This study was done in trained cyclists (who looked to be quite fit already with good VO2 max scores and an experience with HIIT workouts already, going into this study all cyclists apparently already did one HIIT workout a week.
http://www.researchgate.net/publication ... d_approach
The cyclists were split into two groups, and both groups were set to do longer/endurance sessions and then two HIIT sessions a week with one group doing long intervals (4 x 5mins with 2.5 mins rest, with the rest being active rest at 50% wattage of what they were producing during the workout), the second did short intervals of 30 seconds with 15 seconds rest (again, "rest" being active rest), in 3 sets of 13 intervals with 3 mins active rest between sets. The way they ran the workout it seems like they were using the average wattage of each set (so work time and rest time) to see improvement over the weeks. They did this training for 10 weeks and in total the Short interval group average around 9 hours of exercise (including the Aerobic and Anaerobic work) and the Long interval around 11 hours
Interestingly,despite the short workouts and actually having done less overall work, at the end of the 10 weeks the short interval group had a greater improvement to their VO2 max (8.7% over 2.6%), a greater increase in their watts at 4mmol lactate (12% vs 5%) and also showed more improvement in the 30 second, 5min and 40 min tests than the long interval group. The study suggests that the reason the shorter intervals had such an improvement (and why it wasn't seen in other similar studies) could be down to:
- significantly shorter rest periods for the 30 second sprints (keeping your HR and lactate levels higher for longer with little time for recovery)
- Constantly stopping and starting the pieces allowing higher wattages to be reached, encouraging your muscles to develop more than the relatively lower wattages for longer pieces
- This study investigating the long term effects of the training (10 weeks), allowing the differences time to show through
- The fact the subjects were already quite fit, previously investigations on less fit athletes may have "fudged" previous studies results