Sarasvathi. V

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If mechanics is defined as the science dealing with force and motion, then body mechanics is, of course, a science dealing with body forces and motions. The term body mechanics is itself a popularization of kinesiology “scientific study of movements of the human body states Duvall.


Body mechanics is of interest because of a desire to keep the home maker’s costs of work at a minimum. The physical costs of work can be too high in fatigue even for light work if the natural structure and functions of the body are disregarded during work performance. An untrained worker often will work against her body rather than let her body work for her. The physical cost of work can then be out of proportion to the amount of work in the technical sense of foot-pounds, in energy expended, or in the discomfort strain felt.


Definition and Basic Mechanics


In biomechanics analysis, the body segments are assumed to rigid links that rotate about joint centres. Rigid body mechanics or based on new tons laws and deal with the interrelationships among the forces acting upon rigid bodies. Both static and dynamic analysis can be conducted using rigid body.


Statics: Statics is the study of the body at rest statics analysis involves the calculation of forces, moments, and torques, such that the body remains in statics equilibriums.

Dynamic: It is the study of the body and motion with upper- and lower- extremity movement, as frequently analyses very important.

Kinematics: It is the area of mechanics which describes the motion of a body without considering the forces caring the motion. Kinematics variables include linear and angular displacement, velocity, and acceleration. Velocity is the time rate of change in displacement.

Kinetics: Deals with forces change on the body. The terms kinetics refers to the forces that causes the movement. There forces include both internal and external forces. In biomechanics, internal forces are those forces generated by or acting on muscles, ligaments and joints. External forces come from ground (gravitational forces) and others external sources (e.g., an object lifted, a cost pushed unit resistance).


Mechanics of Movement


The laws of mechanics apply to moving the body and its segments just as they do to moving inanimate objects. Knowledge of the force of gravity, of momentum, and of leverage can contribute to the development of efficiency in body motions




Gravity is a force which is constantly acting upon human body and one which the body is constantly combating except when in the prone position. Elftman says, “The only way to profit from gravity is to keep on going downhill, How to maintain the upright posture economically during standing, sitting, walking, and running, climbing and lifting is a base technique to work improvement. The worker’s own body is not only her most important piece of equipment but also greatest weight she normally transports and manipulates. Walking, climbing stairs, lifting the arms all involve the moving of weight through distance, constituting work in technical sense.


Posture: well-balanced and poised position facilitates a good sitting posture for work. muscles and nerves are relieved of all strain as the weight is carried by the bony support of the skeleton. The poise is such that the minimum of altering is needed for such action as the work may demand. Through the middle of the shoulders, hips and seat bones the line of gravity falls. The body is straight from hips to neck and there is on flex or bend at the waistline.


Permanent changes in the spine, in positions of the joints, ligaments, and muscles, and in the location of the body organs is due to poor standing and sitting postures This can increase the fatigue cost of homemaking tasks and can produce strains and tensions. Utilization of the most comfortable body position can ease the body and relieves from being strained while working. Alternating standing and sitting is more soothing than either one sustained for a long period.


Muscular efforts: Physical work is carried out by one’s muscles and is therefore often called muscular work. There are two types:


The muscle stays contracted for a period of time and there is no movement, as in holding a picture against the wall or carrying a bag of shopping. Holding a static or fixed posture can be very strenuous as the muscles do not relax. Heavily contracted muscle squeezes against the blood vessels next to it, restricts their blood flow. This cuts down the supply of oxygen to the muscle and the elimination of a waste product, lactic acid from the muscle leading to muscular aches or pain. These symptoms are noted in any fixed posture .for example, standing to attention or sitting upright.


Dynamic or Rhythmic effort:


Rhythmical contraction and relaxation of a muscle resulting due to movement, as in pulling open a drawer or walking up stairs. Dynamic work is less tiring and more efficient than static work. This is because during dynamic work a muscle contracts and relaxes rhythmically which makes it act like a pump for the flow of blood in the blood vessels, allowing the blood to supply more oxygen and take away more lactic acid than during static work.


