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 theparticular work conditions.

O f th i t k f ti h lth t ti i th id f b k i dOne of the main tasks of preventive health protection is the avoidance of back pain andinjuries 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 allaccepted occupational illnesses were traced back to intervertebral disks problems. Ahealth risk is particularly common for energetic-effective work forms, especially for ones inwhich the handling of loads occurs. The EU Directive 90/269/EWG demands “preventivemeasures for the avoidance of risk due to handling of loads, and that workplaces dealingwith load handling evaluate the risks for their employees”.

In ergonomics, the ideal types of extreme forms of human work are calledphysical/energetic and informational work and are defined either as pure energyinformation transformation. The five types of work (creative, combinative, reactive, motorand 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

h i h ‘t b i ti t d ffi i tl til th t thmechanisms 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 ischaracterized by: (1) change of muscle contraction and relaxation (recovery), (2)response of the muscle blood flow (3) muscle needs oxygen and nutrients (4) possibleresponse of the muscle blood flow, (3) muscle needs oxygen and nutrients, (4) possibleactivity over longer periods.

Activities with dynamic components are for example, cycling or activities with movementsthat use different portions of the muscle. Static work occurs when objects held bymuscular action, lift loads, overcoming frictional resistance or bracing the body againstgravity in particular positions or postures. It is characterized by: (1) continued contractionof muscles over longer periods, (2) a mismatch between oxygen demand and oxygensupply to the muscle, because the blood vessels responsible for the muscle arecompressed during the contraction, (3) rapid fatigue and thus, a continuation of activitiesover extended periods is impossible, (4) adverse biomechanical loading conditions ofbones, joints and ligaments, resulting in premature wear, especially of the spine.

Isometric muscle contraction has two meanings: Foremost, human posture requires thatmusculature perform static work (postural work). Secondly, tools have to be held over aspecific time period (holding work). Static work is the most unfavorable form of musclework and should be avoided!

While hibernation both demand for blood and blood flow are at a constant low level. In contrast, when doing heavy dynamic work 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 While static work the demand for blood and blood flow are higher thanequilibrium state. 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 maximumforce is used for the holding force, no fatigue occurs.

Depending on the load amount, i.e. the degree of exhaustion of the maximum force, themaximum 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 bemaintained for approximately 4 minutes due to the quickly occurring muscle fatigue; at50% of the maximum force only 1 minute is possible.

Muscle force is a physical strength that works through the activity of the muscles withinthe body. There is a difference between static and dynamic muscle force. Static muscleforce is the physical strength that occurs without a change in the length of the muscleduring its activity. Dynamic muscle force, however, occurs during the change in length ofthe muscle in its activitythe muscle in its activity.

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 mobileworkplaces, or statically as own weight.

Action force is a physical strength that works outward from the body. It results from inertiaforce, muscle force, or both. Inertia force and muscle force can reduce or increase theirstrength depending on amount and directionstrength depending on amount and direction.

From the force-releasing body parts the action force is split into e.g. arm, hand, leg orfinger force; from the force direction the action force is split into e.g. vertical or horizontalforce.

The action force is differentiated according to the force of attraction and the force ofpressure from the sense of direction of force.

Referring to the figure it is to differentiate between rather basic-orientedclassification schemes of muscle mass and strength (acting in the body system)and more practical classifications in terms of generated action forces (workingfrom the body to the outside). The correlations are important for the work design.E lExamples are:

The own weights of the bodyparts (inertia forces) are compensated by staticmuscle forces for maintaining a body posture.

Action forces on body support areas can be composed of mass forces of thebody parts and posture forces. This is to be considered e.g. in dimensioningof the restoring force of a pedal.g p

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 orcompletely by the combination of contraction and extension muscle forces(separate muscle groups) (for example, relocating loads).

