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 discs 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-typical extreme forms of human work are referred to as

informational and physical (energetic) work in form of pure information

transformation or else energy transformation. The five specific forms of work

(creative, combinative, reactive, sensorimotor and mechanical) are mixtures 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, impact assessments are only permissible

for the entirety of all acting environmental factors in relation with the work specific

stress factors. However, these causal mechanisms haven‘t been investigated

sufficiently until now, so that the examination for each stress factor still takes place

separately. The next step is to identify the specific effect / impact of the work

environmental factors on defined organismic systems. If the same organismic system

is used more than once, possible bottlenecks must be analyzed.

This method proved its effectiveness in the presence of stressful climatic factors

associated with a high energy load of the human, so-called heat work, for instance.

Both stress factors lead to a higher utilization of the cardiovascular system, which in

this case is considered 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, different forms of work are differentiated:

• 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) adequate oxygen and nutrition

supply for the muscle, and (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

are held by muscular action forces, frictional resistance is overcome or when the body

is braced 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.

The figure shows a mechanical analogous model of a muscle. The length of the

contractile element is denoted with k, while the length of the elastic element is labeled

as s. Using this labels, the lengths of the respective forms of contraction (isometric,

isotonic, and auxotonic) can be compared with and zoned from the non-operating

state. An isometric muscle contraction is present, when a muscle performs solely an

increase in force while the length of the muscles stays thereby constant. As the

distance traveled equals zero, no work is done in a physical sense. The isometric

muscle contraction can therefore merely be regarded as work in the physiological

sense (work = force ∙ duration). If a muscle is contracting isometrically, static work is

present. Static work is required for human body posture, on the other hand for holding

items and tools. Static work should be avoided as it is the unfavorable form of

mechanical work. According to DIN EN 1005-4 it involves high health risks. The

isotonic muscle contraction is marked by a shortening of the muscle length without a

change in force. In doing so, work in a physical sense is accomplished (work= force ∙

displacement). Is the muscle contracting isotonic, one speaks about dynamic work.

Dynamic work is needed when lifting a load, for instance. Dynamic work should be

targeted to lie within a low-frequency range (< 2 load cycles per minute), as this,

according to DIN EN 1005-4, promotes the lowest health risk. The auxotonic

contraction is a combination of isometric and isotonic contraction. Thus, muscle

length and force do change together.

While resting, both demand for blood and blood flow are at a constant low level and

therefore an equilibrium state exists. During heavy dynamic work, however, the

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). However the muscular system is also in an equilibrium state. As well during

static work the demand for blood and blood flow is higher than in rest, because the

blood circulation is significantly reduced. To avoid a performance hit caused by this

non-equilibrium state, 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 a permanent basis.

To maintain the body position when in an inactive body state (that is without

generation of physical work) muscles need to be tensed. This static muscle work is

energy-related especially inefficient, which is due to the lack of movement. In a non-

moving state the muscle circulation results in a much faster muscle weariness, which

in turn leads to an enhanced circulatory-activity. The higher the holding-force, the

lower the holding period. If not more than 15% of the maximum force is expended for

the holding force, then there is no considerable fatigue for the observed time scale. In

dependency of how high the work load is (that is the coverage rate of the maximum

force), the maximum force that is still available after a certain work duration declines

continuously. For example: If 25% of the maximal force is statically demanded, the

force can, due to the fast occurring muscle fatigue, only be maintained for 4 minutes;

for 50% of the maximum force it is only 1 minute.

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

within the body. As previously explained 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 (isometric contraction).

Dynamic muscle force, however, occurs during the change in length of the muscle

during its activity (isotonic contraction).

Inertia force is a physical quantity that acts through the moment of inertia, e.g.

dynamically as accelerating force, force of deceleration, or centrifugal force at mobile

workplaces, or statically as own weight.

Action 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 action force is split into e.g. arm, hand, leg or

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

horizontal force.

With relation to the direction of force, the action force can be differentiated into the

force of attraction and the force of pressure from the sense of direction of force.

Referring to the figure it is important 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 interrelations are important for the work design.

Examples are:

The own weights of the body parts (inertia forces) are compensated by static

muscle forces for maintaining a body posture.