Types of muscle:


There are three types of muscle tissue in our body:


Smooth muscle tissue – located within the walls of the organs of the body Cardiac muscle tissue –heart muscle Skeletal muscle tissue – which constitutes the muscle that make us move. Skeletal muscle is made up of bundles of muscle fibres that contract together in one direction. when a muscle contracts, during a movement the fibres of the muscle reduces in length so the amount of movement that the muscle can produce depends on the original length of these fibres. Number of fibres that a muscle contains and the cross-sectional area of the muscle is the key factor on which the strength of the muscle depends.


Muscular endurance: It is the capacity of a muscle or a muscular group to stay contracted over a period of time. Endurance can be static or dynamic. Static endurance can be resolute by the length of time a limb can remain in a particular position, whereas dynamic endurance can be measured by the number of times a limb can perform a movement against a certain resistance.


Muscular strength: It is the utmost amount of force that a muscle can put forth under maximum contraction. The amount of force that can be applied by the limbs depends on body posture and the direction of force. For example, exertion of more force is possible when standing and pulling backwards requires more force than when pushing forwards.


There are a number of factors that manipulate our muscle strength and endurance:


Age: The strength of a teen and early 20s is always increased, and reaches its maximum by the middle to late 20s, relics at this level for 5 to 10 years, and subsequently begins to decrease steadily.


Sex:, women are said to be about two thirds as strong as men. The reason could be men have greater muscle mass as percentage of body mass compared to women.


Body build: Normally the 95th percentile person of a populace will be stronger than the 5th percentile person, the athletic or muscular person will be liable to be stronger than others;, differences are felt in strength of the people of equal body size is may be due to the amount of muscle tissue, body shape and proportions.


Fatigue: lactic acid in the muscles gets built up due to static muscle work and causes a gradual demur in muscle strength. Fatigue can be deferred by assuming comfortable working postures or changing the posture now and again. Decreasing the intensity or duration of muscular effort, practicing exercises or having adequate rest periods can help in reducing fatigue.


Exercise: increased muscle strength and endurance can be possessed through exercise up to the limits of our maximum physical potential; this is mainly determined by the genes we inherit from our parents.


Heat: Heat, with high humidity, decreases muscular performance and endurance.

   Cold: cold does not affect muscle strength if the worker uses adequate protective clothing, but it may affect their manual dexterity.

Clothing & equipment: This adds on to the overall weight and therefore requires extra muscular energy to move.


Motivation & emotional state: fright, annoyance or enthusiasm can momentarily boost the muscular strength but skill and accuracy may endure.


Nature of our job: manual workers are appreciably stronger than the other types of worker.


Postural aids: backrests amplify pushing strength by directing all the strength forwards. Footrest amplifies pulling strength by letting the worker to brace our legs. The amount of force that can be applied by the limbs depends on the body posture and the direction of force applied.


Optimal need of muscle power:

  • Avoid any kind of bend.
  • Avoid keeping arm outstretched.
  • Work sitting down as much as possible
  • Arm movement should be opposite or symmetrical
  • Working field should be at its best distance from the operator.
  • Hand grip, operating levers, materials should be arranged around workplace. Hand work can be raised above forehand, elbow.

Body mechanics, although concerned chiefly with energy setting includes feeling of comfort and discomfort associated contracting muscles and skeleton. Among its major principles are

(1)   Keeping the body parts in alignment,

(2)   Considering the center of gravity, of both of the body and article that is been handled.

(3)   Using muscles effectively

(4)   Taking advantage of momentum,

(5)   Rhythm in movements.

