1. 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

2. 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
(Henneman’s Size Principle: recruit small
before large)

3. 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)

4. Muscle Function : Force Producing Capacity
decreased muscle force production
begins about 45 years of age
more rapid > 70 years; 25-30% decrease
usually lower extremities > upper extremities
decrease muscle force > 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)

5. Muscle Function : Force Producing Capacity
cadaver studies: cross sections of entire vastus lateralis decrease of 10% in CSA, between years 25-30% decrease in CSA between 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

6. Muscle Function: Endurance Capacity
endurance capacity appears preserved
recovery of contractile properties after fatiguing work slower
(Davies et al, 1983, 1984; Larsson, 1979)

7. 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

8. 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)

9. 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 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)

10. 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)

11. 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)

12. 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)

13. 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

14. Review of Connective Tissue Physiology
Structure and Components:
cells - fibroblast
fibers - collagen
elastin
ground substance - glycosaminoglycans, GAGs
(linked to protein =proteoglycans)
associated proteins - fibronectin and laminin

15. Aging Changes in Connective Tissues
in connective tissue cells: few

16. 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”

17. 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

18. 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

19. Aging Changes on Articular Cartilage Performance
loss of hydrostatic lubrication:
decrease compressibility
increase in subchondral fractures
inflammation - pain / spasm
infection ischemia
septicemia

20. 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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