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Flashcards in Post Midterm 2 Lecture Slides Deck (86):

What does it mean for a problem to be indeterminant?

Can't be solved without more information


What does an increase in moment arm do to the mechanical advantage?

Increases it.


Where do muscles get their mechanical advantage from?

Its moment arm?


Can many muscles rotate a single joint?



What is inverse dynamics?

Calculating the source from the outcome


What are the 3 solutions to solving muscle indeterminacy problems?

  1. Reduction
  2. Optimization
  3. EMG to force processing


explain reduction?

Reduce the number of unknowns by assuming some muscle forces are zero (nonsense!)


Explain optimization?

Assuming that the body tries to meet some performance criterion when it selects muscle forces.


explain EMG to force processing?

Use known mechanical properties of muscle to predict muscle force from electrical activity


What are the assumptions for the reduction method based on?

Assumptions usually based on measured electrical activity (EMG) in a muscle


What are the advantages of the reduction method? What are the disadvantages? (2)

  • Simplified mathematics compared to other methods
  • Co-contraction forces cannot be predicted
  • Assumption rarely holds for real movements


What type of function is chosen for the optimization method? what is meant by this? what is done to this?

  • "cost function"
  • Something the CNS would pay attention to when selecting muscle forces
  • Minimize (or maximize) cost function while satisfying        ΣM = I*alpha


What are some examples of optimization cost functions?

  • minimal total muscle force
  • minimal total squared muscle force
  • minimal total muscle stress (related mostly to injury)
  • minimal total ligament force (excessive force will tear the ligament)
  • minimal total joint force
  • minimal energy expenditure (most popular)


What is special about the force predictions that come from the optimization method?

Can produce force predictions that match EMG activity.


What are the disadvantages of the optimization? (3)

  • Mathematically complex
  • Best optimization criterion has not been identified
  • Optimization doesn't make sense in pathological movements


What does a muscle's force depend on? (3) what does it mean if these quantities can be measured?

  • Electrical excitation
  • Length
  • Shortening velocity

If these quantities can be measured, each muscle force can be predicted individually and an indeterminacy problem can be avoided.


What is motor redundancy? who came up with this concept?

  • Striking point was consistent while joint position varied with each strike.


An infinite number of what? describes indeterminacy?

number of solutions


What are the desirable qualities of muscle? (3)

  • Excellent force-to-weight ratio
  • Built-in safety mechanisms to prevent yielding
  • Self-repairing


What is an origin of a muscle?

proximal attachment of tendon to bone


What is a tendon?

end of the muscle that passively resists applied stretch; made primarily of collagen fibers


What is a muscle belly?

part of the muscle that actively generates force; innervated by motor neurons; also passively resists stretch


What is an insertion of a muscle?

Pistal attachment of tendon to bone


How are muscles classified?

  • Smooth, cardiac, skeletal
  • Fusiform (parallel fibered) or Pennate (pennation angle)


What does pennation allow for? What does pennation decrease?

  • Allows for a large number of fibers in a given volume
  • Decreases tendon force compared to muscle force by factor of cos of the angle (F x CosTheta)


Define mono-articular, multi-articular, and bi-articular.

  • Mono-articular: muscle crosses only one joint
  • Multi-articular: muscle crosses multiple joints
  • Bi-articular: muscle crosses two joints


What is the muscle fiber type split for the soleus and the brachioradialis?

Soleus: 80% slow, 20% fast

Brachioradialis: 40% slow, 60% fast


Do most large muscles have mixed fiber types?



What determines muscle fiber type ratios?

Genetics, with slight room for changes by training (debatable)


What is the order of hierarchy within muscle fibers?

  1. Muscle x 1
  2. Fascicle x 75
  3. Fiber x 1000
  4. Myofibril x 20,000


what is the muscles smallest force generating unit? what are the thick and thin filaments that compose this?

