Civil Engineering Reference
In-Depth Information
breaking point
tolerance limit
Time
sustained or repeated load
acute
FIGURE 14.1 Tolerance limit for maximum force at breaking point depends on the type of muscle, for example,
cross-sectional area, state of training, and age. Further, the contraction mode, that is, static or dynamic, is
significant. For submaximal forces, the tolerance limit decreases with time, the rate of decrease depending on
force magnitude and repetition frequency.
of time before being exhausted. Different muscles show highly different endurance capacity depending on
muscle fiber type, anatomy, and state of training. But for every muscle there is a limit.
Third, in industry many low-level exertions include repeated static exertions or movements at quite
high speed but with little displacement. When observing such tasks, often little attention is paid to
the displacement, which is the cause for such exertions to be assessed as static. Also for intermittent
static as well as dynamic contractions, endurance time curves exist.
2,20
It is for the dynamic contractions
(a)
(b)
8
800
6
600
4
400
2
200
x
x
20
40
60
80
100
20
40
60
80
100
Force (% MVC)
Force (% MVC)
FIGURE 14.2 (a) Shows an endurance time curve for static contractions, which are defined as contractions at
constant muscle length. Of note is the large range of endurance time at low forces, where the end-point of
exhaustion varies significantly, for example, due to the level of motivation. Examples are given for handgrip (D),
shoulder abduction (x), and trunk extension (
). (b) Lower part shows a force-velocity curve for dynamic
contractions, only shown for concentric contractions, that is, during muscle shortening. The maximum muscle
force decreases with increasing velocity of shortening. Examples are given for shoulder movements during floor
cleaning (
†
), and wrist movements during meat cutting
(x). For the latter, it is seen that although the load is only 10% MVC, it is about 20% of the dynamic strength,
since the maximal dynamic strength at this velocity is approximately 50% MVC.
†
), shoulder movements during forking in agriculture (
P
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