Civil Engineering Reference
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scapula
humerus
biceps
triceps
radius
ulna
FIGURE 10.2 Active, passive and, total tension within a muscle at different lengths. (With permission by the
publisher from Kroemer et al., Engineering Physiology. Bases of Human Factors
Ergonomics, 3rd ed., Van Nostrand
/
Reinhold — John Wiley & Sons, New York, 1997. All rights reserved.)
Paradoxically, by contracting the muscle can compress its own fine blood vessels that permeate the
muscle tissues. A strongly contracting muscle generates pressure within itself, as can be felt by touching
a tightened biceps or calf muscle. By such strong pressure, the muscle reduces the cross-section of the
vessels, diminishing or even shutting off its own blood circulation. The interruption of blood flow
through a muscle stops the metabolic energy conversion and allows metabolic by-products to be
accumulated in the muscle tissues; this quickly leads to muscle fatigue, forcing relaxation. Experiencing
such fatigue is painful as it occurs slowly when the muscle is not contracting to its maximum, when
working overhead with raised arms, for example, while fastening a screw in the ceiling of a room.
Muscle fatigue in the shoulder muscles makes it impossible to keep one's arms raised even after a
minute or so, even though nerve impulses from the CNS still arrive at the neuromuscular junctions,
and the resulting action potentials continue to spread over the muscle fibers.
The operational definition of muscle fatigue is “a state of reduced physical ability that can be restored
by rest.” Figure 10.3 shows the relation between static exertion and muscle endurance schematically: a
maximal exertion can be maintained for just a few seconds; 50% of the tension is present for about
1 min; but less than 20% can be applied for long endurance periods.
10.5 Muscle Tension and Its Internal Transmission to the
Point of Application
In common use, the term “strength” may refer to any or all of the
.
Tension within a muscle
.
Internal transmission via body links across joints
.
External application of force or torque by a body segment to an outside object
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