The respective first %-value represents the proportion of surveyed employees (n =

20,000), which are often affected by the particular work conditions. The respective

second %-value represents the proportion of surveyed employees, who feel stressed

by the particular work conditions.

One of the main tasks of preventive health protection is the avoidance of back pain

and injuries that can result from lifting of loads without any aids. According to a report

by the ‘Bundesanstalt für Arbeitsschutz‘ und Arbeitsmedizin (BAuA), in 2006 13,3% of

all accepted occupational illnesses were traced back to intervertebral disks problems.

A health risk is particularly common for energetic-effective work forms, especially for

ones in which the handling of loads occurs. The EU Directive 90/269/EWG demands

“preventive measures for the avoidance of risk due to handling of loads, and that

workplaces dealing with load handling evaluate the risks for their employees”.

In ergonomics, the ideal types of extreme forms of human work known informational

and physical/energetic work are referred to as pure information/energy

transformation. The five types of work (creative, combinative, reactive, motor and

mechanical) are a mixture of the two basic forms.

Based on the fact that the work environment indicators of practical working

processes never occur in isolation but in combinations, the examination should

be include the aggregate of all impacting working environment indicators in

combination with the working specific stress factors. But these causal

mechanisms haven‘t been investigated sufficiently until now, so that the

examination still takes place for each stress factor separately. The next step is

the identification of the specific effect / impact of the work environment

indicators on defined organismic systems. If the same organismic systems is

used more than one time, possible bottlenecks must be analyzed.

This method proved to e.g. the presence stressful climatic factors associated

with a high energy load of the human, so-called heat work. Both stress factors

lead to a higher utilization of the cardiovascular system, which is considered in

this case as a system bottleneck. With energy-effector forms of work the

muscles and the cardiovascular system are mainly stressed. In terms of a

bottleneck analysis, a distinction is therefore to the work:

• Heavy dynamic work

• One side dynamic work

• Universal dynamic work

• Static work

The energetic aspect of work activities is usually in the mobilization of skeletal

muscles. The work possibilities of a muscle can be distinguished according to two

basic forms: static and dynamic muscular work. Dynamic work is the execution of

movements and is characterized by: (1) change of muscle contraction and relaxation

(recovery), (2) response of the muscle blood flow, (3) muscle needs oxygen and

nutrients, (4) possible activity over longer periods.

Activities with dynamic components are for example, cycling or activities with

movements that use different portions of the muscle. Static work occurs when objects

held by muscular action, lift loads, overcoming frictional resistance or bracing the

body against gravity in particular positions or postures. It is characterized by: (1)

continued contraction of muscles over longer periods, (2) a mismatch between

oxygen demand and oxygen supply to the muscle, because the blood vessels

responsible for the muscle are compressed during the contraction, (3) rapid fatigue

and thus, a continuation of activities over extended periods is impossible, (4) adverse

biomechanical loading conditions of bones, joints and ligaments, resulting in

premature wear, especially of the spine.

Isometric muscle contraction has two meanings: Foremost, human posture requires

that musculature perform static work (postural work). Secondly, tools have to be held

over a specific time period (holding work). Static work is the most unfavorable form of

muscle work and should be avoided!

While Hibernation both demand for blood and blood flow are at a constant low level.

In contrast to heavy dynamic work demand for blood and blood flow are at a

maximum, to provide the muscles continuously with oxygen, so that glucose can be

converted into energy (aerobic energy). While static work the demand for blood and

blood flow are higher than in rest as well. To avoid a performance hit, the body adapt

to anaerobic energy generation. Here glucose is converted under the formation of

lactate (lactic acid), leading to acidosis of the muscle on duration.

The greater the holding force, the shorter the holding duration. If 15% of the maximum

force is used for the holding force, no fatigue occurs.

Depending on the load amount, i.e. the degree of exhaustion of the maximum force,

the maximum force still remaining after a certain work duration continually decreases.