  • Sarcomeres
  • Thin: Actin
  • Thick: Myosin


Illustrate the arrangement of a sarcomere. (AI zone, AH zone, I band)

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What is the sliding filament theory? who proposed it? (2)

  • How muscle's contract based on the formation, rotation, and detachment of myosin-actin cross-bridges
  • Proposed in 1954 independently by H.E. Huxley
  • A.F. Huxley won the nobel price in 1963


What is Tetanus?

many muscle fiber twitches close together.

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What is an isometric contraction?

Muscle shortens but stretching of the tendon keeps the length the same


What is an isotonic contraction?

Allegedly it means the same muscle force, but professor shim says moment arm changes so muscle force changes as well.


What is an isokinetic contraction?

The same muscle contraction velocity?


What is a concentric contraction? what is an eccentric contraction?

  • Muscle shortening contraction
  • Muscle lengthening contraction


What does it mean for something to be linked in a series? give some examples. (2)

  • Force bearing elements are linked in series, the forces carried by each are the same
    • Sarcomeres within a myofibril
    • tendon-muscle-tendon


What does it mean for something to be linked in Parallel? what are some examples?

The total force is found by adding force in each

  • Fibers within a muscle
  • Collagen fibers in a tendon


What does "use it or lose it" imply?

When muscle is not used, muscle atrophy occurs.


What is muscle atrophy?

Decrease in cross-sectional areas of fibers; number stays consistent


What is hypertrophy? when does it occur?

  • Increase in cross-sectional areas of fibers
  • Only those fibers that are exposed to high tension experience hypertrophy


What is a less common response to training discussed in class?

Hyperplasia: number of fibers increase


What is exercise specificity? give an example? what does this suggest? (2)

  • Idea that exercise benefits are specific to the method and quality of training
  • Example: cyclists have higher VO2max when cycling than when running
  • Suggests that
    • Muscle adaptations can occur separately from cardiovascular adaptations
    • Training should be similar to the sport being trained for


What is a model? what are three examples? 

an approximation of reality

  • Animal
  • Cadaver
  • Mathematical


What considerations make models especially useful in biomechanics?

Ethical considerations


What is a mathematical model?

Uses equations to predict the behavior of a physical system (like a muscle)


What does T.A. McMahon mean when he says "Muscle is the only known piece of machinery which can be cooked in many ways."

muscles can be modeled in many ways.


A model is only valid for...

the range of data used to build it.


Which is better interpolation of extrapolation?



Explain the passive nature of muscle. (2) what causes this nature?

  • Passive (not contracting) muscle acts like a nonlinear spring
  • Pulling forces increases with applied stretch
  • Caused by collagen fibers that run parallel to muscle fibers becoming stretched tightly


What is active force related to? what does this depend on?

  • Number of crossbridges formed
  • Filament overlap


Explain the active force-length curve for muscle based on crossbridge overlap.

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Active + Passive = ...

Total Force-Length

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How were F-L curves determined? (4) How was the active F-L curve determined? all experiments are done in...


  • Experiments done on living animal muscles
  • Muscle held at constant length while force is measured
    • Passive: when muscle was not stimulated
    • Total: when electrical impulses were delivered to muscles
  • Active-force length curve determined by subtracting passive from total
  • statics


Describe quick-release experiments. (4) what do these experiments show?

  • Done in living muscle
  • Stimulated muscle supports weight
  • Weight suddenly reduced
  • shortening velocity recorded

Show that shortening muscle produces less force than isometric or lengthening muscle

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What is EMG? How is it measured? (2) how does it relate to F-L and F-V curves?

  • Electrical activity of muscle (Electromyographic activity)
    • Electro - electricity
    • Myo - muscle
  • Measured using electrodes
    • placed on skin (surface electrodes)
    • fine wire inserted into muscle with needle (intramuscular electrodes)
  • Active F-L, F-V curves are thought to scale with EMG


What is a motor unit?

Motor neuron on the muscle fibers that it innervates.


What determines the amplitude of a signal in the body?

Being threshold based, amplitude of signals in the body are determined by the frequency of the signal being sent.


who proposed the Hill-type muscle model? what is it used for? what are its three components?