The dependent and independent parameters of the figure refer to an upright standingbody posture with parallel foot position at a foot distance of 30 cm. The indicated valuesof the maximum static action forces were determined at stationary arranged handlesduring short-time maximum force exertion of the working person. A cylindrical handle witha diameter of 30 mm was used which was not supported in anyway Shown are averagesa diameter of 30 mm was used, which was not supported in anyway. Shown are averagesof the maximum achievable static action forces, that are only for a specific collectives ofmen aged 20 to 25 years and are not representative for the total population. Themaximum force is represented in the form of an isodynamic line. For different workingconditions (e.g. in terms of posture or the required force direction), the transferability ofthe data has to be checked. In DIN 33411-3 and DIN 33411-5 for example maximumstatic action 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 shortand longer breaks with steady proportion between work phase and break. Because of theexponential character of the exhaustion and recreation phases it is not functional to workuntil the occurrence of exhaustion. There is a need for disproportionately long recreationphases It is physiologically more favorable to arrange short cyclical work and recreationphases. It is physiologically more favorable to arrange short cyclical work and recreationphases. Human performance has a time limit due to limited energy. The efficientexecution of the business, while maintaining full performance therefore depends criticallyon an appropriate distribution of the burden. In selecting rest breaks, it should be takeninto account the onset of fatigue requires disproportionately long recovery periods. Therequired recovery phase increases disproportionately with increasing work phasedurations, where the recovery from static work requires a longer period than from dynamicwork A quick alternating work and rest regime is therefore more favorable forwork. A quick-alternating work and rest regime is therefore more favorable forphysiological and economic reasons.

An example for the choice of the working process with the highest efficiency is the loadingof an industrial furnace. During the loading of an industrial furnace, the human energydemand can be lowered by reducing the lifting height. If the human work is regarded, apositively directed work has to be performed while lifting and a negatively directed workwhile lowering the workpiece A reduction of the lifting height comes along with awhile lowering the workpiece. A reduction of the lifting height comes along with adecrement of the metabolic rate and therefore with a lower strain of the cardiac circulatorysystem. Simultaneously the shorter movement ways lead to a considerable increase inefficiency.

External loads are not the only ones considered when evaluating job performance, butalso the entire movement and held loads. This especially applies to movements thatinclude 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 135 watts is reached at a generated force of 45 Newton. The contraction speed is then approximately 2,9 m/s. While dynamic work of the muscle alongside the inevitable mass moments of i ti th lidi f th l ’ ti d i fil t l i t tinertia, 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

d i i f t l )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 43muscles.

The state of the spine can be evaluated very accurate by an computer tomographicanalysis, yet by this means only ex-post insights about the effect of strain that occurred inthe past can be determined. A spine overload only becomes obvious after damage of thespine. It is therefore necessary to estimate the risks of spine impairments preventively.

The spine consists of 24 osseous vortices, between which cartilaginous intervertebraldiscs are situated. The intervertebral discs impart to the spine their movability andelasticity. The nutrition of the rubbery invertebral discs is entirely dependent on diffusionbecause there is no blood flow. Sustained compressive loading reduces the pressure-dependent fluid shift and then there is a metabolic impairment in the intervertebral discsdependent 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 ofthe external load and the resulting internal forces. According to the position of the heldload and the diffraction of the back the elastic intervertebral discs get under enormouscompressive stress and are exposed to internal transverse forces. Dangers to health likedamage of intervertebral discs, deforming of vortices or ruptures of muscular fibers canresult. 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% ofthe real value.

