The Mechanics of Lifting

Very simply, manual handling is all about mechanics! Just think back to those physics lessons at school when we learned about forces and levers etc.

Go on – you remember Newton’s Laws of Motion!

Biomechanics is the name given to the study of mechanics as applied to human movement. It looks at the mechanisms involved when we move and the forces that act upon our bodies during specific actions. By having an understanding of these forces it is possible to identify:

1 Those factors that increase loading on the spine and other structures and

2 What is involved in ‘good’ posture when handling

Lifting and handling tasks involve three kinds of risks:

Risk 1 – The risk of accidental injury for example the loss of footing, the load falling on the handler, the load trapping the handler.

Risk 2 -The risk of overexertion

Risk 3 – The risk of repetitive or cumulative damage

The risk involved in the handling of a load will increase as a direct result of:

1 The weight of the load – if it’s heavy there is more risk of injury!

2 The distance of the load from the persons body (either forward or to the sides) – the further away from the body the load is handled e.g. when having to work with the arms outstretched the risk of injury increases with the distance from the body.

Over exertion and therefore injury is more likely when:

1 The strength of the lifting action is less

2 The body is out of balance

3 The back might be bent (flexed)

In order to understand the biomechanics of human movement we need to understand:

The principles of mechanics – gravity, force, friction, stability, stress and strain and levers

Efficient movement of the human body involves the application of biomechanical principles. The better the control of movement and balance we have the less strain is put on muscles and the less static muscle work is required.

Although some ways of moving and handling feel ‘normal’ that does not necessarily mean they are correct or safe. We develop habits that may be potentially harmful. These have to be unlearned and the new, safer ways must replace them.

Centre of Gravity (COG)

The centre of gravity is roughly the centre of the space that a load occupies i.e. the centre of the box.

Unfortunately the human body is not a uniform or regular shape so that the centre of gravity moves as the body changes position and so for a person standing with their arms at the side, the centre of gravity coincides with the middle of the pelvis but if the person raises their hands above their head then the centre of gravity rises into the tummy area.

The further the centre of gravity is from the centre of the body the more effort is needed to keep the body stable so that raising the arms as in reaching and stretching causes the centre of gravity to rise making the body less stable and at more risk of injury.

Principle! Try not to stretch, reach upwards or outwards whilst working

The lower the centre of gravity, the less likely that an object or a person will become unstable.

Principle! Work with “soft” or slightly bent knees

Stability

The line of gravity is the vertical direction down from the centre of gravity.

For a body to remain stable it must retain the line of gravity within its base. The base of support is the area over which the weight of an object or person is distributed and the greater the base of support the more stable the handler becomes.

Offset Foot Position

Working with an offset base i.e. with one foot slightly in front of the other will increase and widen the base of support when handling and will promote stability. As the line of gravity approaches the edge of the base of support and beyond the object or person becomes less stable. Muscles have to work harder to keep us balanced. This often involves static muscle work.

The base of support may also be widened by standing astride this is often the case when moving or sliding a load sideways.

To summarize – feet should be:

Comfortably apart

Offset – one foot should be positioned in front of the other

The leading foot should be positioned where possible in the direction of movement

The rear foot should be positioned where it can give maximum thrust to the body

If someone is sitting or kneeling they are much more stable than someone crouching on the tips of their toes as the base of support is much bigger!

Levers

The word lever comes from the French word “lever” which means to raise. A lever is any rigid object that is used with a fulcrum or pivot point to multiply the mechanical force that can be applied to another object. This is known as mechanical advantage.

Levers can be used to exert a large force over a small distance at one end by exerting only a small force over a greater distance at the other end:

Our bones are like levers. In order for our bodies to move the muscles, tendons and ligaments have to pull on the bones of our arms, legs and backs. The skeletal muscles create motion by pulling on the tough cords of connective tissue which are the tendons. These tendons in turn pull on the bone which creates motion. Muscles move bones through mechanical leverage. As a muscle contracts it causes the bone to act like a lever with the joint serving as a pivot or fulcrum. Muscles can only contract a short distance but since they are attached near a joint the movement at the opposite end of the limb is greatly increased, for example the biceps muscle in the upper arm may contract or shorten only 8 or 9 cm but the hand will move about 60 cm!

The movement that occurs will be rotational (i.e. through an arc or part of a circle) around the pivot point or fulcrum.

Again from school you might remember there are three classes of levers: first, second and third class levers. When talking about levers you can use the following terms:

1 The effort made which is known as the force.

2 The opposing force such as a weight which is to be moved. This is known as the load.

3 The pivot point or fulcrum of the action.

In the human body the effort is the force that your muscles apply to the lever (bone). The load is the weight that resists the pull of the muscles for example the handbag you are trying to lift, the patient you are trying to reposition. The fulcrum is the joint.

First Class Levers

A first-class lever is a lever in which the fulcrum is located in between the input force and the output force. In operation a force is applied (by pulling or pushing) to a section of the bar which causes the lever to swing about the fulcrum overcoming the resistance force on the opposite side. The fulcrum is the centre of the lever on which the bar (as in a seesaw) lays upon. This supports the effort arm and the load.

In the human body an example of a first class lever is the joint between the skull and the atlas vertebrae (first vertebrae) of the spine; the spine is the fulcrum across which muscles lift the head.

Second Class Levers

In a second class lever the input is located to the far side of the bar, the output is located in the middle of the bar and the fulcrum is located on the side of the bar opposite to the input. Examples of a second class lever are a nutcracker, stapler, a diving board and a wheelcbarrow.

