Contractile mechanism does not have a refractory period like action potential.
Repeated stimulation before relaxation has occurred produces additional activation of contractile elements and adds to the contraction already present.
Rapidly repeated stimulation produces activation of the contractile mechanism before ANY relaxation has occurred, and individual responses fuse into one continuous contraction
Complete tetanus - no relaxation between summated stimuli
Incomplete tetanus - some incomplete relaxation between summated stimuli
When a series of maximal stimuli is delivered to skeletal muscle at a frequency JUST BELOW the tetanising frequency, there is a gradual increase in each successive tension developed until the tensions developed reached a steady level.
Also occurs in cardiac muscles.
Thought to be due to increased availability of Ca2+ intracellularly.