According to the text, repeated activation of muscles leading to the accumulation of lactic acid causes fatigue . Tetany is described as rapid spasms or wild contractions due to low calcium ions in body fluids . Tetanus (physiological) refers to a sustained muscle contraction caused by a series of rapid stimuli where the muscle does not relax between them. Although 'Tetanus' is not explicitly defined in the provided snippets, it is the standard biological term for this phenomenon, distinct from the disorder 'Tetany'.