High-intensity interval training (HIIT)

High-intensity interval training (HIIT) alternates exercise at or above about 90% of VO2 max with rest periods.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 453.} Continuous effort above 90% of VO2 max lasts only a few minutes; recovery between intervals lets a client accumulate more time at that intensity.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 455.} The NSCA cites several minutes above 90% of VO2 max per session as necessary for an optimal stimulus.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 584.} Adaptations include a higher lactate threshold, more mitochondria and greater capacity to buffer metabolic by-products.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 455.}

In practice

• Work bouts can last 5 seconds to 8 minutes, with whole sessions of about 5 to 30 minutes and work-to-rest ratios from 1:1 to 1:8.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 455.} Lengthen rest as the client tires.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 455.}
• For long intervals, use 2 to 3 minutes or more at 90% of VO2 max or above, with 2 minutes or less of recovery.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 584.}
• Build an aerobic base first: the client should be able to hold the steady-state target intensity for about as long as the planned interval session.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 455.}

For clients doing concurrent training, HIIT can produce metabolic adaptations similar to longer, easier cardio in less time and may interfere less with strength work.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 58.} High-impact or eccentric-heavy modes carry more injury risk; Helms and colleagues note that sprinters suffer more than twice the hamstring injuries of distance runners.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 58.} They cap HIIT at one or two sessions of 30 minutes or less a week for lifters.^{5Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 59.}