CHANGES IN FUSION INDEX DURING THE FATIGUE TEST OF FAST MOTOR UNITS IN THE MEDIAL GASTROCNEMIUS-MUSCLE OF THE RAT

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 component of teta nus to the peak tension of the last component. In both types of fast u nits (fast fatigable and fast resistant) the changes in the fusion ind ex were similar to those in tetanic tension: first they increased and then decreased. However, the increase of the fusion index was longer t han that of tension and the subsequent decrease in the fusion index wa s smaller than that of tetanic tension. Furthermore, the initial incre ase in the fusion index of fast fatigable motor units was greater than in the tension. The dependence of the fusion index on twitch time ena bles the analysis of the influence of changes in the twitch time on ch anges in tension of unfused tetani observed during activity of fast mo tor units.