Civil Engineering Reference
In-Depth Information
in Figure 11.1. Cumulative trauma, on the other hand, refers to the repeated application of force to a
structure that tends to wear down a structure, thus, lowering the structure tolerance to the point
where the tolerance is exceeded through a reduction of the tolerance limit. This situation was illustrated
in Figure 11.2. Cumulative trauma represents more of a “wear and tear” on the structure. This type of
trauma is becoming far more common in the workplace since more repetitive jobs are becoming
common in industry and is the mechanism of concern for many ergonomics evaluations.
Cumulative trauma can initiate a process that can result in a tissue reactive cycle that is extremely dif-
ficult to break. This process is illustrated in Figure 11.3. The cumulative trauma process begins by expos-
ing the worker to manual exertions that are either frequent (repetitive) or prolonged. This repetitive
application of force can affect either the tendons or the muscles of the body. If the tendons are affected,
the following sequence occurs. The tendons are subject to mechanical irritation when they are repeatedly
exposed to high levels of tension and groups of tendons may rub against each other. The physiologic reac-
tion to this mechanical irritation can result in inflammation and swelling of the tendon. This swelling will
stimulate the nociceptors surrounding the structure and signal the central control mechanism (brain) via
pain perception that a problem exists. In response to this pain, the body will attempt to control the
problem via two mechanisms. First, the muscles surrounding the irritated area will coactivate in an
attempt to stabilize the motion of the tendons or stiffen the structure. Since motion will further stimulate
the nociceptors and result in further pain, motion avoidance is often indicative of the start of a cumu-
lative trauma disorder. Second, in an attempt to reduce the friction occurring within the tendon, the
body will increase its production of synovial fluid within the tendon sheath. However, given the
limited space available between the tendon and the tendon sheath the increased production of synovial
fluid often exacerbates the problem by further expanding the tendon sheath and, in thus, further stimu-
lating the surrounding nociceptors. As indicated in the figure, this initiates a viscous cycle where the
response of the tendon to the increased friction results in a reaction (inflammation and the increased
production of synovial fluid) that exacerbates the problem. Once this cycle is initiated it is very difficult
to stop and often anti-inflammatory agents are required. This process results in chronic joint pain and a
series of musculoskeletal reactions such as reduced strength, reduced tendon motion, and reduced mobi-
lity. Collectively, these reactions result in a functional disability.
Required Manual Exertion
Frequent or Prolonged Muscle Tension
Tendon
Muscle
Muscle Substitution and Fatigue
Mechanical Irritation of Tendons
Muscle Fatigue, Ischemia,
Retained Metabolites, Edema and Heat
Inflammation of Tendon
Muscle Pain
Tendon Pain
Muscle Coactivity
Inflammation of Synovia and Bursa
Immobilization of Joint
1. Fibrous Reaction
2. Muscle Contracture
3. Reduced Joint Mobility
4. Reduced Muscle Strength
5. Reduced Tendon Motion
Chronic Joint Pain
Functional Disability
FIGURE 11.3
Sequence of events in CTDs.
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