Unit 3

Posture and Movement

Posture and Movement: Biomechanical Background - Physiological Background - Sitting - Standing Change of Posture - Hand and arm postures - Movement - Lifting - Carrying - Pulling - Pushing - Repetitive motions - Rapid Upper Limb Assessment (RULA)Rapid Entire Body Assessment (REBA) and Ovako Working Posture Assessment (OWAS) method.

Introduction

• Posture and movement play a central role in ergonomics.
• At work and in everyday life, postures and movements are often imposed by the task and the workplace.
• The body’s muscles, ligaments and joints are involved in adopting a posture, carrying out a movement and applying a force.

• The muscles provide the force necessary to adopt a posture or make a movement.
• The ligaments, on the other hand, have an auxiliary function, while the joints allow the relative movement of the various parts of the body.
• Poor posture and movement - lead to local mechanical stress on the
• muscles,
• ligaments and
• joints,

• It results in complaints of the neck, back, shoulder, wrist and other parts of themusculoskeletal system.
• Some movements not only produce a local mechanical stress on the muscles and joints but also require an expenditure of energy on the part of the muscles, heart and lungs
• Thereafter, possibilities for optimizing tasks and the workplace are presented for common place postures and movements such as
• Sitting and standing,
• lifting,
• pulling and
• pushing.

Biomechanical Background

• In biomechanics, the physical laws of mechanics are applied to the human body.
• It is thereby possible to estimate the local mechanical stress on muscles and joints, which occurs while adopting a posture or making a movement.
• A few biomechanical principles of importance to the ergonomics of posture and movement are outlined below.

Joints must be in a neutral�position

• the joints – must be kept as much as possible in a neutral position.
• In the neutral position the muscles and ligaments that span the joints are stretched to the least possible extent and are thus subject to less stress.
• In addition, the muscles are able to deliver their greatest force when the joints are in the neutral position.
• examples of poor postures

– Raised arms, bent wrists, bent neck and turned head, and bent and twisted trunk

Keep the work close to the body

• If the work is too far from the body, the arms

will be outstretched and the trunk bent over

forward.

• The weight of the arms, head, trunk and

possibly the weight of any load being held

then exerts a greater horizontal leverage on

the joints under stress

Avoid bending forward

• The upper part of the body of an adult weighs about 40 kg on average.
• The further the trunk is bent forwards, the harder it is for the muscles and ligaments of the back to maintain the upper body in balance.
• The stress is particularly large in the lower back.
• Prolonged bending over for long periods must therefore be avoided wherever possible.

A twisted trunk strains the back

• Twisted postures of the trunk cause undesirable stress to the spine.
• The elastic discs between the vertebrae are stretched, and the joints and muscles
• on both sides of the spine are subjected to asymmetric stress.

Sudden movements and forces�produce peak stresses

• Sudden movements and forces can produce large, short-duration stresses.
• These peak stresses are a consequence of the acceleration in the movement.
• It is well known that sudden lifting can cause acute back pain in the lower back.
• Lifting must occur as far as possible in an even and gradual manner.
• Thorough preparation is necessary before
• large forces are exerted.

Alternate postures as well as�movements

• No posture or movements should be maintained for a long period of time.
• Prolonged postures and repetitive movements are tiring and, in the long run, can lead to injuries to the muscles and joints.
• Best to avoid movements that involve regular lifting or repetitive arm movements, Likewise, standing, sitting and walking should also be alternated, and it should be possible to carry out prolonged tasks either standing or sitting.

Limit the duration of any continuous�muscular effort

• Continuous stress on certain muscles in the body as a result of a prolonged posture or repetitive movement leads to localized muscle fatigue, a state of muscle discomfort and reduced muscle performance.
• The greater the muscular effort (exerted force as a percentage of the maximum force), the shorter the time it can be.
• Most people can maintain a maximum muscular effort for no more than a few seconds and a 50 per cent muscular effort for no more than approximately one minute as this causes muscular exhaustion.

Prevent muscular exhaustion

• The muscles will take fairly long time to recover if they become exhausted which is why exhaustion must be avoided.

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• Exhausted muscle needs to rest for 30 minutes to achieve a 90 per cent recovery.
• Muscles in a half-exhausted state will recover to the same degree after 15 minutes.
• Complete recovery can take many hours.

More frequent short breaks are�better than a single long one

• Muscular fatigue can be reduced by distributing the resting time over the task duration or working day.
• It is not sensible to accumulate break times until the end of the task or working day.

Physiological Background

• In physiology, estimates are made of the energy demands on the heart and lungs resulting from muscular effort during movements.
• In addition to fatigue that results from continuous localized muscular effort general body fatigue can develop from carrying out physical tasks over a long period.
• The limiting factor here is the amount of energy that the heart and lungs can supply to the muscles to allow postures to be adopted or movements to be carried out.

Limit the energy expenditure in a�task

• The majority of the population can carry out a prolonged task without experiencing any general fatigue provided the energy demand of the task (expressed as the energy consumed by the person per unit of time) does not exceed 250 W (1 W = 0.06 kj/min = 0.0143 kcal/min).
• The amount of energy, (approximately 80W) which the body needs when at rest.
• At this energy consumption level, the task is not considered heavy, and no special measures such as breaks or alternating with light activities are necessary for recovery.
• Examples of activities with an energy demand of less than 250 W are writing, typing, ironing, assembling light materials, operating machinery, a gentle walk or a leisurely cycle ride.

