Ans: A. 25 HzStimulation frequency at which summation of contractions occurs -Determined by twitch duration of the paicular muscle under study.Example:If twitch duration = 10 ms.Frequencies less than 1/10 ms (100/s) - Cause discrete responses interrupted by complete relaxation.Frequencies greater than 100/s cause summation.Tetany:Continuous contraction of muscle fibers, without latent period & relaxation time.Hence tetanizing frequency depends only on contraction time, i.e. twitch duration.Twitch duration = 40 milliseconds = 0.04 secTetanizing frequency = 1/0.04 = 25 Hz