Lack of relaxation between successive stimuli in sustained muscle contraction is known as:

Tonus$\to$Low-level activity of muscles at rest to maintain posture
Spasm$\to$Sudden involuntary muscle contraction
Fatigue$\to$Decline in muscle activity
Tetanus$\to$Sustained muscle contraction in response to successive stimuli. Tetanus is also known as lockjaw disease and is caused by the bacterium Clostridium tetani.