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Psy Ergonomics Biomenchanics

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					Psy 552 Ergonomics &
Lecture 5
Energy for Muscles
   Energy for muscle contractions if provided for
    by the breaking down of adenosine tri-
    phosphate to adenosine diphosphate ADP.

   When the connecting bond is broken, energy
    is released. This energy is used to rotate the
    myosin head.
ATP sources
   ADP returns to ATP by a reaction with
    phosphocreatine – another high energy

   ATP and phosphocreatine energy stores are
    depleted in 15-30 seconds of strenuous
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
   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

   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

   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:

       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
   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

       Max frequencies vary
           300/second for the eye muscles
           30/second for the soleus, calf muscle
Muscle fiber composition
   There are two major muscle fiber types
       Slow-twitch (Type I)
       Fast-twitch (Type II)
           Fatigue resistant Type IIA
           Nonfatigue resistant Type IIB
Type I muscle fibers
   Are smaller (e.g., soleus)
   Maintain high capacity for aerobic
   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
   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.

   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.
   The tension reducing capabilities of a muscle
    decrease with increased velocity because:
       Inefficient coupling of cross-bridges as filaments
        move passed one another

       Fluid viscosity of the muscle causes viscous
Muscle fatigue
   Serves as an injury prevention function
   Has several causes. The ultimate cause of
    fatigue, however, is very complex.
   Causal factors include:
       Energy depletion
       Accumulation of lactates
       Lack of motivation
Static loads and fatigue
   With a static load, blood can be excluded
    from a contracted muscle
       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
Static loads and blood flow
   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
   At 70% MVC, blood flow to the muscle stops
Blood flow and fatigue
   Without blood flow we get:
       Increased heat
       Inadequate oxygen
       No removal of CO2
       Lactate accumulations
Muscle Arrangements
   Muscle groups are arranged in groups.
    Simply put, there are 2 types of muscles
       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.)
   Muscle forces can be divided into 2 force
       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.

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