Development of activation and rise of tension in an isometric tetanus.

The force-velocity relation was determined at preset times after the beginning of the stimulus volley, either at low isometric tensions, during the raising phase of tetanus and the recovery after a quick release imposed at the plateau, or at the steady state. It is concluded that the characteristics of the force-velocity relation may be used to express the actual level of activation at any time during the contractile process and that the mechanical output of frog twitch muscle fibres implies two distinct steps: (i) activation of the contractile machinery; (ii) production of force.