It takes place when we move or we try to push or pull an object. During muscle contraction, the muscle fibres (the light and dark myofibrils) slide over one another and when the muscle is relaxed, after the performance of the movement, the same muscle fibres slide apart. The moment we perform a movement, the skin covering the muscles stretches and the muscles flex.
When the muscles involuntarily contract for a longer period, we experience spasm. During a spasm, we feel a slight tightness or a knot in a muscle. Spasms are experienced by athletes when they perform strenuous activities. Spasms are also referred as cramps. Generally, cramps are temporary and cause sudden pain, but some cramps, if persist for a longer time, may cause tendon or ligament injury.