In addition to using a repetition-based system of training, the circuit format lends itself very well to time-based training. Instead of using repetitions to establish the workload, individuals train for a specified period of time. For example, work periods of 15 to 30 seconds, coupled with rest periods ranging from 15 to 30 seconds, are frequently recommended for circuit training. From a physiology standpoint, individuals performing a 30-second work period with only 15 seconds of rest will stress their cardiovascular system more than an individual who works with the elastic resistance for 15 seconds and takes 30 seconds of rest. Changing the work rest cycle is a common practice in exercise training. For a tennis player, a work-rest cycle of 15 seconds of work followed by 30 seconds of rest mimics the challenge to the muscular system that is encountered when playing tennis. This is an example of the specificity concept, an important factor to consider when determining what amounts of work and rest to use during circuit training.