Society For Neuroscience | 2009

Discharge characteristics of motor units during long contractions performed by young and old adults Michael A. Pascoe1, Matthew R. Holmes1, Douglas G. Stuart2, and Roger M. Enoka1

179.23

1

Department of Integrative Physiology, University of Colorado, Boulder, USA, 2Department of Physiology, University of Arizona, Tucson, USA

Introduction

1. Experimental setup

3. Motor unit and task characteristics

When a subject performs a submaximal voluntary contraction for as long as possible, the mean discharge rate of motor units recruited at the onset of the task decreases and the coefficient of variation for discharge times often increases (Carpentier et al., 2001; Mottram et al., 2005). Fl

Index finger flexion force Finewire EMG Surface EMG Index finger abduction force

1.2 – 65.2

0.9 – 25.8

5.4 ± 2.9

9.6 ± 7.8 4,852 ± 4,798

10.6 ± 1.9

11.6 ± 3.0

Mean interspike interval (ms)

97.0 ± 15.6

92.1 ± 23.3

Standard deviation of interspike interval (ms)

25.3 ± 5.3

25.0 ± 2.9

Coefficient of variation for interspike interval (%)

26.2 ± 6.3

28.2 ± 9.5

= 0.81

-1.2

R2 = 0.94 = 0.71

-1.7

Time series of interspike intervals -2.2

200

0.5

1

1.5

2

2.5

3

log (n)

mean = 98 sd = 16 α = 0.73

100

*

1.0

0.8

Time series of random numbers

0.6

Time (ms)

mean = 98 sd = 16 α = 0.49

100

0

4. Discharge characteristics over time Young

100

200

300

400

500

600

0.4

0.80

0.74 *P = 0.05

0.2

0

Young

Time (s)

Old

Old

Beginning of long-duration task Instantaneous discharge rate

20

0 2.5

V

Discriminated action potentials

0.0 -2.5 2.5

V

0.16

10 8 6 4 P = 0.102

2

EMG amplitude (V)

Mean force (% MVC)

12

Methods

0

23 ms

Finewire EMG

Conclusions

0.12

· Subjects were able to sustain a relatively constant discharge

0.08 0.04

P = 0.815

0

20%

40%

60%

60%

100%

20%

40%

60%

60%

100%

Surface EMG 1

V

0 -1 9.8

Index finger abduction force

% MVC 0.0

14

10 8 6 4

pps

P = 0.061

0

rate based on audio feedback

· Cessation of the motor unit discharge was preceded by an increase in relative variability of discharge rate

· There were no differences in the relative variability of

30

discharge rate between young and old adults

20 *P < 0.0001

10

2

10 s

· Discharge times were less regular for old adults

0

20%

End of long-duration task

40

*

40

12

CV for ISI (%)

Mean discharge rate (pps)

0.0 -2.5

40%

60%

60%

100%

20%

Time (% duration)

40%

60%

60%

100%

References

Time (% duration)

20

5. Interspike interval histograms

0 2.5

V

Carpentier A, Duchateau J, Hainaut K. Motor unit behaviour and contractile changes during fatigue in the human first dorsal interosseus. J Physiol 534: 903-912, 2001.

0.0

350

2.5

skewness = 1.19 kurtosis = 3.85

0

250

1.2

Skewness

Counts

1

1.6

mean ISI = 84.3 ± 20.3

0.0 -2.5

V

Jordan K, Pascoe MA, Riley ZA, Enoka RM. The regularity of motor unit discharge during steady contractions can vary with age. Program No. 859.13. 2008 Neuroscience Meeting Planner. Washington DC: Society for Neuroscience, 2008. Online.

23 ms

-2.5

V

150

Mottram CJ, Jakobi JM, Semmler JG, Enoka RM. Motor-unit activity differs with load type during a fatiguing contraction. J Neurophysiol 93: 1381-1392, 2005.

0.8

0.4

-1

P = 0.489

50

9.8

All motor units were recruited within 60 s of recovery.

11,958 ± 9,887

0

R2 = 0.99

-0.7

log F(n)

Range (min)

Young Old

200

2. Sample data

pps

Flexion force about the metacarpophalangeal joint was monitored (FlexiForce A201 Sensor) and subjects were required to keep the flexion force minimal.

Time (ms)

8.2 ± 10.2

mean = 98 sd = 16 α = 1.48

100

0

40

The task was terminated when the motor unit stopped discharging action potentials despite the ability of the subject to sustain the force for a further 60 s.

(n = 5)

0

To compare the discharge characteristics of motor units in young and old adults when the task was to sustain the discharge at a low rate for as long as possible.

Subjects received auditory feedback of the discharge times and were asked to keep the rate relatively constant for the duration of the task.

(n = 24)

-0.2

200

20.6 ± 17.9

Mean discharge rate (pps)

Purpose

The task was to maintain the discharge of an isolated single motor unit for as long as possible.

Old

Task duration (min)

Interspike intervals per task

ex iF o rc e

Young

Time (ms)

In addition, it has been shown previously that detrended fluctuation analysis (DFA) can identify subtle alterations in motor unit discharge patterns associated with aging that are not always evident with conventional measures of variability (Jordan et al., 2008).

Subjects exerted a low abduction force (6.1 ± 4.2% maximum) with the left index finger.

Calculation of scaling exponent α

Sine wave

Mean force (% MVC)

The discharge times of single motor units were recorded from the first dorsal interosseus muscle in young (27.9 ± 7.4 yr) and old adults (80.5 ± 6.7 yr).

6. Detrended fluctuation analysis

0

% MVC

0

50

100

150

200

20%

40%

60%

60%

100%

Supported by NIA R01 AG09000 to Roger M. Enoka NIA T32 AG000279 to Michael A. Pascoe

0.0

ISI (ms) 10 s

Time (% duration)

[email protected] - www.mikepascoe.com/cv

SfN 2009 Chicago