Muscle & Body Biomechanics
MUSCLE FIBER ARRANGEMENT
EFFECT ON STRENGTH
The amount of force that a
muscle can generate is proportional to the cross-sectional area of muscle
fibers (a.k.a. muscle cells) attaching to its tendon, i.e., the number of
contractile proteins (actin and myosin) pulling on the tendon and contributing
to
muscle force.
• pennation design increases the number of
muscle fibers (cross sectional area) attached to the tendon
• since force is a function of cross sectional
area - a pennated muscle can generate more force than a comparable muscle with
parallel fibers.
EFFECT ON SHORTENING
In this example, again
consider two muscles - one with parallel fibers the other pennate
Assume each muscle fiber will
contract to 50% of its resting length
Therefore:
• with parallel-arranged muscle fibers the
entire muscle can contract by 50%
• with the pennate arrangement each individual
muscle fiber is pulling at an angle, resulting in reduced overall shortening of
the entire muscle belly.
DEFINITIONS:
LINEAR FORCE:
Force can be broken down into
various vectors.
• Vertical vectors (e.g. the downward forces
due to body weight and the upward forces of the supporting surface)
• Horizontal vectors (e.g. forces exerted to propel
forward and
backward forces to brake
forward motion)
With adequate force and
friction (traction) the body can propel itself forward. (practical application dictates a need for
good traction to allow this forward motion – whoa to those leading a horse on
ice)
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ROTATIONAL FORCE (TORQUE)
Rotational force = force (F)
x distance from fulcrum (d)
Limb rotation = muscle force
(F) x distance from joint (d)
Torque input (muscle
generated) = torque output (limb movement)
Muscles generate forces which
when applied to the skeleton will generate rotation about a joint.
MUSCLE ATTACHMENT EFFECTS
The location of the muscle
attachment (e.g. distance from joint) influences the resultant movement of that
joint
MECHANICAL ADVANTAGE VERSUS VELOCITY ADVANTAGE
Muscles that attach further
from the joint have a mechanical advantage over muscles attached closer to the
joint In the diagram below if muscles #1 and #2 were of equal strength (i.e.,
can generate the same force) then muscle #2 could produce a greater rotational
force because its attachment is at a greater distance from the joint
(rotational force = muscle force X distance from joint).
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Conversely muscles
that attach close
to the point of
rotation are able
to produce faster movement of
the lever arm
than muscle that attach farther from the fulcrum. In
the diagram to
the right if muscle
#1 and muscle #2
both contract 10% during
an identical time period - muscle
#1’s contraction would result in a larger movement of the
lever arm during that
same frame of
time than muscle #2.
In other words,
muscle #1 will result in a more rapid rotation - it has
a velocity
advantage.
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Muscles attaching close to
the joint with their velocity advantage are termed “high gear” muscles and
those with a more distal attachment resulting in a mechanical advantage are
termed “low gear” muscles.
It may be helpful to consider
a similar gear analogy as in a car or bike.
At low gears the output force is relatively large – allowing the vehicle to climb up a steep
hill. High gears on the other hand
generates a lot of speed – as would be
advantageous in passing a vehicle.
JOINT POSITIONING EFFECTS
THE BODY’S LEVER SYSTEM
Unique skeletal features
result from functional adaptations over time.
In the figure below - the
upper diagram is an example of an animal that uses it’s front limbs for
digging; the muscles attached to the point of the elbow (olecranon) are positioned
further from the elbow joint (fulcrum of movement) thereby generating large
forces for digging.
The lower diagram with
muscles attaching closer to the elbow joint is an runner adaptation that can result
in a rapid rotation with muscle contraction
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For the same force and
velocity input (left arrows), note the
relative magnitude of bolded
arrows to the right of the diagrams – a large downward force (F) is generated in
upper diagram and
rapid rotation (V; velocity)
of movement is produced in the
lower diagram.
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