Example: If 25% of the maximum force is statically demanded, then the force can only

be maintained for approximately 4 minutes due to the quickly occurring muscle

fatigue; at 50% of the maximum force only 1 minute is possible.

Muscle force is a physical strength that works through the activity of the muscles

within the body. There is a difference between static and dynamic muscle force. Static

muscle force is the physical strength that occurs without a change in the length of the

muscle during its activity. Dynamic muscle force, however, occurs during the change

in length of the muscle in its activity.

Inertia force is a physical strength that works as a force of inertia, e.g. dynamically as

accelerating force, force of deceleration, or centrifugal force at mobile workplaces, or

statically as own weight.

Applied force is a physical strength that works outward from the body. It results from

inertia force, muscle force, or both. Inertia force and muscle force can reduce or

increase their strength depending on amount and direction.

From the force-releasing body parts the applied force is split into e.g. arm, hand, leg

or finger force; from the force direction the applied force is split into e.g. vertical or

horizontal force.

The applied force is differentiated according to the force of attraction and the force of

pressure from the sense of direction of force.

Referring to the figure it is to differentiate between rather basic-oriented

classification schemes of muscle mass and strength (acting in the body

system) and more practical classifications in terms of generated action forces

(working from the body to the outside). The correlations are important for the

work design.

Examples are:

The own weights of the bodyparts (inertia forces) are compensated by

static muscle forces for maintaining a body posture.

Action forces on body support areas can be composed of mass forces of

the body parts and posture forces. This is to be considered e.g. in

dimensioning of the restoring force of a pedal.

Muscle contraction forces are the partial or full cause of driving forces

(e.g. lifting loads).

Muscle extension forces are the partial or full cause of braking forces

(e.g. take down of loads).

Manipulation forces and actuation forces can be applied partially or

completely by the combination of contraction and extension muscle

forces (separate muscle groups) (for example, relocating loads).

The specifications of the figure applies to an upright free body posture with parallel

foot position at a foot distance of 30 cm. The indicated values of the maximum static

applied forces were determined at stationary arranged handles during short-time

maximum force exertion of the working person. A cylindrical handle with a diameter of

30 mm was used, which was not helped in anyway. These are averages of the

maximum achievable static action forces, that are targeted to specific collectives (e.g.

men aged 20 to 25 years) are valid and not representative of the total population. The

maximum force is represented in the form of an isodynamic line. For different working

conditions (e.g. in terms of posture or the required force direction), the transferability

of the data has to be checked. In DIN 33411-3 and DIN 33411-5 for example

maximum static applied forces for other working conditions were presented.

The illustrated Isodynen apply to males with an average age of 22.8 ± 2.2

years, an average body height of 176.8 ± 5.9 cm, and an average body weight

of 72.73 ± 12.47 kg.

The illustrated Isodynen apply to males with an average age of 22.8 ± 2.2

years, an average body height of 176.8 ± 5.9 cm, and an average body weight

of 72.73 ± 12.47 kg.

The picture shows the behavior of the cardiac frequency during and after work with

short and longer breaks with steady proportion between work phase and break.

Because of the exponential character of the exhaustion and recreation phases it is not

functional to work until the occurrence of exhaustion. There is a need for

disproportionately long recreation phases. It is physiologically more favorable to

arrange short cyclical work and recreation phases. Human performance has a time

limit due to limited energy. The efficient execution of the business, while maintaining

full performance therefore depends critically on an appropriate distribution of the

burden. In selecting rest breaks, it should be taken into account the onset of fatigue

requires disproportionately long recovery periods. The required recovery phase

increases disproportionately with increasing work phase durations, where the

recovery from static work requires a longer period than from dynamic work. A quick-

alternating work and rest regime is therefore more favorable for physiological and

economic reasons.

