When muscles are stretched the muscle fibers are temporary lengthened. Muscles have a unique ability to undergo contraction and lengthening since they are elastic.
When the muscles are warm they are more elastic, therefore, warming up the muscles is very important for the purpose of avoiding injuries. Ir is important to engage in moderate cardiovascular warm-up prior to stretching, which increases the blood flow to the active and thus avoiding injuries to the muscles.
Stretching muscles when they are cold may bring injuries or cause muscle pulls, which are painful.
