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