This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excitation and Action Potiential, Subject name: Physiology
Latent period of muscle twitch is 10 s, contraction period is 40 s and the relaxation time is 50 s. Tetanising frequency would be: 
Select and explain. 
A. 25 Hz, B. 50 Hz, C. 75 Hz, D. 100 Hz
A: 25 Hz
Answer: A: 25 Hz 
Explanation: For tetanus to occur, stimulus must fall repeatedly in the contraction phase that will cause the accumulation of Ca++ in myoplasm leading to sustained contraction. Tetanising frequency = 1 / time of contraction phase = 1/40 = 25 Hz