Low-Level Static Exertions - Fundamentals and Assessment Tools for Occupational Ergonomics - page 329

be causally related to the identified risk aspect. The term “low-level static exertions” will be discussed, fol-
lowed by a presentation of possible short- and long-term physiological responses. Based on this, preven-
tive strategies are presented.

14.2 What Are “Static Exertions”?

Within the area of mechanics “static,” in the strict sense means “no motion.” In the workplace, truly static
work postures are quite rare because most jobs include a number of movements to be performed often by
the upper limbs. Even in supervision jobs, a number of objects have to be handled now and then.
According to the strict definition of static, one might suggest to use observation techniques to quantify
how long time a certain posture is maintained without any movement. But most likely this variable
would fail to show a relationship to musculoskeletal disorders. For instance, lying in bed or sitting
relaxed in a well-supporting chair is hardly considered a risk, although highly static. The reason is, of
course, that no muscle exertions or contractions need to be performed in these conditions. Therefore,
quantifying the true variable “static” when trying to identify risk factors is not sufficient. What we are
looking for is the static muscle contraction that may induce an overload on the musculoskeletal system.
A profile of the muscular load during a period of work may be obtained by analyzing the amplitude
probability distribution function (APDF) of the electromyographic signal (EMG). Such measurements
for analysis of static muscle activity have been widely used in workplace studies, where the static level
is defined as the probability level P 0.1. For instance, a static level of 5% MVC means that the contrac-

tion level of the muscle is 5% MVC or above for 90% of the time, or in other words, only for 10% of the
time is the muscle contraction below 5%MVC. This implies that muscular rest may occur for 10% of the
recording time or less. The interpretation of a static contraction according to the APDF analysis has
caused some confusion because the static level is actually defined in the time domain. Also, this variable
does not give the information that the muscle is really performing a 5% MVC throughout the recording
period; indeed, larger contraction forces may occur. Finally, this variable does not control for length
changes of the muscle, which means that the muscle contractions may well be dynamic. Nevertheless,
redefining “static” in occupational settings has been a great “success.” This is probably due to the time
variable in essence being the real risk. But this was not intentional and no awareness has been paid to
this fact by practitioners. Of note is that the risk factor probably is the sustained contraction.

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14.3 What Is a “Low-Level”?

It may be surprising that low- rather than high-level contraction forces seem to imply a risk. Of course,
high forces can cause ruptures as seen in accidents where bone, ligament, tendon, or muscle are exerted
beyond their breaking point, and in this sense, high forces imply a risk. However, low forces constitute a
corresponding risk if repeated or sustained for a prolonged time. All structures, inert materials as well as
biological tissues, are able to withstand a force characteristic to their structure. At high forces, disruption
will occur when the breaking point is exceeded, and lesser forces repeated over time will eventually cause
fatigue fracture (Figure 14.1). Repetitive force exertions are accumulated and cause eventual disruption,
possibly not of the tissue as a whole but in terms of microruptures.
First of all, when evaluating the force level, the maximum strength of the muscle must be taken into
account. This relates to the muscle's cross-sectional area, age, and state of training; and different muscle
groups and subjects show highly different muscle strength. Therefore, exposure assessment in terms of
force recordings in absolute numbers in Newtons (N) will not give sufficient information regarding
the level of exertion. The MVC force must be recorded as well, and data must be presented in percentage
of MVC as mentioned earlier regarding the EMG data.
Second, endurance time for muscles plays a significant role in this context. The relationship between force
level and the time for which it can be sustained is depicted in the endurance time curve (Figure 14.2). At low-
force levels relative to the maximum strength, the muscle is capable of developing such force for long periods