Muscle

1
Muscle

• Three types of muscle
• smooth
• cardiac
• skeletal

All muscles require ATP to produce movement.
Thus, muscles are chemotransducers
2
Skeletal Muscle

• Muscle organization
• Muscle innervation
• Architecture and structure
• Excitation-contraction
• Fiber type characteristics
• Training adaptations
• Exam 1 (Feb 8)

3
Skeletal muscle organization

• Connective tissue layers
• Epimysium
• Perimeysium
• Endomysium

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Muscle fiber covering

• Sarcolemma
• basement membrane
• plasma membrane
• Plasma membrane has
• membrane receptors
• ion channels
• integrins
• satellite cells
• multinuclei

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Muscle Architecture

• Effect on force output and shortening velocity

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Muscle Architecture
Muscle architecture
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Muscle Architecture
Parallel
Unipennation
Multipennation
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Pennation Effect on Physiological
Cross-sectional Area (PCSA)

• Greater PCSA when fiber is at angle to line of
  force

B
A
A
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Pennation Effect on Force and Shortening
Distance/Velocity
Fiber B
Fiber A
Equal number of sarcomeres in both examples, but
Fiber A has longer fiber and smaller PSFA than
Fiber B, which allows for greater shortening
distance/velocity at sacrifice of force.
12
Identify which muscles are best suited for force
for speed
A
B
C
D
13
Muscle Architecture

• quadriceps and planter flexors designed for force
  production
• larger pennation angles
• large PCSAs
• hamstrings and dorsiflexors designed for velocity
• smaller pennation angles
• intermediate PCSAs

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Muscle Architecture

• Summary
• Muscles designed to fit purpose of joint
• Muscles designed for velocity have longer fiber
  length and small pennation angle
• Muscles designed for force have shorter fiber
  length and larger pennation angle

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Review questions

• Describe the difference between a muscle with a
  fusiform architecture and one with a uni- or
  multipennate architecture. Identify a muscle for
  each type of architecture.
• Discuss how muscle architecture affects force
  output and shortening velocity.  Provide a
  general explanation as to why some muscles are
  designed more for rapid shortening velocity (e.g.
  hamstrings) or higher force output (e.g.
  quadriceps muscles).

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Muscle Innervation

• Motoneurons, neuromuscular junctions, motor units

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Motoneurons

• muscle fibers innervated by large (alpha)
  myelinated nerves
• motoneurons originate from spinal cord
• nerve ending ends at neuromuscular junction
• motor unit composed of motor neuron and all the
  fibers it innervates

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Action Potential

• depolarization influx of Na
• repolarization efflux of K
• refractory period hyperpolarization
• threshold level minimal stimulus required to
  elicit response
• muscle and nerve follow all or nothing principle

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20 0 -20 -40 -60 -80
Membrane potential (mV)
Time (ms)
K
K
K
Na
Na
K
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na channel
K channel
Na-K exchange pump
ATPase
K
K
K
K
K
K
K
K
K
K
ADP
K
K
K
K
K
K
Na
Pi
Na
Na
Na
intracellular
ATP
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Neuromuscular Junction
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Electromyography (EMG)
Describe the relative weights being lifted
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Review questions

• Define the motor unit.
• Describe the events that occur as an action
  potential approaches the nerve terminal.
• Explain the purpose of acetylcholinesterase and
  the consequences of its absence.
• A common agent found in flea powders is a low
  dose of an antiacetylcholinesterase inhibitor.
  Explain the effects that the flea powder would
  have on fleas.
• Explain the interpretation of an EMG tracing.

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Sarcomere Structure
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Skeletal Muscle Structure
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Cross-Sectional View of Skeletal Muscle (X40)
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Skeletal Muscle Structure

• sarcomeres (smallest functional unit) are linked
  end-to-end to form myofibrils
• myofibrils are bunched to form fibers
• sarcomeres are composed of thick and thin
  filaments

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Scanning EM
1
4
2
5
3
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Thick Filament

• composed of numerous myosin protein strands
• flexible heads protrude outward all around
  filament (except center)
• myosin heads attach to active sites on actin
  (thin) filament
• myosin heads contain ATPase to break down ATP

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Myosin filament
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Myosin Filament
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Thin Filament
Composed of three proteins

• actin - two protein strands twisted around each
  other, contain active sites
• tropomyosin - thin strand laying in actin groove
  that covers active sites
• troponin - attached to actin and tropomyosin
  strands has strong affinity for Ca2

