Sustained muscle contraction without relaxation between successive stimuli is called

Tetanus

Explanation:

Tetanus is prolonged/sustained contraction without relaxation resulting from repeating stimulation when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. This is also known as tetanic contraction. This is sometimes needed to achieve and maintain some postures/positions for a long time such as crouching and holding up heavy objects etc.

Sustained muscle contraction without relaxation between successive stimuli is called Tetanus

Explanation:

Tetanus is a bacterial infectian that is characterized by sustained muscle contraction without relaxation between successive stimuli. It usually starts with spams in some muscles. The responsible bacteria is called Clostridium tetani. It can be prevented with vaccination. There are different treatment procedures regarding the symptoms you may have.