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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.
Acta Neurobiologiae Experimentalis
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1999
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vol. 59
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issue 1
37-43
EN
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.
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Changes in fusion index during the fatigue test of fast motor units in the medial gastrocnemius muscle of the rat

100%
Celichowski J., Grottel K., Bichler E.
Acta Neurobiologiae Experimentalis
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1996
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vol. 56
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issue 4
881-887
EN
Changes in the fusion of tetani were investigated in fatigue tests of 50 fast motor units of the rat medial gastrocnemius muscle. Fusion of the tetani was measured using the fusion index, being the ratio of the tension to which motor unit relaxed before the last of tetanus to the peak tension of the last component. In both types of fast units (fast fatigable and fast resistant ) the changes in the fusion index were similar to those in tetanic tension: first they increased and then decreased. However, the increase of the fusion index was longer than that of tension and the subsequent decrease in the fusion index was smaller than that of tetanic tension. Furthermore, the initial increase in the fusion index of fast fatigable motor units was greater than in the tension. The dependence of the fusion index on twitch time enables the analysis of the influence of changes in the twitch time on changes in tension of unfused tetani observed during activity of fast motor units.
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Changes of motor unit contractile output during repeated activity

88%
Lochynski D., Celichowski J., Korman P., Raglewska P.
Acta Neurobiologiae Experimentalis
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2007
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vol. 67
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issue 1
23-33
EN
The aim of the study was to evaluate changes in the motor unit output and to determine changes in the optimal stimulation frequency (i.e., giving the maximal output per one pulse) during prolonged contractile activity when, successively, potentiation of force and fatigue developed. The influence of these phenomena was studied on three types of motor units: fast fatigable (FF), fast resistant (FR) and slow (S) in the rat medial gastrocnemius muscle. The motor units were isolated by a method of splitting of L5 ventral root into very thin bundles of axons which were electrically stimulated 10 times with repeated series of 10 trains of stimuli at duration of 500 ms and progressively increasing (1?150 Hz) frequency. The initial (the first series of stimulating trains), potentiated (the second series), as well as fatigued (the tenths series) force recordings were compared. The motor unit output was expressed as the area under the force-time record in response to one stimulus measured at a plateau phase of the tetanic force. The stimulation frequency when the force-time area per one pulse was maximal was accepted as the optimal frequency. In fast motor units, the maximal contractile output increased with potentiation and was reduced with fatigue, and the optimal frequency decreased and increased, respectively. Nevertheless, the fusion degrees of the optimal tetanic contractions were similar in initial state, potentiation and fatigue independently of the changes in force. The applied stimulation protocol had almost no influence on the mechanical activity of slow motor units. The study highlights the physiological importance of force potentiation induced by preceding contractile activity for the economy of motor performance. The observed changes of the optimal stimulation frequency are consistent with the known changes in the motor unit firing rates during voluntary activity when the two phenomena develop.
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