In between big workouts, I'm back on the treadmill doing shorter intervals. Very short in fact! Today's sprint workout was a warmup followed by 15 sets of 1 minute sprints. After each sprint I walked for 2 minutes to get my heart rate back down. While my effort level was definitely Zone 4, it takes a while for the heart to ramp up to a really fast beat, so my data shows that I would just touch Zone 4 by the end of the sprint. I was running at 9.4 mph, which is about my fastest mile pace. (Told you I wasn't a fast runner). Next time I have to hold that pace for 2 minutes...bet my heart rate will get a bit higher in the zone then!