Other principles includes

  •  Remain close to the object
  •   Use short lever arms for better control & efficiency (with less strain)
  •  Maintain the COG close to the object’s (or patient’s) COG
  • Widen the BOS (Base of Support) and position the feet according to the direction of movement one will use to perform the activity
  •  Use the largest & strongest muscles of one’s arms, legs and trunk
  •  Avoid twisting the body when one lifts
  •  When possible, push, pull, roll, or slide an object rather than lifting it

When any part gets out of line, muscular effort is required to maintain body balance in addition to whatever work the body is doing. Strains may also result. When there is a problem of maintaining balance, a border base of support is necessary. The feet may be wide apart or parallel with one in advance of the other.


Energy cost of homemaking activities:


The performance of each home making task requires several types and combinations of effort. For instance some mental effort is necessary to direct the doing of any task even the routine ones such as dressing, sweeping, and dishwashing, which are done almost automatically by most homemakers. Although one is seldom conscious of it, visual effort is required in most activities, since the eyes must direct the movements of the body. Muscular movements of the eyes and adjustment of the vision to different distances and lighting conditions are continually taking place. No small amount of manual effort-reaching, raising, lifting, holding, carrying, stretching, pulling, and pushing is necessary. Such task as preparing meals, setting and cleaning the table, dishwashing, laundering requires this type of effort. Torsal effort – bending, leaning, rising, turning, stooping, sitting, and kneeling- is necessary in doing some of the more strenuous tasks such as those connected with the care of the house, the garden and the yard. Pedal effort-walking, moving, and standing is an essential part of many homemaking and recreational activities.


Thus while the various homemaking activities require different combinations of effort, most tasks require mental, visual, manual, and torsal effort of some kind and a large number also require pedal effort.


The human energy required for the performance of any task is made up of several different parts. A certain amount of energy is needed for the maintenance of muscular tension and for the natural body processes such as respiration, circulation, secretion, and excretion. This is known as resting metabolism. In addition there is the energy used in moving about and in the actual doing of the task.


Different types of effort used in home making activities


The field of physical ergonomics can be summarized in a series of principles.


Principle 1 -Work in Neutral Postures


Maintain the “S-curve” of the spine


Posture provides an excellent initial point for evaluate the tasks that one do. The best pose in which one can comfortably work is keeping the body “in neutral.” Maintenance of the natural S-curve of the back is of vital importance whether sitting or standing. The prime part of this “S” is situated in the lower back, that means that it is good to keep a slight “sway back,” When standing, putting one foot up on a footrest aids one to keep the spinal column in proper alignment. Good lumbar support often lends to maintain the proper curve in the small of the back. The “Inverted V-curve” poses a greater strain on the back. Bending over creates a great deal of pressure on the spine, even when a load is not been lifted.Use of a lifter or tilters is always preferable.


Neck alignment to be maintained


Spinal column consists of the neck bones and thus is prone to the same requests of maintaining the S-curve. Neck being twisted for prolonged period and bent postures can be as stressful as its equivalent for the lower back. Adjusting the equipment is the best way to make changes such that the neck is in its neutral posture.


Keep the elbows at sides


Keeping the elbows at the sides is the neutral posture for the arms at this point the shoulders are relaxed. This is actually apparent once, if one thinks about it, but we don’t always do it.


Keeping the Wrists in Neutral position


Wrist posture is never given a good thought. keeping the hand in the same plane as the forearm would enable it to be in the neutral position.


Principle 2: Excessive Force Reduction


Unwarranted force on one’s joints can generate a potential for fatigue and injury. In practical terms, the action item is for one to recognize precise instances of unnecessary force and think of ways to make improvement. For example, pulling a heavy load might produce undue force for one’s back. To make it easier and comfortable, t the wheels on the cart can be sufficiently large, and that there are good grips on the cart, or use a power trigger, which could make it easier. Having the handhold lessen the exertion of one’s hands need to bear the same amount of weight.


