ECSS%202008 TR[1]
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Transcript of ECSS%202008 TR[1]
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Differences in time to failure are not
explained by the work performed by theelbow flexor muscles
Rudroff T, Justice J, Matthews S, Enoka RM
Department of Integrative Physiology
University of ColoradoBoulder, CO, USA
University of Colorado
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Muscle fatigue is defined as an exercise-induced reduction inthe ability of muscle to produce force or power whether or notthe task can be sustained.
100%
Exercise Fatigue
Before After
University of Colorado
Force
What is muscle fatigue ?
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Muscle fatigue begins immediately eventhough the task can continue
Mosso (1891) Fatigue
Task
Failure
University of Colorado
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What is Muscle Fatigue?
MotorCommand
DescendingDrive
SpinalActivation
NeuromuscularPropagation
Excitation-ContractionCoupling
Metabolism
IntracellularMilieu
ContractileApparatus
Blood Flow
Afferent Feedback
University of Colorado
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Maintain a submaximal force Maintain elbow angle
PositionTask
ForceTask
Identifying the impairment
University of Colorado
1402 728 s 702 582 s
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Indirect measures indicate that the elbowflexors may perform more external work
during the Position task
30 N
2 ms-2
2 min
Acceleration
Force
Rudroff et al. 2005,2006
University of Colorado
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A more direct way to estimate work
Force Task Position Task
University of Colorado
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If external work is the limiting
factor, then:
University of Colorado
As force fluctuations increase, the differencein time to failure between the Force and
Position Tasks should increase
Force fluctuations should be greater in thetask with the shortest duration (Position Task)
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Difference in time to failure for theForce and Position tasks are not explained byexternal work.
University of Colorado
Hypothesis
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Low-Force (20% MVC) High-Force (60% MVC)
University of Colorado
40 N 120 N
To compare the time to failure and forcefluctuations in the Force and Position tasks at
low (20%MVC) and high (60%MVC) forces.
Aim
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No difference in time to task failure in
High-Force group .
Tim
e(s)
0
200
400
600
800
1000Force Task
Position Task*
Low Force(20% MVC)
High Force(60% MVC)
- 35% MVC for both tasks
- 34% MVC for both tasks
University of Colorado
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Time (s)
0 100 200 300 400 500 600 700 800
CV
ofFor
ce(%)
0
1
2
3
4
5
6
Time (s)
0 20 40 60 80 100 120
0
2
4
6
8
10
Low Force (20% MVC) High Force (60% MVC)
Position
Position
Force Force
University of Colorado
Similar relative force fluctuations inForce and Position Tasks
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S
D
ofForce(N)
0.0
0.51.0
1.5
2.0
2.5
3.0
3.5
Low-Force
(20% MVC)
High-Force(60% MVC)
Absolute force fluctuations are greater inHigh-Force group
Force TaskPosition Task
University of Colorado
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Time (s)
0 100 200 300 400 500 600 700 80015
20
25
30
35
40
45
Time (s)
0 20 40 60 80 100 120
40
50
60
70
80
90
100
Force
ForcePosition
Position
Low Force (20% MVC) High Force (60% MVC)
aEMG
(%MVC)
University of Colorado
Similar increase in EMG activity inForce and Position Tasks
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Greater external work is not the primarycause of the briefer duration of thePosition Task at low forces.
University of Colorado