An example for the choice of the working process with the highest efficiency is the

loading of an industrial furnace. During the loading of an industrial furnace, the human

energy demand can be lowered by reducing the lifting height. If the human work is

regarded, a positively directed work has to be performed while lifting and a negatively

directed work while lowering the workpiece. A reduction of the lifting height comes

along with a decrement of the metabolic rate and therefore with a lower strain of the

cardiac circulatory system. Simultaneously the shorter movement ways lead to a

considerable increase in efficiency.

External loads are not the only ones considered when evaluating job performance,

but also the entire movement and held loads. This especially applies to movements

that include body weight and small loads.

The figure shows the dependancy of the average bending force and the angle

of the cubital joint. The maximum of the bending force is achieved at a angle of

the cubital joint about 100 degrees.

The Figure shows the relationship between force and contraction speed with

thus calculated power output. The maximum power of about 180 watts is

reached at a generated force of 80 Newton. The contraction speed is then

approximately 2 m/s. While dynamic work of the muscle alongside the

inevitable mass moments of inertia, the gliding of the muscle’s actin- and

myosin-filaments plays an important role. Because using (analogous to an

internal friction) a speed-dependent part of the total force, the maximum force

decreases with increasing the contraction speed of the muscle‘s length (so-

called Hill‘s force-velocity relation).

Avoiding activities that impose repeated static load on the internal structure

lead to a considerable relief for the organism. One way is to replace static

work by dynamic work (e.g. moving a lever instead of pushing a device for

fixing a work object), another way is to install appropriate retaining devices

(e.g. weight reducing suspension of tools).

The diagram shows the stress in the static case. Dynamic forces (e.g.

acceleration forces, as they arise, when pulling tools) should be considered,

when analyzing workplaces and for the installation of suspension.

The human body contains more than 600 muscles. The muscle mass represents

approx. 40% a men‘s body weight and approx. 26% of a women‘s body weight. The

face has 43 muscles.

The state of the spine can be evaluated very accurate by an computer tomographic

analysis, yet by this means only ex-post insights about the effect of strain that

occurred in the past can be determined. A spine overload only becomes obvious after

damage of the spine. It is therefore necessary to estimate the risks of spine

impairments preventively.

The spine consists of 24 osseous vortices, between which cartilaginous intervertebral

discs are situated. The intervertebral discs impart to the spine their movability and

elasticity. The nutrition of the rubbery invertebral discs is entirely dependent on

diffusion because there is no blood flow. Sustained compressive loading reduces the

pressure-dependent fluid shift and then there is a metabolic impairment in the

intervertebral discs. During handling of loads, the spine is heavily stressed because of

the leverage effect of the external load and the resulting internal forces. According to

the position of the held load and the diffraction of the back the elastic intervertebral

discs get under enormous compressive stress and are exposed to internal transverse

forces. Dangers to health like damage of intervertebral discs, deforming of vortices or

ruptures of muscular fibers can result. Damages to intervertebral fabrics are

irreversible.

For the sample calculation a 50th Percentile man is used and the assumption is

made, that the weight force, acting on the intervertebral disc, reduced by

approximately 50% of the real value.

The estimate of the applied forces on the spine is based on empirically tested

biomechanical models. When considering a load, usually the spinal area L5-S1 is of

main interest because it is a common injury point (95% of all disc damage accounts

for the three lowest lumbar intervertebral discs). The body parts (i) above the lumbar

sacrum transition L5-S1 themselves each apply a moment around the point of

reference for the calculation. The lever arms (ai) are independent of the position of the

body and therefore represent variables as a function of time during the execution of

movements. If there is not only a body movement executed, but also a manipulation

of a “load“, then there additionally occurs a restoring force (FA), that applies a moment

on L5-S1 over the lever arm aA and hence raises the stress. As opposed to this, the

abdominal pressure constitutes a certain help: Through holding one’s breath an

abdominal pressure can be set up so that the solidified abdomen builds a supporting

force for chest and spine (pABD). Besides to the moments, the forces take effect on the

spine, it constitutes a measure for the stress of the spine. On one hand the weight of

the body parts above the lumbar sacrum transition leads to a compression of the

intervertebral discs and transverse loads occur because of the ascent even in upright

body position. On the other hand additional forces are set up by muscles, e.g. by the

back muscles. These forces build up a counter torque against the moments

mentioned above.

