text: A<unk>  (also known as a<unk>  twitch or simply twitch) occurs when a<unk>  fiber generates tension through the action of actin and myosin cross-bridge cycling.  While under tension, the<unk>  may lengthen, shorten or remain the same. Though the term<unk> ' implies a shortening or reduction, when used as a scientific term referring to the muscular system<unk>  refers to the generation of tension by<unk>  fibers.  Locomotion in most higher animals is possible only through the repeated<unk>  of many<unk> s at the correct times.<unk>  is controlled by the central nervous system, which comprises the brain and spinal cord.  Voluntary<unk> s are initiated in the brain, while the spinal cord initiates involuntary reflexes.
option_0: Muscle contraction
option_1: Heat transfer
option_2: Excretion
option_3: Drug metabolism
option_4: Blood flow
option_5: Photosynthesis
option_6: Thermoregulation
option_7: Cell signaling