Design, Development and Calibration of Isometric Vertical Leg Strength Measuring Device
2.2 Background
Near about 58% of Indian population is engaged in agriculture spread over 640,000 villages, representing ~10% (225 million) of global agricultural workforce (Nag and Nag, 2004). Despite countrywide advancement of farm mechanization during the recent decades in Indian agriculture, human muscular strength is still extensively used for operating various push-pull type farm tools and equipment such as manual ridgers, rotary dibblers, rice transplanters / seeders, push / pull weeders, field rakes, long-handled tools, chaff cutters, groundnut / castor decorticators etc. (Tiwari et al., 2010; Agrawal et al., 2010). Leg and foot operated controls on machinery and equipment such as the foot operated sprayers, threshers and dibblers are prominently used in India (Yadav et al., 2010). These activities impose a critical extent of psychophysical exhaustion on farm workers. With an aim to reduce the human exertion considerably, 16 strength variables were identified and recommended by All India Coordinated Research Project on Ergonomics and Safety in Agriculture, India (Gite and Chatterjee, 1999) for ergonomic design of such farm tools, as appended hereunder.
1 Right hand grip strength, 2 Left hand grip strength, 3 Preferred hand grip torque,
4 Left hand push strength in sitting posture, 5 Right foot strength in sitting posture, 6 Left foot strength in sitting posture, 7 Right leg strength in sitting posture, 8 Left leg strength in sitting posture, 9 Left hand pull strength in sitting posture, 10 Both hand push strength in standing posture,
11 Both hand pull strength in standing posture, 12 Right hand push strength in sitting posture, 13 Right hand pull strength in sitting posture,
14 Preferred hand torque strength in standing posture, 15 Torque strength in standing posture-both hands, and 16 Both hand torque strength in sitting posture.
Unlike the fairly extensive strength database existing for Western populations (Xiao et al., 2005; Yadav et al., 2010), availability of strength data of Indian agriculture workers are quite limited (Mehta et al., 2007; Agrawal et al., 2009; 2010; Yadav et al., 2010; Tiwari et al., 2010; Gite et al., 2009; Dewangan et al., 2010) and even the available information is seldom utilized by the manufactures. It is an accepted point that any efficient and ergonomic design takes both the anthropometric dimensions and strength parameters of intended users into account. Strength database have paramount importance for ergonomic design and development of farm tools (Vyavahare and Kallurkar, 2012). The required amount of force developed by the human musculoskeletal system depends on factors like race, gender, age, body weight and lifestyle (Gite and Singh, 1997). The amount of force exerted depends on the following factors:
Strength of muscle, Strength of bone,
Structural geometry of the body,
Extent of physical exhaustion of the person, and
Psychological factors like willingness / motivation of the person.
Normative / standardized database of muscular strength of any population (equally applicable for agricultural workers also) stands to be the fundamental requirement to determine effective percentile force for operating any tools / equipment. Depending on application, a certain percentile of strength value can therefore ensure that, majority of the given working population should be able to perform an assigned task (like clutch and brake, gear-shift lever, gear control lever, accelerator pedal, human powered push-pull equipment etc.) without undue fatigue and discomfort. Greater and repetitive demand of muscle strength to perform any physical task is a potent contributor for development of any kind of musculoskeletal problems.
The relationship between the physical demands of the task and the competencies of the user is of immense practical importance for designing tools, equipment, and consoles.
Compatibility of tools and equipment according to physical capacity of users is frequently endorsed for manual materials handling activities. This controls injury by reduction of muscle overloading. Strength of thigh and calf muscles plays an important role in employing the vertical force required to operate a pedal-operated or gear-based paddy thresher. The thigh muscle contracts and calf muscle extends while pushing the pedal downward. During this process, the leg extends and the angle between leg and thigh (knee angle) increases from its initial value. The knee angle is shown in Fig. 2.1 while measuring force in standing posture. Therefore, knowledge of human strength capabilities and understanding of the key elements involved is an important consideration in design (Mital and Kumar, 1998).
Any agricultural activities with a repetitive nature of execution should ergonomically be so designed that, the requirement of working force will never exceed 30% of the 5th percentile value of maximum strength of corresponding female working population. This will ensure the force to be executed well within the safe limits. The force exerted may be raised up maximum to 50%, till the effort applied is below 5 min (Agrawal et al., 2009;
Gite et al., 2009; Tiwari et al., 2010).
Fig. 2.1 Measurement of leg strength in standing posture at 1300 knee joint angle
In some instances, where the physical tasks are performed by male agricultural workers (like operation of tractor clutch, brake and steering etc.) only, required working force
should be set at 5th percentile of maximal strength value of male workers. For example, the sickle is used by both male and female workers with characteristic pull / sawing mode of operation for more than 5 min. Therefore, 30% of 5th percentile value of pull force with right hand in sitting posture for female workers was considered using 30% criterion. Gite et al. (2009) recommended pull force with right hand in sitting posture 12 N for Indian population. On the other hand, for the tractors mainly operated by male workers, required working strength of males should only be considered for design of various controls in the tractor, such as clutch, brake, steering wheel and gear lever. These controls are operated frequently for short durations (less than 5 min) and therefore, required effective force may be calculated to be 30% of the 5th percentile of male strength value. The baseline information on various tools and equipment provided by Patel et al. (2014) would be helpful for design or design modification of agricultural tools – pertaining to the executed working force fairly within safe limit.