.3 36.0 09.7 12.2 15.6 — — —

(1.1) (1.08) (1.1) (1.53) (1.63) (0.63)

VO2work (l/min) 0.54 0.62 0.70 0.45 0.47 0.50 0.09 0.15 0.20

(16.67) (24.19) (28.57)

DVO2 (l/min) 0.30 0.40 0.47 0.21 0.23 0.26 — — —

(0.02) (0.03) (0.03) (0.03) (0.03) (0.03)

EER (kJ/min) 11.28 12.95 14.62 9.40 9.81 10.44 1.88 3.14 4.18

(16.67) (24.25) (28.59)

ODR (0–10 scale) 1.5 2.0 3.0 0.5 1.0 1.5 1.0 1.0 1.5

(0.07) (0.13) (0.18) (0.07) (0.23) (0.12) (66.67) (50.00) (50.00)

70

75

80

85

90

95

100

105

110

115

120

0.2 0.4 0.6 0.8Forward speed, m/s

Hea

rt r

ate,

bea

ts/m

in

With seat

Without seat

Fig. 2. Variation in HR of power tiller operators during

rotatilling operation with and without operator’s seat.

0.4

0.5

0.6

0.7

0.8


Oxy

gen

cons

umpt

ion

rate

, l/m

in With seat

Without seat

Fig. 3. Variation in oxygen consumption rate (VO2) of power

tiller operators during rotatilling operation with and without

operator’s seat.


seat. It was also observed that the oxygenconsumption rate reduced by 16.67–28.57% withthe provision of operator’s seat on power tiller asthe energy required by leg muscles for walking ontilled land reduced.

The trend was statistically validated by comput-ing the ANOVA for the effect of operator’s seatand forward speed of operation on DHR andDVO2 and the results are presented in Table 4. Paircomparison technique was used to compare thedifference in the DHR and DVO2 at different

forward speeds of operation. The Bonferronimultiple comparison tests based on Student’s t-statistics showed that there was a significantdifference in DHR and DVO2 at different forwardspeeds at 0.1% level.

3.2. Psychophysical measures

Fig. 4 presents the variation in ODR whileoperating power tiller with and without operator’sseat at different levels of forward speed. It showed

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Table 4

ANOVA for the effect of operator’s seat (S) and forward speed

(V) on DHR, DVO2 and ODR of power tiller operators

S.No. Source of variation Description P

1 Effect on DHR

Operator’s seat (S) F(1,30) ¼ 1095.15 o0.001

Forward speed (V) F(2,30) ¼ 126.56 o0.001

S�V F(2,30) ¼ 19.17 o0.001

2 Effect on DVO2



S�V F(2,30) ¼ 10.19 o0.001

3 Effect on ODR



S�V F(2,30) ¼ 7.38 o0.005

0

1

2

3

4


Ove

rall

disc

omfo

rt r

atin

g

With seat

Without seat

Fig. 4. Variation in ODR of power tiller operators during

rotatilling operation with and without operator’s seat.


a linear relationship between ODR and forwardspeed with and without the operator’s seat.

The results of ANOVA for the effect ofoperator’s seat and forward speed of operationon ODR is given in Table 4. The Bonferronimultiple comparison tests based on Student’st-statistics showed that there was a significantdifference in ODR at different forward speeds atthe 0.1% level. The overall discomfort ratingsreduced by 50–66.67% with the operator’s seat onpower tiller. It indicated that the operators felt morecomfort with the operator’s seat on power tiller ascompared to that without the operator’s seat.

4. Discussion

The study indicated that the operator’s seat onthe power tiller significantly affected the HR ofpower tiller operators. Since HR may be taken as ameasure of overall stress due to work beingperformed and the stress due to environment itmay be concluded that there was a reduction inoverall stress on the operators with the provisionof the operator’s seat. This was mainly because theprovision of the operator’s seat on power tillercompletely eliminated the walking on tilled land,which reduced the cardiac cost of power tilleroperation. Taking HR responses as the basis forwork classification (Astrand and Rodahl, 1986),the operation of power tiller with the operator’s

seat was under the category of light work (up to90 beats/min) while without the operator’s seat itwas under the category of moderate work(90–110 beats/min).The study also indicated that the provision of an