In the human body an example of a second class lever is the Achilles tendon which attaches the calf muscles to the heel to result in movement of the foot, pushing or pulling across the heel of the foot.

Third Class Levers

In the third class lever, the force is between the fulcrum and the load:

In third class levers the input effort is higher than the output load which is different from the first and second class levers. The input effort moves through a shorter distance than the load. Thus it still has its uses in making certain tasks easier to do. In third class levers the effort in the centre while the output load is on one side… raising the load on the opposite end. Examples of third class levers are a garden hoe, tweezers, a shovel, a fishing rod and a hockey stick.

In the case of third class levers there is no force advantage – force is NOT increased. In fact a larger force is actually needed to move a smaller weight, so there is a force disadvantage. The use of this lever is in the gain in speed of movement of the weight and so this class of lever is used when we wish to produce large movements of a small load or to transfer relatively low speed of the force arm to high speed of the load arm. For example when a hockey stick or a baseball bat is swung, a third class lever is in effect. The elbow acts as a fulcrum in both cases and the hands provide the force (hence the lower arm becomes part of the lever. The load (i.e. the puck or the ball) is moved at the end of the stick or bat.

An example of a third class lever in the human body is the elbow joint. When lifting a book the elbow is the fulcrum across which the biceps muscle performs its work.

The spine can be used like a lever although it was not designed to be used like a crane. When bending forward the spine becomes a very long lever arm with a load being handled at the end. The back muscles then have to support not only the weight of the upper body as it leans forward but also the weight of the load being handled. The force exerted by the spinal muscles can be up to TEN TIMES GREATER than the actual weight of the load.

Think about helping a person to stand up!

When handling loads try to remember the following simple rules relating to levers to reduce the risk of a back injury!

· When handling a load, the closer the load is to the person’s body the less strain and effort is required and so the potential for injury is reduced.

· When the load is further away which might be the case when the work layout is poor, the work area cluttered or the handler does not move his/her feet or use an offset base, there is greater strain on the person doing the handling.

· Muscles work most efficiently when a joint is in its mid range or the middle third of their range of movement e.g. the biceps muscle is at its strongest when the elbow is bent at right angles. As the elbow straightens or bends further the biceps become increasingly less efficient. When a joint is working at the end range of its movement the muscles have to work harder. In the legs for example, when we crouch to the ground to pick something up the muscles in our thighs have to work significantly harder to enable us to stand up.

Principle! Try to work with short levers – if possible work with the elbows bent (flexed)

Principle! Don’t use your spine like a crane

Principle! Try to keep your load as close to your body as possible

The Movement of Loads

In physics, force is an influence that may cause a load or an object to move or accelerate. It may be experienced as a lift, a push or a pull. The actual acceleration or movement of the body is determined by the total of all forces acting on it. Force may also cause rotation or deformation of the object.

Force should be applied in the direction that you want the object to move in e.g. horizontal when pushing a trolley and vertical when digging with a spade. If an action is not efficient in terms of force and friction then more muscular effort is needed, spinal loading is increased and the potential for injury is increased.

Pushing, pulling and resistance to movement is affected by the angle at which force is applied and any resistance to movement e.g. friction.

Friction

Friction is a resistant force which holds surfaces together. The more friction that is exerted the more difficult it is to move an object therefore reducing friction enables an object/person to be moved more easily.

Reducing friction or low friction when moving a load reduces loading on the spine and other soft tissues e.g. sliding a VDU from the back of a desk rather than lifting it or using a slide sheet to re-position a patient or client in bed.

High friction resists movement. This can be beneficial when handling e.g. a slip resistant surface on shoes gives a better grip on the floor and therefore reduces some of the force required by the person. It is possible to get slip resistant cushions to prevent patients or clients sliding in their chairs or beds. High friction can also be detrimental, carpets and rough flooring will increase friction and make the pushing of trolleys, wheelchairs and hoists more difficult.

If force and friction are applied inefficiently the spine will be under greater loading and compression and at greater risk of injury.

Gravity

Gravity is a phenomenon whereby everything on earth is pulled towards the centre of the earth. Every load or mass has a centre of gravity and it creates a force by gravity acting on it. A load is said to have an inertial force i.e. a force that keeps it still and will not move unless acted upon by a greater force.

When pushing a trolley up a ramp or a gradient gravity will be acting against the load and increased effort of force will be required.

Gravity can be used to assist in the movement of a “load”, when sliding a heavy patient or a client on a slide sheet up a specilaist bed, the bed can be tilted so that gravity assists in moving the person.

Principle! Consider the forces of friction and gravity when undertaking a manual handling task.

Twisting and Asymmetry

Twisting and asymmetry during manual handling operations can cause soft tissue strain through the generation of large muscle forces and loads on the spinal discs. Soft tissues are then exposed to a range of forces resulting in damage and injury.

Handling in an uneven or lop-sided manner places additional forces and loading on the spine and associated soft tissues

Try to reduce twisting movements and asymmetrical postures. Try to move feet rather than twisting the back.

If possible re-organise the workplace to eliminate the need to turn around particularly when handling heavy loads.

Principle! Try to reduce twisting and asymmetrical postures when handling.

Pushing and Pulling

Pushing is generally preferable to pulling. Pushing allows the worker to use large muscle groups and apply more force to the load. Pulling carries a greater risk of strain and injury.

Principle! Push rather than pull wherever possible.

See Principles of Good Posture to see some of the theories above in practice.