Civil Engineering Reference
In-Depth Information
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A range of expected strength exertions is, obviously, that between the considered minimum and
maximum. The infamous “average user” strength is usually of no design value — see the
Chapter 9 on “Engineering Anthropometry” in this handbook.
Most data on body segment strength apply to static (isometric) exertions. They provide reasonable
guidance also for slow motions, although they are probably too high for concentric motions and a bit
too low for eccentric motions. Of the little information available for dynamic strength exertions,
much is limited to isokinematic (constant velocity) cases. As a rule, strength exerted in motion is less
than that measured in static positions located on the path of motion.
The usual statistical treatment of measured strength data assumed that they fall into a normal distri-
bution, which allows describing them in terms of averages (means) and standard deviations. This also
allows the use of common statistical techniques to determine data points of special interest to the designer
— as discussed in detail in the Chapter 9 on “Engineering Anthropometry” in this topic. In reality, data
describing body segment strength often appear in a skewed rather than in a bell-shaped distribution. The
actual shape of the distribution is not of great concern, however, if the data points of special interest are
the extremes. We can determine the maximal forces or torques, which the equipment must be able to bear
without breaking, as those above the strongest measured data points. We can identify the minimal exer-
tions, which even “weak” persons are able to generate at the low end of the distribution; again, see the
“Anthropometry” Chapter 9 for procedures to calculate or estimate the design values.
10.7.1 Designing for Hand Strength
The human hand is able to perform a large variety of activities, ranging from those that require fine
control to others that demand large forces. (However, the feet and legs are capable of more forceful exer-
tions than the hand.)
One may divide hand tasks in this manner:
Fine manipulation of objects with little displacement and force. Examples are writing by hand,
assembly of small parts, adjustment of controls.
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Fast movements to an object requiring moderate accuracy to reach the target, but there is fairly a
small force exertion. An example is the movement to a switch and its operation.
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Frequent movements between targets, usually with some accuracy but little force; such as in an
assembly task, where parts must be taken from bins and assembled.
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Forceful activities with little or moderate displacement (such as with many assembly or repair
activities, for example, when turning a hand tool against resistance) and forceful activities with
large displacements (e.g., when hammering).
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Texts book by Karwowski (2001), Kroemer et al. (2001), and Salvendy (1997) contain guidelines for
design for accuracy and for displacement. For strength exertion, the designer keeps in mind:
Of the digits of the hand, the thumb is the strongest and the little finger the weakest. Finger forces
depend on the finger joint angles, as listed in Table 10.3 and Table 10.4. Table 10.5 provides detailed
information about manual force capabilities of male students and machinists. Female students
developed between 50 and 60% digit strength of their male peers, but achieved 80 to 90% in
“pinches.”
Gripping and grasping strengths of the whole hand depend on the coupling between the hand and the
handle — see Figure 10.6. The forearm can develop considerable twisting torques. Large force and torque
vectors are available with the elbow at about the right angle, but the extended arm can exert the strongest
pulling
away from the shoulder, especially if the trunk braces against a solid
structure. Torque about the elbow depends on the elbow angle as depicted in Figure 10.7 and, in
more detail, in Figure 10.8. Obviously, body posture and body support have great effects on the strength
exerted with the arm and shoulder muscles.
pushing forces toward
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