Stair stepper

My latest workout was 1:30 on the stair stepper with 4 sets of 15 minutes at 155 bpm. For the first set, I had the machine at levels 14 and 15. For the second set, level was almost sufficient to keep my HR up, though I occasionally had to pop it up to level 15 at the beginning of the set and drop it to level 13 near the end of the set. For the third and fourth sets, respectively, I kept the level around 14 and then around 13. Despite the varying levels, I hit the target 155 bpm on every set (averaging 156 bpm on the first three sets and 155 bpm for the fourth). Though my calves were pretty tired, it wasn't as difficult a workout as I had anticipated.