  • Proposed by A.V. Hill in 1938
  • Accurate for shortening muscle w/ constant force
  • Three components
    • Series elastic (SE): a spring representing tendons and sarcomeres
    • Parallel elastic (PE): another spring representing connective tissue in muscle
    • Contractile element (CE): represents response to stimulation

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What is the Hill equation? What does it describe? Was does plotting the Hill equation give you?

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What is the viscoelastic effect? give an example? How is this concept relevant to biomechanics?

  • Observed when a material exhibits time-dependent force-length behavior
  • memory form mattress
  • Stretching exercises may reduce the stiffness of muscle


What is a muscle model?

Equations that predict force from EMG, length, and/or velocity


What is energy? What is the unit for energy?

  • The capacity to do work
  • Joule, 1 J of energy is needed to lift a 1N weight to a height of 1m.


What are five forms of energy? can it be converted between these forms?

  1. Mechanical (kinetic or potential)
  2. Chemical: stored in chemical bonds
  3. Light
  4. Heat
  5. Electricity



What has kinetic energy? what are the two types and their accompanying formulas?

  • Any body that moves or rotates has KE
  • Translational KE: 1/2*m*v2
  • Rotational KE: 1/2*I*omega2



What happens as a body loses KE? (3)

  • it is coming to a stop
  • Its KE is transformed to another form of energy
  • It does work on another body


What is potential energy? what are the two types and their accompanying equations?

  • Stored energy that does not require movement and depends on the position of the body
  • Gravitational PE depends on the mass and height above the floor.
    • PE = mgh
  • Elastic PE is stored in a bdoy that deforms but returns to its normal shape
    • PE = 1/2*K*x2, K constant is the stiffness of the spring


Explain the changes in kinetic energy of a ball thrown straight up into the air.

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Describe the energy transfer in walking.

Nearly inverse relationship between KE and PE when walking

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Explain the energy transfer of a bouncing ball

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Explain the energy transfer in running? At the lowest energy point in the graph where is energy stored? why is this important?

during running, the kinetic and potential energies are at their smallest at the same time.

  • in the tendons as elastic potential energy
  • Tendon stretch allows the body to save energy by giving the muscles less of a need to shorten, lengthen.

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Explain the 6 steps of energy transfer in a pole vault.

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What is power? what is the unit for power? what is joint power? what is joint power a product of? (2)

  • Rate at which energy is produced/used.
  • Unit is the watt; 1 W represents 1 J of energy created/used in 1 s
  • Joint power tells how fast energy is produced or absorbed by muscles crossing a joint
  • Product of angular velocity and joint movement;
    • Power = M*omega


Define a fluid. what satisfies this definition?

  • Any substance that continuously deforms in response to an applied shear stress (parallel to surface)
  • Gases and liquids both satisfy this definition


When applied shear stress, what is the difference between the water at the top of the application and the water at the bottom?

water at the surface moves more, and the water at the bottom moves less.


Define buoyant force.

upward force that acts on a submerged or partially submerged body


What is buoyant force magnitude equal to? Where does the buoyant force act?

  • Weight of displaced food
  • At the body's center of volume


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When is COV the same as COM?

when the density is constant throughout the entire body



What is drag? what is its formula (define all variables)

  • Force that resists motion through a fluid
  • Fdrag = 1/2*CD *Density*A*v 2
    • CD is coefficient of drag which depends on shape
    • A is projected area in direction of flow
    • v is relative velocity, determined as difference between body and fluid velocities


What does the Bernoulli principle state? What is the result of these differences? (2)

  • Areas of high velocity (v) flow are subject to low pressure (p) and vice-versa.
  • Velocity differences result in an airfoil passing through a fluid
  • Different shapes result in different velocities and pressures and cause lift to occur


is a curve ball path parabolic?



What force causes a curve ball to curve? explain how this force works.

  • Magnus force: spinning object moving through fluid medium can create transverse forces that alter its path


What are the two theories explaining how the magnus effect occurs in a baseball? how do stitches affect this?

  • Ball like a wing: similar to Bernoulli effect, velocity different translates into pressure difference, causing lift
  • Ball like a rocket: air is expelled downward, ball moves upward by third law
  • stitches enhance the magnus effect.

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