Physiology of Aging Muscle and Connective Tissue

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Physiology of Aging Muscle and Connective Tissue

• Jessie VanSwearingen, PhD, PT
• Associate Professor
• Department of Physical Therapy
• University of Pittsburgh
• School of Health and Rehabilitation Sciences

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Muscle Physiology Force Production
Motor unit motor neuron, motor axon, and all of
the muscle fibers innervated by the motor
axon Motor Unit Recruitment - to Increase
Force - increase the number of active motor
units - increase the firing rate of active motor
units (Hennemans Size Principle recruit small

before large)
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Body Composition Changes and Muscle Mass

• 60 of body K highest ratio of nitrogen

by tracing these ions, determined the protein
loss in aging is largely skeletal muscle
protein loss (Cohn et al, 1980)

• evidence suggests decrease in muscle mass
• with aging accounts for
• decrease in BMR
• decrease in VO2 max
• (BLSA, Tzankoff and Norris, 1978)

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Muscle Function Force Producing Capacity

• decreased muscle force production
• begins about 45 years of age
• more rapid gt 70 years 25-30 decrease
• usually lower extremities gt upper extremities

• decrease muscle force gt decline in cross
  sectional area(adductor pollicis, ankle plantar-
  and dorsi- flexors)

• Frontera et al (1991), corrected for muscle
  mass no difference in MVF / CSA for old
• versus young (knee
  flexors)

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Muscle Function Force Producing Capacity

• cadaver studies cross sections of entire vastus
  lateralis decrease of 10 in CSA, between 30-50
  years 25-30 decrease in CSA between 50-80 years
  (Lexall et al, 1988)

• CONCLUDE
• maximal force / unit area remains constant
• MESSAGE
• suggests the quality of the muscle
  intrinsically muscle fibers are able to produce
  
• force in old as in
  young

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Muscle Function Endurance Capacity

• endurance capacity appears preserved

• recovery of contractile properties after
  fatiguing work slower
• (Davies et al, 1983, 1984 Larsson, 1979)

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Muscle Morphology Fiber Type Distribution

• (previous lit.) muscle biopsy studies (Gollnick
  et al, 1972 Green, 1986, rev.)
• 10-30 increase in slow twitch fiber number
• selective fast twitch fiber loss

• (recent lit.) cadaver whole muscle studies
  (Lexall et al, 1988, 1989) surgical
  resectionings (Sato, 1984 Grimby et al , 1982,
  1984)
• no preferential loss of
  fiber type
• number with ageing

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Muscle Fiber Type Distribution

• total muscle fiber number reduction of about 25
  by 70 years (likely result of loss motor units)
• Brooks and Faulkner (1994) suggest motor unit
  loss leads to reinnervation, preferentially by
  slow motor neurons, with an increase in the
  proportion of slow versus fast muscle fibers
  (biopsy studies)

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Muscle Fiber Size
(biopsy and whole muscle cross sectional studies
Lexall et al, 1988) slow twitch
fiber (Type I) area maintained fast
twitch fiber (Type II) area decreased 25
between 20-80 years greatest loss in fast
fatigueable (Type IIb)
CONCLUDE decrease CSA related to decrease in
fast twitch fiber type size - (atrophy, loss of
muscle protein,blood, enzymes)
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Muscle Blood Flow

• Capillarization - few studies
• appears little changed in active old
• decreased in sedentary old

• decrease effectiveness of vasodilation with
  activity -- shunting of blood to active tissue
• (??decrease sensitivity to circulating
  norepinephrine and epinephrine)
• (??decrease ability of muscle to exchange
  metabolites across thickened basement membrane)

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Muscle Metabolic Activity

• ANNAEROBIC
• little change in glycolytic enzymes (3-15 or
  less)
• little change in high energy phosphates (CP)
• AEROBIC
• oxidative enzymes
• little or no change in active older people
• 20-40 decrease in sedentary older people
• (Coggan et al, 1992
• Meredith et al,
  1989)

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Microscopic Changes in Muscle

• sarcolemma leakage
• thickening of the sarcolemma
• disorganization of myofibrils

Little evidence for myopathic changes in aging
muscle.
(except dehydration, K moves out, muscle
function declines recovery from damage DOMS)
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Neuromuscular Changes in with Aging

• decrease number of motor units (25-30 decrease
  in spinal cord motoneurons)
• prolonged contraction time
• lower threshold firing rate (for remaining units)

Result
Increased EMG for a given
level of force production
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Review of Connective Tissue Physiology
Structure and Components cells -
fibroblast fibers - collagen elastin grou
nd substance - glycosaminoglycans, GAGs
(linked to protein proteoglycans) associated
proteins - fibronectin and laminin
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Aging Changes in Connective Tissues
in connective tissue cells few
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Aging Changes in Connective Tissues
in fibers collagen - decreased solubility,
reducible cross linkages stabilize, increased
rigidity elastin - decreased production,
increased fragmentation, rupture, loss of
rebound
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Aging Changes in Connective Tissues
in ground substance Aggrecan proteoglycan of
articular cartilage, binds a lot of water
changes in GAGS decreased
chondroitin-4-SO4 , changed to
chondroitin-6-SO4 increased
keratan SO4
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Aging Changes in Connective Tissues
Aggrecan - degradation of protein core
Hyalauron - smaller size, less link
protein Result more unbound GAGS - smaller
fragments, diffuse into joint fluid
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Aging Changes on Articular Cartilage Performance
loss of hydrostatic lubrication decrease
compressibility increase in subchondral
fractures inflammation - pain /
spasm infection ischemia
septicemia
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Aging Changes on Articular Cartilage Performance

• loss of boundary lubrication
• cracking and fibrillation
• disrupts binding of fibronectin and laminin
• exposes cartilage to degredative enzymes

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