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Thin Filament
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Cytoskeleton (structural) proteins

• M-band located in middle of thick filament
  provides structural support to myosin filaments
  contains creatine kinase (CK)
• Titan connects myosin filament to Z-disk
  stabilizes myosin in middle of sarcomere.
• Z-disk thin filaments attachment composed of
  several cytoskeletal proteins

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Actin-myosin orientation
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Transverse Tubule

• in human skeletal muscle, each sarcomere has two
  transverse tubules running perpendicular to fiber
  
• T-tubules extend through fiber and have openings
  at sarcolemma allowing communication with plasma
• cardiac fibers have only one T-tubule which lies
  at Z-line

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Sarcoplasmic Reticulum (SR)

• made up of terminal cisternae and longitudinal
  tubules
• serves as a storage depot for Ca2
• terminal cisternae abut T-tubules
• longitudinal tubules cover myofibrils and connect
  terminal cisternae

40

• 1. On what component does Ca2 bind to?
• Sarcoplasmic reticulum
• Myosin heads
• Troponin
• Tropomyosin

• 2. What protein returns Ca2 to the sarcoplasmic
  reticulum?
• Myosin head
• Ca2 pump
• Ca2 channels
• tropomyosin

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Review questions

• Describe the myosin filament of a skeletal muscle
  fiber.  Include a detailed description and
  function of the myosin head.
• Describe the thin filament of a skeletal muscle
  fiber.
• Describe the cytoskeleton proteins and their
  functions in the sarcomere.
• Describe the sarcoplasmic reticulum and its role
  in excitation-contraction.

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Excitation-Contraction

• How muscle contracts

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Excitation-Contraction Coupling

• action potentials, generated at neuromuscular
  junction travel around sarcolemma and through
  T-tubules
• T-tubules signal SR to release Ca2 into
  sarcoplasm (cytosol)
• Ca2 saturates troponin (in non-fatigued state)
• troponin undergoes conformational change that
  lifts tropomyosin away from actin filament

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E-C Coupling (cont.)

• myosin head attaches to active site on actin
  filament
• after attaching to actin, myosin head moves
  actin-myosin complex forward and releases ADP and
  Pi
• ATP binds with myosin head, which releases actin,
  and returns to original position
• in resting state, myosin head contains partially
  hydrolyzed ATP (ADP and Pi)

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E-C Coupling Schematic
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E-C Coupling (cont.)

• entire cycle takes 50 ms although myosin heads
  are attached for 2 ms
• a single cross-bridge produces 3-4 pN and
  shortens 10 nm
• as long as action potentials continue, Ca2 will
  continue to be released
• when action potentials cease, SR Ca2 pumps
  return Ca2 ceasing contractions
• skeletal motor units follow all or nothing
  principle

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• Excitation-Contraction
• AP causes vesicles to release Ach
• Muscle AP travels down t-tubules
• SR releases Ca2 into sarcoplasm
• Ca2 binds to troponin
• Myosin heads bind to actin mysoin ATPase splits
  ATP
• ATP binds to myosin heat releases from actin
• Crossbridge action continues while Ca2 is
  present
• When AP stops, Ca2 pumped back to SR
• Tropomyosin covers active sites

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EC Coupling

• QuickTime Movie of sliding filaments
• http//www.sci.sdsu.edu/movies/actin_myosin.html
• Click on Link
• Click on Actin Myosin Crossbridge 3D Animation

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• 3. What will happen if ATP is depleted in muscle?
• Nothing
• Muscle will relax
• Muscle will not relax
• 4. What will happen if sarcoplasmic reticulum of
  fiber is enhanced?
• Fiber will develop tension more quickly
• Fiber will relax more quickly
• Nothing
• Both a and b will occur

51
Review questions

• Discuss the signaling process of the T-tubules
  that leads to Ca2 release by the sarcoplasmic
  reticulum. 
• Describe ATP hydrolysis by the myosin filament. 
• Discuss factors that could affect the rate of ATP
  hydrolysis by the myosin head as well as factors
  that affect tension development.

