Civil Engineering Reference
In-Depth Information
(a)
(b)
Internal Load (F)
Internal Load (F)
F
.
.0254 m = 44.5 N .305 m
44.5 N .305
m
.0254 m
F
.
.0127 m = 89 N .1525 m
89 N .1525
m
.0127 m
F =
F =
F = 1068 N (240 lbs.)
F = 534.35 N (120 lbs.)
Internal
Load (F)
Internal
Load (F)
.0127 m
External Load
External Load
.0254 m
.305 m
.1525m
44.5 N
89 N
FIGURE 11.4 An example of an anatomical third-class lever (a) demonstrating how the mechanical advantage
changes as the elbow position changes (b).
force acts in between the two. An example of this system in the human body is the elbow joint and is
shown in Figure 11.4.
11.2.4 External and Internal Loading
Two types of forces can impose loads on a tissue during work. External loads refer to those forces that
are imposed on the body as a direct result of gravity acting upon an external object being manipulated
by the worker. For example, in Figure 11.4a, the tool held in the worker's hand is subject to the forces
of gravity, which impose a 44.5 N (10 lb) external load at a distance from the elbow joint of 30.5 cm
(12 inches). However, in order to maintain equilibrium, this external load must be counteracted by an
internal load that is supplied by the muscles of the body. Figure 11.4a also shows that the internal load
(muscle) acts at a distance relative to the elbow joint that is much closer to the fulcrum than the exter-
nal load (tool). Thus, the internal load or force is at a biomechanical disadvantage and must be much
larger (534 N or 120 lb) than the external load (44.5 N or 10 lb) in order to keep the musculoskeletal
system in equilibrium. As shown in this example it is not unusual for the magnitude of the internal
load to be much greater (typically 10 times greater) than the external load. Thus, it is typically the
internal loading that contributes mostly to the cumulative trauma of the musculoskeletal system
during work. The sum of the external load and the internal load define the total loading experienced
at the joint. When evaluating a workstation the ergonomist must not only consider the externally
applied load but must be particularly sensitive to the magnitude of the internal forces that can load
the musculoskeletal system.
11.2.5 Factors Affecting Internal Loading
The previous discussion has discussed the importance of understanding the relationship between the
external loads imposed upon the body and the internal loads generated by the force generating mechan-
isms within the body. The key to proper ergonomic design involves designing workplaces so that the
internal loads are minimized. One can consider the internal forces as both the component that loads
the tissue as well as a structure that can be subject to over-exertion. Muscle strength or capacity can
be considered as a tolerance measure. If the forces imposed on the muscles and tendons as a result of
the task exceed the strength of the muscle or tendon an injury is possible. In general, three components
of the physical work environment (biomechanical arrangement of the musculoskeletal lever system,
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