Download Skeletal Muscle Mechanics

PHYSIOLOGY 1
LECTURE 18
SKELETAL MUSCLE
MECHANICS - MODEL
Skeletal Muscle
Mechanical Model
Objectives: The student should;

1. Learn the basic model of
skeletal muscle contraction.
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2. Learn about isometric and
isotonic tension development
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3. Learn that opposing muscle
groups imply that both muscles must
develop tension.
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4. Learn the relationship of
velocity of shorting to load.

Skeletal Muscle
Mechanical Model

A skeletal muscle contraction may be
considered in terms of a mechanical
model. The elements of the model
consists of contractile elements
(sarcomere), series elastic elements
(Tendons), and parallel elastic elements
(Connective tissue - endomysium).
Skeletal Muscle
Mechanical Model

Based on this model any muscle
contraction can be considered to
consists of four phases an isometric
contraction phase (muscle tension
development), an isotonic contraction
phase (muscle contraction), an isotonic
relaxation phase (muscle elongation),
and finally an isometric relaxation phase
(developed muscle tension is
dissipated).
Skeletal Muscle
Mechanical Model
A. Definition of terms
 Isometric - (SAME LENGTH) - Muscle
contractile machinery contracts but the
series elastic element stretches an
equal amount, therefore, the muscle
fiber itself does not change length.
This is the tension development or
dissipation phases of muscle
contraction.

Skeletal Muscle
Mechanical Model

Isotonic - (SAME TENSION) - The
tension developed by the muscle does
not change as the muscle fiber
contracts - Basically this is the
movement phases of muscle
contraction.
Skeletal Muscle
Mechanical Model


Isometric - (SAME
LENGTH) Muscle contractile
machinery contracts
but the tendons
stretch an equal
amount, therefore,
the muscle fiber
does not change
length.
Skeletal Muscle
Mechanical Model


Isotonic - (SAME
TENSION) The tension in the
muscle fiber remains
constant but the
fiber shortens as the
contractile
machinery continues
to contract.
Skeletal Muscle
Mechanical Model
B. Tension development does not
necessarily imply muscle contraction,
but may involve muscle lengthening
(Eccentric Contraction) or no movement
at all (Isometric Contraction).
 C. Velocity of contraction is dependent
on the load. The greater the load the
slower the contraction.

Skeletal Muscle
Mechanical Model



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Mechanical Model SEE - Series Elastic
Element (Tendons)
PEE - Parallel Elastic
Element
(Endomysium)
CE - Contractile
Element
(Sarcomere)
Skeletal Muscle
Mechanical Model Phases
Any muscle contraction consists of four
phases.
 1. Isometric Contraction Phase
 2. Isotonic Contraction Phase
 3. Isotonic Relaxation Phase
 4. Isometric Relaxation Phase

Mechanical Model Phases
Isometric Contraction Phase
During the isometric contraction phase
 SEE - Lengthens (Stretching tendons)
 CE - Shortens (Sarcomere shortens)
 PEE - Shortens (Endomysium is
compressed)
 Whole Muscle - Stays the same length

Mechanical Model Phases
Isometric Contraction Phase
Mechanical Model Phases
Isotonic Contraction Phase
During the isotonic contraction phase
 SEE - Stays the same length
 PEE - Shortens
 CE - Shortens
 Whole muscle shortens

Mechanical Model Phases
Isotonic Contraction Phase
Mechanical Model Phases
Isotonic Relaxation Phase
During the isotonic relaxation phase
 SEE - Stays the same length
 PEE - Lengthens
 CE - Lengthens
 Whole muscle lengthens

Mechanical Model Phases
Isotonic Relaxation Phase
Mechanical Model Phases
Isometric Relaxation Phase
During the isometric relaxation phase
 SEE - Shortens
 PEE - Lengthens
 CE - Lengthens
 Whole muscle stays the same length

Mechanical Model Phases
Isometric Relaxation Phase
Sarcomere Length - Tension
Relationship (Active Tension Curve)

III. The sarcomere length tension
relationship or active tension curve is
generated by placing a muscle in an
isometric condition, attached to a strain
gage then applying stretch to the
muscle fiber. By stimulation a muscle
contraction the tension developed for a
given degree of sarcomere stretch can
be measured.
Sarcomere Length - Tension
Relationship (Active Tension Curve)

These experiments result in a bell
shaped curve where the greatest
tension developed for skeletal muscle
occurs at the peak of the tension curve.
The drops in tension development on
either the left or the right hand limb of
the curve has to do with the numbers of
cross-bridges being formed, if the
sarcomere is too long no cross bridges
etc.
Velocity of Shortening - Load
Relationship
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IV. For any active muscle group it is clear
that at zero load the muscle is capable of
generating it”s greatest velocity of shortening
then it is also clear that at some point the
load will become so great that the muscle will
not be able to lift it. The velocity of
shortening to load relationship is not linear
due to pathway the weight must follow,
changes in the angle of the muscle to the
bone, and the elastic properties of the
muscle.
Mechanical Model
Summary
 1. What are the four phases of the
mechanical model, describe each
phase?
 2. What is an isotonic, isometric
contraction?
 3. What is an active tension curve,
velocity of shortening to load curve?
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