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Skeletal Muscle Fiber Types

• generally categorized by histochemical criteria
• innervating nerve is primary determinant of fiber
  type
• motor units composed of homogenous fibers
• all human muscles contain mixture of three
  general fiber types
• slow twitch (ST, oxidative, red, Type I)
• fast twitch (FTa, fast-oxidative, white, Type
  IIa)
• fast twitch (FTb, glycolytic, white, Type IIx
  often called IIb)

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• stained for myosin ATPase (pH 10.3) (dark
  stained)
• stained for myosin ATPase (pH 4.3) (light
  stained)
• stained for SDH
• (dark stained)

Type I
Type IIa
Type IIx
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Muscle Twitch Characteristics

• frontalis/orbicularis oculi (15 ST)
• first dorsal interosseous (57 ST)
• soleus (80 ST)
• extensor digitorum brevis (60 ST)

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Fiber Type Characteristics
Performance characteristics affected by

• size of motoneuron
• size of muscle fibers
• amount of SR
• Ca2-ATPase
• myosin ATPase
• aerobic capacity (amount of mitochondria)
• anaerobic capacity (amount of glycolytic enzymes)

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Be able to explain the differences in the force
responses between motor units.
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Fiber Type Performance Characteristics
Compare fiber types responses for the following
AND provide a reason for your response

• (absolute relative) force output
• time-to-peak tension
• relaxation time
• shortening velocity
• fatigability

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• 5. Which fiber reaches peak tension most
  quickly?
• Type I
• Type IIa
• Type IIx
• 6. What is the reasoning for your response to
  Q5?
• faster myosin ATPase
• more Ca2 channels
• more Ca2 pumps
• faster action potentials
• none of the above are correct

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Exam 1 Thu, Feb 8

• Begin preparing for exam NOW!
• Use posted learning objectives as basis for
  studying
• Read text to clarify material
• Initially, study by self, then study with
  classmates
• Teach each other course material question
  accuracy/completeness of others explanations
• See me if questions remain
• You may start the exam at 745 am
• Bring the medium-sized RED scoring sheet (sheet
  that enables you to bubble in your name)

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Motor Unit Recruitment Pattern Size Principle
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Quiz 1

• c
• b
• e
• b
• No ATP available for myosin head to detach from
  actin.
• c
• d
• a
• a

• ST fibers have less SR, thus Ca2 release and
  uptake are slower.
• c
• a
• e
• Force was decreasing.
• Decreasing EMG represents decreasing motor unit
  recruitment.

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Muscle Movements

• isotonic develops tension while changing length
• isokinetic resistance to muscle changes with
  muscle length to ensure equal tension development
• isometric (static) develops tension but no
  length change
• concentric develops tension while shortening
• eccentric develops tension while lengthening

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Muscle Performance Characteristics
Force and power development dependent on

• number of muscle fibers recruited
• muscle architecture
• angle of pull
• length of fiber
• velocity of shortening
• load place on muscle

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Length-Tension Relationship
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Length-Tension Relationship
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How sarcomere length affects force outputThis
explains the length-tension relationship
68

• At which length would force output by the biceps
  muscle be greatest?
• When the arm is in full extension
• When the arm is flexed at 90-100º
• When the arm is at full flexion
• Strength (force) would be the same throughout the
  entire range of motion

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Force-Velocity Relationship
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• How would the EMG activity to a leg squat during
  the lowering (eccentric) phase compare to the
  upward (concentric) phase.
• EMG activity would be the same for both phases.
• EMG activity would be greater for the concentric
  phase.
• EMG activity would be greater for the eccentric
  phase.

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EMG comparison of concentric and eccentric actions
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Muscle Spindles (sensitive to stretch)
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Golgi tendon organs(sensitive to strain)
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Resistance Training Adaptations

• dependent on neural and physiological adaptations
• training specificity determines adaptations

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Strength Training Adaptations
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Neural Adaptations

• increased motor unit recruitment
• decreased neural inhibition of motor unit
  recruitment
• decreased antagonist muscle recruitment
• increased neural coordination

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Muscle Fiber Adaptations

• increased fiber size (both types)
• increased hypertrophy (1º)
• increased hyperplasia (2º)
• occurs more to FT fibers than ST
• little or no change of fiber types
• testosterone explains only part of larger muscle
  mass in males

78

• How does
• an untrained individual increase strength?
• a trained individual further increase strength?
• neuromuscular adaptations
• hypertrophy
• both neuromuscular adaptations and hypertrophy

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Exercise-Induced Muscle Damage and Soreness
Unaccustomed exercise stimulates sequence of
events that

• diminishes performance
• causes ultrastructure damage
• initiates inflammatory reaction
• causes delayed-onset muscular soreness (DOMS)

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Muscle Damage/Repair Overview

• damage occurs during lengthening (eccentric)
  movements
• damage commonly occurs to sarcolemma, Z-disk
  (streaming), T-tubules/SR, myofibrils,
  cytoskeleton
• initial muscle damage followed by
  inflammatory-induced damage
• produces muscle swelling
• affects FT fibers more than ST fibers
• repair begins 3 d post-exercise

