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2 Acta Neurobiologiae Experimentalis

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The area under the record of contractile tension: estimation of work performed by a contracting motor unit

100%

Celichowski J. , Grottel K. , Bichler E.

nr 2

165-168

The area under twitch tension records was measured for motor units in rat medial gastrocnemius. These measures were compared to measures of tension. The tension varied in significantly larger range than the area. The area of slow motor units was similar to the area of fast resistant units, whereas their tensions differed significantly. The area depended mainly on the amplitude of contraction and to a smaller degree on its time course. The measure of area under the tension record gives a more exact evaluation of the work performed by contracting motor units than the measure of tension alone. The obtained results show that motor units in mammalian muscle are less variable in their ability to perform contractile work and moreover, that slow motor units play a more significant role during contractions than was supposed based on tension measures.

Fatigability of motor units estimated by changes in work performed during their repetitive stimulation during the fatigue test

100%

Celichowski J. , Grottel K. , Bichler E.

nr 1

37-43

In numerous studies resistance to fatigue is evaluated by measuring the peak tension of motor units in muscle. In the present study, the work performed within successive tetani during the fatigue test of rat medial gastrocnemius motor units was estimated by assessing of the area under the tension record. Resistance to fatigue was evaluated by a modified fatigue index which is expressed as the ratio of the area under a tetanus recorded two minutes after maximal potentiation of tension has been reached to the area under this maximally potentiated tetanus. The values of this modified fatigue index were compared to the standard fatigue index which was taken as the ratio of peak tensions for corresponding tetani. For fast fatigable units, values of the modified fatigue index were significantly lower than those of the standard index. This observation resulted from changes in the shape of unfused tetani accompanying developing fatigue. These changes strongly influenced the area under the tension record whereas the peak tension of these tetani diminished less significantly. For slow and a part of fast resistant to fatigue units (with the standard fatigue index above 0.85) the modified fatigue index was slightly higher than the standard one although the difference was not significant. This phenomenon was due to the prolongation of relaxation which was visible in the last part of the fatigue test. It is being concluded that the modified fatigue index describes more precisely the fatigue-induced changes in tetani during the fatigue test than the standard fatigue index, especially in fast fatigable units.