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Psy 552 Ergonomics &
Biomechanics
Lecture 5
Energy for Muscles

Energy for muscle contractions if provided for
by the breaking down of adenosine triphosphate to adenosine diphosphate ADP.

When the connecting bond is broken, energy
is released. This energy is used to rotate the
myosin head.
ATP sources
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ADP returns to ATP by a reaction with
phosphocreatine – another high energy
phosphate.

ATP and phosphocreatine energy stores are
depleted in 15-30 seconds of strenuous
exertion.
ATP sources (cont.)

ATP is generated using two processes:

Anaerobic metabolism of sugar (glucose) which
provides energy for 30 to 90 seconds. This
process produces lactate as a by product

Aerobic metabolism (aka Krebs or citric acid
cycle) which provides energy stores for activity
lasting longer than 90 seconds. This process
yields water and CO2 as by products
ATP sources (cont.)

During moderate activity levels, the oxygen
supply is sufficient to created the needed ATP
– a condition called a steady state

At high activity levels, there can be
insufficient oxygen that cause ATP production
via the anaerobic process yielding an oxygen
debt and muscle fatigue.
Types of Muscle Contractions
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Isometric (aka a static load): muscle length
does not change during a contraction.

Isotonic (concentric shortening) a very rare
type of contraction in the workplace. Here the
muscle length does change but the load
remains constant. While it is true the load is
present, changes in geometry change the
consistency of the load.
Muscle Contractions (cont.)

Eccentric contraction (aka lengthening
contraction) occurs when the external force is
greater than the internal force as occurs when
lowering a load. The muscle control but does
not initiate the movement.
Muscle Contractions (cont.)

Isokinetic (aka constant force) muscle
contractions where motion velocity is kept
constant

Isoinertial contraction: a contraction against a
constant load where the measurement system
considers acceleration and velocity.
Muscle response to stimulation

Twitch: occurs when a muscle is stimulated by a
single nerve action potential

Latent period: the interval between the stimulation
(action potential) and the contraction
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Contraction period: the time of muscle shortening

Relaxation period: the time the muscle lengthens to
a resting state.
… response to stimulation (cont.)

The response of a muscle depends on:
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The size and frequency of the stimulus
The fiber composition of the muscle
The length of the muscle
The velocity of the muscle contraction
Size and frequency
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As neural stimulation increases, additional
motor units will be recruited until a maximum
contraction is achieved.

If a second nerve impulse is delivered before
the end of the prior impulse a greater
contraction force will be created. A process
called temporal summation
Size and frequency (cont)

A maximal contraction, call tetanus, occurs
when the frequency of impulses reaches its
maximum.

Max frequencies vary
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300/second for the eye muscles
30/second for the soleus, calf muscle
Muscle fiber composition
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There are two major muscle fiber types

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Slow-twitch (Type I)
Fast-twitch (Type II)

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Fatigue resistant Type IIA
Nonfatigue resistant Type IIB
Type I muscle fibers

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Are smaller (e.g., soleus)
Maintain high capacity for aerobic
metabolism
Good at low levels of exertion over short
periods of time.
Have low peak tensions
Have long rise time to peak tension
Type II muscle fibers
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The bicep brachii is a type II muscle fiber
They rely on anaerobic metabolism
Have large peak tensions
Have short rise times to peak tension
Have short peak durations
Are associated with high intensity activity
Muscle length

The ability to contract is directly related to the
cross bridging of actin and myosin fibers.

The maximum number of cross bridges exists
when the muscle is in approximately the
resting position.
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No tension is created when there is no overlap
Muscle length (cont.)

Tension reduces when the muscle shortens
and there is an overlap between the actin
fibers on the opposite side of the sarcomere.

The tension a muscle can produce is also
dependent on the stretch of connective tissue.
Velocity
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The tension reducing capabilities of a muscle
decrease with increased velocity because:
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Inefficient coupling of cross-bridges as filaments
move passed one another
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Fluid viscosity of the muscle causes viscous
friction
Muscle fatigue
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Serves as an injury prevention function
Has several causes. The ultimate cause of
fatigue, however, is very complex.
Causal factors include:

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Energy depletion
Accumulation of lactates
Lack of motivation
Static loads and fatigue

With a static load, blood can be excluded
from a contracted muscle
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The intramuscular pressure associated with a
static contraction of the quadricep at 25% of the
maximum voluntary contraction exceeds the
systolic blood pressure -- no blood flows into the
muscles
Static loads and blood flow

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At 20 to 30% maximum voluntary contraction
(MVC) blood flow will increase in response
to the contraction.
At > 30% MVC, blood flow decreases to the
muscle
At 70% MVC, blood flow to the muscle stops
Blood flow and fatigue
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Without blood flow we get:
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Increased heat
Inadequate oxygen
No removal of CO2
Lactate accumulations
Muscle Arrangements
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Muscle groups are arranged in groups.
Simply put, there are 2 types of muscles
functions:
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Agonists – prime movers
Antagonists that relax when the agonist contracts
Several muscles often work together as
synergists. One might stabilize a joint while
another moves the distal end.
Muscle Arrangements (cont.)
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Muscle forces can be divided into 2 force
vectors
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One moving parallel to the bone -- produced by
shunt muscles that cause compression at the joint
and promote joint stability
One moving perpendicular to the bone –
produced by spurt muscles that cause rotation of a
limb around the joint.