A weight of 10 kg held close to the body is equal to a load more than 10 times as high

while standing upright on the intervertebral discs in the lumbar vertebra region,

according to the law of levers.

Even a load 6-7 times as heavy and carried on the head would not result in a greater

internal load. A load of only 10 kg already results in a 300 kg intervertebral strain.

Such a strain would only occur with a 230 kg heavy load directly on the head.

The mechanical effects inside the abdomen also have to be regarded. On lifting

heavy loads the air is held in the lungs by pressure breathing and is highly

compressed inside the body. A pressure like this is required for stabilization of the

trunk, but not without danger. Therefore it is highly demanded to keep the body in an

upright position on the lifting of heavy loads. Only with an upright position is a

consistent pressure of the intervertebral disc reached. The spine should only be

strained axially, in no case eccentrically. For this case of strain, high surface

pressures occur at the margin of the intervertebral disc. With inflected spine and a

lifted load of ~50kg the surface pressure affects the intervertebral discs inconsistently.

Additionally, the pressure strain at the right margin reaches values around 300 N/cm2

(R), while with straight stance only a surface pressure half as strong results (Rohmert,

1983).

This illustration shows how the strains of the spine increase or decline with variation

of the masses of the lifted loads under otherwise equal conditions. The pressure

forces for the dynamic, two-handed lifting of loads with a mass of 0 kg up to 50 kg are

therefore illustrated in the figure. The overall duration for the leverage operation was

supposed to be 1.5 sec. With the 0-kg-graph strikes that even in an upright body

position the pressure forces are not zero, this attributes to the weight of the body parts

above L5-S1. With increasing load, the curves get more peaked, resulting of the

increasing influence of movement-related shares because of the mass inertia of the

load.

The figure on the right illustrates how the strain of the spine varies if a load is lifted in

different body positions.

The critical value given by NIOSH of 3400 N insufficiently accounts for the different

physiological width (age, gender, etc.). The limit hence applies only to healthy

persons below the age of 50. The limits suggested by JÄGER differentiate between

various age and gender groups. For older people, the NIOSH-limits are already

considered to be too high. In case of ascent, torsions, sudden jerky movements or

asymmetric loads the strain on vortices and intervertebral discs increases. According

to this, the listed percentages are to be set against the denoted load limits. The

denoted values are the consequence of a biomechanical view, not the premise Safety

Guidelines.

Several methods for the assessment of stress respectively strain of the

musculoskeletal-system during manual load handling have been developed over the

recent years.

The illustration shows different gradations of methods for different areas of use that

can be distinguished according to the level of detail of the examination. These

gradations allow the selection of methods for approximate assessments, for extended

examinations or methods for specific problems depending on the field of application.

The NIOSH approach was developed by the National Institute for Occupational Safety

and Health (NIOSH) in the USA as a method to estimate the maximum permissible

load.

The NIOSH approach is based on the calculation of the Recommended Weight Limit

(RWL). The RWL is defined for a specific set of task conditions as the load nearly all

healthy workers can work with over a substantial period of time (e.g., up to 8 hours)

without an increased risk of developing lifting-related lower back pain. The RWL is

calculated by multiplying the six mentioned influencing factors and a so-called load

constant (LC).

Model basis:

Dynamics:

Calculation of movements on the basis of the force progression in individual muscles, resp.

calculation of the force progression in muscles by means of movements Model of the

muscular-skeletal system (including muscles, tendons and insertion points at the bone)

Functionality

Positioning of marker points (for movement recording)

Optimization functions for muscle recruitment and motion sequences (e.g., balance)

Programming occurs with the AnyScript scripting language

Analysis possibilities (graphically, numerically) e.g., direct calculation of muscular strain