operator’s seat on power tiller significantly af-fected the oxygen consumption rate. Since oxygenconsumption rate has a direct co-relation withworkload on the subjects it may be concluded thatthe workload on the subjects reduced with theprovision of the operator’s seat. This was mainlybecause the operator did not require to walk overthe tilled field with the provision of the operator’sseat resulting in reduction of the activity of the legmuscles as compared to the power tiller withoutthe operator’s seat. Taking oxygen consumptionrate as the basis for work classification (Astrandand Rodahl, 1986), operation of power tiller withthe operator’s seat was under the category of lightwork (up to 0.5 l/min) while without the operator’sseat it was under the category of moderate work(0.5–1.0 l/min).Saha et al. (1979) suggested that a physical

activity consuming 35% of VO2max might beconsidered as an acceptable workload (AWL) forIndian workers, which gave the mean value ofoxygen consumption rate as 0.78 l/min for thesubjects who participated in the study. At thislevel of work, the HR and energy expenditurerate of the subjects would be 112 beats/min and16 kJ/min, respectively. Comparing the mean

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values of HR, oxygen consumption rate andenergy expenditure rate with the AWL limits, itappears that rotatilling by power tiller with orwithout the operator’s seat might be carried outcontinuously for 8 h even at the highest testedspeed without exceeding the AWL limits. How-ever, in practice, it is not possible to continue theoperation beyond an hour or two, because ofpostural discomfort. It can be seen from the ODRdata of a 15-min trial at a speed of 0.63m/swithout the operator’s seat, the mean rating was3.0, which could have increased to the extremelevel with the passage of time. On the other hand,mean ODR at the speed of 0.62m/s with theoperator’s seat was 1.5, which indicated thatpower tiller with the operator’s seat could havebeen operated continuously for a longer durationin comparison to that without the operator’s seat.Owing to less frequent rests while operating thepower tiller with the operator’s seat, more workoutput on per day basis could be achieved.

Further, with the provision of operator’s seat onpower tiller the machine may be operated at highforward speeds without excessive physiologicalload on the operator, which could ultimatelyimprove the overall work output of man–machinesystem. However, the rotor speed must be syn-chronized with the forward speed of the operationto get the same quality of tilth. For proper soilpulverization, the ratio of peripheral speed of rotorand forward speed of the power tiller should be 4–8(Bernacki et al., 1972). The conventional powertillers have been designed such that the ratio falls inthe above range while operating the power tiller atthe walking speed. However, if we operate the tillerat higher forward speeds (in high gears) withoutincrease in rotor speed the above ratio will be lowerthan 4 and thus, the soil pulverisation will be poor.Therefore, to operate the power tiller with theoperator’s seat at high forward speeds, the designmodifications have to be made in the power tiller toobtain high rotor speed.

5. Conclusions

The metabolic cost of the power tiller operationwith or without an operator’s seat increased

linearly with an increase in forward speed ofoperation. Mean HR and oxygen consumptionrate with the operator’s seat increased from 81.7 to87.6 beats/min and from 0.45 to 0.50 l/min, respec-tively with an increase in forward speed from 0.28to 0.62m/s. The mean HR and oxygen consump-tion rate without the operator’s seat increasedfrom 94.9 to 108.0 beats/min and from 0.54to 0.70 l/min, respectively with an increase inforward speed from 0.29 to 0.63m/s. ODR onthe 10-point VAD scale varied from 0.5 to 1.5 withthe operator’s seat and from 1.5 to 3.0 withoutthe operator’s seat for a trial duration of 15min.The provision of an operator’s seat on powertiller reduced human energy expenditure by16.67–28.59% in comparison to that without theoperator’s seat. It was concluded that for uplandcultivation a power tiller with operator’s seatmight be considered as a good alternative to apower tiller without operator’s seat. The provisionof an operator’s seat on power tiller will reducedrudgery during its operation, making it anattractive source of power and increasing workoutput of man–machine system on per day basis.

Acknowledgements

The authors are thankful to Dr. Nawab Ali,Director and Dr. V. V. Singh, Head, AgriculturalMechanization Division at Central Institute ofAgricultural Engineering, Bhopal for their helpand encouragement during the study. Thanks arealso due to the subjects for their wholeheartedcooperation during the experiment.

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