Low-Level Static Exertions - Fundamentals and Assessment Tools for Occupational Ergonomics - page 331

that the contraction level cannot be described only by the force in N or percent MVC. The force-velocity
relationship must also be taken into account, the relative load being higher when a specific force is devel-
oped with increasing speed (Figure 14.2). For instance, keyboard operators may press 200,000 keys a day
or 500 per minute and a piano player strikes the tangents with finger movements at very high, maybe
sometimes maximum, speed. For the unloaded limb, the EMG activity increases linearly with the velocity
of the movement. 21 If, in addition to the movement velocity, there is an external force to overcome
during the work tasks, this could imply maximum effort at high velocities even if the force level is
low. Thus, low-level cannot be assessed only in terms of percent MVC, but the mode of contractions
must also be taken into account. Ideally, the maximum dynamic voluntary contraction forces should
be assessed and the work task evaluated in relation to the corresponding maximum force-velocity
relationship.
In short, the term “low-level” in the context of work-related static exertions refers to a working con-
dition in which a muscle is activated at a level that can be maintained for a long period. This may be a
true static contraction, sustaining a constant force and posture or varying in force within a limited range
and without any movement. But even performing intermittent static or dynamic contractions (con-
centric eccentric) at submaximal force velocities with small displacements and at intensities that can

be maintained for a long time may be considered low-level static exertions in occupational settings. 22
Actually, when such exertions are measured by electromyography and analyzed by the previously men-
tioned APDF of the EMG, “static” levels of 5% MVC or more may be found. This means that a low-level
static exertion is to be considered in the time domain and is characterized by workers being able to
perform it for hours. The main feature is that the exertion is sufficiently low so that it can be sustained
for a prolonged time and the duration probably implies the risk.

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14.4 Which Work Requirements Induce “Low-Level
Static Exertions”?

Examples of jobs in which low-level static exertions are frequent are presented in Table 14.1 and
Table 14.2. Additional job titles are numerous in the literature. 28 It is important in risk assessment to
identify generic work requirements that induce these exertions. At random, requirements such as pre-
cision, speed, visual demand, and mental load can be mentioned. 29 Also monotony or lack of variation
is a characteristic that concerns working posture and movement as well as mental challenge. The same
task is repeated over and over again most often by the hands. When operating with fast precise move-
ments with the hands, there is a demand to stabilize the shoulder girdle. One reason is that the shoulders
are the reference point for the upper limbs and if they move, the hands will be repositioned with respect
to the motor control pattern for the upper limbs. Similarly, to control the position of the eyes, fixation of
the neck is needed, and stable eye position is a prerequisite for most visual demands in industry. Inter-
estingly, the fastest repositioning of the eyes can be performed when the neck and shoulder muscles are
contracted upto 30% MVC. 30 When performing tasks at high speed, the stiffness of the musculoskeletal
system must be increased. For this purpose, cocontractions are performed, which means that antagonistic
muscles, that is, muscles on each side of a joint, are contracting. This is especially common for the
shoulder muscles. One reason is, as mentioned earlier, that the shoulder must be the stable fix point
and “take-off ” for arms and hands. Also the anatomy of the shoulder is such that it has the greatest mobi-
lity of all the joints in the body. It is a joint that is highly dependent on muscle stabilization, including
cocontractions. These cocontractions have been shown to increase with increasing speed and precision
demands. 21,31,32

14.5 Why Do “Low-Level Static Exertions” Imply a Risk?

As discussed earlier, it is not necessarily because exertions are static or at a low level that they imply a risk,
but because such exertions are often sustained for prolonged periods of time. Additionally, often no