Muscle Histology
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Transcript Muscle Histology
Aging & Skeletal Muscle
Fatigue
David W. Russ, PT, Ph.D.
Ohio University
School of Physical Therapy
Skeletal Muscle
“Without skeletal muscle, there is no
physical therapy.”
--Eugene Michels
Muscle Fatigue
Definitions:
#1 Change in
maximum forcegenerating capacity
of muscle with use
#2 Ability to
maintain required
or expected force
during repetitive
and/or prolonged
use – Task Failure
Muscle Fatigue: Definition 1
Maximum Effort
Or electrically-stimulated
Isolated muscle or
muscle group
Isometric or dynamic
Sustained or intermittent
Fixed time of exercise
Relative (percentage)
Degrees of fatigue
Top: Lanza et al, 2004
Bottom: Stevens et al, 2001
Muscle Fatigue: Definition 2
Typically submaximal
Potentially any
functional or exercise
task
Output is kept fixed,
time to task failure is
principal variable
Fatigue is binary
For certain protocols,
Definitions 1 & 2 can
be combined
Cheng et al, 2003
Loss of Force
Common factor in each definition
How is muscle force generated?
Pretty complicated…
Central Drive
Recruitment
Rate Coding
Peripheral
Excitation
Signal
Modulation
Ascending/
Descending
inputs
Crossbridge Formation
FORCE!
N.M. Transmission
T-tubule Propagation
Calcium Release
TnC Binding
Muscle Fatigue
“Fatigue makes cowards of us all.”
• V. Lombardi
Multiple sites of failure
Multiple potential mechanisms
Task specificity
Single mechanism not likely
Impact of Muscle Fatigue
Quadriceps strength
Transient loss of
was a significant
strength
factor in completion of
Reduced muscular
ADLs in 16 frail
endurance was
elderly (Brown, et al., 1995)
significantly
Lower extremity
associated with a
power positively
history of falls in
older women (Schwender predicted functional
et al., 1997)
independence in
community-dwelling
elderly (Bean et al., 2002, Suzuki et
al., 2001)
Functional Outcomes
Strength is associated with higher
performance on tests that are used as
predictors of function (6 min walk, Timed
get-up-and-go, etc.)
Petrella et al, 2004
Visser et al, 2000
Judge et al, 1996
Studies of Muscle Fatigue
Older subjects
(65-85 yrs)
Matched for
physical activity
Dorsiflexors
Isometric
• Submax – “ramp”
• MVC
• 50% and 70%
duty cycle
Dynamic
• Isokinetic (90 s-1)
Outcome measures
MVC force
• Also power for dynamic
Central Activation
Peripheral
Excitability/NMJ
Contractile Properties
Age-related
differences
Baseline
Changes with fatigue
Fatigue Data
Russ et al., 2008
Kent-Braun et al.,
2002
Lanza et al., 2004
Central Activation Testing
200
2.00
170
1.50
Force (N)
0.50
110
0.00
80
-0.50
50
-1.00
20
-10200
-1.50
700
1200
1700
2200
2700
-2.00
2.00
140
1.50
1.00
0.50
80
0.00
50
-0.50
-1.00
20
-1.50
-10200
1200
2200
3200
4200
-2.00
EMG (mV)
110
EMG (mV)
1.00
140
Force (N)
No study showed age-related
deficits
Testing is not simple to do in
the clinic
Peripheral Excitability
M-wave
Compound Muscle
Action potential
(CMAP)
Amplitude & Area
Again, age appears
unimportant
12.00
8.00
4.00
0.00
250
350
450
550
650
750
850
-4.00
-8.00
emg (mV)
6.00
3.00
0.00
70
-3.00
90
110
130
150
170
Contractile Properties
Stimulated contractions
Twitches or trains
Contraction time
Half-relaxation time
Maximum rates scaled
for force
• force development
(+df/dt)
• relaxation (-df/dt)
Twitch Potentiation
Contractile properties and
muscle fiber type
Contractile property data are consistent
with global shift to slow, Type I myosin
heavy chain
• Correlation between fatigue resistance and force
relaxation
Type I fibers are fatigue resistant
Also slower, reduce power
Evidence for increase in Type I fiber area
&/or number with age
Likely muscle specific
Generalizability
Healthy, older
subjects
Minimal medications
No co-morbidities
Sedentary, but
activity matched
Muscle specificity
Results corroborated
in other muscles
• Stevens et al 2001;
Allman & Rice, 2004
Few data in upper
extremities
Task specificity &
Function
Increased time to
task failure with age
(endurance)
• Hunter et al., 2005
May not relate to
whole body exercise
• Reduced cardiac
output with age
So why do older adults
complain of fatigue?
Reduced physical activity
Muscle oxidative capacity maintained
relative to young when activity is
comparable
Strength relative to function in the
environment
Although more fatigue-resistant, elders
are weaker (15-25% MVC deficits)
Absolute vs. relative tasks
Strength, Fatigue &
Functional performance
Younger subject
Quads produce
800N
Needs 300N to
stairs
Fatigues 60%
Can produce 320N
and still perform
task
Older Subject
Quads produce
400N
Needs 300N to
stairs
Fatigues 30%
Can only produce
280N – task cannot
be performed
Aging & Muscle Fatigue
Studies that control for physical activity
tend to indicate that older subjects
fatigue no more, and perhaps less than
young subjects.
Submaximal and functional fatigue tasks
may require a greater percentage of
exercise capacity of older subjects and
produce greater fatigue/earlier task
failure
Exercise Interventions
Endurance Exercise:
May not be an issue from the aspect of
muscle fatigue
• Older muscle tends to be fatigue resistant
• Will mitigate the effects of disuse, but probably not
aging per se
Plenty of other good reasons to do it
• Cardiovascular
• Insulin Sensitivity
Strength training is probably more of an issue
Exercise Interventions
Focus on strength not size
Sarcopenia is real
However weakness tends to exceed loss of
mass
Capacity for hypertrophy persists with age
• Blunted – work more for smaller gains
• Resistance exercise increases mixed muscle
protein synthesis (Balogapal, et al., 2001; Hasten et al., 2000; Yarasheski et
al., 1993)
• “Functional Resistance” protocol increased
myofibrillar area (Cress et al., 1996)
Central Drive
Recruitment
Rate Coding
Peripheral
Excitation
Signal
Modulation
Ascending/
Descending
inputs
Crossbridge Formation
FORCE!
Potential
areas of
action
Calcium Release
TnC Binding
N.M. Transmission
T-tubule Propagation
Many Thanks
Jane Kent-Braun
Ian Lanza
Danielle Wigmore
Ted Towse