Why do cramps occur? Mainly, they arise when the metabolic processes are disturbed, which leads to a decrease in the amount of ATP. If there is a lack of ATP, muscles are unable to relax on their own, and a convulsive contraction occurs. It usually lasts a few seconds, sometimes a few minutes. What is the underlying cause? There are actually many of them. Stress, excessive muscle strain, dehydration, lack of Mg, Ca, vitamin D3 and some other micronutrients. And at night it can be triggered by an uncomfortable posture. Most often cramps occur in the lower extremities, and especially in the calf muscle. But a cramp in the abdominal muscles or a short spasm (as in the animation) is quite rare