Society For Neuroscience Poster 2008

Muscle function differs with load type and body posture during fatiguing contractions with the knee extensor muscles Jamie Justice, Thorsten Rudroff, Stephen Matthews, Rena Zuo and Roger Enoka 671.2

Department of Integrative Physiology, University of Colorado, Boulder, USA Experimental Setup 1200

The time to failure of a sustained submaximal contraction in the elbow flexor (Rudroff et al. 2007), first dorsal interosseus (Maluf et al. 2005), and dorsiflexor muscles (Hunter et al. 2007) is less for a task that requires supporting an inertial load (position task) compared with exerting an identical net torque against a rigid restraint (force task). However, the relative durations of the tasks varies with limb posture. For example, the elbow flexors can sustain the force task longer than the position task when the forearm is horizontal and neutral, but there is no difference in the two durations when the forearm is vertical and neutral (Rudroff et al. 2005).

Position Task

Force was measured with a force transducer in series with the cable that connected the leg to the external restraint (force task) or to the inertial load (position task). Electromyographic activity (EMG) of the agonist muscles (vastus medialis longus, vastus medialis obliquus, vastus lateralis, rectus femoris) and one antagonist muscle (biceps femoris) were recorded during the fatiguing contractions with bipolar surface electrodes. Rating of perceived exertion and mean arterial pressure were recorded during all fatiguing contractions.

Force Position

10

100

Rating of Perceived Exertion Force Position

8

RPE

90

6 4

80

2

70

600

0

50

100

150

200

250

0

50

Position

400

Mean Arterial Pressure 130

Force

200

Position

Supine

Subjects 7 7 8 8

Seated Force Seated Position Supine Force Supine Position

Representative Data

Knee Extensors

Rectus Femoris

65

Vastus Lateralis

Seated Posture

Force Position

aEMG (%MVC)

50 45

35

Force Position

8

110

6 4

100

0

200

400

600

800

1000

0

200

50 0

50

100

150

200

250

Conclusions · Time to task failure was briefer for the position task than the force task when subjects were seated and supine.

for both tasks, but was lower in the supine posture.

0

50

100

150

200

250

Time (s)

· Rating of perceived exertion (RPE) increased similarly

References

Knee Flexors

Force Position

40

Force Position

36

Enoka RM, Duchateau J. J Physiol 586.1: 11-23, 2008. Hunter SK et al. J Appl Physiol 105: 463-471 2007. Maluf K et al. J Neurophysiol 167: 165-177, 2005. Rudroff T et al. Muscle Nerve 31: 78-87, 2005. Rudroff T et al. J Appl Physiol 102: 1000-1006, 2007.

32

40

28

30

24

20 10

50

Time (s)

· Mean arterial pressure (MAP) increased at a similar rate

Knee Extensors

50

Biceps Femoris

1000

increased at a greater rate for the position task in the supine posture.

Seated Posture

Vastus Lateralis

Vastus Medialis Obliquus

800

for force and position tasks in both postures.

60

Vastus Medialis Longus

600

· Muscle activation (EMG) for the knee extensor muscles

1 min

Rectus Femoris

0

Force Position

Time (s)

Position Task

Force (N)

0

400

indicating that the subjects may have experienced less fatigue with the knee extensors.

12

8

250

· The decline in MVC was least in the seated position task,

16

40

Biceps Femoris

Force (N)

% MVC Decline 26 ± 19 % 10 ± 11 % 32 ± 19 % 31 ± 13 %

20

55

200

Rating of Perceived Exertion

10

120

90

Knee Flexors

24

Force Position

60

Vastus Medialis Obliquus

Supine Posture

150

2

Muscle Activation (EMG)

ForceTask

Vastus Medialis Longus

Pre-task MVC 257 ± 57 N 251 ± 54 N 180 ± 37 N 171 ± 41 N

100

Time (s)

Time (s)

Time (s)

aEMG (% MVC)

The net muscle torque exerted by each subject during the submaximal contractions to failure was similar for the force and position tasks in the supine (15 ± 4 N·m) and seated (21 ± 3 N·m) postures.

Force

800

Seated

To compare the times to failure of the force and position tasks performed with the knee extensor muscles when the subjects were seated and supine.

Eight subjects (5 men; 30 ± 10 yrs) performed force and position tasks with the knee extensors at 20% of maximal voluntary contraction (MVC) until task failure.

1000

0

Purpose

Methods

110

MAP (mmHg)

Force Task

Mean Arterial Pressure

Supine Posture

RPE

Seated Posture

Measures of Exertion

MAP (mmHg)

Supine Posture

The mechanisms that cause muscle fatigue are specific to the task being performed. As a consequence, the time a submaximal contraction can be sustained varies with the load type and posture (Enoka & Duchateau 2008).

Time to Failure

Time to Failure (s)

Introduction

20

0

200

400

Time (s)

600

800

1000

16

0

200

400

Time (s)

600

800

1000

Acknowledgements. The authors thank Jakob Dideriksen and Martin Baeckgaard (Aalborg University, Denmark) for assisting with MATLAB programming, and Mike Pont for assistance in design and construction of the experimental apparatus.

1 min

Supported by NINDS NS43275 to RME