What causes your muscles to become fatigued or cramped when you exercise too vigorously?

Answer 1

Fatigued muscle

When you exercised, a pain called ischemic pain occurs. A major cause of fatigued muscle is inorganic phosphate, its concentration in your skeletal muscle increases during intense exercise due to the broken down of Crp and may be a leading cause of fatigued muscle.

Contrary to popular belief, lactic acid does not cause muscle fatigue. Lactic acid is washed away by the bloodstream to the liver, and is converted into pyruvic acid and then to CO2 and water. Also, your muscles do not use oxygen in cellular respiration, so they cannot be deprived of it. You will still die without O2 though, because all other cells use it. That "need oxygen" feeling that you get when you exercise is actually caused by your need of oxygen to convert the lactic acid in the liver into pyruvic acid

Muscle spasms or cramps

Cramps that occurred in the night after a intense exercise are call Nighttime calf cramp, as a result of contracting the calf muscles by suddenly pointing your toes or by lying with the feet in that position. As you exercised strenuously earlier in the day, your muscles may tighten while you sleep and hence cramp.

Athletes' cramps occur during intense exercise for several reasons. The imbalance of minerals called electrolytes (potassium and sodium) in our blood, which often results from excess sweating (dehydration) may cause muscles spasms. Another common cause is overexertion during exercise, marked by excessive tightening or the muscles and/or a build-up of lactic acid in them.