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Z-line streaming
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Muscle Fiber Damage Sarcolemma damage
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Exercise-Induced Muscle Damage

• extent of injury more related to length than
  force or velocity
• weaker fibers become overstretched, which become
  damaged (Morgan, 1990)

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Elastic filaments only linking thick filaments
Total tension is 80 of maximal tension
sarcomere is on descending limb of length-tension
relation.
Popping-Sarcomere Hypotheses
Additional elastic element
When half of sarcomere is over-stretched, tension
is increased on additional elastic element, which
increases passive tension.
Proske Morgan, J Physiol, 2001
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Stages of Muscle Damage
1. During exercise

• Mechanical (strain) damage results in
• sarcolemma damage
• SR damage
• myofibrillar damage
• Ca2 influx

2. After exercise
Inflammatory response causes
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Effects of Elevated intracellular Ca2

• activates proteases
• damages cytoskeleton proteins
• activates phospholipases
• generates free radicals
• damages plasma membranes

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Acute Phase Response

• Promotes clearance of damaged tissue and
  initiates repair
• ? circulating neutrophils (w/in 1-12 h) and
  monocytes (w/in 1-3 d)
• enters injury site and phagocytizes damaged
  tissue
• release cytotoxic factors (e.g., oxygen radicals)

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Typical Times of Peak Effects

• Ultrastructural damage ? 3-d postexercise
• DOMS ? 1-2 d postexercise

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Effects of Eccentric Arm Curls(on a scale of 0
to 6)
Kolkhorst et al., ACSM, 2003
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Effects of Eccentric Arm Curls
Kolkhorst et al., ACSM, 2003
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CK from 60-min Downhill Running
Kolkhorst, unpublished observations
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Effects on Performance/Soreness

• greater damage to FT fibers
• prolonged strength loss
• primary cause ? failure of SR-Ca2 release
• ultrastructure damage secondary cause of strength
  loss
• muscle swelling/DOMS
• DOMS caused by tissue breakdown products that
  sensitize pain receptors

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Muscle Repair

• macrophage infiltration required for activation
  of satellite cells
• satellite cells located between basement membrane
  and plasma membrane
• in response to signal from injury site, satellite
  cells migrate to injury
• differentiate into myoblasts, which fuse into
  myotubes

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Muscle repair
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Immediately after crush injury
2 days

• At 2 d, damaged fibers have undergone necrosis,
  with digestion/removal by macrophages.
• At 5 d, several newly formed myotubes are
  visible.
• At 10 d, myotubes have transformed into fibers,
  many of which have linked up with fibers stumps
  on either side.

5 days
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Adaptation to Eccentric Exercise

• adaptation occurs w/in 1 week
• ? number of sarcomeres?
• increases fiber length,
• Allows sarcomere to work at shorter lengths

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Quiz 2

• e
• d
• Increased motor unit recruitment
• Measure EMG during max lift before and after
  training. Post-training EMG should be greater.
• e
• c
• d

• a
• d
• c
• Maximal number of cross-bridges occur at this
  muscle length
• b
• c
• If fiber contracts, it develops its maximal
  tension
• Yes, but fibers develop more tension during
  eccentric movement

98

• Which type of activity would likely cause the
  most severe DOMS or muscle damage?
• level running (involves about half concentric and
  half eccentric movements)
• rowing exercise (involves mostly pulling motion,
  a concentric movement)
• running down stadium stairs (involves more
  eccentric than concentric movements)
• cycling (entirely concentric movements)
• none of the above would cause DOMS

99

• Eccentric exercise
• causes the greatest damage at the shortest muscle
  lengths.
• causes the greatest damage to ST fibers.
• initiates an inflammatory response that causes
  further myofibril damage.
• stimulates macrophage infiltration to the damaged
  area, which is essential for muscle repair.
• both c and d are correct

100

• The greater the load placed on a muscle during a
  shortening movement, the _____ it can shorten.
  This illustrates the _____ relationship of
  skeletal muscle mechanics.
• slower power-load
• slower length-tension
• faster length-tension
• slower force-velocity
• faster force-tension

101

• According to the Force-Velocity relationship, how
  does force output of a fiber when shortening
  compare to when it is forced to lengthen?
• force output is greater when it is allowed to
  shorten
• force output is equal regardless of shortening or
  lengthening
• force output is less when it is allowed to shorten