Rest is necessary after heavy tasks

• If the energy demand during a task exceeds 250 W, then additional rest is necessary to recover.
• Rest can be in the form of breaks or less demanding tasks.
• The reduction in activity must be such that the average energy demand over the working day does not exceed 250 W.

Anthropometric Background

• A table height, which is suitable for a person of average stature, can be unsuitable for a tall or short person.
• – A table height that is adjustable over a sufficient range is the solution if the table is to be used by several people.
• In designing a control panel that has to be reached with the arms. In other cases, such as in choosing a door height, only the tall users have to be considered instead.

Alternate sitting with standing and walking

• Many manual activities carried out while seated (e.g., writing or assembly work) require the person to keep the hands in view.
• Neck and back are then subjected to prolonged stress
• The back is subject to further stresses if the trunk has to be twisted and the seat cannot swivel
• Working with unsupported raised arms, which can lead to shoulder complaints.
• The heights of the seat and backrest of the chair must be adjustable

– It must be possible while sitting to adjust the height of the seat in a continuous, smooth motion rather than in steps.

• Limit the number of adjustment possibilities
• Provide proper seating instructions

The work height depends on the task

During most tasks the hands have to be used and viewed simultaneously.

– Then, the work height is a compromise between the optimum height for the arms and the optimum position of the head and trunk.

– In the first instance, a low table is better since the arms have to be raised to a lesser extent and it is easier to apply a force.

– In the second instance, a high table is better because it means less bending forward and a better view of the work.

Standing

• Activities where considerable force has to be exerted or where the workplace has to be frequently changed should be carried out in a standing position.

The height of the work table must be adjustable

– must have an adjustment range of at least 25 cm in order to cater to individual differences in body size.

• Do not use platforms

• Select a sloping work surface for reading

tasks

Movement

• Various tasks require moving the whole body, often while exerting a force.
• Such movements can cause high, localized mechanical stresses that in time can lead to bodily aches and pains.
• Movements can also be stressful in the energetic sense for the muscles, heart and lungs.

Lifting

• Restrict the number of tasks that require displacing loads manually.
• Create optimum circumstances for lifting.’
• If manual lifting of heavy loads (up to 23 kg) is necessary, then lifting conditions have to be optimized:

– It must be possible to hold the load close to the body

– (horizontal distance from hand to ankles about 25 cm)

• The initial height of the load before it is lifted should be about 75 cm.
• The vertical displacement of the load should not exceed 25 cm.
• It must be possible to pick up the load with both hands.
• The load must be fitted with handles or hand-hold cut-outs.
• It must be possible to choose the lifting posture freely.
• The trunk should not be twisted when lifting.
• The lifting frequency should be less than one lift per five minutes.
• The lifting task should not last more than one hour, and should be followed by a resting time (or light activity) of 120 per cent of the duration of the lifting task.
• Ensure that people always lift less, and preferably much less, than 23 kg
• Recommended weight limit = 23 kg × HM × VM × DM × FM × AM × cm

Lifting- Do’s

• Foot and legroom must be sufficient to allow a stable
• Position for the feet and to be able to bend the knees.
• Twisting the trunk should not be necessary.
• The height and location of the load on the work surface must be such that when lifting the load or setting it down, the hands are at the optimum height of approximately 75 cm and are close to the trunk.
• Loads should be fitted with handgrips
• Ensure that the load is of the correct shape
• Use correct lifting techniques
• Heavy lifting should be done by several people
• Use lifting accessories

Carrying

• After a load has been lifted, it must sometimes be moved manually. In general, walking with a load is both mechanically stressful and energetically demanding.
• As a result of holding the load, the muscles are subjected to continuous mechanical stress; this particularly affects the muscles in the arms and back. Displacing the whole body and the load consumes energy.
• Limit the weight of the load
• Hold the load as close to the body as possible
• Provide well-designed handgrips
• Provide well-designed handgrips
• Avoid carrying loads with one hand
• Use transport accessories

Pulling and Pushing

• Many types of trolleys have to be moved manually. Pulling and pushing trolleys places stress mainly on the arms, shoulders and back.
• Limit the pulling and pushing force
• When setting a trolley in motion by pulling or pushing, the exerted manual force should not exceed approximately 200 N (about 20 kg-force).
• If the trolley is kept moving for more than one minute, the permissible pulling or pushing force drops to 100 N.

Use the body weight when pulling or pushing

• Provide handgrips on trolleys
• A trolley should have two swivel wheels
• Ensure that the floors are hard and even

Change of Posture

Rapid Upper Limb Assessment (RULA) 

• To provide a method of screening a working population to assess exposure to significant risk of work-related upper extremity disorders.
• To identify the muscular effort which is associated with working postures and excessive forces while performing static or repetitive work, and which may contribute to muscle fatigue.
• To provide a simple scoring method with an action level output that identifies an indication of urgency.
• To provide a user-friendly assessment tool that requires minimal time, effort, and equipment.

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Limitations- RULA

• Does not consider the duration of the task, available recovery time, or hand-arm vibration.
• Only allows the evaluator to assess one employee’s worst-case posture at one point in time, requiring the use of representative postures.
• Requires separate assessment of right and left sides of the body, although in most cases you will be able to quickly determine which side of the body has the greatest exposure to MSD risk.

REBA – RAPID ENTIRE BODY ASSESMENT

FORCE/LOAD SCORE=+1

Total Score : 6+1=7

Coupling Score = 1

Total Score=10+1=11

Ovako Work Posture Analysis- (OWAS)