The nature of muscular contractions are such that they involve many of the bodys systems including the nervous system, the respiratory system, tangentially the endocrine system and also most visibly the circulatory system which reacts to muscular contractions by causing famous pump. Contractions can become so intense that the flow of blood to the muscle is reduced. This may be circumvented to some by taking a brief rest pause between contractions-usually 10 seconds or less. In my experience less than 2 seconds is too brief to have any effect and more than 10 seconds allows the muscle to cool off too muchand may allow the muscle too much time to recover.