Journal of Physiology, “Short term sprint interval versus traditional endurance training: Similar initial adaptations in human skeletal muscle and exercise performance.” Sept 2006, Vol 575 Issue 3.
Study specifics- comparison of 20 minutes of interval training (30 sec sprint/ 4 minute rest) with 90-120 minutes in the “heart rate zone.”
Results- same improvement in oxygen utilization. One hour per week vs. 4.5-hours per week.
http://jp.physoc.org/cgi/content/full/575/3/901