This principle is redundant with posture in many instances, but it facilitates to evaluate a task from this specific view.One concept involved in this is “reach envelope.” A semi-circle that the arms construct as one reaches out. Frequently used things should ideally be within the reach envelope of the full arm. Rearranging the work area and moving things closer to the worker solves many problems. Identifying the things that one reaches a lot for and change the location of things is as simple as mentioned if one works with presence of mind. Of course it is a matter of habit that worker could be easily move closer the things that he/she continually reaches for often. If in case the work surface is just too big, leading the worker to reach across to get something, then the favourable option is just to get a smaller surface.


Principle 4: Proper work Heights


Right working height can make things easier. Performing most work at elbow height is advisable, whether sitting or standing.


Exceptions to the Rule


Lower than elbow height is suitable for heavier work. A height above the elbow is opted for precision work or visually intense work.


Adjusting heights of the work table can be done by extending the legs or cutting them down. If that is not feasible, then one can either put a work platform on top of the table (to raise the work up) or stand on a platform.


Principle 5 – Excessive Motions reduction


The number of movement one makes, with the fingers, their wrists, the arms, or their back throughout a day. Use of power tools whenever likely is the simplest ways to reduce manual repetitions. Changing the layouts of equipment to eliminate motions is also opted for. Uneven surfaces or lips in the way can be changed and thereby one can eliminate motions. Static load is holding an object in the same position for a period of time. This is possible to create fatigue and discomfort that definitely can interfere with work. A good example would be writer’s cramp. It is needless to hold onto a pencil very hard, for long period. The muscles exhaust after a time and start to hurt. This act can be eliminated if worker prefer to use a fixture.


Principle 7 Minimize the Pressure Points


“Contact stress” is commonly used term for pressure points. Squeezing hard is best example to be cited for. Onto a tool, like a pair of pliers having a cushioned grip and contouring the handles to suit the hand could solve this problem. Inclining/ pressing one’s forearms against with the hard edge of furniture make a pressure point. Padding the edge of the furniture solves this problem. Similarly when standing on hard surface same points can be felt, that the heels and feet can begins to hurt and the entire legs begins to tire. The best solution to this would be is anti-fatigue matting or special insoles in their shoes.


Principle 8 Provide Clearance


Essentially work areas needs to be set up such that one has enough room for their head, their knees, and feet. This is the concept that relates to providing clearance. In order to prevent the worker from having to work in contorted postures or reach work area must have sufficient space. This enables the worker from having to bump into things all the time. Another perspective to this principle would be that, equipment built and tasks set up are in such a way that nothing blocks their view.


Principle 9 Move, Exercise, and Stretch


Healthy body requirements are exercise and stretching which means that the muscles needs to be loaded such that their heart rate needs periodic elevation. Based on the type of work one does, different exercises on the job can be useful.

  • Stretch and warm up before any strenuous activity is useful for physically demanding job
  •  Taking a quick “energy break” every now and then and to do a few stretches is sufficient for a sedentary worker.
  •  shifting postures is ideal for one who sits for long periods,
  • Adjusting the seat up and down throughout the day can also be of additional help.
  • Move, stretch, and change positions often to reduce fatigue.

Principle 10 Maintaining a Comfortable Environment


This principle involves working in a suitable environment. It should be a worker friendly environment. Lighting would be the commonly faced issue.


Highly polished computer screen reflects every stray bit of light around always poses a problem in computerised office. Similarly insufficient lighting can also affect the worker very badly. Glare, shadow and just insufficient light can produce a lot issue on the worker. It drastically affects the performance of the worker. Task lighting had been an advantage to such problems in many instances.




Vibration is an additional frequent problem, as an example, vibrating tools can be dampened to reduce its effects.

  1. Make displays and controls explicable
  2. Advancement of work organization



It is far easier to prevent injuries than it is to fix them after they occur. It is the responsibility of the employer to provide a safe, “work-friendly” environment. It is the responsibility of employees to use good body mechanics and maintain healthy life-style habits to help prevent injury.