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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

8/14/2019 ECSS%202008 TR[1]

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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