2. Straining your muscles
Your muscular fibers can take a certain amount of effort, depending on how developed they are and the preceding warm-up, and if they perform an effort higher than that, there will be short and sometimes long-term consequences. Pain and fatigue are expected when the fibers perform movements requiring more resistance and/or more strength than they have, and both of these symptoms disappear after some rest.
However, a higher exigence on the muscle can physically hurt the tissue and alter the electroconductivity of the neuromuscular union between the end of a nerve and the muscular fibers. Some of these fibers might rip or become displaced due to excessive strain. The excessive contractions to which strained muscles are subjected can cause an involuntary reaction that will lead to cramps. If you exercise your legs too much, for example if you run fast for a long amount of time, or perform leg strength exercises more than you should, you could have leg cramps.