Interval training, defined as continuous exercise with intermittent work variation cycles, has numerous benefits for exercisers ranging from athletes to people concerned with general fitness. Interval training has profound effects on improving aerobic capacity because it is a greater stimulus than prolonged steady-state endurance exercise used to promote improvements in VO2max. The perception of higher levels of unaccustomed stress triggers General Adaptation Syndrome (GAS) response at a level of about 24% that of stead-state training with consistent work volumes. The mitochondrial density, oxidative enzymes, and capillary/fiber ratio increase in response to the greater intensity that interval training provides. In addition, sophisticated studies using labeled isotopes reveal that hard intervals, defined by high heart rates, produce very large concentrations of lactate via an increased recruitment of fast-twitch muscle fibers. This, in turn, conditions the body to better tolerate lactic acid accumulation and improves the lactic acid removal mechanisms. Furthermore, neuromuscular efficiency improves as the body learns how to become more economical while performing at higher levels of intensity. For example, swimmers who perform intervals at high intensities learn how to obtain a more efficient stroke in order to swim faster, versus a swimmer who only swims long slow distances.

In addition to improving performance and aerobic capacity, interval training makes it easier to burn a greater number of calories than prolonged exercise at lower intensities. This is because the high intensity causes elevated rates of metabolism and subsequent greater fuel utilization, even during the recovery periods where oxygen dept, and excess post-exercise oxygen consumption (EPOC) continues at a higher than normal rate of aerobic metabolism to support heart and respiratory muscle activity; replenishment of anaerobic fuel sources (used outside of steady state); and respond to endocrine factors. In other words, the hard work during the actual intervals “pays off” as you continue to burn more calories during the rest periods between intervals and after the exercise bout is over. Studies have shown post-excess oxygen consumption to persist for 9-12 hours post exercise.

To perform interval training there are a few general guidelines to keep in mind. There is an inverse relationship between interval intensity and recovery time, and typically it is difficult to maintain an intense interval for longer than 5 minutes, so intervals should generally not surpass that duration. Extremely intense intervals will usually consist of a work/rest interval of 1:2-3; e.g. hard effort for 1 minute followed by 3 minutes of recover. More traditional intervals which include a high effort just over the lactate threshold can consist of work:rest ratios around 1:1. Difficult workouts with moderately hard intervals and short rest periods help increase the accumulation of lactate concentration in the cell because recovery is brief. These types of interval sessions simulate conditions experienced during competition since most athletes will be performing at a constant intensity around lactate threshold; therefore these intervals include a work/rest of ~3:1. Sometimes random intervals, often referred to as Fartlek (speed-play) workouts, are ways to make a workout more creative, where you mix interval distances and recovery times. An example would be to run hard to a traffic light, jog to the next mailbox, run hard to the street sign, jog to the lamp-post, etc…where you focus on things in front of you and make them the destination for the interval.

Interval training can help you or your client, meet fitness, health, or weight loss goals in shorter periods of time than traditional training methods. To prevent injury or overtraining, use progressions consistent with the abilities of the participant and be sure to follow the standard dose acclimation guidelines to avoid too much, too often. Additionally, varying activities can help to avoid overuse injuries and usually lends itself to reduced boredom associated with doing the same activities all the time. It usually takes approximately 12 doses for notable improvements in efficiency. Using premeditated formats will best serve the appropriate implementation of the principles of progressive overload and specificity as well as allow for easy tracking methodology and identify any necessary modifications.