The estimate of the action forces on the spine is based on empirically testedbiomechanical models. When considering a load, usually the spinal area L5-S1 is of maininterest because it is a common injury point (95% of all disc damage accounts for thethree lowest lumbar intervertebral discs). The body parts (i) above the lumbar sacrumtransition L5-S1 themselves each apply a moment of force around the point of referencetransition L5-S1 themselves each apply a moment of force around the point of referencefor the calculation. The lever arms (ai) are dependent on the position of the body andtherefore represent variables as a function of time during the execution of movements. Ifthere is not only a body movement executed, but also a manipulation of a “load“, thenthere additionally occurs a restoring force (FA), that applies a moment of force on L5-S1over the lever arm aA and hence raises the stress. As opposed to this, the abdominalpressure constitutes a certain help: Through holding one’s breath an abdominal pressurecan be set up so that the solidified abdomen builds a supporting force for chest and spinecan be set up so that the solidified abdomen builds a supporting force for chest and spine(pABD). Besides to the torque, the forces take effect on the spine, it constitutes a measurefor the stress of the spine. On one hand the weight of the body parts above the lumbarsacrum transition leads to a compression of the intervertebral discs and transverse loadsoccur because of the ascent even in upright body position. On the other hand additionalforces are set up by muscles, e.g. by the back muscles. These forces build up a countertorque 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 highwhile standing upright on the intervertebral discs in the lumbar vertebra region, accordingto the law of levers.

Even a load 6-7 times as heavy and carried on the head would not result in a greateri t l l d A l d f l 10 k l d lt i 300 k i t t b l t i S hinternal load. A load of only 10 kg already results in a 300 kg intervertebral strain. Such astrain 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 heavyloads the air is held in the lungs by pressure breathing and is highly compressed insidethe body. A pressure like this is required for stabilization of the trunk, but not withoutdanger. Therefore it is highly demanded to keep the body in an upright position on thelifting of heavy loads Only with an upright position is a consistent pressure of thelifting of heavy loads. Only with an upright position is a consistent pressure of theintervertebral disc reached. The spine should only be strained axially, in no caseeccentrically. For this case of strain, high surface pressures occur at the margin of theintervertebral disc. With inflected spine and a lifted load of ~50kg the surface pressureaffects the intervertebral discs inconsistently. Additionally, the pressure strain at the rightmargin reaches values around 300 N/cm2 (R), while with straight stance only a surfacepressure half as strong results (Rohmert, 1983).

This illustration shows, how strew exerted on the intervertebral disc L5-S1 increases ordeclines with variation of the masses of the lifted loads under otherwise equal conditions.The compromise forces for the dynamic, two-handed lifting of loads with a mass of 0 kgup to 50 kg are therefore illustrated in the figure. The overall duration for the leverageoperation was supposed to be 1 5 sec With the 0-kg-graph strikes that even in an uprightoperation was supposed to be 1.5 sec. With the 0-kg-graph strikes that even in an uprightbody position the pressure forces are not zero, this attributes to the weight of the bodyparts above L5-S1. With increasing load, the curves get more skewed, resulting of theincreasing influence of movement-related shares because of the moment of inertia of theload.

The figure on the right illustrates how the strain of the spine varies if a load is lifted indifferent body positions.

The critical value given by NIOSH of 3400 N insufficiently accounts for the differentphysiological width (age, gender, etc.). The limit hence applies only to healthy personsbelow the age of 50. The limits suggested by JÄGER differentiate between various ageand gender groups. For older people, the NIOSH-limits are already considered to be toohigh In case of ascent torsions sudden jerky movements or asymmetric loads the strainhigh. In case of ascent, torsions, sudden jerky movements or asymmetric loads the strainon vortices and intervertebral discs increases. According to this, the listed percentagesare to be set against the denoted load limits. The denoted values are the consequence ofa biomechanical view, not the premise Safety Guidelines.

Several methods for the assessment of stress respectively strain of the musculoskeletal-systemduring manual load handling have been developed over the recent years.

The illustration shows different gradations of methods for different areas of use that can bedistinguished according to the level of detail of the examination. These gradations allow theselection of methods for approximate assessments, for extended examinations or methods forselection of methods for approximate assessments, for extended examinations or methods forspecific problems depending on the field of application.

The DIN EN 1005-2 is based on the calculation of the Weight Limit (RML). The RML

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 RML is

l l t d b lti l i th ti d i fl i f t d f l dcalculated by multiplying the mentioned influencing factors and a reference load (Mref).

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-k l t l t (i l di l t d d i ti i t t th b )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