Action forces on body support areas can be composed of gravitational 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 in the above diagram apply to an upright standing body posture

with parallel foot position at a foot distance of 30 cm. The indicated values of the

maximum static action forces were determined at stationary arranged handles during

short-time maximum force exertion by the working person. A cylindrical handle with a

diameter of 30 mm was used, which was used without supporting tools. Shown are

averages of the maximum achievable static action forces, that are valid for specific

collectives, e.g. men aged 20 to 25 years, and therefore are not representative for the

total population. The maximum forces are represented in the form of “isodynes”. For

different working conditions (e.g. concerning posture or required force direction), the

transferability of the data has to be checked, that is contours of equal maximum

exerted action force. For example, in DIN 33411-3 and DIN 33411-5 maximum static

action forces for other working conditions were presented.

The illustrated isodynes (contours of equal maximum exerted action force) 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 isodynes also 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 figures show the behavior of the heart rate during and after work with shorter and

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

process is almost purely energetical and consists of cycling on a bicycle ergometer.

The average power generated through cycling is 200 [W]. 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 is limited in time due to limited energy

sources. The efficient execution of the work, while maintaining full performance

therefore depends critically on an appropriate distribution of the physical workload. In

selecting rest breaks, it should be taken into account that 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. Regarding human work,

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 lengths lead to a

considerable increase in efficiency as well as to a higher effectiveness when

activating leg- and trunk musculature.

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

but also the entire movement loads and the mass of the involved body parts. This

especially applies to movements that include large body parts and small loads.

The figure shows the dependency of the average bending force on the angle of the

cubital joint. The maximum of the bending force is reached when the cubital joint is at

an angle of about 100 degrees.

The diagram above shows the relationship between contraction force and velocity of

contraction with the thereout calculated power. The maximum power of about 135

Watt is achieved with a generated force of only 45 Newton. The velocity of contraction

then amounts to approx. 2.9 m/s.

When considering the dynamic work forms of the muscle, not only does the

unavoidable moment of inertia play a role, but also the gliding of the muscle’s actin-

and myosin-filaments. As a velocity dependent part of the total force occurs

(analogous to an internal friction), the maximum force decreases with increasing

velocity of contraction of the muscle length (so-called Hill’s force-velocity relation).

Avoiding activities that impose repeated static load on the internal structure leads to a

considerable relief for the organism. One possibility is to replace static work with

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 related to the three design alterations 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 has over 600 muscles available. The muscle mass constitutes

approx. 40% of men’s body mass, while for women it is 26%. The face alone has

more than 43 muscles. A health hazard for energetic-effectoric work forms emerges

especially when handling loads. Here, especially the spine is mechanically

vulnerable, which is shown schematically on the right hand side in the figure. At this,

the lumbar spine, the thoracic spine and the cervical spine can be differentiated. As

for the consequences of possible impairment, in 1993 the list of occupational disease

was extended with spinal disc related ailments of the lumbar spine through carrying

and lifting heavy loads (BK 2018) and also with longstanding carrying of loads on the

shoulders.

For the assessment of force proportions of the spine, mainly specific biomechanical

models (e.g. “Der Dortmunder”) are used. The inspection of the validity of such

models (especially regarding non-linearities, idealizations, coefficients standards and

so on) as well as the establishment of the load-carrying capacity limit of the spine is

only possible empirically. Because of the difference in load, the consideration of every

single vortex shows a complex picture.

The state of the spine can be evaluated very accurately by a computer tomographic

analysis, yet by this means only ex-post insights about the effect of past strains can

be determined. A spine overload only becomes noticeable 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 their agility and elasticity to the

spine. 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 can lead to a metabolic impairment in the intervertebral

discs. When handling loads, the spine is heavily stressed because of the leverage

effect of the external load and the resulting internal forces. Dependent on the position

of the held load and the diffraction of the back the elastic intervertebral discs are

exposed to enormous compressive stress and are strained by internal transverse

forces. Health risks include 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 force related to the body mass, acting on the intervertebral disc only

through the upper body parts, is reduced by approximately 50% of the real value.

