Intermittent exercise is a phrase used to describe a variety of different physical training types. The terms "intermittent," which means to stop and start at intervals, and "interval," as in interval training, are used somewhat interchangeably. In most circumstances, interval training will be conducted as a high intensity exercise activity.
By its nature, exercise is not aimless; it involves physical exertion that is directed to the development, increase, or maintenance of physical fitness. Intermittent exercise is both a description of the intensity of the activity as well as its nature.
Intermittent exercises of various types are best known where they have been employed as components to endurance sports. Disciplines such as distance running, road cycling racing, and mountain biking require the body to produce the energy necessary for physical performance through the aerobic energy system, which primarily utilizes stores of carbohydrate products, in the form of glycogen, reduced as energy is required, to the sugar glucose. To generate energy, the body—through the cardiovascular system—transports oxygen and other nutrients essential to muscle function. The greater the ability of the heart to power blood volume to the muscles, the likely more efficient the production of energy and the removal of wastes such as carbon dioxide will be.
Intermittent exercise programs will tend to increase the oxygen transporting capacity of the body, often referred to by the shorthand VO2max. As a further general rule, the more intense the intermittent period of training, the greater the VO2max. By illustration, suppose two equally athletically talented and physically fit cyclists are monitored over a training period of six months. One cyclist maintains a set exercise program of 60 minutes per day. The second cyclist rides the same distances at the same speed as the first for four days per week; his or her remaining three workouts are higher intensity, intermittent workouts of four 15-minute segments—each separated by rest intervals of five minutes. The intermittent training cyclist would expect to obtain an increase in measured VO2max levels in the ranges of 5-15%.
The maximum oxygen uptake of an endurance sport athlete is not a guarantee of competitive success. Physical techniques, tactics, the strength of the musculoskeletal system generally, and other fitness factors will all play a role. What is virtually certain in these sports is that all other factors being equal, the athlete with the best ability to power blood through the cardiovascular system and process oxygen has the best chance for competitive success.
As a further general proposition, the more intense the work performed in the intermittent period, and the shorter the rest interval, the greater the impact upon the VO2max. Intermittent training also tends to produce a heightened ability in the body to rid itself of lactic acid, a byproduct of aerobic energy production and a performance inhibitor. There have been a number of high level scientific studies conducted in an effort to resolve the question as to whether intermittent exercise tends to reduce the fat stores in the body more efficiently that regular single duration exercises. It is believed that during the post-exercise recovery period that the body does not utilize fat stores any more readily than it would after regular forms of exercise; it is likely that the noted fat losses in many of these studies are actually confirmation that the appetite of an athlete involved in intermittent exercise programs will be more readily suppressed, resulting in a reduction in fatty foods being consumed.
Sports that are powered by the anaerobic energy systems also benefit from intermittent exercise programs with an aerobic emphasis. In a sport such as rugby, all of the body's energy will be produced by the anaerobic alactic system, where the ATP is available in the muscle for short bursts of activity up to approximately ten seconds, and it is stored in a fashion that requires the ATP to be replenished very frequently. In the anaerobic lactic energy system, the body must sustain an energy level in segments of play greater than approximately 10 seconds and less than 90 seconds. Intermittent exercise training will assist the athlete in increasing the anaerobic threshold, which is the speed at which the athlete may function without drawing unduly upon lactic system. The aerobic system is the measure of the athlete's ability to recover from intense activity through the increased transport of oxygen.
The best specific measure of the recovery powers of an athlete in intermittent exercise is the ability of the athlete to attain a resting heart rate after the interval is completed. The biathlon, the traditional winter Olympic event involving intense cross-country skiing and the calm precision of target shooting, is an example of a discipline where the heart rate of the competitor may be pushed to levels very close to the maximum rate during the skiing portion of the event, followed by a required settling of the athlete's physiology to shoot at a target from a prone position. If the athlete is unable to reduce the heart rate for the shooting component, the ability to deliver the shots will be compromised.