HIIT Science

 All information is from https://hiitscience.com

HIIT Science will be a classic in exercise physiology.  It breaks down high intensity interval training into 6 different types of training, depending on energy system (aerobic, anaerobic, neuromuscular) to be trained:

"Envisage receiving a stressful hit on your neuromuscular/ musculoskeletal system – day in day out – session after session. As per Figure 1, these would all be HIIT Types #2, 4, 5, & 6. I’m sure you can imagine how this would likely result in a train wreck for most individuals. More often than not, we need substantial periods of recovery (often days) between such sessions to allow the neuromuscular/ musculoskeletal system to adapt appropriately (think how long it takes for muscle soreness to go away)."  https://hiitscience.com/why-the-hiit-types/

Each type is built around specific intensities and interval times:

"Traditionally, HIIT is performed by running or cycling....When such equipment is unavailable, HIIT can even be performed skipping using a jump rope or by completing stationary movements (e.g., squats, frontal or lateral lunges, push-ups, jumps) during the interval period (Table 1). These latter HIIT variations require more familiarisation and are likely more demanding on the neuromuscular system (6), but they can also be effective through their ability to target both cardiovascular and neuromuscular systems simultaneously (5)."  https://hiitscience.com/high-intensity-interval-training-the-key-strategy-to-maintain-fitness-during-periods-of-reduced-activity/

They argue that passive recovery is more beneficial than active, since it allows for more work during each interval.  https://hiitscience.com/recovery-bout-intensity-of-hiit-active-or-passive/

Bottom line:

- Shorter rests increase HR and leads to more cardio conditioning

- Longer sets increase burn and lead to more anaerobic conditioning

- Higher intensity/weights increase neuromuscular stress and lead to more muscle growth

- Focus on all three increases stress and leads to exhaustion, longer time between workouts, and loss of fitness