Focus, commitment, maximum level of performance, and "pushing to the max" are all commonly recognized markers of intensity in exercise. Intensity may be defined as a maximum level of physical effort directed to a specific activity or movement in sport; as a matter of safety, the development of an athlete's physical skill will be the foundation of intense training, as intense efforts built upon poor technique often leads to injury and frustration.
Intensity is a marriage of mind and body. The execution of high intensity exercise programs requires a progressively greater mental and emotional commitment to the objectives of the training to which the athlete is subjected; to continually bear physical discomfort that will invariably accompany high intensity exercise, the activity must be relevant in the mind of the athlete.
High intensity exercise may refer to a general overall physical training program; it may also describe a specific or isolated routine or set of exercises with a high intensity focus. Racing in a multi-day cycling competition and a 30-minute sprint training session on the track of a velodrome are both high intensity exercises, as each will demand a maximum level of performance. All sports require a high intensity at some phase of competition.
Intensity levels can never be assessed in a vacuum when considering their relative relationship to sports performance. Intensity with which exercise is performed must be measured in conjunction with both the duration of the exercise in a given session, as well as the frequency during a given training period. The formulation of a successful high intensity exercise program will require the variables of intensity, duration and frequency to be adjusted on an ongoing basis to account for athlete fitness, training gains and objectives.
High intensity exercise has several proven physiological benefits apart from the specific training and performance objectives of individual exercise programs. As a general proposition, the greater the intensity of an aerobic training program, the greater is the ability of the athlete to inhale and transport oxygen in the cardiovascular system to working muscles as a part of the production of energy (known as the VO2 max calculation). The greater the VO2 max capability in an athlete, the generally greater the ability to succeed in aerobic sports such as cycling and distance running.
The second general consequence of high intensity exercise is the greater loss of subcutaneous fat (body fat) by athletes who area part of a program that includes high intensity exercise components. The body, when participating in high intensity exercise, will utilize fat stores more readily it will do in regular exercise systems. The anaerobic and aerobic systems will by pass the usual glycogen transport and metabolization of glucose into the energy fuel adenosine triphosphate (ATP), instead converting fats into fatty acids for the same ultimate purpose.
Further, consistent with the rules of physics, if two athletes of approximately the same size and weight run the same 6 mi (10 km) distance, they will be expected to use almost the same amount of energy, even when one runner travels at a rate of 6 minutes per mile (1.6 km) and the other moves at 10 minutes per mile. The faster runner will expend more energy to cover the distance than the slower runner; the slower runner will be expending the lesser amount of energy over a longer period. However, if the two athletes trained at these levels over time, the faster runner would develop a significantly higher VO2 max, with an accompanying greater physical ability to perform at a higher level.
High intensity exercise can be subdivided into two general areas: high-intensity strength training and high intensity aerobic training. There are crossover training effects between many of the exercises found in each of these groups, where the exercise has a multiple application.
High-intensity strength training was once the exclusive domain of weightlifters, body builders, and athletes in sports that placed a premium on muscular strength, such as American football. The principles and the techniques used to develop effective weight training exercise programs are now well understood to have wide application in any sport where muscle strength, the cross training of the musculoskeletal system, and the development of the mental strength to push physical limits in training will enhance ultimate athletic performance.
The ratio of the interrelated factors of intensity, frequency, and duration is of heightened importance in the development and ongoing assessment of any high-intensity strength training program. Strength training involves muscle contractions that are often explosive in their execution, with a correspondingly greater risk of injury. The recovery period after any high intensity workout will be in direct proportion to the intensity of the workout and how often the workout has been performed in the period.
The following high-intensity strength program is modeled upon a weightlifting training regime; the principles that are the foundation of this model are applicable to any high-intensity strength exercises, such as a plyometrics jumping program as may be employed by basketball or volleyball players, or the strength-building exercises specific to the equipment in sports as diverse as boxing and kayaking.
A well-crafted high-intensity strength training program has a number of built-in protections against one of the scourges of athletic development: overtraining. Overtraining will often arise when the training loads placed on the athlete, particularly in high intensity situations, exceeds that which the body can bear over time. Many athletes and coaches fall into the sports quagmire that begins with the path marked "if a little more intensity in the workouts is good, more intensity must be better." Overtraining, also referred to as staleness or burnout, is not a training state so much as it is an outcome of a high intensity program. Break periods in the exercise routine and changes in the established routine are often useful components to high intensity work of any type.
As distance running became a popular racing fixture in the late nineteenth century, training methods employed were typically centered on relatively high mileage churned out at steady speeds. Emil Zatopek, the Czech legend who won the never-duplicated triple championship of the 5,000-m, 10,000-m, and marathon events in the 1952 Olympics, was the first notable runner to employ high intensity interval running techniques in his training program. Zatopek discovered through personal trial and error that he could run faster in the long distances through running short, very hard, and fast interval segments. Over the next fifty years, intense interval training became a staple for elite and recreational runners the world over. Similar high intensity training principles have been applied to all aerobic sports, including cycling, swimming, and cross-country skiing.
High-intensity exercise employed in aerobic training is founded upon the principle of periodized training. Periodization is a concept that is similar to the schedule outlined in the high-intensity strength exercises, with the key difference that it is formulated over much longer time intervals; a year is a typical periodization time frame, with broad time periods such as a three-month interval having correspondingly general interval training objectives. The broad time periods are then broken into much smaller training modules, with specific interval proscribed. Periodization has demonstrated benefits in aerobic sports in addition to increased VO2 max and reduction of body fat. As an example, in their build up to the competitive road-racing season (often four to six months in duration), members of the Kenyan national distance running team will spend three months engaged in very intense, interval-based track focused workouts. The road races will be routinely 10-km to half-marathon events (6 mi to 13 mi); the workouts involve running 40 400-m intervals, with a 60-second rest period between each; each 400-m segment will be run in under 60 seconds. The intense demands of such workouts, which total 16 mi (26 km) in a single session, enhance both speed and recovery ability.