This is an excerpt from Essentials of Strength Training and Conditioning 4th Edition eBook With Web Resource by NSCA -National Strength & Conditioning Association.
High-intensity interval training (HIIT) involves brief repeated bouts of high-intensity exercise with intermittent recovery periods. High-intensity interval training typically incorporates either running- or cycling-based modes of exercise and is an efficient exercise regimen for eliciting cardiopulmonary (23) and metabolic and neuromuscular (24) adaptations. In fact, Buchheit and Laursen (23) stated that HIIT "is today considered one of the most effective forms of exercise for improving physical performance in athletes" (p. 314). High-intensity interval training is often discussed in terms of duty cycles involving a high-intensity work phase followed by a lower-intensity recovery phase. It has been suggested that nine different HIIT variables can be manipulated to achieve the most precise metabolic specificity (23), including
- intensity of the active portion of each duty cycle,
- duration of the active portion of each duty cycle,
- intensity of the recovery portion of each duty cycle,
- duration of the recovery portion of each duty cycle,
- number of duty cycles performed in each set,
- number of sets,
- rest time between sets,
- recovery intensity between sets, and
- mode of exercise for HIIT.
The authors (24) indicate, however, that the intensities and durations of the active and recovery portions of each duty cycle are the most important factors to consider. To optimize HIIT training adaptations for athletes, HIIT sessions should maximize the time spent at or near O2max. More specifically, the cumulative duration and intensity of the active portions of the duty cycles should equate to several minutes above 90% of O2max (24).
The benefits of a HIIT protocol designed to repeatedly elicit a very high percentage of O2max are primarily the result of the concurrent recruitment of large motor units and near-maximal cardiac output (6). Thus, HIIT provides a stimulus for both oxidative muscle fiber adaptation and myocardial hypertrophy. Additional HIIT adaptations include increases in O2max, proton buffering, glycogen content, anaerobic thresholds, time to exhaustion, and time-trial performance. For example, Gibala and coworkers (63) reported equivalent improvements in muscle buffering capacity and glycogen content for HIIT at 250% of O2peak during four to six 30-second cycling sprints compared to continuous cycling for 90 to 120 minutes at 65% of O2peak over six total training sessions. In addition, 750 kJ cycling time trials decreased in both groups by 10.1% and 7.5% in the HIIT and long, slow endurance training groups, respectively. Thus, HIIT provided performance and physiological adaptations equivalent to those of long, slow endurance training, but in a time-efficient manner.
The strength and conditioning professional should consider a number of factors when designing a HIIT program. For example, a 400 m sprinter would need a HIIT program geared toward anaerobic-based durations and intensities more than a 2-mile (3,200 m) runner. Other considerations for the desired training adaptations are periodization, similar to that for resistance training, and the number of exercise sessions per day and week. Periodization allows for the general development of aerobic and anaerobic systems during the preseason with transitioning to sport-specific HIIT sessions during the competitive season. In addition, HIIT sessions in conjunction with other training sessions (i.e., team practices) may result in greater stress and risk for injury as a result of overtraining. Therefore, careful consideration is warranted in determining the appropriate number of HIIT sessions when concurrent with other sport-related activities.
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