aging & skeletal muscle fatigue david w. russ, pt, ph.d. ohio university school of physical...
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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 FatigueDefinitions:
#1 Change in maximum force-generating 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 fatigueTop: Lanza et al, 2004Bottom: 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 ForceCommon factor in each definitionHow is muscle force generated?Pretty complicated…
Central DriveRecruitmentRate Coding
Signal Modulation
Ascending/DescendinginputsPeripheral
Excitation
N.M. TransmissionT-tubule Propagation
Calcium ReleaseTnC Binding
Crossbridge Formation
FORCE!
Muscle Fatigue
“Fatigue makes cowards of us all.”• V. Lombardi
Multiple sites of failureMultiple potential mechanismsTask specificitySingle mechanism not likely
Impact of Muscle Fatigue
Transient loss of strength
Reduced muscular endurance was significantly associated with a history of falls in older women (Schwender et al., 1997)
Quadriceps strength was a significant factor in completion of ADLs in 16 frail elderly (Brown, et al., 1995)
Lower extremity power positively predicted functional 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
Russ et al., 2008
Lanza et al., 2004Kent-Braun et al., 2002
Fatigue Data
Central Activation Testing No study showed age-related
deficits Testing is not simple to do in
the clinic-10
20
50
80
110
140
170
200
200 700 1200 1700 2200 2700
Forc
e (N
)
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
EMG
(mV)
-10
20
50
80
110
140
200 1200 2200 3200 4200
Forc
e (N)
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
EMG
(mV)
Peripheral Excitability
M-wave Compound Muscle
Action potential (CMAP)
Amplitude & Area Again, age appears
unimportant
-8.00
-4.00
0.00
4.00
8.00
12.00
250 350 450 550 650 750 850
emg (mV)
-3.00
0.00
3.00
6.00
70 90 110 130 150 170
Contractile PropertiesStimulated contractions
Twitches or trainsContraction timeHalf-relaxation timeMaximum 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 resistantAlso slower, reduce power
Evidence for increase in Type I fiber area &/or number with ageLikely 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 environmentAlthough 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 InterventionsFocus on strength not size
Sarcopenia is realHowever weakness tends to exceed loss of
massCapacity 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 DriveRecruitmentRate Coding
Signal Modulation
Ascending/DescendinginputsPeripheral
Excitation
N.M. TransmissionT-tubule Propagation
Calcium ReleaseTnC Binding
Crossbridge Formation
FORCE!
Potential areas of action
Many Thanks
Jane Kent-BraunIan LanzaDanielle WigmoreTed Towse