Civil Engineering Reference
In-Depth Information
length-strength relationships, and temporal relationships) can be manipulated in order to facilitate this
goal and serve as the basis for many ergonomic recommendations.
11.2.5.1 Biomechanical Arrangement of the Musculoskeletal Lever System
The posture required by the design of the workplace can affect the arrangement of the body's lever
system, and thus, can greatly affect the magnitude of the internal load required to support the external
load. The arrangement of the lever system can influence the magnitude of the external moment imposed
upon the body as well as dictate the magnitude of the internal forces and the subsequent risk of cumu-
lative trauma. Consider the biomechanical arrangement of the elbow joint that is shown in Figure 11.4.
In Figure 11.4a, the mechanical advantage of the internal force generated by the biceps muscle and
tendon is defined by a posture that keeps one's arm bent at a 90
angle. If one palpates the tendon
and inserts the index finger between the joint center and the tendon, one can gain an appreciation for
the internal moment arm distance. One can also appreciate how this internal mechanical advantage
can change with posture. With the index finger still inserted between the elbow joint and the tendon
and if the arm is slowly straightened one can appreciate how the distance between the tendon and the
joint center of rotation is significantly reduced. If the imposed moment about the elbow joint is held con-
stant (as shown in Figure 11.4b by a heavier tool) under these conditions, the mechanical advantage of
the internal force generator is significantly reduced. Thus, the internal moment must generate greater
force in order to support the external load. This greater force is transmitted through the tendon and
can increase the risk of cumulative trauma. Therefore, the positioning of the mechanical lever system
(which can be accomplished though work design) can greatly affect the internal load transmission
within the body. The same task can be performed in a variety of ways but some of these positions are
much more costly in terms of loading of the musculoskeletal system than others.
8
11.2.5.2 Length-Strength Relationship
Another important relationship in defining the load on the musculoskeletal system is the length-strength
relationship of the muscles. Figure 11.5 shows this relationship. The active portion of this figure refers to
structures that actively generate force such as muscles. The figure indicates that when muscles are close
to their resting length (generally seen in the fetal position), they have the greatest capacity to generate
force. However, when the muscle length deviates from this resting position the capacity to generate force
is greatly reduced because the cross-bridges between the components of the muscle proteins become ineffi-
cient. Hence, when a muscle stretches or when a muscle attempts to generate force while at a short length the
ability to generate force is greatly diminished. Note also, as indicated in Figure 11.5 that passive tissues in the
Active
+
Passive
Passive
Active
50
100
150
200
% of Rest Length
FIGURE 11.5 Length-tension relationship for a human muscle. (Adapted from Basmajian, J.V. and De Luca, C.J.,
Muscles Alive: Their Functions Revealed by Electromyography, 5th ed., Williams and Wilkins, Baltimore, MD, 1985.
With permission.)
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