Interval Training
The key to developing the cardiovascular system without an unacceptable loss of strength, speed, and power is interval training. Interval training mixes bouts of work and rest in timed intervals. Below table gives guidelines for interval training. We can control the dominant metabolic pathway conditioned by varying the duration of the work and rest interval and number of interval repetitions.
Representative Guidelines for Interval Training 
Note that the phosphagen pathway is the dominant pathway in intervals of 10–30 seconds of work followed by rest of 30–90 seconds (work:recovery 1:3) repeated 25–30 times. The glycolytic pathway is the dominant pathway in intervals of 30–120 seconds of work followed by rest of 60–240 seconds (work:recovery 1:2) repeated 10–20 times. And finally, the oxidative pathway is the dominant pathway in intervals of 120–300 seconds of work followed by rest of 120–300 seconds (work:recovery 1:1) repeated 3–5 times. The bulk of metabolic training should be interval training.
Interval training need not be so structured or formal. One example would be to sprint between one set of telephone poles and jog between the next set, alternating in this manner for the duration of a run.
One example of an interval that CrossFit makes regular use of is the Tabata interval, which is 20 seconds of work followed by 10 seconds of rest repeated eight times. Dr. Izumi Tabata published research that demonstrated that this interval protocol produced remarkable increases in both anaerobic and aerobic capacity.
It is highly desirable to regularly experiment with interval patterns of varying combinations of rest, work, and repetitions.
Some of the best resources on interval training come from Dr. Stephen Seiler. His articles on interval training and the time course of training adaptations contain the seeds of CrossFit’s heavy reliance on interval training. The article on the time course of training adaptations explains that there are three waves of adaptation to endurance
training. The first wave is increased maximal oxygen consumption. The second is increased lactate threshold. The third is increased efficiency.
According to the CrossFit concept, we are interested in maximizing first-wave adaptations and procuring the second systemically through multiple modalities, including weight training, and avoiding completely third-wave adaptations. Second- and third-wave adaptations are highly specific to the activity in which they are developed and can be detrimental with too much focus to the broad fitness that we advocate and develop. A clear understanding of this material has prompted us to advocate regular high-intensity training in as many training modalities as possible through largely anaerobic efforts and intervals while deliberately and specifically avoiding the efficiency that accompanies mastery of a single modality. It is at first ironic that CrossFit’s interpretation of Dr. Seiler’s work was not his intention, but when CrossFit’s quest of optimal physical competence is viewed in light of Dr. Seiler’s more specific aim of maximizing endurance performance, CrossFit’s interpretation is powerful.
Dr. Seiler’s work, incidentally, makes clear the fallacy of assuming that endurance work is of greater benefit to the cardiovascular system than higher intensity interval work.
This is very important: with interval training, we get all of the cardiovascular benefits of endurance work without the attendant loss of strength, speed, and power.
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