Research by Seiler1 shows that intervals of 4x8 minutes at 90% of Max HR gives better results than 2x16m or 8x4m. However shorter duration intervals at a higher intensity, as short as 15 seconds, may be even better but more research is needed. These kinds of efforts are too short to measure using heart rate.
Seiler and Sandbakk found that accumulating approximately 30 to 45 minutes at around 90% of HRmax twice per week was a more effective HIIT prescription than accumulating 15-20 min at around 95% HR max . Stepto et al also found superior adaptations to a 4x8 min interval prescription compared to shorter duration protocols conducted with higher intensity.
Studies therefore suggest that higher work intensities are better but a slight intensity reduction in the HIT-range (say from 95 to 90% HRmax ) makes it possible to increase the duration of the effort giving better overall adaptive responses in recreational to well-trained athletes.
Hudson suggests a balanced approach where quantity and quality are given equal weight. He proposes one HIIT interval session and one threshold session per week. This means you must be relatively rested for these sessions. However because quantity is also important athletes should exercise on the other days insofar as it doesn’t get in the way of the quality workouts.
1.) Seiler, Stephen. (2012). Training Intensity Distribution.
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