Nothing does as good job of proving adaptation to stress, than a power meter. To wit, a week ago I did a set of one leg intervals. They were pitiful- 119 watts average, per leg. (Call this my untrained benchmark) Four days later, I was able to do two sets, same gear (39x19), separated by a 5 minute recovery (1 minute between each leg) Average watts were high 130s in the 1st set, low 130s for the 2nd set. Speeds and distance also way up compared to the workout. My next session of one leg intervals will be in the 39x17, and go 2 full minutes. The data of the 1st 90 seconds will be fun to watch. These intervals will increase in duration by 30 seconds every couple of weeks, until I reach 3:00 minutes. They are indeed evil as Brendan correctly pointed out. Those last 10 seconds really turn you inside out. Another interesting observation: leg is stronger than the right.