The determination of the action forces on the spine in industry is usually 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 generate a moment of

force around the point of reference for the calculation. The lever arms (ai) depend on

body position 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 an action force (FA). That in

turn applies a moment of force on L5-S1 over the lever arm aA and as a consequence

the strain increasesHowever, the abdominal pressure also 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). Alongside the

torque, the forces that take effect on the spine also constitute a measure for the stress

of the spine. On the 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 an 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 greater

internal load. A weight of only 10 kg already results in an intervertebral disc strain that

corresponds to a mass of 300 kg directly compressing the disc. Such a strain

corresponds to 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. It is therefore necessary to keep the body in an upright

position when lifting heavy loads. Only with an upright position there is an evenly

distributed pressure of the intervertebral disc. 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

contact stress at the right margin reaches values at around 300 N/cm2 (R), while with

straight stance the stress is only half as much (Rohmert 1983).

This illustration shows, how strew exerted on the intervertebral disc L5-S1 increases

or declines with variation of the masses of the lifted loads under otherwise equal

conditions. The compressive forces acting on L5-S1 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. It is

apparent that for the 0-kg-graph even in an upright body position the forces are not

zero, which can be attributed to the weight of the body parts above L5-S1. With

increasing load, the curves get more skewed. This results from the increasing

influence of movement-related shares because of the moment of inertia of the load.

The figure on the right illustrates how the force on L5-S1 varies if a load is lifted in

different body positions.

The limiting value of 3400 N given by NIOSH insufficiently accounts for the difference

in physiological attributes such as age and gender. Therefore the limit applies only to

healthy individuals below the age of 50. The limits suggested by JÄGER however do

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 limit suggested by Jäger are the consequence of biomechanical

modeling, not a pure risk consideration.

If loads are too heavy and/or operated with a high frequency over a long period of

time and/or are handled in unfortunate body positions, there is a high risk of damage

to the musculoskeletal system for the manual handling of loads. Ailments of the

musculoskeletal system are prevalent in whole Europe. Risks emerge when the work

place does not correspond to the ergonomic principles of design. The European

standard DIN EN 1005-2 is a method for safety-related occupational medical care on

the basis of the European guideline 90/269/EWG „about the minimum regulations

regarding the safety and health protection for the manual handling of loads which

carries a risk to damage the lumbar column” and the load handling-regulation.

The model for risk assessment, which was presented in the European standard DIN

EN 1005-2, encompasses three processes. These processes repose on the same

foundation, however they differ in the complexity of application. The first process is a

quick rough analysis. The second process is easier to handle and has to be used

whenever the rough analysis indicate risks. Process 2 additionally considers a few

risk factors. Process 3 is a comprehensive thorough evaluation process, which

considers additional risk factors that are not included in process 1 and 2. These 3

processes possess different complexities. For logical reasons the risk assessment

starts with process 1 (the easiest process). Process 2 and 3 are only used when the

requirements or the loading cases from process 1 do not apply. The European

standard DIN EN 1005-2 apply for the manual handling regarding lifting or else

lowering of loads with a mass of a minimum of 3 kg and the carrying over a distance

of less than 2 m. The European standard DIN EN 1005-2 does not apply for holding of

property without walking, for pushing or pulling of property, for manually operated

machines or for handling load manually while sitting.

The actual mass of the work object that needs to be handled also comprises its

packaging or batteries, as well as the (for the manual handling needed) technical

auxiliaries. The position of the center of mass is determined by the property’s mass

distribution. The center of mass should ideally be in the middle between both hands

and as close as possible to the body. A shift of the center of mass within the work

object during the handling should best be avoided. The work object should be

constructed as compact as possible. Based on the risk index-calculation the design

recommendation can be derived, so that a low level of risk for the manual handling of

loads can be achieved; these recommendations include reducing the actual mass and

to hold it low from the very beginning (e.g. for the configuration of new workstations).

The weight limit RML, for a particular set of working conditions, is defined as the

weight of a load that virtually all healthy employees can manage for a particular time

(e.g. 8 hours) without an inacceptable risk of back injuries. The weight limit RML

determines by means of a multiplicative connection of reference load Mref and the

nine shown multipliers a maximum load with a low level of risk. The vertical distance

V between the center of both hands and the floor can be varied between 60 cm and

90 cm, the ideal value accounts for 75 cm. The horizontal distance H between the

center of both ankles and both hands as well as the vertical lifting distance D should

not exceed 25 cm. The asymmetry angle A should be 0 degrees and the grasping

quality should be judged as good. The lifting frequency F should be less than or equal

to 0.2 strokes per minute. Single handed work, working in pairs and spare-time work

are to be avoided.

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