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

Lack of relaxation between successive stimuli in sustained muscle contraction is known as tetanus. With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation has occurred, and the individual responses fuse into one continuous contraction. Such a sustained contraction is called tetanus. 

Tonus is especially used to refer to muscle tone, the continuous and passive partial contraction of the muscles. 

A spasm is a sudden involuntary muscular contraction or convulsive movement.        

Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.