Isometrics and Muscle Fibre Action

[Guest guest]

What is not often known is that strenuous isometric (or quasi-isometric)

activity produces not only obvious fatigue, but also definite changes in

muscle fibre electrical activity. The following studies discuss this issue:

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Jpn J Physiol 1994;44(1):75-88

Recovery of conduction velocity of muscle fiber action potential after

strenuous isometric contraction.

Hanayama K

The change of conduction velocity of muscle fiber action potentials of

individual motor units after loading of 50% maximal voluntary contraction was

examined in biceps brachii muscles of twenty healthy male subjects. A surface

electrode array, consisting of fifteen electrodes spaced at 5 mm intervals,

was used to detect EMG signals during minimal isometric contraction which is

adjusted so as to generate action potentials in a single motor unit.

Conduction velocity was measured from the recorded signals of 14 channels.

Mean conduction velocity of all subjects was 3.33 ± 0.28 m/s (n = 20).

A significant decrease of the conduction velocity was observed after

contraction maintaining 50% of maximal voluntary contraction torque as long

as possible. From the results of conduction velocity decrease during 6 min

minimal contraction following the contraction at 50% of maximal strength,

subjects were divided into two groups, having endurance times exceeding 50 s

and less than 50 s, respectively. The former showed significant recovery

during minimal contraction after loading, while the latter showed no apparent

recovery.

It was demonstrated that the conduction velocity of muscle action potentials

obtained with minimal contraction decreased after strenuous contraction and

the extent of its recovery was related to the endurance time of contraction

which the subjects could maintain 50% of maximal strength.

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Electromyogr Clin Neurophysiol 2000 Apr-May;40(3):151-61

Muscle fiber conduction velocity during isometric contraction and the

recovery period.

Sakamoto K, Mito K.

Changes of muscle fiber conduction velocity (MFCV) detected by surface array

electrodes during an isometric contraction and the recovery period were

evaluated. The location on skin measured for action potentials of muscle

fiber in m. biceps brachii was a distance of 5 mm and 30 mm from the

end-plate to the distal tendon. The MFCV was evaluated by averaging raw EMG

waves.

The MFCVs at both locations declined gradually during the loads of sustained

isometric contractions of 30, 50, and 70% of the maximum voluntary

contraction (MVC). The degree of the decrease of the MFCV was extremely

intense during a sustained contraction of 70% MVC. The values of the MFCV at

the location of 5 mm from the end-plate in the period close to the

exhaustion state showed a significant decrease compared with the values in

the initial period during the contractions for the above three kinds of

loads, while the decrease of the MFCV at the location of 30 mm from the

end-plate was not found to be so significant during the contractions.

In the recovery period, the contraction of 5% MVC was maintained, and the

resultant MFCVs restored gradually to the value of the initial period. At

the location of 5 mm from the end-plate, the MFCVs at 15 minutes after the

end of the load were significantly higher than those just after the end of

the load. At the location of 30 mm from the end-plate, the increases of the

MFCVs during the recovery period did not show significant changes. Changes

of the MFCV during the isometric contraction and the recovery period depend

greatly on the location of the electrodes measured for the action potentials

of the muscle fibers.

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Dr Mel C Siff

Denver, USA

http://groups.yahoo.com/group/Supertraining/