[The force of an isometric twitch contraction and the phases of tetanus]

The isometric twitch contractions of human flexor digitrum sublimis muscle were recorded after the unfused 9.5 pps tetani of different durations. The peak twitch force was plotted against the number of preceding pulses both for this experimental situation and for the usual staircase phenomenon on the frequencies of 2-4 pps. The dependence of the twitch force on the number of the preceding pulses in tetanus differs from that in staircase. In the staircase twitch reaches maximal level after 100-120 repetitions and remains fully potentiated during 800-1000 repetitions more. In 9,5 pps trains potentiation begins to decline immediately after reaching peak on 100th-120th pulse and the decline is accompanied by the slowing of twitch relaxation. So we can speak of the two stages of the post-train twitch potentiation. This has some implications to the process of the force maintenance in unfused tetanus. Initial stage of increased twitch force and quick relaxation corresponds to that phase of prolonged unfused tetanus which is characterized by big oscillations of tension. Second stage of decreasing twitch force and slowing relaxation give rise to the gradual increase of tetanic tension and to the diminishing of its oscillations. It is supposed that sustained tetanic contraction leads to the slowing of the cross-bridges cycling thus impairing the ability of muscle to develop sufficient tension in twitches.