1. Aging & Skeletal Muscle Fatigue
David W. Russ, PT, Ph.D.
Ohio University
School of Physical Therapy

2. Skeletal Muscle
“Without skeletal muscle, there is no physical therapy.”
--Eugene Michels

3. Muscle Fatigue Definitions:
#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

4. 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

5. 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

6. Loss of Force Common factor in each definition
How is muscle force generated?
Pretty complicated…

8. Muscle Fatigue “Fatigue makes cowards of us all.”
V. Lombardi
Multiple sites of failure
Multiple potential mechanisms
Task specificity
Single mechanism not likely

9. Impact of Muscle Fatigue
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)
Transient loss of strength
Reduced muscular endurance was significantly associated with a history of falls in older women (Schwender et al., 1997)

10. 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

11. 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

12. Fatigue Data Russ et al., 2008 Kent-Braun et al., 2002
Lanza et al., 2004

13. Central Activation Testing
No study showed age-related deficits
Testing is not simple to do in the clinic

14. Peripheral Excitability
M-wave
Compound Muscle Action potential (CMAP)
Amplitude & Area
Again, age appears unimportant

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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)

23. Potential areas of action

24. Many Thanks
Jane Kent-Braun
Ian Lanza
Danielle Wigmore
Ted Towse