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Tissue Injury 101:
How do Muscles, Tendons, or Ligaments
Get Injured?
Reducing the risk of injury in the work place is a common
goal of all who are involved with
workplace health and safety. Typically
these efforts involve making some sort
of change which results in a reduction
of an individual’s exposure to known or
suspected injury risk factors. This is
a good thing, for if you decrease
exposure, you decrease risk.
Understanding the mechanism of
how tissues get injured (e.g. muscles,
tendons, ligaments) provides a
foundation for better understanding risk
factors which permits you to put forth
better, more effective counter measures
in the workplace..
So, how do tissues get injured? Tissue injury
occurs when the tissue experiences an
applied force that is greater than the
tissue’s tolerance. Mathematically,
this mechanism can be expressed as:
Applied Force > Tissue Tolerance
There are three very exciting things about this mechanism.
The first is that it applies to all
tissues (not just muscles, ligaments and
tendons). Secondly, it applies to all
injuries. And finally, it provides us
three possible approaches to preventing
tissue injury: a) decreasing the applied
force, b) increasing the tissue
tolerance, or c) some combination of a)
and b).
But before we start thinking about possible approaches to
the prevention of tissue injury, we need
to explore the applied load and tissue
tolerance relationship a little bit
further so that we better understand the
Modes of Tissue Injury.
Modes of Tissue Injury
High Force Injury
This type of injury is associated with a tissue
experiencing a single high force event
(e.g. slip, trip, fall, impact with,
struck by). You can illustrate this
type of injury by using some popsicle
sticks and performing the following
demonstrations.
Demonstration #1:
Hold the ends of a Popsicle stick (the
tissue) between the thumb and forefinger
on each hand. If you slowly apply a bit
of force with your thumbs you can put a
slight bend in the Popsicle stick.
Release the force and the Popsicle stick
returns to its original shape. Now, it
is really important to observe here that
“nothing happened” to the Popsicle
stick! This result is identified by the
circled 1 in Figure 1. Releasing the
applied force (solid line) before the
(tissue) tolerance level (dashed line)
of the Popsicle stick was reached
created a “safety margin” and no tissue
injury occurred, which is our desired
outcome in the workplace!
Demonstration #2:
This is similar to
Demonstration #1 but instead of slowly
applying a small force, you increase the
force until the popsicle stick cracks.
Congratulations, you’ve now created a
tissue injury! Note that the tissue can
still bear a load, as it hasn’t been
completely ruptured (separated) , but as
a result of the injury it certainly
cannot return to its original shape.
This result is identified by the circled
2 in Figure 1. The moment the applied
force (solid line) reached the (tissue)
tolerance level (dashed line) the injury
occurred.
Demonstration #3:
You’ll need another
popsicle stick and this time you’ll
apply a large force very quickly so that
you can completely break the tissue into
two pieces. This result is identified
by the circled 3 in Figure 1. The
moment the applied force (solid line)
reached the (tissue) tolerance level
(dashed line) the injury occurred. Note
that although the magnitude of the
injury produced in Demonstration 3 is
more severe than in Demonstration 2, the
mode of injury is still the same.
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Figure 1:
An illustration of a high force mode
of tissue injury.
Low Repetitive Force Injury
These types of injuries are associated with repetitive
movements (e.g. Repetitive Strain
Injury). You can create this type of
injury by using a coat hanger and
performing the following
demonstration.
Demonstration #4:
Hold the coat hanger the same way you
did the Popsicle stick and try to
“injure” the tissue as you did in
Demonstration #3. The single high
force that you can produce just isn’t
enough to exceed the tissue
tolerance. To injure this tissue you
will need to repeatedly bend the coat
hanger back and forth. After a few
repetitions you should observe that
the metal is getting easier to bend
(softer) and that it is also getting
warm. These changing “tissue
properties” are illustrated in Figure
2 by the Tissue Tolerance curve
(dashed line) which decreases over
time. After a few more repetitions
you’ll eventually get to the point
where the applied load (solid line) is
greater than the tissue tolerance
(dashed line) and the coat hanger
(tissue) will become injured (break in
two) as identified by the circled 4.
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Figure 2:
An illustration of a low, repetitive
force injury
Low, Constant Force Injury
This type of injury is associated with holding an awkward
posture for a sustained period of
time. You can create this type of
injury by using Silly Putty (or
plumber’s putty), a coffee mug and
performing the following
demonstration.
Demonstration #5:
Roll the Silly Putty until you make
a snake about 8 inches long. Loop
the snake of putty through the
handle of the mug and make sure it
is thick enough to lift the mug.
Raise the mug about twelve inches
above the table and just hold it in
this suspended position. After a
few moments you’ll see that the
silly putty starts to stretch and
gets thinner in the middle. These
changing “tissue properties” are
illustrated in Figure 3 by the
Tissue Tolerance curve (red line)
which decreases over time. In this
example the Applied Load (solid
line) is constant (flat) because the
weight of the mug doesn’t change.
Eventually the putty (tissue) gets
so thin that it breaks due to the
weight of the mug as identified by
the circled 5.
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Figure
3:
An illustration of a low, constant
force injury
Putting it Into Perspective:
So how do Popsicle sticks, coat hangers and silly putty
relate to the real, working world? Many
of us have heard an injured worker
exclaim, “I’ve done this my entire life
without injury, why would I get hurt
now?” One of the big advantages that
humans have, unlike the inanimate
objects used in the demonstrations, is
that over time we can change the tissue
tolerance level. This can occur as a
result of rest (providing time for
tissues to recover back to their
starting level) or training (tissues
become stronger so tolerance level
increases). Also, humans have the
ability to heal once a tissue becomes
injured. However, a natural outcome of
the aging process is that tissue
tolerance decreases as we get older so
that eventually our tissue tolerance
levels will decrease compared to our
“younger years”. Figure 4 uses all of
these observations to help explain our
injured workers situation.
After a single shift of work, the
worker’s tissue tolerance has decreased
but there is still a safety margin due
to the applied load being less than the
tissue tolerance. An evening of rest
restores the tissue tolerance back to
its original level. After a week of
work we see the tissue tolerance has
decreased even further but there is
still a margin of safety present. A
relaxing weekend restores the tissue
tolerance but eventually it doesn’t
return to its original level. Then,
after a year or two, or perhaps well
into a career, the tissue tolerance has
decreased to a level where the applied
load, the size of which hasn’t changed
over this time period, is now at a point
where it exceeds the tissue tolerance
and an injury occurs!
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Figure
4:
An example of tissue
injury produced over a career due to
decreased tissue tolerance
Putting it Into Practice:
The next time you make a change in the
workplace try to identify the effect the
change will have on the safety margin
for the tissues in the body part of
concern. If the change is expected to
increase the safety margin how will this
be accomplished? Will, a) the tissue
tolerance increase, or, b) the applied
load decrease, or c) will it be a
combination of the two?
Knowing how the change will specifically
affect tissue injury will allow you to
provide this information as further
support for change and to explain the
impact of the change to those involved.
If your evaluation identifies that the
change isn’t going to have an impact on
either a) or b) then you should
re-evaluate the proposed change for it
appears that there is a good chance that
it may not produce the desired outcome.
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