it depends on the individual.....

Sometimes we expect scientific research to give us clear answers and prescriptions. Unfortunately science is not always that helpful. Take this research - the conclusion is basically that ..... "it all depends on the individual!"

Still, note that there are again clear benefits from high intensity interval exercises. I'll try to get hold of the full paper and see what else it says.

Nonconsecutive- versus Consecutive-Day High-Intensity Interval Training in Cyclists.
APPLIED SCIENCES

Medicine & Science in Sports & Exercise. 39(9):1666-1671, September 2007.
GROSS, MICAH; SWENSEN, THOMAS; KING, DEBORAH

Abstract:
Purpose: We compared the effects of a high-intensity interval training (HIT) program completed on three consecutive or nonconsecutive days per week for 3 wk on V[spacing dot above]O2peak, peak aerobic power output (PPOa), and 5-km time trial (TT5k) performance in trained cyclists.

Methods: Fifteen trained cyclists completed a TT5k and an incremental test to exhaustion for V[spacing dot above]O2peak and PPOa determination before and after training. Pretraining TT5k times were used to form groups, one of which (N = 9) performed three HIT sessions per week on consecutive days (CD), while the other (N = 6) did so on nonconsecutive days (NCD). Each interval session consisted of up to eight 2.5-min intervals at 100% of PPOa, separated by 4 min of active recovery. Pre- and posttraining TT5k performance, V[spacing dot above]O2peak, and PPOa were compared using 2 x 2 (group x time) ANOVA with repeated measures on time.

Results: HIT significantly improved V[spacing dot above]O2peak, PPOa, and TT5k performance in both groups across time (P < 0.05); there were no differences between groups. In both groups combined, V[spacing dot above]O2peak and PPOa increased by 0.2 +/- 0.2 L[middle dot]min-1 (5.7%) and 23 +/- 15 W (7.2%), respectively, and TT5k velocity and power output increased by 0.9 +/- 0.8 km[middle dot]h-1 (2.6%) and 17 +/- 19 W (6.9%), respectively. Despite comparable group changes, the individual response varied widely.

Conclusion: CD and NCD similarly improved TT5k performance, V[spacing dot above]O2peak, and PPOa, but the individual response varied widely in each group. Thus, athletes should experiment with both designs to discern which one optimizes their training.

(C)2007The American College of Sports Medicine